# Modal Ringing



## Skylinestar

*n00b Bass sweep*

I am having serious bass ringing as shown in REW waterfall. The setup is in my living room that's open to kitchen & dining area. The house has concrete-brick wall, with hard concrete-tiled floor & soft plaster ceiling. I know the house construction is bad for acoustic. 

What can I do to improve this?


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## robbo266317

I have only played with waterfalls a little bit. However that does not look like ringing, it looks more like background noise. :scratch:
What do you get if you run a sweep without the amp turned on? Do you still see the 23, 26 and 34 Hz bumps?


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## Skylinestar

That's not ringing? The bass takes along time to decay (as shown in the waterfall).

What do you mean "run a sweep without the amp turned on"? Switching my system off? There will be no sound.

Dear moderator, I'm not sure if this thread is suitable in here (all about acoustic), or it should be moved to REW section.


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## robbo266317

I will move it to the REW forum for you. 
To me the results at 50 Hz could be ringing but the stuff around 22 to 36 Hz likes unrelated to the signal as the signal drops off but you then get some signal rising at different frequencies. 
I live within several km of a coal loader and in our old house it clearly showed up in the waterfall results.


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## Wayne A. Pflughaupt

Skylinestar said:


> What can I do to improve this?


Bass traps are about the only thing that’ll improve ringing, but they will take up a lot of space in the room. And they’re expensive, unless you make your own. 

Is there anything audibly objectionable to the way your bass sounds, or is it just that the graph looks scary? If the bass does sound bad, you might simply try parametric EQ for the sub (assuming that’s what this is a graph of). That’s usually enough to satisfy most folks, and it’s a cheap fix.

Regards, 
Wayne


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## Skylinestar

Can bass trap help with frequency below 100 Hz? My local AV shop sells generic DST bass trap at reasonable price but I'm wondering about how deep they go. Or are they only good at helping with mids?

I do walk around my living room and I notice that areas close to walls and corners do have stronger, boomy and muddy bass. If these bass were to be absorbed by bass traps, frequency response at main listening position will be improve, am I correct? So, in a perfect listening room, even the room corner doesn't sound boomy?


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## Wayne A. Pflughaupt

It’s probably best to pose these questions at our Acoustics Forum. That said, it’s true that any seating close to boundaries will exhibit increased bass response, but that’s not necessarily a bad thing. You only have to be concerned with locations where you actually have seating. I can’t see any practically reason to aim for even response anywhere in the room, unless you like to wander around the place while you’re watching a movie?  

And not even every seating location, necessarily. Those that are extremely off-axis from the screen, they aren’t getting a good audio experience anyway. No reason to fret over getting fabulous bass response for a seat that’s so far off to the right (for instance) that it’s a mere 6 ft. away from the right speaker.

Regards, 
Wayne


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## Skylinestar

I'm not worried about other seating. I'm only concern about the sweet listening spot aka MLP.

I have zero experience with bass trap, therefore I'm asking the question regarding the effectiveness, whether it is suppose to tame the whole room, or just improving the response at MLP.

This the the bass trap that my local AV dealer is selling: DST Bass Trap. Do you think it is effective?


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## aackthpt

I can't open the page for that acoustical device, so no opinion. Perhaps post a picture if others have the same problem? A search for "DST bass trap" turns up the Auralex Lenrd. This is a decent device, but not nearly as large as necessary for the frequencies we are discussing here. You would need the MegaLENRD from the Auralex line to start doing what you are talking about. Alternately would have to build something like a Studiotips Superchunk (use the goggles).

Properly implemented LF damping devices will improve the sound in the entire room, not just at the MLP. At LF it is not really possible to improve the sound at the MLP without improving it everywhere in the room (this is possible at MF/HF).

Effective LF devices are LARGE (depending on one's sense of the term "large"), and many if not most devices often thought of as "bass traps" are not really good LF damping devices. Also, unless one takes care to limit their action to LF/MF, it is easy to create a device that improves LF but reduces sound quality at HF. It can be a challenge and a sacrifice (mostly in terms of room space) to build devices that are effective at 50 Hz, but it is possible. To build devices that do much below ~40Hz becomes highly technically challenging.

But I will second other posters and say that it isn't really clear from the waterfall you posted what may be necessary. It looks like possibly some modal activity is in evidence, but also maybe some background noise. Maybe other things going on too. The person asking about making a sweep with the sound off was serious - he is trying to get a measurement of background noise in your environment which will help tell us how much of the waterfall you posted is useful.


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## Wayne A. Pflughaupt

Adding to what aack said, the signal for generating a meaningful waterfall graph needs to be about 50 dB above the room’s noise floor. In the average residential setting that will be in the 30-35 dB range (assuming there are no background noise issues as discussed above). The average peak level in the graph you posted is only ~75 dB. So, you need to get your signal level up at least 10 dB, and lower the horizontal axis of your graph to 35 dB.

Regards, 
Wayne


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## Skylinestar

@Wayne
How do I create a graph with nominal 85dB? If I were to increase the volume of audio out (the green bar goes higher), the audio picked up with be too high, and REW will give me a warning regarding clipping.

@aackthpt
Here's a screenshot of the said product


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## crom0123

Hi Skylinestar,
The DST bass traps *are not going to help you at all* for taming the modes under 100Hz. 
(save your money, those are not *REAL* bass traps!)
In order to tame the modes under 100Hz you have 4 options:
1 BFD eq - price and space effective
2 two subwoofers or 4 subwoofers carefully placed-costly
3 Helmotz rezonator tuned on specific frequency-not easy to tune
4 REAL BASS TRAPPS- take off a lot of real estate/not practical and costly
Regards,
crom


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## aackthpt

Let's start from the beginning on the levels. Do you have an SPL meter, and have you used it to "calibrate" the level of the SPL meter in REW? This is what sets the displayed level within REW. The number displayed doesn't really matter, what matters is that your test signal is enough above the room ambient noise and that the input channel doesn't clip.

If you follow the REW instructions, you will set the signal to ~75 dB SPL with your external meter (I use 80 dB SPL, actually), then you set input gain so that the signal is between -12 and -30 dBFS. If you choose to use an 85dB SPL test signal, you just lower the input gain on your preamp so that the signal is still within that same dBFS range when you do "check levels" in REW.

If you have calibrated the SPL meter in REW, that value is directly linked to your input gain so it must be recalibrated if you change the input gain.

The picture you show is the regular size LENRD since it is 12" on a side. That is too small. To be much effective at LF you need devices that are 16" along the wall, preferably 24" along the wall (if triangles, but if you make them full squares rather than triangles they can be on the smaller side and still work well). The MegaLENRD (link) goes 24" along the wall. Some examples demonstrating these ideas are the Superchunk, the GIK tri-trap, and the GIK soffit trap.


