# Traps vs Equalizers vs Placement?



## hrpschrd

I have a 13' x 29' room with a sub in the front corner and bass traps in almost all corners (RealTraps). As you can see, I still have deep troughs around 73Hz and 146Hz (wide). I can't measure any bass build-up in the room with a meter so I can't see the point of getting more traps. That probably leaves getting an equalizer or moving the sub and measure again. I have learned that changing the sub cross-over and volume has no effect on the trough frequencies and intensities (no surprise).

Would moving the sub (or the listening position) be the next step before buying more stuff?


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## dyohn

Move the sub first.


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## hrpschrd

I should have pointed out that the sub cross-over is 32 Hz. This brings up the question of which trough is the fundamental. The 73 Hz matches the width of the room (13ft) and it is doubled at 146 Hz. 

It appears to me that the 73/146 set is stronger. Does that imply that the resonance is created by the mains rather than the sub?


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## bpape

That then appears to point to the seating position and/or the distance of the speakers from the wall beside them and behind them

Bryan


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## laser188139

hrpschrd said:


> I should have pointed out that the sub cross-over is 32 Hz. ...


Yes it certainly suggests that your issues are with your mains. You probably want to take separate measurements of each. It could be that the wide dip around 140Hz is a combination of different problems with each front speaker. 

In my room I was able to move my mains far enough from the wall that I had a dip a little below 70Hz, rather than a little above, and I then set the sub crossover to 80Hz so it covered up the hole in the mains. 

Good luck,
Bill


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## SierraMikeBravo

Greetings,

May I inquire as to why you have your sub crossed over at such a low frequency? There are very valid reasons why we use 80 Hz generally as a crossover, and the fact that the mains may be able to go that low is not one of them. :T 

Also, placing your subwoofer in the corner is perhaps the worst thing you can do. It excites all modes evenly which is an uphill battle. There are four elements to sound quality management, and I have yet to run into a room that did not need some form of all four: Speaker placement, seating placement, treatment and electronic correction (equalization). Good luck!


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## hrpschrd

Thanks Bill, I think you may have it. The speakers are planar and it may be that the one that is in front of the corner with the sub needs to be moved a little. I may need to move the sub out of that corner and replace it with a bass trap on the floor tri-corner. Off to experiment! This is fun stuff.


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## glaufman

I agree. But, at 73 Hz, your mains are still going to contribute energy even with an 80Hz crossover, so scan the two mains separately so we can start to determine where the problem is coming from, before we go about recommending solutions. Also, a sketch of the room would be nice.


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## hrpschrd

Sure enough, the two mains are quite different and the biggest problem is the Left which is in front of equipment and a corner. The Right is in front of a opening in the room. For this graph the sub is off. The speakers are Magnapan planars (1.6R) so they are radiating front and back. In the (poor) photo you can see the two black panels (speakers), the four thin white panels (high freq. traps) and four thicker white panels (bass traps - RealTraps). The sub is in the front right corner.


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## glaufman

Wow. That left speaker is nasty from 140-175 Hz. Any chance that speaker is 2ft from the front wall?


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## hrpschrd

No kidding!
Left Main is 43" from back, 38" from side. Right speaker the same.


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## glaufman

Well, this one might be a bit over my head, better left for Brian or Shawn, but I might try moving the trapping around to fill the gaps in that corner, and/or looking for a sdie/ceiling reflection that would cause a 3.4-3.9ft path difference...


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## laser188139

glaufman said:


> Wow. That left speaker is nasty from 140-175 Hz. Any chance that speaker is 2ft from the front wall?





hrpschrd said:


> No kidding!
> Left Main is 43" from back, 38" from side. Right speaker the same.


I'm thinking along the same lines you are, Greg. If we take those as perpendicular distances to the walls, then the distance from the speaker to the corner is 4.78'. So the predicted cancellation waves would be 59.1Hz and 177.3Hz. One could match the data in the graph if the actual distances to the driver were slightly higher, i.e., if the given distances are from the corners and not from the center of the speakers. Maybe the wide dip in the 140-165 range comes from the nature of the magnaplanars, if the position of the driver is effectively dispersed. 

Bill


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## bpape

Until you either kill that corner or get the rear wave of the speaker not to fire into the corner, you're going to have those anomolies. 

Also, remember that while doing individual channels is a good thing to identify issues, the only thing that matters really is both channels together - unless you listen to your speakers one at a time 

Bryan


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## hrpschrd

laser188139 said:


> I'm thinking along the same lines you are, Greg. If we take those as perpendicular distances to the walls, then the distance from the speaker to the corner is 4.78'. So the predicted cancellation waves would be 59.1Hz and 177.3Hz. One could match the data in the graph if the actual distances to the driver were slightly higher, i.e., if the given distances are from the corners and not from the center of the speakers. Maybe the wide dip in the 140-165 range comes from the nature of the magnaplanars, if the position of the driver is effectively dispersed.
> 
> Bill


How do you get 59.1 and 177.3? I get 205Hz from 4.78ft. I am sure you are right but how? And yes the distances are perpendicular. I have taken a better photo. The result of moving the square bass trap in the tri-corner to mid-way up the wall is zero. I would have expected a poorer result. Since I have both tri-corners trapped and only the middle not covered, should the troughs be that bad?

Also when I scan the room with an SPL meter using bass frequency pink noise I don't find any other source of excess bass except in that corner (+10dB). 

I guess my overall question is why if I have mostly "killed" the corner is the response so bad? Really unfortunate speaker placement?


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## laser188139

hrpschrd said:


> How do you get 59.1 and 177.3? I get 205Hz from 4.78ft. I am sure you are right but how? And yes the distances are perpendicular. I have taken a better photo. The result of moving the square bass trap in the tri-corner to mid-way up the wall is zero. I would have expected a poorer result. Since I have both tri-corners trapped and only the middle not covered, should the troughs be that bad?
> 
> Also when I scan the room with an SPL meter using bass frequency pink noise I don't find any other source of excess bass except in that corner (+10dB).
> 
> I guess my overall question is why if I have mostly "killed" the corner is the response so bad? Really unfortunate speaker placement?


Using the freq/distance calculator from RealTraps, it gives 4.78' as being 1/4 wavelength at 59.1Hz and 3/4 wavelength at 177.3Hz. So, if one were sitting straight out from the corner, one would have a 1/2 wave or 3/2 wave reflection at those frequencies. And being an odd half wavelength, the reflections would be exactly complementary. Of course, the analysis is a little trickier to calculate reflections off the other surfaces. 

Of course, this analysis is typical of "normal" speakers, with a driver facing one way, treating the driver as a point source and the wave into the corner starting in phase with the wave out from the speaker. Magnaplanars could be different, if they are driving the air in both directions, and the sound toward the corner starts out 180 degrees out of phase. I had not considered that. 

I'm sure Bryan can give more advice about traps and treatment. I've not yet gone there, but I have played a lot with placement. One can spend hours moving one speaker or the other, in one dimension or in the perpendicular dimension, to see how curves change. As Bryan suggests, one cannot forget to look at them together as well. If an anomaly appears only with the speakers together, you know it is an interference of one against the other, and is probably very location and frequency specific. But except for localized signals that would be in one channel and not the other, evenness in one speaker can cover a dip in the other. 

Bill


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## hrpschrd

Thanks Bill. I am a strong advocate of statistically designed experiments (which show interactions between variables) so I appreciate the two speaker interaction.

What I am hoping is that someone here with experience in trapping and placement will give me a direction to start in based on my spectra. Changing traps that are hung on the ceiling is not trivial (and a two person job) and moving speakers is pretty tedious if done correctly. 

