# New user with a few questions about measurements



## burzel2 (Nov 24, 2008)

First let me say that REW is fantastic. Since I discovered it last weekend I can't stop using it! I have a DIY sub with a 12" Dayton driver and a 300 watt amp in a 2 cubic foot sealed enclosure. F3 using WinISD is about 38 Hz. I'm using the older analog RS sound level meter. The first measurement below was taken with a 90 dB target with the preamp crossover set at 60 Hz. The second measurement was taken with a 95 dB target. There's always a spike at 45 Hz which is exactly the first order resonance of the width of the room. The second spike is around 90 which is perhaps the second order resonance. Here are my questions:

Why does the spike at 45 Hz get less pronounced at higher volumes? At 75 dB target the spike is huge.
Why do the measurements not look anything like WinISD? I don't see a distinct F3. Is that room gain?
How the do you minimize a first order resonance without putting the sub in the middle of the room or adding a second subwoofer? 

Any help is greatly appreciated.


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## DrWho (Sep 27, 2006)

If you want your graph to look like WinISD, then you'll need to take your sub outside to do the measurements since that's the environment that REW is predicting in. You might also see if your sub's frequency response doesn't change with output level outdoors as well....that will allow you to see what is room gain and what is the driver.

Btw, are these plots smoothed, or EQ'd? Can you post the raw responses?


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## burzel2 (Nov 24, 2008)

Great suggestion. Is testing outside similar to an anechoic chamber? I'm not sure my laptop has a good enough sound card, but I'll look into it. To answer your question the plots have the 1/3 octave smoothing applied. The plots below do not have any smoothing. I've also included a third plot showing a 75 dB target from a previous measurement. I think the crossover is disabled in that plot (ignore the target curve). It shows the pronounced spike at 45 Hz and again at 90. I have no EQ in my system.


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## DrWho (Sep 27, 2006)

Yea, outdoor ground planes are how all the speaker companies achieve "anechoic" measurements at lower frequencies.

For what it's worth, I'm not seeing any of your spikes changing with output level. Instead, what I think you're seeing is non-linear room gain. Basically, as the lower frequencies get louder and mask the fact that you've still got a ~45Hz spike.

Could you provide more information about your setup? Like room size and the subwoofer you're measuring and where the microphone is located?

The 45Hz stuff looks like it might be two different frequencies and/or some of the polar response effects of having a subwoofer surrounded by walls. Looks like your sub is about 2ft from the walls? (or your microphone is). I would expect to see about a 6dB rise in response starting at around 60-70Hz....kinda like a shelf-filter until the driver response starts to naturally roll-off.

Are you opposed to plotting your WinISD predictions too? (preferably with the same scale).


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## burzel2 (Nov 24, 2008)

My sub is a DIY 2.0 cu ft sealed enclosure using a Dayton RSS315HF-4 and Bash 300W amp. The room is 20'W X 12.5'L X 7.5'H. The couch is against the longer wall. The right side of the room has stairs and two open door ways. Behind the couch is a bay window. The sub is currently to the left of the front left floor standing speaker (Paradigm Studio 60s) and is near the back wall and about 5' from the left wall. I can reduce the 45 Hz spike by moving it to the center of the room, but there's a fireplace along the side wall. Experiments against the back wall were worse. The mic was located at the listening position in the middle of the couch.

I very much appreciate your help. I enjoy learning the technical aspects of this hobby.


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## DrWho (Sep 27, 2006)

Are you setting your "microphone" on the couch when taking the measurement? And do you get the same results if you feed your signal directly to the sub amp instead of through your receiver?

If you use the all-measured tab, you could provide 10 measurements at 10 different SPL's...maybe start at 70dB and move up in 3dB steps? I think this will show the trend with SPL....I'm wondering if it's not voice coil temp rise / room gain / power compression? There are examples of this performed by Ilkka in the subwoofer measurement forum (it's down on the list somewhere).

Btw, are you more curious about the source of the behavior you're seeing, or are you trying to tweak things to sound better? or both?