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## aackthpt

crom0123 said:


> 1 BFD eq - price and space effective
> 2 two subwoofers or 4 subwoofers carefully placed-costly
> 3 Helmotz rezonator tuned on specific frequency-not easy to tune
> 4 REAL BASS TRAPPS- take off a lot of real estate/not practical and costly
> Regards,
> crom


On the other hand multiple DIY subwoofers are not all that costly (if you have to have one anyway). Also "real bass trapps" do not have to be that costly. You can construct some devices similar to the GIK soffit trap for fairly little using materials from the local home and fabric stores. But if you're not dedicated enough to DIY any of it then you should try equalization and if you are still not satisfied with the sound then call GIK who could outfit you with either type of acoustical device.


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## crom0123

aackthpt said:


> Let's start from the beginning on the levels. Do you have an SPL meter, and have you used it to "calibrate" the level of the SPL meter in REW? This is what sets the displayed level within REW. The number displayed doesn't really matter, what matters is that your test signal is enough above the room ambient noise and that the input channel doesn't clip.
> 
> Thanks for your advice. I have already did all that you advise me to do and it simply doesn't work. The problem is much more complicated...:dontknow::coocoo:


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## aackthpt

insert song here "I second that emoticon"


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## crom0123

aackthpt said:


> insert song here "I second that emoticon"


I meant "I'm going coo-coo" with this issue, no offense.

Regards,:wave:
crom


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## aackthpt

Show us your soundcard calibration curve, and run the sweep with the stereo off so we know where your room noise is. The problem isn't going to be solved without providing more data.

Is there an audibly observed problem or do you posit there is a problem based only on an REW result?


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## crom0123

I don't want to entirely hijack this thread.
Please see my own thread:
http://www.hometheatershack.com/forums/rew-forum/58851-radio-shack-spl-meter-broken.html
Thanks,
crom


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## Skylinestar

Here's the REW waterfall for my ambient noise.

, my fridge is loud!

All this while I've been running sweep with the fridge on.:duh:

Attached is a frequency response that shows pre and post Audyssey MultEQ XT of the Rythmik FV15HP.

...and finally the waterfall of the subwoofer sweep.


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## aackthpt

Sounds like you're getting the idea. At this point your frequency response looks pretty decent, and post-500ms your waterfall looks pretty much like your ambient noise. Do you have more questions / are you happy with the sound now?


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## Skylinestar

I thought my living room was dead quiet (all doors and windows closed) until I saw the REW graph. How is that possible (noise at 60dB?)

I do have more questions.

Attached here is the REW sweep showing 100Hz to 5 KHz.
It looks like comb filtering to me. Should I be worry about that? Does the REW graph look scary to you?
I've emailed GIK before and the experts told me that I have a very live room (shiny tiled floor, no carpet, with brick wall).


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## aackthpt

Skylinestar said:


> I thought my living room was dead quiet (all doors and windows closed) until I saw the REW graph. How is that possible (noise at 60dB?)


Simple: Fletcher-Munson curves. We aren't very sensitive to the lowest LF until 90-100 dB, until then we pretty much don't even sense it. And most people don't realize how much noise is in their environment if they have never stood in an anechoic chamber or somewhere else nearly noiseless. Even those who have are trained not to notice. What humans can acclimate to is remarkable.

This is precisely the reason that dedicated theatre rooms (and real theatres, and music studios) are constructed in such a way to keep the outside sound out--and why we need to measure rather than listen. Our ears, or perhaps I should say our brains, deceive...



> Attached here is the REW sweep showing 100Hz to 5 KHz.
> It looks like comb filtering to me. Should I be worry about that? Does the REW graph look scary to you?


Yes, it looks like comb filtering. No it does not look scary - quite typical in fact (even typical of a fully acoustically treated environment at higher frequencies). I wouldn't worry about it other than possibly the stuff below 400 hz, probably everything above there definitely not. Also, according to some theories (e.g. Toole) if you have wide-dispersion speakers such that the reflections causing these have the same FR as your direct signal then these are not deleterious in a home listening environment - and might even be beneficial. If you don't have such speakers or sit very close to boundaries then you might want to do something about them. Opinions on this vary in the acoustical community though.

The first thing to do in making a decision is to view the graph at different degrees of smoothing. Standards for professional AV control rooms use 1/3 octave smoothing, and this graph will look much better that way. Experienced acousticians view the data at a variety of smoothings though. For more information, there is much in this thread that is useful, but it goes pretty deep into the rabbit hole. You can cut to the chase and view pro standards for mixing and surround rooms in this post.

The proper way to view and quantify these effects, technically, is to do a full-range sweep and use time-domain tools such as the ETC. It all depends on how much you are willing to do, and how deep down the rabbit hole you are willing to go.


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## Skylinestar

Thanks for the information.

FYI, my main speakers are Klipsch Synergy B-3. As far as I know, horn design is narrow dispersion. I do have plain, reflective surfaces everywhere in my living room (shiny floor tiles, glass-top coffee table, plain wall).

Can the bass traps which I'd posted previously fix the peaks and dips <500Hz?


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## robbo266317

Skylinestar said:


> Here's the REW waterfall for my ambient noise.
> 
> , my fridge is loud!
> 
> All this while I've been running sweep with the fridge on.:duh:
> 
> .


That's what I was trying to see when I mentioned running a sweep of REW with no connection to the amp.
I ran an RTA sweep in my shed a while back and you can clearly see the 14.7 Hz rumble from the nearby coal-loader.

http://www.hometheatershack.com/gallery/index.php?n=1488


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## Skylinestar

I tried adding in my old Klipsch SW450 subwoofer...setup as dual subs, hoping for a smoother response. The position/location of FV15HP remained the same, at the front left of my room. The smaller SW450 is currently placed opposite, right, aft of my couch.
I'm happy with everything below 55Hz, but there are nulls at 70Hz and 99Hz.:doh:


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## aackthpt

Skylinestar said:


> As far as I know, horn design is narrow dispersion.
> 
> Can the bass traps which I'd posted previously fix the peaks and dips <500Hz?
> 
> I'm happy with everything below 55Hz, but there are nulls at 70Hz and 99Hz.


Horn designs are actually usually wider dispersion. Most of the post-Toole Harman stuff has a "waveguide" / horn tweeter, so you might be better off here than you think.

Your results with the two subs combined are strange. Fantastically better up to 55 Hz but above there everything goes gazoinkers. The first things you should experiment with are the "phase" (actually delay) of the subs and their placement. What crossover frequency are you using? If you are using 100 Hz then the dip at 99 Hz is probably caused by a phase reversal at the crossover frequency. Also, make sure there is only one crossover/LPF in the system--if your subs have them and you are using the sub-out from an AVR then turn the ones in the subs off or to the highest frequency they can be set to.