Another question I have is whether bass traps are over-hyped. I assumed that putting traps in all the corners should have pretty well stifled that problem but look at that huge hole in my bass. All I get from RealTraps is "more traps will solve it." I can only see one space left and moving a trap in-and-out had no effect whatsoever. 

An alternative is equalization (thus the thread) but I hate to potentially add expensive distortion so it is my last resort besides a new room(!).


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## hrpschrd

I can't find a place in the room or hallway that registers more than 70dB* (70dB reference at the listening position) except for 73-74 dB in the Left corner where there is no mid-wall trap. So how is energy building up to create a large trough in my room response at 150-175Hz?

Now nothing makes sense. Help please.

* low-frequency pink noise CD (RealTraps)


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## laser188139

hrpschrd said:


> ... So how is energy building up to create a large trough in my room response at 150-175Hz? ...


You are obviously looking at the question totally different than I was. 

I was suggesting that you are seeing a reflection out of the corner 180 degrees out of phase with the sound from the speaker in that range, but lower in amplitude because of absorption and dispersion over the increased distance. So this complementary wave would subtract at that range over much of the area radiating out from that corner. As it is only over a limited range, I'm not sure whether that would show up on a broad SPL measure. 

When you are talking about energy buildup, you seem to be thinking of a modal frequency, that might have peaks in the corners and troughs somewhere else in the room. 

If you are dealing with a simple reflection, moving the left front speaker, particular in toward or away from the corner, should change the frequency of the trough. Changing your seating position should have little effect. 

If you are dealing with a modal issue, moving the speaker small distances in the same area might change the amplitude, but not the frequency. As well, changing your seating should have a significant effect on amplitude. 

The traps may be having a significant effect for you. I don't know. But I would expect to see that effect in the delays in waterfall charts. That is where you would expect to see the benefits of not letting modal energy "build up", i.e., persist. 

At least that's my understanding. But I am relatively new at this. 

Have fun experimenting,
Bill


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## hrpschrd

I obviously understand acoustics only enough to be dangerous. My understanding is that modes are set up at multiples of room dimensions and one can "trap" them where they reflect in corners. Cancellations can be eliminated by not letting them build up. That is why RealTraps' Ethan Winer offers the pink noise file - so you can find the worst corners and put traps there. This makes sense to me although it may be wrong. I didn't find any high intensities in any corners without traps.

So it seems that I might have a node at 73Hz (doubled at 146Hz) and at 80Hz (doubled at 160Hz). My room is about 13ft wide so these two nodes could be standing waves between the side walls. My solution was to put traps in all the available corners hoping the standing waves would be reduced. It obviously isn't working.

It would seem to me that this trapping idea should work even though I am throwing in- and out-of-phase waves from the front and back of my speakers, respectively. It doesn't matter what the phase is if it is absorbed effectively.

I suppose the next thing is to measure different listening positions and measure different speaker placements and see if I can find nodes at different frequencies. I really don't like having a broad 20db hole in my mid-bass!

(Incidentally, the high frequency traps work beautifully. There are just no echoes in the room from first reflections.)


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## glaufman

Well, part of the trouble may be the absorption coefficients of your traps at these frequencies. All commercially available broadband absorbers that I've seen (admittedly a small sample size, so forgive me if I'm wrong here) drop off significantly below 100Hz. I think corner placement extends that somewhat, where one of your tri traps (for argument sake) may have better coefficients in the tri-corner than it would on the wall. That being said, the main reason people treat corners and tri-corners for bass is it allows you to attack more of the significant modes simulataneously, but it's not quite to assume that because you've treated them that there won't be problems. 

As always with acoustics, my instinct is to listen to Bryan's advice, where he said kill the corner. My own strengths are logical thinking and and planning of experimentation. So, with that being said...

If you think that's not working as well as you hoped, then we need to figure out why. Possibilities
include:

1) Trapping ineffective at the frequencies in question
2) Problems not related to axial modes.
3) Tangential modes?
4) Oblique modes?
5) Side/Back/Ceiling reflections?
6) Speaker interactions?

Personally, I don't think it's 6 because you scanned each speaker separately. Bryan's of course right that what you care about isn't the indifidual scan as much as the combined scan, but we didn't like the combined scan, so we ran the separate scan, and found one speaker OK and making up for the lack of the other speaker. So we're attacking the other speaker. 

BTW, what's the height of the ceiling?

Bill- good job, somehow forgot about 3/4 wavelength and was focusing on the 1/4 wavelenght. And I thought the magnapans fired in phase, but I could be wrong.


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## hrpschrd

Thanks Greg. I am both interested in "fixing" what appears to be a large hole in my mid-bass and in better understanding acoustic "treatments". 

I may be wrong in assuming that I have fundamentals at 70-80Hz because normally the fundamental is stronger than the higher harmonic. The trough at 70-80Hz is only 8dB down whereas the mid-bass (140-155Hz) is down as much as 18dB. I suspect that if I "kill" the 70-80 trough, I may not kill the 140-155 trough. I would be pretty happy to just kill the mid-bass trough. I might get greedy though.

Here are the absorption graphs for my RealTraps MiniTraps. They seem to be at the right frequencies. If so, then I don't know how else I can "Kill the Corner". I could put one more 2x2 in the corner but that's all there is room for. If removing all the traps would be worse, why is this room such a nightmare?! (8 ft ceiling)

My larger question is why didn't the traps kill the troughs in the first place? Bryan? Anybody?


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## bpape

Well, unfortunately, not all bass problems can be solved with corner treatments only. There are lots of other places that can cause issues. 

Why didn't they kill the problems to start with? Could be it's not the whole cause. Could be what you used doesn't reach low enough to get the fundamental. Could be that there's just not enough surface area. And/or some combination thereof.

Bryan


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## hrpschrd

Shawn:

Why is it most people suggest putting a sub in the corner for greatest efficiency? Why is it bad to excite all room modes equally? I have moved my sub out of the corner but I would like to learn more about this. Any references?

I use a low cross-over because the bass of the Magnapans is so much cleaner than any cone subwoofer. I use the sub just to add a little low-frequency help. The Maggies go down pretty well to 45Hz without the sub.


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## hrpschrd

Bryan;

So what would go low enough? What other places that cause issues? Is this room just too small or badly proportioned? 18dB is pretty bad. No hope?


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## laser188139

hrpschrd said:


> ... If removing all the traps would be worse, why is this room such a nightmare?! ...
> 
> My larger question is why didn't the traps kill the troughs in the first place? ...


I think you are developing a lot of guesses, but are not yet doing enough experimenting to see what applies to your room. 

For example, you suggested that some of your dips could be room modes, as your room is small enough that the modes might be at these frequencies. So you have two hypotheses: the dips are a result of reflections from the corner, or the dips are a room mode where you seating position might be in a null. You could distinguish these two by moving a speaker in toward or away from the corner and see how the frequency response changes, then you would have some evidence for which is really happening. 

If you are using the REW software to generate your plots, it has waterfall charts that will confirm for you where the modal frequencies are in your room, as these should appear as having a longer duration. Of course, with the treatments you have already made, these may already be attenuated. And that would be a good thing. 

You expect that moving the traps would make things worse. You could try this by removing the trap at floor level behind the left front and measuring the change. This would confirm or deny that the trap is impacting the frequencies of interest. It could be that the trap is helping, but that unlike traditional speakers the Magnapan generates bass tones through its entire height. So the trap is preventing reflections from the bottom of the corner, but you are still seeing reflections from higher up in the corner. As Bryan suggested, you could be seeing reflections off other surfaces. After removing the trap, you could reintroduce it in different positions. If re-introducing the trap in another position also has a beneficial effect on the frequency response, then you are probably seeing reflections off that covered surface. 



hrpschrd said:


> ...
> I use a low cross-over because the bass of the Magnapans is so much cleaner than any cone subwoofer. I use the sub just to add a little low-frequency help. The Maggies go down pretty well to 45Hz without the sub.