Also, you can model VC temp rise in WinISD....basically, the resistance of the VC rises and causes a voltage-divider against the non-linear impedance response of the speaker.


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## burzel2 (Nov 24, 2008)

My cheap mic is on a tripod pointing horizontally right behind where your head would be. I did a little experimenting with having it vertical and in a few other locations, and the results were marginally different. 


> Btw, are you more curious about the source of the behavior you're seeing, or are you trying to tweak things to sound better? or both?


 Definitely both.

First plot below is through my preamp (Rotel RSP-1066):








The next plot is directly to the sub amp. I would say the results are very similar to the previous plot:








This sweep has the source fixed and I'm changing the sub amp volume knob:








This sweep is through the preamp and includes the main speakers:


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## Wayne A. Pflughaupt (Apr 13, 2006)

burzel2 said:


> My cheap mic is on a tripod pointing horizontally right behind where your head would be.


What "cheap mic" are you using? Do you have a calibration file for it? If not, none of your measurements are valid.

Regards,
Wayne


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## burzel2 (Nov 24, 2008)

I'm using the Radio Shack 33-2050 with the proper calibration file. Sorry for the confusion.


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## Ricci (May 23, 2007)

Looks like you are compressing on your highest level measurements. Either the sw itself or otherwise.


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## burzel2 (Nov 24, 2008)

I admit I had to Google subwoofer compression. I read about thermal compression and power compression. They seem to have the similar definitions. I'm not familiar with this so bear with me. My understanding is power compression occurs when the voice coil heats up, the impedance goes up. So as the power goes up the coil heats up, impedance rises, and less power is delivered to the driver. But my plots show compression at lower levels. Is my understanding of this correct?


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## Wayne A. Pflughaupt (Apr 13, 2006)

Ricci said:


> Looks like you are compressing on your highest level measurements. Either the sw itself or otherwise.


That was the first thing that crossed my mind. But then it came to me that the peak is acoustic, not electronic. IOW, the amp does not "know" that there is a peak at 45 Hz and therefore would not be trying to flatten it at higher volumes.



burzel2 said:


> I admit I had to Google subwoofer compression. I read about thermal compression and power compression. They seem to have the similar definitions. I'm not familiar with this so bear with me. My understanding is power compression occurs when the voice coil heats up, the impedance goes up. So as the power goes up the coil heats up, impedance rises, and less power is delivered to the driver. But my plots show compression at lower levels. Is my understanding of this correct?


That's a different issue. What Ricci is talking about, some subs have a limiting feature that restricts their output at a pre-determined level in order to prevent damage. It's most noticeable in that at a certain point, the sub just won't put out any more volume, irrespective on how high you crank the receiver's volume control.

Regards,
Wayne


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## DrWho (Sep 27, 2006)

Well at first I was going to mention that the Bl isn't linear with excursion and that is a common source of compression...basically, as the voice coil starts to leave the magnetic gap, the amount of force it can generate goes down and thus the cone doesn't move as far. The suspension can also get stiffer as the excursion increases too.

However, if this was the problem, then you would see everything compressing below 40Hz especially in a sealed box because the excursion goes up with lower frequencies.

Instead, I think you might be clipping your SPL meter....or something in the measurement chain (and REW is probably not noticing). It looks like the compression starts happening near the same SPL at other frequencies once they get that high.


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## robbo266317 (Sep 22, 2008)

If its the SPL meter then moving it closer and running the same sweep at the same level again will tell you. (ie don't change any settings, just move the meter closer to increase the level it "hears")


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## JohnM (Apr 11, 2006)

You can look at the scope tab to see whether the input signal is externally clipping, if you are leaving your meter on the 80dB range for all those tests then the meter will be the limiting factor. If the meter needle reaches the upper end stop you need to increase the meter range. You can either use a higher meter range for all the tests and live with reduced signal/noise at the lower levels, or switch the meter setting as the level increases and offset the measured curves accordingly (though the steps on the meter will not be exactly those indicated depending on component tolerances).