If that doesn't fix the issues, then to determine if you need LF dampers to fix these issues you should do two things:
(1) post a waterfall, because even if the frequency response is smooth you may still have modal ringing (FR can be smooth at a mode if you simply were fortunate with mic location for that test). Additionally, stored energy that rings on is a telltale sign of a mode. You should also take a few sweeps with the mic in different locations to see all the modes--especially tri-corners.
(2) dwell the signal generator of REW at the frequencies you believe are modes, turn on the SPL meter, and walk around the room with the mic--making sure to raise it high and low also. If the SPL varies significantly around the room, then yes you have a modal issue.

...and if you don't have a modal issue at a given frequency where you have an anomaly then you have a non-standing-wave reflection-combining issue which you need to attack in a different fashion.

One way to help figure this out is to use Bob Golds Room Mode Calculator. One portion of the output will have data like this:
Frequency Regions:
- No modal boost: 1hz to 31hz
- Room Modes dominate: 31hz to 123hz
- Diffraction and Diffusion dominate: 123hz to 492hz
- Specular reflections and ray accoustics prevail: 492hz to 20000hz
For that room, then, almost certainly anomalies above 492 hz are not modes, almost certainly anomalies below 123 Hz are modes, everything between is indeterminate and requires diagnostics.

Typically problems at 70 Hz are caused by the floor to ceiling mode of an 8 foot ceiling.


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## Skylinestar

I have 9-foot ceiling. Main L+R speakers are crossed at 120Hz.

I've tried to adjust the Delay adjustment knob on the FV15HP sub... manage to reduce the nulls at 70Hz & 99Hz by a little...but also added a dip at 120Hz.

Before = Blue
After = Green


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## Phillips

Skylinestar said:


> I have 9-foot ceiling. Main L+R speakers are crossed at 120Hz.
> 
> I've tried to adjust the Delay adjustment knob on the FV15HP sub... manage to reduce the nulls at 70Hz & 99Hz by a little...but also added a dip at 120Hz.
> 
> Before = Blue
> After = Green


Have you got a distance setting in your receiver.
I had a problem just with the main speakers, measured seperately, Eq, but when combined there were two very severe dips.
After playing around with the distance settings in the receiver those dips have been dealt with (biggest 3db). 
The sound is totally different.


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## aackthpt

Skylinestar said:


> I have 9-foot ceiling. Main L+R speakers are crossed at 120Hz.
> 
> I've tried to adjust the Delay adjustment knob on the FV15HP sub... manage to reduce the nulls at 70Hz & 99Hz by a little...but also added a dip at 120Hz.


9 foot ceiling first mode will be about 63 Hertz, so probably not the problem. Are there other areas of the connected spaces that are 7-8 feet, even wall-to-wall (I have seen modes caused by standing waves between a regular wall and a half-height wall in a friend's listening room).

Also, I suspect the delay adjustment needs to be in the OTHER sub. If it doesn't have one, get its delay right alone using the AVR adjustment, then adjust the delay on the one that does have it (alone, WITHOUT adjusting the AVR) to get it right, then see what the combination is. Do this first with the subs within ~3ft of each other. If we can get that right, and you post a waterfall so we know if there is energy storage somewhere, then we move on 

Your sweep needs to be one channel only-- I don't think you have mentioned what exactly your sweep is yet.

Why are you crossing at 120, do you have pathetically small mains?

One way to attack the problem would be to cross the SW450 using its internal crossover, since it helps below 55hz but hurts you above that.


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## Skylinestar

I've done some tweaking and finally manage to reduce the null. Regarding the 120 x-over, I've tried with 80Hz and 100Hz but both created dips at x-over region. 120Hz gives me the smoothest of all. The sweeps include both subs and mains...wanna make sure that the x-over region is good.

I've also read audioholics guide and decided to go back to single sub. The SW450, being the inferior sub, tends to make the bass slightly muddy at the expense of smoother frequency response. Having 2 similar subs will be easier to dial in too.

Is it worth the cash to spend on another FV15HP sub (dual FV15HP setup  )? Or shall I save the money and just buy an Antimode 8033?

Anyway, here's the REW waterfall for the single FV15HP sub.


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## hjones4841

Your waterfall will look "prettier" if you decrease the SPL to 75db or so...

And to answer an earlier question, yes bass traps make a lot of difference below 100Hz.


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## aackthpt

Skylinestar said:


> 120Hz gives me the smoothest of all.
> 
> Is it worth the cash to spend on another FV15HP sub (dual FV15HP setup  )? Or shall I save the money and just buy an Antimode 8033?


Smooth is OK but if you can localize the sub at 120 Hz cross then it's no good. If you can't, great, run with it.

Sweep should include ONE main and the sub, not both. Feed the left channel or the right, not both together. It won't make a difference at the sub frequency but above there it may make your results essentially useless.

Are you happy with the sound? There's no need to buy anything if you are. Also, I expect if Audyssey MultEQ XT didn't take care of problems then Antimode won't do a lot more, but as I've no experience with either product I could be wrong. Nonetheless, from looking at your waterfall your biggest acoustical problem is probably coupled spaces (ignoring the ambient noise issues we discussed earlier i.e. refrigerator).

There are differing opinions about multiple subwoofers in the acoustical community so it's difficult to say anything other than try it and see if you like it. More subs gives more headroom, no question. Welti and Devantier (working for Toole) say multiple symmetrically placed subs smooths bass around the room smooth the sound around the room, but that assumes a symmetrical room which I bet you don't have. Geddes likes to run speakers that go pretty low and three subwoofers at varying strengths, one with high placement even--and claims this smooths sound around the room best--but he also has a fairly complex procedure and equipment requirements (for the average person) for implementing his method.

Diagnosing anything further is going to be near impossible without much more work and information e.g. sweeps with the mic and sub in corners, room layout, mdat file containing sweeps of left channel and right channel. So let's start with... how do you like the sound?


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## Skylinestar

I've watched a bootleg DVD copy of WOTW (in Dolby Digital) at master volume of -10dB. I'm not amazed by the amount of LFE. Perhaps the DTS or Bluray version will sound a lot better?
(I remember seeing a graph where DD track has lower SPL while DTS has higher SPL).

I've also watched Megamind. The final building collapsing scene was nice. Loved it.
I also loved the LFE in The Dark Knight.

I have emailed SVS before. The recommendation was to have at least two PC12-Plus subwoofers for my big space (listening area about 3000 cu ft but fully opened to kitchen and dining area, totalled about 8000 cu ft).


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## aackthpt

I've heard DTS mixes tend to have LFE goosed a bit, but I've never seen evidence of it, so I dunno.