It isn't a question of how low the Magnapans go in an anechoic environment. Looking at your charts, for example, the left front has a dip around 40-50Hz. If the crossover were set at 60Hz, you would see less of an effect from this, as the sub would provide more content in this range, and the contribution from the mains would be reduced. But as you say, you may have a tradeoff here between freq resp and other characteristics of the bass. 

By the way, I could not help but notice that your .pdf chart used a linear range for frequency, and not log. This could bias your judgment of which problem is the more important to address. I'm not saying it is, but you want to be careful. 

I think you will answer a lot of your questions if you start trying different things and see what changes. 

Have fun,
Bill


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## hrpschrd

Thanks for the catch Bill. I used Excel and forgot to take the log of the axis.

I am starting the measurements but they are very tedious because I seem to have an intermittent problem with REW. On the REW forum I have posted the problem and gotten some help but haven't solved it and as a result I can get only about 2 measurements per session and don't know when it will work again. I was hoping to understand the problem better and get feedback from the experts here to narrow down the experimental space.


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## glaufman

hrpschrd said:


> I may be wrong in assuming that I have fundamentals at 70-80Hz because normally the fundamental is stronger than the higher harmonic. The trough at 70-80Hz is only 8dB down whereas the mid-bass (140-155Hz) is down as much as 18dB. I suspect that if I "kill" the 70-80 trough, I may not kill the 140-155 trough. I would be pretty happy to just kill the mid-bass trough. I might get greedy though.


I think you're overanalyzing a little. You're thinking about this as a classic Fourier series, but that doesn't necessarily apply here.


> Here are the absorption graphs for my RealTraps MiniTraps. They seem to be at the right frequencies. If so, then I don't know how else I can "Kill the Corner". I could put one more 2x2 in the corner but that's all there is room for.


Well, notice how they're absorption rolls of as you get lower. I don't know if it would do any good, BUT you could switch to Mondos. But I wouldn't advise that until you determine that's the problem. 


> If removing all the traps would be worse, why is this room such a nightmare?! (8 ft ceiling)


That could be why. 1st mode at 8ft would be about 70Hz. Of course, going back to the corner, you also get better performance out of chunk style corner traps, but again, until it's determined that the corner's the problem...



> My larger question is why didn't the traps kill the troughs in the first place? Bryan? Anybody?


Have you scans without the traps? Without that, you don't know how much good/not the traps are doing...



hrpschrd said:


> Shawn:
> 
> Why is it most people suggest putting a sub in the corner for greatest efficiency? Why is it bad to excite all room modes equally? I have moved my sub out of the corner but I would like to learn more about this. Any references?


I'm not Shawn, nor am I a reference, but conceptually, the corner has tradeoffs. One is boundary gain. You can get up to 6dB of gain from putting the sub in the corner as the walls reflect the energy radiated at them back to the LP, effectively increasing the energy that gets to the LP. Unfortunately, those same reflections (or at least similar ones) produce that gain only at the frequencies where they're in phase with the directly radiated waves... at other frequencies the reflected waves cancel the direct ones. this gives rise to peaks and dips, so pulling the sub out of the corner gives better flatness. In another related area, near the walls are always peaks in the modal response, so putting the sub in a tri-corner excites ALL axial modes, whereas pulling it out can leave one or more less-excited, again flatenning the response. If you're sub is gain-challenged, you're room open, you're just crying for bass, no matter how sloppy it is, you may see benefit from putting the sub in the tricorner. BUT, it's my opinion that many times this is not the best advice.


> I use a low cross-over because the bass of the Magnapans is so much cleaner than any cone subwoofer. I use the sub just to add a little low-frequency help. The Maggies go down pretty well to 45Hz without the sub.


As mentioned by others, many times the higher xover is used not because the speakers are incapable of going lower, but because the best place for a bass radiator is different that the best place for the mid and treble radiators. I know what you're thinking. It bothers me too. My mains go down to 37Hz pretty well, and it pains me to think of not using them that way. But sometimes the truth hurts. Not necessarily true in your case. Even with an 80Hz xover, your mains still need to put out good energy at 70Hz where they have problems, so that's not a silver bullet for you...


hrpschrd said:


> I am starting the measurements but they are very tedious because I seem to have an intermittent problem with REW. On the REW forum I have posted the problem and gotten some help but haven't solved it and as a result I can get only about 2 measurements per session and don't know when it will work again. I was hoping to understand the problem better and get feedback from the experts here to narrow down the experimental space.


Well, keep in mind that once you save a scan, you can reload it and generate the waterfall and IR later.


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## hrpschrd

I don't know if the settings for this waterfall are correct. It looks like the worst ringing is at 60Hz which is a room length mode. What does this say about the bad null at 157Hz?


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## laser188139

hrpschrd said:


> I don't know if the settings for this waterfall are correct. It looks like the worst ringing is at 60Hz which is a room length mode. What does this say about the bad null at 157Hz?


You did not specify, but it appears this is the waterfall from the left front speaker. 

If you calculate that 60Hz is a room mode lengthwise, that says that your room is about 18ft long, right? 

Besides the 60Hz mode, the lump at 40Hz would correspond to the half wave across your 14ft room. 

If 60Hz corresponds to the full wavelength the length of the room, then you would expect to see a resonance at the 30Hz half wavelength, too. I expect you don't see that in this chart, as the front speaker probably does not put out enough energy that low, but that you would see it in a waterfall that included the sub. If it's not there, that would say your traps are absorbing 30Hz, which I doubt, or that your couch/measurement point is near the middle of the long distance, which could be a null in the standing wave if the sub is next to the front wall. 

Since you are not seeing anything at 157Hz, that is consistent with the the null there being a reflection from the corner, and not a modal effect. Of course, the initial level at time zero being relatively low there would also explain the tails dropping off below the displayed floor. As I mentioned above, you should be able to tell the difference now by moving the measurement and the speaker independently. If moving the speaker toward or away from the corner moves the frequency, then you are seeing reflections from the corner. If moving the microphone toward or away from the speaker changes the nature of the curve, then you are seeing some modal null at your measurement position. 

At least, that's what I saw on first glance this morning, and I haven't had any more ideas since then.

Bill


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## glaufman

hrpschrd said:


> I don't know if the settings for this waterfall are correct. It looks like the worst ringing is at 60Hz which is a room length mode. What does this say about the bad null at 157Hz?


Well, usually we ask for 45-105dB, but this isn't too far off from that. Truth be told, I don't think it looks too bad given the original FR plot. there's not much that's ringing out too long. Tough to say with certainty as there could be more ringing being cut off by the floor...



laser188139 said:


> If you calculate that 60Hz is a room mode lengthwise, that says that your room is about 18ft long, right?


Not necessarily... First mode at 9.4ft would be 60Hz. 

What the dispersion pattern on these look like? I'm still thinking there could be reflections off the rear or right wall that could provide just the cancellation we're looking at... I'd love to see a dimensioned sketch and an IR graph.


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## laser188139

glaufman said:


> ...
> 
> 
> laser188139 said:
> 
> 
> 
> ...
> If you calculate that 60Hz is a room mode lengthwise, that says that your room is about 18ft long, right? ...
> 
> 
> 
> Not necessarily... First mode at 9.4ft would be 60Hz.
> ...
Click to expand...

True, but that's why I asked the question. We have been given the width of the room as 14ft, and the guess that the 60Hz lump was a "room length mode". As most people use length to describe the longer dimension, my guess was that the length was 18ft. (And you're right, I should have rounded 18.8 up to 19.)