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## burzel2 (Nov 24, 2008)

It appears to be a measurement issue (fantastic advice from everyone by the way!). The following plot has the mic right next to the sub. I set the meter to 80 and ran the sweep.









For the next test I adjusted the meter as it got louder. I re calibrated each time and verified by testing at the same volume as the previous run. The meter appears to be accurate even when the needle becomes pegged. However it becomes inaccurate beyond a certain point. For example when it's pegged during almost the entire sweep. Here's the plot with it adjusted and calibrated. I also set the crossover to 60 Hz on this run.









I'll re run the sweeps at the listening position this weekend. It's too loud to test with the family around. Some quick tests show the traces look identical at different volumes. Looks like there is no compression happening. My 45 Hz spike just goes up and up.


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## brucek (Apr 11, 2006)

> The meter appears to be accurate even when the needle becomes pegged.


As John suggested above, you only need to examine the Scope to see if there is distortion from the meter.

Are you familiar with interpreting the Scope information?

brucek


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## DrWho (Sep 27, 2006)

Ah good, I'm glad to see things are starting to make sense :T

Btw, I was looking at your plots earlier....I bet you can get that 70Hz dip to move around by changing the height of your meter.


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## burzel2 (Nov 24, 2008)

> Are you familiar with interpreting the Scope information?


Negative. I looked at it, but didn't dive into the help section. Any quick pointers?



> I bet you can get that 70Hz dip to move around by changing the height of your meter


Unfortunately it's at the same height as my head in the main listening position. But I'll try some tests to see the effects of height. I did a few tests with the sub on it's side and at various heights. Some things looked better and some looked worse. I'm thinking of building another sealed enclosure that's a bit smaller with a front facing driver. 

The 70 Hz dip is smoothed out a bit when driven with the mains, but both the sub and mains have that spike at 45 Hz. I can't get rid of it and have the sub in a wife approved location! I might have to bite the bullet and build two smaller subs.


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## brucek (Apr 11, 2006)

> interpreting the Scope information


The scope is available after each measure and is lost when you do the next measure, so you do have to look directly after a measure.

It simply gives a picture of the output and input signal.

See a typical measure below where the output is the purple signal from the soundcard and the red is the input signal once it's received by the soundcard. See I have one peak there received that is about 80% of full scale (FS). That's a good sweep signal as long as the meter itself wasn't clipping.









Hard to see anything when you look at the entire horizontal axis of the sweep, so........

Below is an expanded horizontal scale version and you can see the output and input are not clipped. If I increased the input level control so that the VU meter on the Settings page showed more than 0dBFS, then the red signal would be as large as the output signal and you would see the red signal clipping (flat top). If on the other hand, the signal from the meter was clipping, you would see a flat top or other distortion of the input signal without it being 100%FS. This is likely the case when you leave an SPL meter on a lower scale and measure high SPL readings (your case). This is one of the other advantages of a microphone over an SPL meter for measuring audio.









brucek


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## burzel2 (Nov 24, 2008)

Wow. That was more than a quick pointer. Thanks for the visual aids. I definitely had an issue with the measuring tool. The plot below has the meter about 2" from the sub's cone. IMO it shows the subwoofer performing as it should. However my room needs some help. The big 60 Hz dip has something to do with the bay window. Only when the mic was not in front of it or the sub more to the side did it go away. I'd like to thank everyone again for all your help. REW is a great tool!


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## DrWho (Sep 27, 2006)

In regards to 60Hz, check for boundaries that are about 4.7ft within the mic or the speaker...
http://www.peavey.com/support/technotes/soundsystems/boundarycancellation.cfm

You might try some outdoor measurements just to see what this looks like so that you can identify it in your room.


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## burzel2 (Nov 24, 2008)

> You might try some outdoor measurements just to see what this looks like so that you can identify it in your room.


I missed winning an external USB sound card on Ebay by $1.00!! I need to get one soon for some outdoor testing. Also I'm considering an upgrade on measuring equipment. The RS meter is apparently only accurate down to 24 Hz.