It sounds like you're generally happy with it, so why tweak further? Or if you're semi-unhappy with it then tweak the sub level up by a dB, try various source material, lather, rinse, repeat. Research shows that most people like a FR that rises toward LF and you haven't posted a full-range-enough sweep to know if you have that or not. In any case, acoustical testing tools are tools for diagnosing and improving acoustical issues, they don't tell you if a system sounds good. If you think it sounds good then put it away and enjoy! We get caught up with too much tweaking in the AV community and the result is often under-enjoyment, IMO.

That is a pretty large volume for one sub. I'm working on a theatre in a similar volume and planning to build two THT-LPs for it, as well as to run an existing small ported sub - and possibly the mains full range if I choose to try the full Geddes style.


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## aackthpt

Also, in case you're interested, http://www.data-bass.com/data?page=system&id=51
Quite a good performing subwoofer, I must say. As one would hope given the price, I suppose.


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## Skylinestar

When it comes to awesome bass experience, people have been talking about wall shaking, pants flapping, etc. The weird part is I'm not experiencing any of those.:scratch:


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## aackthpt

Skylinestar said:


> When it comes to awesome bass experience, people have been talking about wall shaking, pants flapping, etc. The weird part is I'm not experiencing any of those.:scratch:


Have you ever experienced those things at the theater?


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## Phillips

Skylinestar said:


> When it comes to awesome bass experience, people have been talking about wall shaking, pants flapping, etc. The weird part is I'm not experiencing any of those.:scratch:



Depends if it is in the sound track + how accurate is it?


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## Skylinestar

I think my area is too big...impossible to pressurize such a big space.:sad:

I have a few question regarding REW:
In the Check Levels part of REW guide/help, the screenshot and descriptions are generally based on WindowsXP. There's no WAVE volume control in Windows7 or Vista. Should I worry about that? Currently my volume (in Windows Vista) is at about 50.

I'd also uploaded the whole REW file (full sweep) here ... measured using my RS analog SPL meter with Line-in connection on my Dell Studio 15 laptop. The high frequency seems too much. I know that RS SPL meter is not accurate at higher frequency (>3KHz), but can I trust that graph or just take it as a pinch of salt?


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## aackthpt

Skylinestar said:


> I think my area is too big...impossible to pressurize such a big space.:sad:


Our point here was that you only get those kind of effects if the sound is inaccurate in some way. And even if you wanted them, you can't pressurize a room using a ported subwoofer. It may get low but it simply cannot make the to-DC response you can make with sealed or IB subs (or fan subs). Which as you have noted is likely moot in your situation anyway.



> Should I worry about that? Currently my volume (in Windows Vista) is at about 50.


It's fine as long as the output levels are reasonable and the dBFS was right on the input side.

I'll have a look at your file later...on android at the moment.


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## JohnM

Skylinestar said:


> I'd also uploaded the whole REW file (full sweep)


That measurement has very high distortion levels. The distortion could be a result of clipping somewhere (e.g. in the meter or the soundcard input) or something being overdriven. Your measurement notes mention "Dyn EQ", any dynamic (i.e. signal dependent) effect applied during a measurement would mean the measurement had limited value.


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## Skylinestar

May I know how do you identify the distortion? Any tips in analyzing the graph?
I do have another sweep run at lower level. I'd uploaded it here.
What do you mean by "measurement had limited value"?


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## JohnM

Why not just attach your mdat files to your posts? The latest sweep shows similar distortion levels.

If you are using V5.01 beta the distortion levels are shown on the distortion graph, distortion is also visible on the impulse response graph as scaled down versions of the main peak stretching back to its left, each copy is a specific distortion harmonic - easiest to see with the vertical scale set to dB FS.

If some form of dynamic EQ is applied during measurement you cannot distinguish between the effect of the EQ and other effects on the measurement such as the response of the room.


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## Skylinestar

Are you referring to this? (I'm a newbie in impulse graph:scratchhead
What are the reasons behind this distortion?


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## JohnM

Skylinestar said:


> Are you referring to this? (I'm a newbie in impulse graph:scratchhead


The peaks are further back, see below.










> What are the reasons behind this distortion?





JohnM said:


> The distortion could be a result of clipping somewhere (e.g. in the meter or the soundcard input) or something being overdriven.


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## Skylinestar

The X scale on the graph is time in second? Distortion before 0 second?:scratch:

How to check if it's the soundcard?

Attached are the files related to my laptop's IDT soundcard.


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## aackthpt

Skylinestar said:


> The X scale on the graph is time in second? Distortion before 0 second?:scratch:


LOL, I think we just convolved, or maybe convoluted, his brain... :bigsmile:

Here is a paper that discusses how the distortions fold back into the t<0 period. I'll have to read it in detail myself, but the pictures on the next to last page are enough to get the idea.

I actually didn't realize this about the folding back, nor that there is a distortion measurement for a sweep. So I'm learning from this discussion also--please do go on.

Edit: found another paper discussing errors in log-sine-sweep IR testing--here.


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## JohnM

Skylinestar said:


> How to check if it's the soundcard?
> 
> Attached are the files related to my laptop's IDT soundcard.


Soundcard measurement is fine. The most likely places for distortion to happen are in the speakers/subwoofer, especially if playing the test signal at a high level, or in the RS meter, especially if the sound level during the measurement exceeds the meter's range setting.


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## Skylinestar

Attached are some sweeps (20-200Hz only) run in the past. Are they showing serious distortion too?
(The peaks looks different from those in full range sweep).

By the way, since my laptop soundcard is up to the task, is there anything that can interface between a dedicated mic (eg EMM6) and my laptop line in minijack port? Thinking of something cheap just to provide the phantom power.

If there's none, is it worth to spend on a Tascam US-122 or M-Audio Fast Track (both with a new EMM6 mic) considering that I'm just at newbie level in acoustic science? Or an SPL meter is good enough for newbie's basic understanding?

How accurate is the RS SPL meter in high frequency (>2KHz range)? Is the deviation more than 5dB even with the cal file loaded in REW?


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## Wayne A. Pflughaupt

Something like the  Behringer Xenyx 502  will work with your sound card. But what’s your objective? If it’s only measuring your subwoofer, then the Radio Shack SLM is fine. The only reason to get a mic like the EMM6 is if you’re interested in full-range measurements, for “FYI” purposes. However, if you’re interested in full-range measurements for the purpose of performing manual equalization (which means you have outboard equalizers), then you need a calibrated mic. A generic calibration file won’t do for that.

Regards, 
Wayne


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## JohnM

Skylinestar said:


> Attached are some sweeps (20-200Hz only) run in the past. Are they showing serious distortion too?


Install V5.01 beta and look at the distortion graph



> How accurate is the RS SPL meter in high frequency (>2KHz range)?


It is not accurate by any normal use of the word



> Is the deviation more than 5dB even with the cal file loaded in REW?