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## hrpschrd

This is a great discussion and just what I was hoping for besides helping me with my specific problem.

On the first page, I describe my room as 30ft long, 13ft wide, 8ft high (added later). Sorry the dimensions got scrambled somewhere. When I use the RealTraps modal calculator I don't see any frequencies that match up with my graphs.

I know the recommendation is 45-105dB but I can't seem to get the scaling right. I can use the +/- and the slider on both axes but that doesn't give me exact control. I have attached a new measurement waterfall for L main with a 2x2 trap in the floor tri-corner. There is a lot more low bass ringing. What's an IR graph? Sketch coming.

I would like to compare with/wo traps, speaker placement, etc., but am having trouble with intermittent REW.


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## bpape

Please post the graph with a full 60db decay so we can see what's static, what's modal, what's travelling, etc. 

To treat into the range below 60Hz, you simply need thickness or something narrower and tuned to that range.

Again, you can treat corners till you're blue in the face and it will help decay time, but, if the issues are height or width related at the seating position, corners won't do it. Might have to do directly overhead, directly to your sides, or more likely directly behind you.

Bryan


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## glaufman

hrpschrd said:


> I can use the +/- and the slider on both axes but that doesn't give me exact control. I have attached a new measurement waterfall for L main with a 2x2 trap in the floor tri-corner. There is a lot more low bass ringing. What's an IR graph? Sketch coming.
> 
> I would like to compare with/wo traps, speaker placement, etc., but am having trouble with intermittent REW.


In the upper right corner you should see an icon marked "Graph Limits" that should give you complete control. IR=Impulse Resposne, it's the "Impulse" tab.

Sorry to hear you're still having REW problems. I've been following your thread in the REW sub-forum, it's certainly a curious one at best. Unfortunately, I have no new ideas to help at the moment.


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## laser188139

hrpschrd said:


> ...
> On the first page, I describe my room as 30ft long, 13ft wide, 8ft high (added later). Sorry the dimensions got scrambled somewhere. When I use the RealTraps modal calculator I don't see any frequencies that match up with my graphs.
> ...


Sorry, I saw a 14' number later and missed the 29' length in the very first post. If I plug 29' into the freq/distance calculator, I see that it is a half wave multiple of 19.5Hz, 39.0Hz, 58.4Hz, 77.9Hz, 97.4Hz, ... To my eye, these do seem to match well with the longer decaying lumps in the waterfall graph. 

It is awkward that the room length is so close to being a multiple of the width. It makes it a little harder to infer what is happening from the measurements.


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## hrpschrd

If I understand correctly, having a "full 60dB decay" means adjusting the left slider to put the "floor" at about 45dB (105dB-45dB). I also increased the time range to 500ms.

Here are three waterfalls (impulses on a separate post) that represent, respectively, the Left main only:
- no 2x2 trap behind it (except in the ceiling tri-corner)
- a 2x2 trap in the floor tri-corner
- a 2x2 trap in the middle of the wall corner

My interpretation is not a huge difference but it seems the floor trap is a little better in some areas than the middle trap or no trap. Your analyses? 

By the way, is there such a thing as "a reasonably small amount of ringing" and how would you quantify that?


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## hrpschrd

Impulse Responses for previous post
- Left only no trap
- Left only floor trap
- Left only middle trap

I have no idea what you are looking for here.


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## bpape

What I was looking for is 60db from your highest point in response. What you posted will be fine though. At 120Hz, there's something going on in the room that's not speaker related. If I saw the same thing at 60Hz, I'd say it's a ground loop hum for example.

By color in your waterfalls, which is which? 

If it turns out the floor is best, then it's likely at least partially boundary related and a panel behind the speaker will also help.

Bryan


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## hrpschrd

And now that I am getting good measurements again, here are adjustments of traps, speakers, and listening positions.

- Moving the L main 6" forward or to either side did not improve the 150Hz null much
- Moving the LP back about 3" made a significant difference.
- The least null at 150Hz was with no trap

The graphs below are:
- L only with no floor or middle trap original LP
- L and R with no floor or middle trap new LP

Obviously I have a new null at 80Hz (from the R main) to work on now.


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## laser188139

hrpschrd said:


> And now that I am getting good measurements again, here are adjustments of traps, speakers, and listening positions.
> 
> - Moving the L main 6" forward or to either side did not improve the 150Hz null much
> - Moving the LP back about 3" made a significant difference.
> - The least null at 150Hz was with no trap
> 
> The graphs below are:
> - L only with no floor or middle trap original LP
> - L and R with no floor or middle trap new LP
> 
> Obviously I have a new null at 80Hz (from the R main) to work on now.


That the response moved that much from a change in listening position, but not in speaker position, is really cool. I was thinking this morning that I need to make the same experiment in my room, to move the microphone over a wider area and see how the response changes. But are you sure you are not seeing the effect of the change in position and not effect of combining both speakers? The right speaker did not have the deep dip the left showed, so adding it should cover much of that range. 



bpape said:


> What I was looking for is 60db from your highest point in response. What you posted will be fine though. At 120Hz, there's something going on in the room that's not speaker related. If I saw the same thing at 60Hz, I'd say it's a ground loop hum for example. ...
> Bryan


I was just noticing the same thing as Bryan. If your computer is a laptop, you might try running it off battery instead of plugged in. For some, this eliminates at 60Hz component and its 120Hz harmonic. The 120Hz one looks particularly suspicious. 

Bill


----------



## hrpschrd

Here are the waterfalls for the previous two graphs. I am still a little confused by the "floor". When I use the Graph Limits and set 45-105 it seems to set the "floor" under the peaks. Is that correct to leave it there?

If so, it seems that I have improved the null at 150Hz but really added ringing. Probably to be expected with two speakers now. So the next move is to leave both speakers in the mix and work on the null at 80Hz as well as the ringing? Did I hear that traps are good for removing ringing?


----------



## hrpschrd

That 60/120Hz ringing is in everything. I use the laptop on batteries in all of them. Where else do I look for a ground loop? All my equipment is plugged into a surge strip. Lights?


----------



## hrpschrd

Bryan; the graphs are in the order listed in the text above them.


----------



## bpape

Left to right first or top left, bottom left, top right?

Your range is fine. You can scroll up and down to move where that range falls within the db scale on the left.

Bryan


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## hrpschrd

bpape said:


> Left to right first or top left, bottom left, top right?
> 
> Bryan


Sorry, I don't understand. The first (top) graph is "Left main only with no trap behind it and at LP #1"
The second (bottom) graph is "Left and Right mains with no trap behind and at LP#2".

Could the hum be from the cabling?

laser: I should have posted the direct comparison between listening positions. I didn't want to flood the Forum with graphs. The first below is at LP #1 and the second is at LP#2 which is actually about 5 inches behind it (away from the speakers). There isn't much difference. The biggest difference was the speaker interaction (L&R).


----------



## bpape

The way they show for me is:

Top left is light blue
Top right is light purple
Bottom left is aqua.



hrpschrd said:


> Here are three waterfalls (impulses on a separate post) that represent, respectively, the Left main only:
> - no 2x2 trap behind it (except in the ceiling tri-corner)
> - a 2x2 trap in the floor tri-corner
> - a 2x2 trap in the middle of the wall corner


Based on your 2 graph posts, I'm assuming:

Top left is light blue - no trap except in ceiling tri corner
Top right is light purple - 2x2 trap in the floor tri-corner 
Bottom left is aqua - 2x2 trap in the middle of the wall corner

In small room acoustics, we really don't use RT60. It's just nice to see as some things show steady state which are usually outside room issues. Some go up and down which can be a variety of things but not normally axial modes, etc.

Adjust the positioning for the best response overall with the fewest bad nulls and we can get the decay times down later once everything is positioned.