Yes


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## Barleywater

Yes, your space is an echo chamber. Thanks for posting data directly. I looked at impulse response as %FS, as this is waveform display. Real impulse starts about 6ms before maximal positive peak that REW selects as zero time ref. for FV15HP solo, and 16ms for two subs together. This is typical behavior of situation for LF in hard space. Reflected energy much stronger at microphone than direct energy. For two subs time alignment is off significantly. FR may look smoother, but the mud is only getting thicker. Porting IR to Cool Edit Pro and examining further with indicates decimated IR preference was checked. I haven't investigated how well this is done in REW. 

Nonetheless I do all windowing with Blackman-Harris windowing and my results question the capability of your microphone/soundcard abilities at low frequency.

Is this primarily a music listening system? If for this purpose DRC well executed can produce fantasic results. DRC may work quite well with home theater as well but require ability to delay video for synchronizing lag introduced by correction.

What sweep parameters have you been using for measurements? To get a better handle on what you've got going I suggest a couple of settings: To get better idea of what your SPL meter's microphone is capable of do a near field measurement. Tightly stuff the ports of FV15HP and place microphone about 1 inch from center of driver. Run sweep 2Hz to 150 Hz of 1M samples (23.8s), single sweep. You appear to be using 256k (5.9s). In Preferences->Analysis tab my preference would be to uncheck all boxes and set all windows to Blackman-Harris 4. 2nd measurement with ports open. 3rd measurement with microphone positioned at listening spot.

I look forward to your results

Regards,

Andrew


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## Skylinestar

@ John
I have the RS SPL meter range at 70. My Denon 3312 master volume was at -5dB. Laptop audio out master volume was at 50 and input volume was at 3. The lower level REW sweep shown in post #43 did occasionally hit 80-85dB, although REW didn't give out any warning regarding clipping. I'm not sure if the needle on the meter was fully deflected to the max as I wasn't paying my attention to the meter but I believe it did since it was registering >80dB.

@ Andrew
My room is an echo chamber? I've played the "clap track" from the XLO Test/Burn-in CD but I don't hear any serious ringing/echo. I do agree that my area has lots of reflective surfaces with glass top table between the TV and my leather couch.
I believe the FV15HP is a renowned subwoofer and it's response is definitely flat on its own. The whole setup is primarily a home theater setup. 100% for movies.


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## aackthpt

On the other hand I'd say your room is not an echo chamber. If anything can be made out of your final sweep file (which is questionable due to the distortion), your decay time is reasonable and probably quite pleasant down to ~120 Hz though below that it goes above where you'd want it very quickly (or else you just have a of a lot of LF ambient, which seems likely given the earlier ambient sweep you posted). The frequency response, when viewed with 1/3 smoothing, is quite strange--it rises significantly below 200Hz and at 3k-9k above which it drops off precipitously. That looks pretty unusual to me. You do have a small bit of extra energy visible in the midrange around 1s, but it's pretty low level so nothing to really worry over. Looking at the impulse response you do have some clutter at ~0.7ms so maybe your speakers are sitting on a hard surface or maybe this is from a corner of the speaker. Other than that the time response actually looks decent for a home listening environment.

I presume by "Dyn EQ" in your sweep file you mean Audyssey Dynamic EQ. This is OK to listen with but don't run acoustical tests with it on.


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## Barleywater

From standpoint of low frequencies, even wall to wall carpeted room is echo chamber. I've seen performance data for FV15HP outdoors that is quite flat, but this is not typical listening room. I just saw another review test mentioning limiter function. Does your sub have this feature? Limiters and compressors show up in sweep measurements as incredible distortion. Limiters and compressors by very nature are not linear time invariant.

On the other hand, I run a sub as combination of 12" sealed on top of 12" set up as "U" frame each with 400watts available. Combo forms cardioid pattern. Big high power sweeps rattle windows, shelving, wood paneling, and my butt. 24% distortion. Impulse response looks much like yours. With DRC impulse response is beautiful, and sound is amazing. LF burst sequence sounds like locomotive throttling up under load. 

DRC approach leads to lag delays necessitating delay for video to get synch. I'm a music person, but understand that some theater processors have delay facility. 

A few choice PEQs may be helpful...

Regards,

Andrew


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## aackthpt

Barleywater said:


> From standpoint of low frequencies, even wall to wall carpeted room is echo chamber.


When you say "echo chamber" it implies all frequencies. Also, "echo" is not really an accurate description of small room LF problems. The intent of saying it is correct in that echoes only happen off hard surfaces (the same condition required for standing wave formation) but "echo" implies discrete reflections from surfaces which is not really a good description of how a small room behaves below the Schroeder frequency, or at the very least what causes the "overhang" phenomena. In other words, modal energy storage is not the same as a long reverberation time.

Unless he is running a test well above normal listening volume, I'd think a limiter would be doing little. But if it is possible to disable it while testing, it's a fine idea.


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## Barleywater

Tile on concrete floor, no carpet, brick walls, and plaster ceiling with energy bouncing around for over one second isn't an echo chamber....Just a highly reflective space that energy has a hard time getting out of.










The horizontal banding shows strong broadband reflections. Lack of vertical banding shows lack of standing waves and lack of modal behavior. Not that it isn't there: The sweep is too fast and too short for size of room to have these features build significantly. 20Hz to 200Hz in 5.9s is roughly 0.56 octaves/sec.

Andrew


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## aackthpt

Barleywater said:


> The horizontal banding shows strong broadband reflections. Lack of vertical banding shows lack of standing waves and lack of modal behavior.


I disagree with your definition of "broadband" if you base such an observation on a sweep that ends before 300 Hz. At all frequencies higher his decay times are reasonable. However, thank you for mentioning this interpretation of spectrogram viewing. I can see how it makes sense and had not heard of it before. Nonetheless his room is fine above 300 Hz (unless he is trying to create an RFZ or something).


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## JohnM

Barleywater said:


> The sweep is too fast and too short for size of room to have these features build significantly.


The duration of the sweep and the response of the room are entirely unrelated, the only requirement is not to end the signal capture before the decay time of the lowest frequencies to avoid truncation artefacts. Sweep length only affects signal to noise.


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## Barleywater

Broadband is used in context of percentage of bandwidth used in measurement. In this case frequencies 20Hz to 80Hz are coherently reflecting 500ms into measurement. That's a large proportion of the 20Hz to 200Hz measurement bandwidth. In this circumstance I have little trouble in describing this as broadband reflection.

I've only seen spectrum above 300Hz of post #22, of which analysis in post #23 I agree.

Andrew


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## Barleywater

Hello John,

Linkwitz has great stuff on duration of sweep and steady state transfer function:

http://www.linkwitzlab.com/AES'98/aes-98.htm

"

2.2 Steady-state transfer function

The steady-state response at the listening position for a radiator in a room may be measured with a sine wave sweep or calculated from the measured impulse response. The frequency sweep must be sufficiently slow for the room response to reach steady-state behavior. Each resonance in the response must be traversed much slower than the rise time for that resonance. "

Andrew


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## JohnM

You misunderstand his meaning. "Measured using a sweep" in that context means simply using the recorded amplitude of a sweep as the rooms frequency response, REW is calculating the impulse response.