Bryan


----------



## hrpschrd

Bryan;
Yes, you have the right assignments. I should be more clear with colors! It would be nice if you could overlay the frequency graphs in REW.

"In small room acoustics, we really don't use RT60. It's just nice to see as some things show steady state which are usually outside room issues. Some go up and down which can be a variety of things but not normally axial modes, etc."

I don't follow that paragraph at all. Sorry, just an amateur here. RT60?

At what point do you stop tinkering with frequency response? Everything +/- 3dB?
At what point do you stop tinkering with ringing? Everything < 300ms?


----------



## laser188139

bpape said:


> The way they show for me is:
> 
> Top left is light blue
> Top right is light purple
> Bottom left is aqua.
> ...
> 
> Bryan


I was seeing three images, one after the other. But after making my Firefox window really wide, it reproduces the picture Bryan describes, with the first two graphs side-by-side. 

Bill


----------



## bpape

Ah yes - another browser issue - gotta love it.

RT60 is the time it takes a sound to decay by 60 decibels after the tone stops playing. It's primarily used for large space acoustics but is useful to plan toward and extrapolate from in balancing a decay time curve across the spectrum. 

The target decay time range is higher in the low frequencies and lower as you go up. It also matters what you're using the room for (2 channel, HT, speech, etc.) 

If you can get anywhere close to +/-3 you're doing great. Real world residential rooms usually you're doing VERY well to get to +/- 5db with no EQ.

Target decay in that room will likely be (RT60) about .4 in the low end falling to the low .2x in the highs. You're down 45 db on average by about .33 ms. 

Bryan


----------



## glaufman

hrpschrd said:


> laser: I should have posted the direct comparison between listening positions. I didn't want to flood the Forum with graphs. The first below is at LP #1 and the second is at LP#2 which is actually about 5 inches behind it (away from the speakers). There isn't much difference.We The biggest difference was the speaker interaction (L&R).


Well, this isn't so surprising... we knew the R fills it in somewhat, but we started down this road to fix the dip you see in even the cimbined graph.


----------



## hrpschrd

bpape said:


> Target decay in that room will likely be (RT60) about .4 in the low end falling to the low .2x in the highs. You're down 45 db on average by about .33 ms.
> 
> Bryan


Thanks Bryan. 
I could use just a little more help with the last two sentences. Are you saying that my waterfall drops an average of 45dB by .33ms (typo for 0.33s)? If so, how does that compare to ".4 in the low end"? Is that the same as dropping 60dB in 0.4s?


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## bpape

Sorry. Right now, from the top of a peak, down to the low cutoff is approx 45 db. When you look at where (front to back on the right scale) it disappears into the 'floor' of the graph, it's at about 33ms or 0.33 seconds.

When I say 'high end', I'm talking about above 2kHz

Your entire graph is the 'low end'

Bryan


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## bpape

Deleted - duplicate


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## hrpschrd

And were you saying that "target decay (RT60)" means that a good value is 60dB drop in 0.4s?

Thanks


----------



## bpape

Yes. That's correct. That's why I asked that you scroll the scale so that the peaks are at the top of the window so we can see a full 60db of your plots vs just 60db of window.

Bryan


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## hrpschrd

So here is my waterfall on the best frequency scan conditions. Looks like a huge amount of ringing in the bass even though the sub is off. What is the order of most useful in removing ringing:
speaker placement, bass traps, listening position, sub cross-over, other?


----------



## bpape

If the sub is off, you're still running signal through your mains at full range assuming you turned the sub off at the processor. If not, then definitely check your setup because something is wrong in the xover if the mains are still full range.

MODAL ringing can be helped slightly by seating position changes. Other than that, it's purely room treatment.

Bryan


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## hrpschrd

I am primarily interested in music reproduction rather than theater so my sub (REL) is only connected to the same power amp outputs to the mains. The sub is turned off in the above graph and the mains are at full range (not limited by the processor).

"something is wrong in the xover if the mains are still full range"

When I turn the sub on could that improve the ringing? Wishful thinking.

"it's purely room treatment."

Two questions please:
1. What is the result of ringing? Muddy bass? Exagerated bass?
2. Since I have several bass traps already (maybe not the very best, but pretty good), and there aren't many more corners left for me to fill, am I just stuck with an unfortunate room?


----------



## bpape

Part of the problem is that both the mains and sub are reproducing below 80 Hz. The mains aren't likely in the best place for bass reproduction. The sub can be moved around to help with that. However, when they're both on, then there are interactions between them. You're not going to get good bass response both ways in most cases.

Excessive deay time in the bass can result in:

- Masked dialog clarity
- Lack of detail in music
- Muddy, boomy, indistinct bass
- Reduced bass etension
- "one note" bass

What are the treatments you have and where are they placed specifically? What size, thickness, material, and quantity? There's a lot more that just corners to deal with to get the bass right. They're just efficient places to start.

Bryan


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## hrpschrd

In the first photo on Page 1 you can see four bass traps in corners and four thinner HF traps (for reflections) - all from RealTraps (your competitor). In the second photo you can see that I have moved the sub out of the left front corner and moved a 2x2 trap to the bottom tri-corner. Now that I have moved the sub I might want to put in another 2x2 trap in the left corner. The bass traps are 4" thick so they are not the most efficient model but not bad.

"Part of the problem is that both the mains and sub are reproducing below 80 Hz. The mains aren't likely in the best place for bass reproduction. The sub can be moved around to help with that. However, when they're both on, then there are interactions between them. You're not going to get good bass response both ways in most cases." (Bryan)

I'm not sure what this paragraph means. If I am not going to get good bass response, is that because of the dimensions of the room? Should I move everything to the long walls? I don't show them but I have two more Maggies at the back of the room for surround (you can see the center channel).

I am measuring the addition of the sub today and will post what I learn. I am inclined to use a low cross-over (<50Hz) in preference to the tighter bass of the planars.


----------



## bpape

OK. So you have Mini-Traps in the corner(s)? If you want to stay with Real Traps, try switching to the Mondo in that corner. 2 high would be great actually so you get both the tri-corners.

Even with a low xover, both speakers and sub are going to reproduce frequencies outside that range due to xover slopes (don't know how steep yours are) that will interact with each other.

With a 32Hz xover, it's < 1.5 octaves to 70ish Hz. Standard 12db/octave slope on the sub would still only be down about 14 db in the 70's. No slope on the planars and they'd likely be down about the same by the time you get to the xover point. 

Bryan


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## hrpschrd

Here is the addition of the sub on the left wall. Both mains with the 2x2 trap in the middle not floor.
First impression is the strength of the interaction between sub and mains and room. 

The cross-over was changed in about 10Hz increments and the volume is dereased in some when it boosts the bass too much in the area between 80-100Hz. Nulls move and new ones appear. I believe the nulls get too bad above 100Hz when the cross-over is >50Hz. In order (32-6 means 32Hz, volume at 6):
1. 32-6
2. 41-6
3. 49-6
4. 60-4
5. 69-4
I did more and I am going to make up a DOE matrix to evaluate the variables of x-over, volume, and placement of the sub and mains later. The real trick is evaluating the results. How do you choose between a null at 80Hz vs a null at 60Hz vs more ringing at 45Hz? I don't really know at this point. I am clearly not a professional at this. I suspect Bryan would do it very differently.


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## bpape

When things move like that purely by changing xover point, that's exactly the type of interaction I'm speaking of. Some are worse than others as you get overlapping boundary effect cancellations based on the addition of sub position.

My setup procedure would be:

Set the speakers for the best overall response and imaging.

Add the sub with a xover of at least 24 db/octave slope no higher than 80Hz.