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## Skylinestar

@ John, regarding your comment on post #55,
My main speakers are rested on dedicated speaker stand. I do notice the overall curve that shows lower mids. Perhaps it's due to wrong mic placement during Audyssey setup. I'll perform Audyssey again when I'm free. Even my local AV rep notice that just by hearing. He said there's something missing in the mids and asked me to re-do Audyssey. I doubt that it's speaker design. I belief no manufacturer will produce a speaker with such weird response.

@ Andrew, regarding your comment on post #56,
My FV15HP was run with the limiter on as a safety measure when watching movie.

--------------------------------------------------------------

Anyway, back to the issue with distortion. I have updated my REW to v5.01. Looking at the Distortion tab of the full range sweep. I can see THD, 2nd, 3rd, 4th, etc harmonic distortions in the graph. I'm a noobie in it. What do they represents? Bad thing I guess. The 3rd harmonic seems to be highest. How low should the distortion be?

When I increase the volume of my AVR (Denon 3312) to a high value (-5dB or higher), I can hear slight hissing. Is that distortion? How about room rumbling when bass hits? Could that be picked up by REW? Or my speakers are not efficient enough to be played back at this volume distortion-free?

I've attached my main speakers sweep which I've managed to dig out, measured at listening position (although it's just 20-200Hz sweep as I was concentrating on bass. No Audyssey applied). For your perusal. Distortion?


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## Barleywater

Really need some fresh data taken using beta version that has distortion tab. Posted mdat for your mains only showing 2nd Harmonic, and THD figures. Both show bad distortion:as in >4%, At reasonable volumes distortion should be significantly <1%. That you are getting bad distortion from both sub and mains strongly suggests meter/sound card issue. Per previous post, must measure FV15HP with limiter off. Sure, use it for movies. I'd still like to see some sweeps of it (and mains as well, based on current post) up close at low levels so we can get a sense of distortion being from speaker or meter. Without limiter start with low volume and repeat sweeps with increasing volume. You should be able to hear sound quality decay long before volume is loud enough to cause bottoming of driver.

Andrew


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## Skylinestar

Barleywater said:


> Really need some fresh data taken using beta version that has distortion tab.
> Andrew


You mean sweeps taken using REW v5.00 are not compatible with v5.01?


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## Barleywater

Concerning distortion plots, yes. Version 5.10 beta 9 has distortion plots. Distortion information is contained in extended tail left of impulse response peak when it is generated using exponential swept sine. In prior versions of REW this information was not used, and mostly trimmed away. 2nd harmonic info using shorter sweeps was not trimmed, this is what I could see in your previous post.

Andrew


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## Skylinestar

Barleywater said:


> Concerning distortion plots, yes. Version 5.10 beta 9 has distortion plots. Distortion information is contained in extended tail left of impulse response peak when it is generated using exponential swept sine. In prior versions of REW this information was not used, and mostly trimmed away. 2nd harmonic info using shorter sweeps was not trimmed, this is what I could see in your previous post.
> 
> Andrew


If this is true, why are the 3rd, 4th, 5th, 6th...distortions visible for a full range sweep, but not visible in 20-200Hz sweep?:scratch:


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## Barleywater

It's all in the details of how REW processes bandwidth limited sweeps. 

Have you done any measurements without limiters and dynamic EQ?

Andrew


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## Skylinestar

Barleywater said:


> It's all in the details of how REW processes bandwidth limited sweeps.
> 
> Have you done any measurements without limiters and dynamic EQ?
> 
> Andrew


Currently I only have the REW sweep file without Audyssey (and no DynEQ) but with limiter ON for the sub.
Sweep was done with FV15HP and B3.


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## Skylinestar

A month has passed. Finally found a free time to do the REW sweep. I have made fresh sweep using the latest REW V5.01 Beta 9 Build 2596. Attached here is the frequency response of my main L+R speakers. No subwoofer involved. No audyssey.

I need some comments regarding the distortion plot. Is it bad? I'm still totally newbie in this matter.


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## JohnM

Distortion plot looks fine.


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## Skylinestar

JohnM said:


> Distortion plot looks fine.


May I know how do you define "fine"? Any parameter to look out for?


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## JohnM

THD is around 1-2% across the band, for an in-room measurement that is nothing to be concerned about - particularly as you used a Radio Shack meter as the input, which adds distortion of its own, and measured both mains together. Note that the RS meter does not give reliable results above a couple of kHz.


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## Barleywater

Looking at THD, 50Hz peak region has values <2%, this is generally very good. Figures in 66Hz dip region increase peak at about 11%, which is quite expected as null is 17dB down from 50Hz peak. Without lots of driver surface area, non linearity is very hard to avoid. Many listeners regularly listen to THD of 25% in bass region, and think it sounds great.

Signal to Noise of measurement also looks fine.  Impulse peak is >60dB above apparent noise floor.

Regards,

Andrew

P. S. John outposted me!


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## Skylinestar

JohnM said:


> THD is around 1-2% across the band, for an in-room measurement that is nothing to be concerned about - particularly as you used a Radio Shack meter as the input, which adds distortion of its own, and measured both mains together. Note that the RS meter does not give reliable results above a couple of kHz.





Barleywater said:


> Looking at THD, 50Hz peak region has values <2%, this is generally very good. Figures in 66Hz dip region increase peak at about 11%, which is quite expected as null is 17dB down from 50Hz peak. Without lots of driver surface area, non linearity is very hard to avoid. Many listeners regularly listen to THD of 25% in bass region, and think it sounds great.
> 
> Signal to Noise of measurement also looks fine.  Impulse peak is >60dB above apparent noise floor.
> 
> Regards,
> 
> Andrew
> 
> P. S. John outposted me!


May I know where is the percentage of distortion shown? The distortion plot has dB-Hz scale. Or is it something to be calculated manually?


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## Barleywater

Controls for Distortion plots gives choice for display as Percent, and also the number of harmonics to display.

Regards,

Andrew


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## Skylinestar

Thanks. New knowledge for me. It's my first time looking at distortion figure.

Anytime when there's a null in the frequency response, the THD percentage goes higher? 

Can room acoustic affect the reading or it's purely amp-speaker-mic capability?


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## JohnM

Skylinestar said:


> Anytime when there's a null in the frequency response, the THD percentage goes higher?


Distortion is shown as a percentage of the fundamental level, when the fundamental is very low the distortion readings, which include the contributions of noise, become higher.



> Can room acoustic affect the reading or it's purely amp-speaker-mic capability?


The room is linear in its effect, so doesn't affect or cause distortion, but it alters levels differently at different frequencies so can alter the distortion figures REW shows that way - for example, rooms often have higher absorption at higher frequencies, so the levels of harmonics are reduced more by the room than the fundamental which makes distortion appear lower. The boosting/cutting effects of modal resonances can also skew results at the corresponding frequencies.


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## Skylinestar

Attached picture shows the waterfall for lower bass region.
I'm thinking of getting an Antimode 8033 or SVS AS-EQ1. Do you think the EQ device can do wonder in reducing the long bass decay time that I'm having here?


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## Wayne A. Pflughaupt

Hey sky,

Here’s what you can expect with an equalizer with regards to long low frequency decay times (aka “ringing”).

* An equalizer will reducing ringing in a room mode, but only to the point of (and not beyond) what the room is naturally exhibiting.
* If you desire a reduction in ringing beyond that point, absorption is required.
* An equalizer will not reduce ringing in areas of the frequency spectrum where there is no room mode. For that, absorption is required.

For more info, see this thread.

Regards, 
Wayne


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## Barleywater

Post #80: Is this of SVS sub? No limiter?

Please post mdat of measurement, I would like to take a closer look.

Regards,

Andrew


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## Skylinestar

Subwoofers are dual Rythmik FV15HP. 14Hz, Low damping, AVR/12 setting, Rumble filter off (1 port mode). Limiter on. That's the recommended setting to run Audyssey calibration.

Brian's (Rythmik designer) comment on limiter and REW:


> It is best leave limiter on during sweeps.


Attached here is the mdat file.
Sweep is done with subwoofers and mains together from 20-500 Hz...just wanna check out how's the total integration in low, mid and upper bass. Mains crossover is temporary set at 200Hz. I know it's kinda high but I have SBIR issue (at 100-120Hz) with my current mains. Therefore, I set the crossover higher so that much of the frequency gets to the subwoofer, and that's where Audyssey MultEQ XT can do much of it's magic later due to more filters available for subwoofer instead of mains. I've tried setting the crossover to 80Hz & 100Hz after Audyssey but the graphs just doesn't look as nice as at 200Hz.

If adding bass traps can fix my SBIR issue, I'll drop the crossover to 100Hz later. But that's still a long way to go.

For L+R mains sweep alone, refer back to Post #71

There's null at 85Hz and 107Hz. I've tried to vary the delay/phase adjustment on one of the subwoofers, which can only shift the nulls to other points but not eliminate the. Adjustment also creates bigger dip around 25-35Hz region. After much adjustment, that's the best phase setting I've got.


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## Skylinestar

Wayne A. Pflughaupt said:


> Hey sky,
> 
> Here’s what you can expect with an equalizer with regards to long low frequency decay times (aka “ringing”).
> 
> * An equalizer will reducing ringing in a room mode, but only to the point of (and not beyond) what the room is naturally exhibiting.
> * If you desire a reduction in ringing beyond that point, absorption is required.
> * An equalizer will not reduce ringing in areas of the frequency spectrum where there is no room mode. For that, absorption is required.
> 
> For more info, see this thread.
> 
> Regards,
> Wayne


Looks like traditional bass trap is the way to go first before adding EQ device. I've read too many positive reviews about Antimode, where everyone has gain benefits in non-treated room. But most of them have really nasty peaks, +20dB room mode issue, unlike mine where the peak is not that much. Some also mention that bass traps can only fix upper bass & leave the rest to EQ device. Hence, I'm wondering how much magic can Antimode do for me.

Currently, I do have priority in fixing the SBIR, lowering the crossover, then the bass decay issue (which I initially thought it's the easiest to fix by getting Antimode).


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## Barleywater

> Looks like traditional bass trap is the way to go first before adding EQ device.


I wouldn't jump to conclusions. I recall your space has 9ft ceiling, brick, and tile. I don't recall room dimensions, or major opening to other rooms.

I exported impulse response from post #83 and brought it into software that allows work with room correction EQ. An EQ was derived and a hard low pass applied, since current interest is bass. The finished EQ was applied to raw impulse response, and the result exported, opened in REW and a waterfall plot generated:









In fine tuned room correction EQ, deep notches would have been skipped. Here they show up as artifacts at 90Hz, 250Hz, and 325Hz. Extended smear <30Hz decays 50dB in about 300ms. Performance here is limited because real low frequency response below 20Hz is missing, in part due to sweep range used, and in part due to real limits of microphone. Also a lot of smear is inherent to driver and enclosure, and much of this is corrected too.

I run woofers and subs with similar correction methodology, deep notches corrected, and get real measured results similar to these. I've seen posts of decent Audyssey correction, but it appears to limit corrections, likely for ease of use by general users.

Even at moderate volumes I can feel snare drum hits. Response is super fast.

Filters such as these will run on JRiver Media Center software, which accommodates movies and audio synch. A decent, quiet media PC, and acalibrated microphone worthy of room correction technique offers better performance/price than likely had with Antimode product.

Regards,

Andrew


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## Skylinestar

Thanks Andrew.

Off to other topic which doesn't involve other tools/money/materials...

How do I check or verify if the 2 subwoofers are correctly timed/phased with respect to each other? Audyssey XT cannot timed 2 subwoofers individually. Therefore, I gotta check it myself.


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## Barleywater

How many measurement points are you using with Audyssey?

How are two woofers hooked up? Is woofer signal from AVR being split?

What are your before and after Audyssey results using REW? Please post mdat.

Sound at any point in room is sum of each individual source. 

Here is example of two very different woofers. A series of three measurements are taken from the same location:

One measurement is of a sealed woofer:











The second measurement is of an open baffle (OB) woofer:










The third measurement is of both woofers driven:










Features of the first two pictures are clearly present in the third picture. The impulse response of the sealed woofer and OB woofer are summed, adjusted for level and an overlay picture made of frequency response with response for two drivers working together:










The results are virtually identical.

Similarly correction filters may be generated for individual drivers, or a single correction filter may be generated for both drivers working together. The single correction filter turns out to be sum of individual correction filters:

Sealed woofer corrected:










OB woofer corrected:











Correction of sealed woofer and OB woofer working together:











But here is joy of very complex math: If either driver is measured using correction filter designed for both drivers working together, the result is far from flat. This is your situation with Audyssey. If well used Audyssey will come up with decent solution for both of your subs when they share the same signal, and the result will look decent. If you use setting and measure each sub independently, the individual results may not look sensible, but when summed up with REW math features, should produce virtually identical results as measurement of both subs working together.

Andrew


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## Skylinestar

Barleywater said:


> How many measurement points are you using with Audyssey?


Full 8 (max for Audyssey XT)



Barleywater said:


> How are two woofers hooked up? Is woofer signal from AVR being split?