Move the sub around the room with the speakers off and find it's best response position.

Now turn them both on and see where the interference is. If it's in the range between 45 and 80Hz, adjust the xover accordingly. Also, make sure you set the phase control every time you move anything.

When you get to a semi-final decision point, pick the one with the fewest deep nulls and the one whose nulls are not below 50Hz where it's very very difficult to deal with via treatments. From what I see of those graphs, the light blue plot is by far the one I'd go with.

Bryan


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## hrpschrd

I should have included no sub:


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## bpape

Ah! So, the null at 80 is related most likely to speaker/boundary interaction. The lack of that same null in the light blue graph says that something in the overlap of the 2 whether position, phase, or both is at play. Nothing wrong with that at all. Many times, that's the easiest way to deal with a problem is to deliberately create one anomaly that's an inverse of one you're trying to deal with.

Your response really doesn't look that bad without the sub. Try moving the mic 6" forward or back with just the speakers on and see what happens. If that helps, then I'd suggest a lower xover point even than the 40Hz and use a moderate slope on the xover. For the most part, you're +/-5 down to about 35Hz with no sub.

Bryan

Bryan


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## Lonely Raven

I love when Bryan gets set loose on threads like this...I learn so much just reading his responses to other peoples questions. 

Thanks Bryan!


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## bpape

You're making me blush  Seriously, this is a very misunderstood and often ignored topic that's very important to address. The more people know and understand, the better off we all are.

Bryan


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## hrpschrd

I'm sure glad you feel that way, Bryan, because I'm treading water only barely here but this is a fascinating area not to mention practical. I have spent too little time and energy on room treatments but am trying to make up for it. First step is understanding, so I am going to ask stupid questions until you stop answering.

I have more measurements from your suggestions. Will post asap.


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## bpape

No problem. It's a good thing to do in open forum for others to follow. Just remember, there's always some trial and error involved. No room is perfectly rectangular, perfectly rigid, completely empty, etc. so all of the tools and formulas just point you in the right (or sometimes wrong) direction.

Bryan


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## glaufman

Lonely Raven said:


> I love when Bryan gets set loose on threads like this...I learn so much just reading his responses to other peoples questions.
> 
> Thanks Bryan!


Agreed. Sometimes I have to remind myself to just shut up and let him teach me!


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## hrpschrd

Here is some data. To see the room itself, go to the first page of the thread. The speakers and sub are drawn in.
Here is a sketch of the room, the calculated room modes, and the modes to the listening position.
Next I will post some recent experiments with adding the sub and changing the cross-over.


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## hrpschrd

I like your idea of finding a good spot for the sub before adding in the speakers. However, before I read that I tried a couple of runs by just adjusting volume at a given "roll-off frequency" (the REL manual does not call it cross-over). Apparently the frequency of nulls is strongly dependent on volume! I didn't expect that! I am thinking it is time for a multi-variate analysis rather than one variable at a time. Adjust the sub position, roll-off frequency, and volume in a matrix. The problem is what to measure? A mathematical sum of the null peaks?

black is 32Hz vol4, red is 32Hz vol5, blue is 32Hz vol6

PS: what is the difference between cross-over and roll-off?


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## hrpschrd

And here are the rest of those trials with the sub. Seems a lower roll-off reduces the severe nulls. (I still want a room curve that has a little stronger bass in the <80Hz region.)

purple = 28Hz vol6
green = 25Hz vol6
red = 25Hz vol 7


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## brucek

> And here are the rest of those trials with the sub


It's somewhat difficult for most people to get a feel for the graphs when you continue to post them with LIN (linear) axis. Would it be possible to use the LOG (logarithmic) axis. 

brucek


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## hrpschrd

Aack! I didn't notice. I am just saving the graphs from REW by clicking on the disk icon at the bottom left. I don't see a menu that gives me a choice of linear or log. What am I doing wrong and I will fix them all.


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## brucek

The FreqAxis button is in the top right corner of REW. It toggles between LOG and LIN.

brucek


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## hrpschrd

I knew that but I guess I've just been careless. I'll fix them, thanks.


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## JerryLove

This continues to be a very enlightening thread for me. Accoustic treatments is still something I'm very much a neophyte at. Thank you all for all of this wonderful information. 

Jerry


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## bpape

Something is very wrong. Changing the volume (gain) of the sub should do absolutely nothing in terms of shifting nulls if frequency response. Are you sure you're not adjusting phase instead?

Bryan


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## hrpschrd

Very sure. Shall I shift phase to see the difference?


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## glaufman

bpape said:


> Something is very wrong. Changing the volume (gain) of the sub should do absolutely nothing in terms of shifting nulls if frequency response. Are you sure you're not adjusting phase instead?
> 
> Bryan


And how! Mic position/orientation isn't moving? At all?


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## hrpschrd

Fair enough, I re-ran those measurements starting from scratch. Mic position might be slightly different from first three but not touched during these three. Nothing touched except volume control. Phase and frequency constant.
blue = 32Hz vol 4
purple = 32Hz vol 5
green = 32Hz vol 6


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## bpape

With just the sub running, it shouldn't matter - only when both speakers and sub are on. That's very strange indeed. Is this the volume on the sub or on your preamp/processor?

Bryan


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## glaufman

And it looks like the crossover is disabled again.


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## bpape

That's really bizarre. The only thing I can think of is that there is something resonating in the room that's only excited at higher SPL's. 

Bryan


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## glaufman

So, this is going to sound bizarre, but.... how is the microphone being held in place? Any chance that whatever that is is moving/resonating at higher SPL?
Or a dirty connection or control? Play a single tone and try wiggling all the connections, sweeping the dials... listen for something irregular...


----------



## hrpschrd

Yes, the volume control is on the sub not the processor. The processor volume is set to maintain Check Levels at 70dB. The numbers represent a range of 0-12 with 6 the middle point so I am not near the extremes of the range and there is no clipping.

The microphone (RS analog SPL) is on a tripod sitting on a sofa at the listening position. Since my tests found a small difference (if any) in listening position, I doubt that is the issue. Multiple measurements at the same settings gives identical traces.

I have checked the cabling for erratic effects and (after replacing one cable recently) it is stable. Even if there were some mechanical issue it wouldn't change the frequency dispersion.

I think this is real, whatever the reason.
(Equipment: Classe CAV500 amp, Classe SSP30 processor, Magnepan 1.6 speakers, REL Strata III sub)


----------



## hrpschrd

glaufman said:


> And it looks like the crossover is disabled again.


I have no idea what that means.

BTW: notice that the same peaks are in each spectrum just at different intensities. Seems like the room just sets up different resonances at different volumes.


----------



## laser188139

bpape said:


> With just the sub running, it shouldn't matter - only when both speakers and sub are on. That's very strange indeed. Is this the volume on the sub or on your preamp/processor?
> 
> Bryan


I think the graphs show the combined sub/main curve, not sub alone. 

My hypothesis is that the null comes from the sub contribution being out of phase with the mains at the point of the null. As the null is appearing above the crossover, the sub is not the primary contributor, but increasing its volume should increase its effect on the mains. 

I'm not suggesting here that the phase of the sub should be changed. In my limited experience, doing so just moves the frequency where the interference appears. The general advice to set the phase so the sub and the mains are in phase at the crossover frequency worked for me in yielding the best overall curve. 

Bill


----------



## hrpschrd

A question that is missing an answer:
Apparently (see waterfall) I have a "ground loop problem". What causes should I be looking for? Most equipment is plugged into a surge strip and is three wire.

Second question: why do I have ringing at 25 and 32Hz when none of my room nodes are there? (see room node graph)


----------



## srckkmack

For the ground loop problem you can temporarily use a cheater plug on one piece of equipment at a time and then rerun your sweep to see if you found the culprit.