Denon 3312 has internal splitter, thus has 2 subwoofer outputs. Sub1 connector to SW1. Sub2 to SW2. Both subs are level matched before running Audyssey.



Barleywater said:


> What are your before and after Audyssey results using REW? Please post mdat.


Refer Post 83 for the attached 20-500.zip mdat file. It is after Audyssey. Is that OK? Are the subwoofers in good phase? Or totally in bad shape, out of phase? I'm not into perfection but I'm trying to get just a decent level.



Barleywater said:


> But here is joy of very complex math: If either driver is measured using correction filter designed for both drivers working together, the result is far from flat. This is your situation with Audyssey.


I understand this is the limitation of Audyssey XT as it measures 2 subs as a whole single unit. 



Barleywater said:


> If well used Audyssey will come up with decent solution for both of your subs when they share the same signal, and the result will look decent. If you use setting and measure each sub independently, the individual results may not look sensible, but when summed up with REW math features, should produce virtually identical results as measurement of both subs working together.


If I understand correctly, you're saying that the best graph is achieved by measuring each subs individually, correcting each sub, then measure everything as a whole and correct again...which I assume is the job from the more expensive feature of XT32 or other EQ device.


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## Barleywater

What is physical distribution of Audyssey microphone locations for correction measurements?

My understanding is that you are primarily interested in single sweet spot, and would guess that keeping eight measurements within 12-18" around listening position may produce good result.

Single measurement from post 83 is not enough information for analyzing your setup. With microphone at listening position, loopback timing reference used, and full range sweep 2Hz-24000Hz 256k long: Measure each sub solo, both subs together, left main solo, and right main solo. Additionally, mains together may be measured. All measurements are performed with single volume setting. From here is possible to look at where phases cross, amplitudes cross, and to do trace arithmetic to check various sums and differences.

Andrew


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## BOTBOTCHAI.

Skylinestar said:


> Can bass trap help with frequency below 100 Hz? My local AV shop sells generic DST bass trap at reasonable price but I'm wondering about how deep they go. Or are they only good at helping with mids?
> 
> I do walk around my living room and I notice that areas close to walls and corners do have stronger, boomy and muddy bass. If these bass were to be absorbed by bass traps, frequency response at main listening position will be improve, am I correct? So, in a perfect listening room, even the room corner doesn't sound boomy?


Yes , but you need alot to trap the frequency below 100hz . I have 17 basstrap and 8 difusser in my HT room . Below are my graph . You can see how the basstrap reduces the room ringing.


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## Skylinestar

@BOTBOTCHAI
I have no idea which is the before and after graph. What type of bass traps are you using?
-----------------------------------------
I re-arranged my subs (due to WAF issue) and forced to add the Antimode 8033 into the chain. This is what I've got. Thank god.


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## BOTBOTCHAI.

@SKYLINESTAR

My both graph are after Audyssey XT32 calibration . Basstrap are DIY. Can you post a waterfall graph in 1000ms . Look like the Antimode only make the frequency flat but didn't help in decay time . Maybe you can consider treat your room with basstrap. Cheers


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## Skylinestar

I don't have the same chart anymore because I've accidentally deleted it. But based on my observations on the waterfall last time, it does reduce the decay time by a little. The main advantage is the peak cut.

My only gripe is the super duper long calibration of the Antimode 8033-SII. 20 minutes NON-STOP is the norm. The first time I did, it took more than 40 minutes. Then I realized that the subwoofer volume is just a little high. The Antimode prefers a VERY LOW volume to begin with.

I'm worried with the long & non-stop calibration because I fear that car or motorbike passing by my house could have actually cause a wrong unreliable EQ correction.

-------------------------------------------------

I wonder how is the calibration for SVS AS-EQ1. Is the test tone non-stop too? Or just a poop-poop sound like a typical MultEQ XT correction with pauses in the middle?


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## D770

I am new here and couldn't find the best place to post this question. A quicksearch of "decay times" didn't bring what I was looking for. So my question is, What is a good decay time? Is there a goal? 200 ms? 300? 400? etc. I have some areas that are in the 1,200-1,700ms, how to reduce these? Is it even necessary? What does reducing these times do for the sound? Any help is greatly appreciated or if you could point me to the correct thread. I'm also considering buying a BFD. But all my times would need to be reduced under 70hz. Bass traps or BFD first? What bass traps would absorb in this freq range. I would love some DIY suggestions if you have any or prebuilt is fine too. Thanks Dustin


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## JohnM

To have a significant effect below 70 Hz requires an awful lot of bass trapping. As a rough guide, around 300 ms is a good target for a domestic space, and that can increase below about 125 Hz without having much subjective impact, even if it is 600ms or more by 50 Hz. There is a good guide on getting the bass right, includign bass traps, in a HiFiZine article by Paul Spencer here.


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## Skylinestar

It's been a long time since my last update.
Here's a chart showing the response of my mains LR with subwoofers. Blue: No EQ, Green: with AudysseyXT & Antimode, and Red: Audyssey & Antimode with subwoofer distance corrected.
I read the subwoofer distance adjustment guide posted by someone from AVS. The adjustment in subwoofer distance finally fixes the huge null problem at the XO region of 100Hz-120Hz region. :bigsmile:

Also uploaded is the final waterfall chart. Seems I can't do anything about the long decay due to the full concrete construction of my room.


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## Skylinestar

I've added 6" thick bass traps (standard 4'x2') to the front left and right wall-wall corners and 4" thick bass traps on the sidewall-floor corners. The bass traps greatly reduce the decay time till 60Hz. I'm happy with that.

I hope bass traps can go lower (till 20Hz). Is that possible with 2' thickness?


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## Skylinestar

*RT60*

Is my RT60 terrible? This is a concrete room. How can I improve my RT60 ? Advice needed.


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## JohnM

*Re: RT60*



Skylinestar said:


> Is my RT60 terrible? This is a concrete room. How can I improve my RT60 ? Advice needed.


How good or bad the figures are depends on how big the room is, the bigger the room the higher the RT60 can (and should) be. Changing RT60 is mostly about the materials on the room's surfaces and what is in the room, best advice on that is to be found in the Home Audio Acoustics forum.


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## Skylinestar

*Re: RT60*



JohnM said:


> How good or bad the figures are depends on how big the room is, the bigger the room the higher the RT60 can (and should) be. Changing RT60 is mostly about the materials on the room's surfaces and what is in the room, best advice on that is to be found in the Home Audio Acoustics forum.


My room size is about 15'x25'x9'.


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## smygolf

Over here i Sweden we often use sono tubes to build a quarter-wave tube to treat room modes.
Its easy to build, the cost is low and they work really god. 3 tubes often lower a peak with 8-9 db.
I have a calculator somewhere if anybody likes to try, but its in Swedish


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