-Steve


----------



## glaufman

hrpschrd said:


> A question that is missing an answer:
> Apparently (see waterfall) I have a "ground loop problem". What causes should I be looking for? Most equipment is plugged into a surge strip and is three wire.


While it's a good thing to look into, the noise youhave a 60/120Hz isn't necessarily ground loop induced... but it's reasonable to investigate whether it is first...


> Second question: why do I have ringing at 25 and 32Hz when none of my room nodes are there? (see room node graph)


Well, the calculator DOES show one mode between 20-30Hz, even if it's not the exact frequency your seeing... also keep in mind that the calculator is only so good as dimensions you enter into it, and it makes certain assumptions regarding shape being a perfect rectangle (it doesn't include your "opennings" in the front right by the stairway...)...
Then there's always, I agree it looks like a mode, but it still may not be... use the "find peaks' function in REW... it specifically looks for peaks that have the characteristics of room modes... if it doesn't pick that one out, maybe it's not... Lastly, I'm not sure about Ethans calculator... does it do axial modes only? Or tnagential and oblique modes as well?



srckkmack said:


> For the ground loop problem you can temporarily use a cheater plug on one piece of equipment at a time and then rerun your sweep to see if you found the culprit.


Key word here is "temporary"... using a cheater plug presents a potentially serious safety concern. Acceptbale (using caution) for a 5 minutes test. Very bad long term.


----------



## laser188139

hrpschrd said:


> A question that is missing an answer:
> Apparently (see waterfall) I have a "ground loop problem". What causes should I be looking for? Most equipment is plugged into a surge strip and is three wire. ...


In the process of elimination, keep in mind that it could be external to the sound system. It could be from the computer/sound card driving the measurement. It could be from the heater/AC system. 



hrpschrd said:


> ...
> Second question: why do I have ringing at 25 and 32Hz when none of my room nodes are there? (see room node graph)


At 32Hz, you are looking for a distance of about 18'. Your pictures of the front of the room left open the possibility of a partition in the back dividing the room into sections. The drawing you made suggests that the apparent stairs to the right of the picture are wider than one might expect. So perhaps this is a resonance across the listening area into the stair cavity. 

As Greg suggested, openings make calculations difficult.


----------



## brucek

> Apparently (see waterfall) I have a "ground loop problem".


Or it can be a low level hum from a power supply - even the PC itself can cause it (if it shows at 60Hz).

In fact, always be aware that response to the swept stimulus doesn't just suddenly appear out of nowhere and charge out of the waterfall like a train tunnel. When you see that sort of situation in a waterfall, you know it's an external signal and not part of how your room responds to the swept signal.

Take a look at your waterfall below. The green circle show where a signal suddenly appears after many slices of the waterfall at that frequency dropping quite well behaved - then suddenly a tunnel of signal shoots out of the waterfall. You can safely assume that's not a room mode or the response to the swept signal stimulus.

On the other hand, look at the signal decay at ~26Hz. See how closely the spacing is each slice - that's a very slow decay and is probably a room mode. Look beside that red line at ~40Hz. See how wide the spacing is, or at ~80 and ~90Hz you have wide spacing even though the signal started out at a peak - it decays very nicely and quickly. Not so at 26Hz.











It's also wise to take a no signal spectrum of your room before measuring and look for noises such as furnaces, cars, 60 cycle hum, etc, etc. Set up the Spectrum Analyser levels as if you were about to take a reading of THD, but then don't use the stimulus, simply look at the noise floor. 

Below is a noise floor showing my furnace, 60 cycle hum with its associated harmonics from my PC, and even the 15.750Hz horizontal oscillator of my old RPTV that was 3 rooms away.











brucek


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## hrpschrd

Wow, great information guys!

I am going to assume that the 60Hz/120Hz is likely noise floor from the environment. It is also not very strong and since I'm not aware of it when music is not playing, I will ignore it. It may be something I am used to hearing and have filtered out (like furnace drone).

I am also going to assume the 26Hz is a room mode from some irregularity or open space. Since it is so low, it will only affect organ music. And who minds having your belly jiggled by Bach.

Can I also assume that the rest of the spectrum doesn't have any peaks, nulls, or ringing that will badly distort the instrumental relationships? Maybe I am done!


----------



## brucek

Yeah, there's not a lot wrong with that waterfall or your response.

brucek


----------



## hrpschrd

Disappointing in the sense that there is a lot left unanswered. I will keep reading this forum to see how other's problems are solved. Really interesting subject, acoustics. I may still do the multi-variable experiment just for grins.
Thanks everyone.


----------



## glaufman

For the record, I agree with everything Brucek has said, but... I'm still confused/concerned about your wide dip from the left speaker at ~150Hz... even thought the right is covering it nicely, it's going to hurt imaging somewhat... I'd love to see you continue to experiment to find what's causing that and eliminate it...


----------



## bpape

One can only do so much to address a severe lack of symmetry. There have been some suggestions made to try to address it better. There area also other issues going on that are at this time undefined so it's hard to address how to resolve them.

Hang in there.

Bryan


----------



## hrpschrd

I am still curious about the volume affecting the null resonance frequency between 60-80 Hz. It's not obvious to me why that should happen but I have not heard why that should be inherently impossible either. It does add a level of difficulty in all acoustic measurements if it is real.

I'm not sure where the bass is being absorbed to create that null around 150Hz but that may be a complex answer considering the width of it! If it keeps raining here and golf is out, I may attack it. In theory, I should be able to play the bass pink noise CD and find a concentration of energy in some corner, shouldn't I?


----------



## hrpschrd

One more question. I may have a relatively uneventful frequency response but the bass does not have a rising "room curve". Because of that, the very low bass in music and movies is pretty dissapointing. 

To bring up my bass to about 8dB above "flat" at 20Hz, I can't just turn up the volume of the sub with a cross-over of 25Hz. I need to emphasize the bass at 40-140Hz too. I may have to start over.


----------



## bpape

8db at 20Hz is a ton. IMO, more of what you're missing is 40Hz and down. Very little at 25Hz realistically.

IMO, the volume related dip has to be something in the room physically resonating more as volume is louder - which is in turn, causing a cancellation at that frequency range which doesn't occur at lower volumes which don't cause the item to resonate.

Bryan


----------



## hrpschrd

Some general help with my next experiments please.

I have been assuming that I should have a "house curve" along the lines published here by Wayne. That is where I got the 8dB at 20Hz. There just isn't any music down there, I know. But I still want the boost in bass that Wayne convincingly recommends. I do listen to pipe organ music and watch movies with low bass. (I DONT want to over-do low bass for movies that will spoil the music however.) My question is:

What sub-bass frequencies should I experiment with, using the sub cross-over, that would best produce a "house curve" boost?


----------



## bpape

Organ stops pretty much at 32Hz. If you're not cutting off the mains, I wouldn't go any higher than maybe 40-45

Bryan


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## hrpschrd

So I should stick to 25-45Hz cross-over range?

Where is the low bass of a piano? A bass fiddle? The sound effects in movies?

When you have the cross-over fixed, does an increased volume on a sub broaden the frequency range of the sub, or is the curve shape constant?


----------



## glaufman

hrpschrd said:


> I am still curious about the volume affecting the null resonance frequency between 60-80 Hz. It's not obvious to me why that should happen but I have not heard why that should be inherently impossible either. It does add a level of difficulty in all acoustic measurements if it is real.
> 
> I'm not sure where the bass is being absorbed to create that null around 150Hz but that may be a complex answer considering the width of it! If it keeps raining here and golf is out, I may attack it. In theory, I should be able to play the bass pink noise CD and find a concentration of energy in some corner, shouldn't I?





hrpschrd said:


> So I should stick to 25-45Hz cross-over range?
> 
> Where is the low bass of a piano? A bass fiddle? The sound effects in movies?
> 
> When you have the cross-over fixed, does an increased volume on a sub broaden the frequency range of the sub, or is the curve shape constant?


I don't vouche for the accuracy of this, but...







As for your volume-related-frequency, I have a hunch that when you find the source of the dip, you'll know why it changes frequency. WRET 150Hz, keep in mind that the bass probably isn't being absorbed per se, just cancelled.


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## bpape

If you aren't crossing over the mains, then most likely yes, stay low. If you go higher, then you get a hump where you don't really want it as well as more potential cancellations. Here is a little graphic showing the range of various instruments. Movies can go down to 10Hz in some cases.










Bryan


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## hrpschrd

Here are some new measurements with both speakers and the sub on the left wall. Sorry the log scale on the bottom is not labeled- having trouble with Excel. 

(Is there any way to stack graphs in REW?)

As you projected, Bryan, as I move up the cross-over there are more cancellations in the mid-bass.


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## brucek

> (Is there any way to stack graphs in REW?)


Yes, use the Measured tab and select the traces you want to overlay at the bottom of the graph.

brucek


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## bpape

What happens if in the same position, you move the sub say 1' closer to the seating? Don't change anything else.

Bryan


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## hrpschrd

Thanks guys for those charts. Very interesting how many instruments go so low. Harpsichord? You can tell by my moniker I have one and I don't hear much energy there.

Bryan: I will try moving the sub 1' closer but I may have to make it also 1' closer to the front wall (sofa in the way. What are you expecting, and why?

Another general question: My room response is not terribly far from flat in the bass but my recordings now have very little low bass. The obvious explanation is that I had the sub too strong and high before but the recordings sounded more realistic (as compared with live instruments). Is it possible the difference is due to a room too small to reproduce the long wavelengths? If so, why does the microphone pick up substantial energy at 20-40Hz? Do the recordings (high quality classical) not have the bass in them? Would two subs be any different?


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## bpape

If the response is flat, then it's flat. It can't be flat and not have much low bass. You can have less than you like or less than you had before. This is why some people like a house curve. I personally couldn't handle 8db at 20Hz but to each their own. 

What I was trying to accomplish is to see if we could get by with a higher xover for you by introducing complementary cancellations to what you're getting now.

Bryan


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## hrpschrd

Bryan: Here is the best I could do.
The pink is the sub in it's position near the wall by the sofa (photo on thread page 1). The center point of the sub (8") is 10 inches from the wall and is the reference point.
The blue curve has the sub moved 2 ft towards the front wall (and speakers).
The green curve has the sub 2 ft to the front + 1 ft to the middle of the room (towards the LP).
The Xover = 25Hz.

So when you say flat, you mean flat to the microphone. Wayne's flat is when 30Hz and 100Hz are equal to his ears. It seems that your flat corresponds to the signal at the recording microphone, right?


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## bpape

OK. But, we were trying to see what happened with a higher xover point. Set it at 40 or so and repeat. 1' and 2' don't tell a lot - try prime fractions (1/7, 1/11, 1/5, 1/'13, etc.)

What I originally asked for was JUST to move it 1' forward so we only had 1 variable to deal with.

Bryan


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## hrpschrd

Here is the data on volume vs. null movements. Each graph compares an increased volume setting at the same xover frequency. Nothing else changed.
The graphs are 25hz, 27Hz, 28Hz, 30Hz, 32Hz, and 34Hz in order. In every case the shift of a null around 60Hz to lower frequency (say, 50Hz) is the higher volume. The rest of the trace is nearly identical with the expected exception of higher energy from 20-50Hz with higher volume.


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## hrpschrd

bpape said:


> OK. But, we were trying to see what happened with a higher xover point. Set it at 40 or so and repeat. 1' and 2' don't tell a lot - try prime fractions (1/7, 1/11, 1/5, 1/'13, etc.)
> 
> What I originally asked for was JUST to move it 1' forward so we only had 1 variable to deal with.
> 
> Bryan


You can see in the graph that moving the sub forward 2ft created a nasty null. What I didn't include was a move of the sub backwards which did nothing at all. I was unable to move the sub directly towards the LP as you suggested because of a sofa in the way. That is why I made two moves (two variables). The complex move is nearly identical to the single move forward. My conclusions (for small distances) that the movements towards the front wall made a new null but movements sideways and backwards made no difference.

Are the primes you mention fractions of the length (or width) of the room?


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## laser188139

It strikes me that what you are seeing in the latest graphs are the same effect as the earlier graphs where you changed the volume level. By moving the crossover higher, you are implicitly increasing the strength of the sub signal at frequencies above the crossover. Apparently, around the dip the sound from the sub is out-of-phase and subtracting from the mains. 

Have you tried recently inverting the phase at the sub and seeing what effect it has on the frequency response? I suspect you will see a dip somewhere else, but I could be wrong, and it could be less annoying than the dip you are trying to remedy. 

Good luck,
Bill


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## glaufman

I think these might be telling. I don't think your dip is moving at all. As you go up in SPL, the dip is getting deeper, and wider, and more complex. I'm looking at the last graph, the biggest dip on the blue trace is right at 60Hz. Going to the first graph, there indeed is a little dip at exactly 60Hz. I'm guessing that this 60Hz is simply not as sever at low SPL as the one just a little higher than it, but much more severe at higher SPL. Indeed, each and every big dip in the last graph has a corresponding smaller dip in the previous graphs. This could indicate that they're being caused by different phenomena. Such as one being a characteristic of the main speakers, with the other being a characteristic of cancelling between the speakers and their reflections (first or even multiple) which at SPL isn't so sever because the reflected sound is attenuated by the time it gets to the microphone (perhaps indicating it's bouncing around a lot before getting there...)

Or I could just be guessing randomly again.:huh:

Edit: OK. After thinking more, I would be hard pressed to say that at the frequencies in question there's enough distance involved to attenuate anything that much... but still, I see what I see WRT the dips. I don't think they're shifting in frequency, just in magnitude. Unless Bryan says I'm wrong, that's one step closer.


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## bpape

The prime fractions I mentioned are for all 3 dimensions.

Bryan


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## hrpschrd

I think this is what you were asking for:
graph 1 is 25Hz, vol6, 0 phase (green), 180 phase (blue)

graph 2 is 40Hz, vol3, 0 phase (blue), 180 phase (green) - the volume had to be reduced to avoid clipping.

Pretty interesting variable! This is such a fun exercise. Of course it makes perfect sense that phase should be so important in whether speakers/sub cancell each other. (I am quite sure that my two main speakers are in phase with each other.)
Of the choices, I think 25Hz - 0 phase is most flat.

By the way, what was your (or anybody's) answer about the effects of a second sub?


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## acoustat6

Harpsicord are you still out there? How come this thread died? It was just getting interesting. Any updates on you tests?:scratch:

Bob
PHP143


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## frd

hrpschrd 
you cant run away from your fundamental room modes :
1- they can make gain and ringing in bass region 
2- they can make sound cancellation in bass region 
3- you move chair ,sub , spk which only shifts the problem 

when we are talking about bass region below 100hz always bear in mind it cant be done by distributed mass acoustics easy ...(it has lots of energy)

the realtraps cant do any good in that region ...
if you are yet serious to solve it there is a chance 
you may need to waste few cash on it tho ... 
let me know about it


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## glaufman

For the record, moving things around can and does minimize the effects of modes ... depending on the construction of the room... the modes are still there, you can just change if/how they're excited.

That being said, you should know you're posting in a fairly old thread that likely has been forgotten by the individual you're addressing.


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