Mike, if I get a good outdoor measurement, can I then compare it to the room measurement and draw some conclusions on what the sub is doing vs. the room modes? I've also learned that my Bash 300 has a 1 dB boost at around 28 Hz. This apparently is also a high pass filter at 17.7 Hz. It can be changed by replacing a few resistors. I assume this will change the response vs. WinISD as well. Do you guys ever send a high level signal to the sub to bypass the amp for comparison purposes? Below is my latest measurement with the SPL meter properly set and the sub in a better location (no more 60 Hz dip).


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## brucek (Apr 11, 2006)

> The RS meter is apparently only accurate down to 24 Hz


It's not particularly accurate in general, but it's fine for home use and can be used down to 10Hz with our calibration files - You are using a calibration file?



> if I get a good outdoor measurement, can I then compare it to the room measurement and draw some conclusions on what the sub is doing vs. the room modes?


Certainly.

brucek


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## burzel2 (Nov 24, 2008)

I am using the calibration file. 

One other question. The plot in post #21 was taken with the meter about 2" from the cone. Does this method minimize room gain? I'm asking because f3 is at about 25 Hz. Much lower frequency than predicted by WinISD. I need to measure this thing outside!


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## Wayne A. Pflughaupt (Apr 13, 2006)

> Does this method minimize room gain?


If you asking if it will minimize the effects of the room, the answer is no.

Regards,
Wayne


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## brucek (Apr 11, 2006)

> If you asking if it will minimize the effects of the room, the answer is no.


Do you not feel the near-field measure overwhelms the effect of the room on the reading at the mic?

Is this not the purpose of near-field measurements?

brucek


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## Wayne A. Pflughaupt (Apr 13, 2006)

Sorry, my response was admittedly a bit hair-triggered. Sure, close-micing will get you a different response reading than what you'd get at the listening position, but it's not going to totally eliminate the effect of the room. This will be obviously apparent once he performs his outside measurements.

The only exception would be a quick gated reading. I seem to recall some discussion as to whether or not REW could do this, but I don't recall the details...

Regards,
Wayne


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## brucek (Apr 11, 2006)

> The only exception would be a quick gated reading.


For sub testing, you don't want to alter the IR gating, since the gating puts a limit on the lowest frequency and the resolution of the response. For example, if you were trying to limit reflections from surfaces 1 meter away, you would use a 6msec gate (d=(time*speed)/2), but this would limit the lowest frequency of usable response information to ~167Hz (1/gate time). Not much use in this case. The default windows are much better for low frequency testing. Gating is reserved for measuring mains.

Moving a sub to the middle of the room and taking a near-field response is a pretty good method of removing the room. One of the problems that arises is that it isn't very practical with a ported sub, since both the driver and port contribute at different frequencies, and so setting the mic to get a decent mixing point places you outside of the area you would consider near-field. In that case it's better to drag it outside.

brucek


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## Wayne A. Pflughaupt (Apr 13, 2006)

> Moving a sub to the middle of the room and taking a near-field response is a pretty good method of removing the room.


 I had forgotten about the graph Brian showed us in post #21, which pretty well supports your contention; I can’t imagine an outside measurement showing anything different other than less extension (which would be from a lack of room gain). Sorry to sidetrack the thread. 










Regards,
Wayne


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## DrWho (Sep 27, 2006)

Well you're getting proximity gain on the microphone by putting it right up next to the cone (in other words, changing the frequency response of the mic). I dunno how big of a deal it is with the ratshack meters. You'll also be getting some effects of room gain as well. Really, the only way to know is to do some outdoor measurements 

Btw, the WinISD predictions are very accurate in terms of power response in half-space (middle of a field). Sometimes the shape of the cabinet and the arrangement of the driver(s) can affect the on-axis frequency response. Adding other boundaries (like putting the sub into a corner) will change the on-axis frequency response too. But in all these cases, the power response is essentially the same....you're just redirecting where it goes (and hopefully keeping it in phase).

If your subwoofer is "small" (relative to the wavelengths), then the polar response will be nearly omnidirectional - in which case, the power response and on-axis frequency response will have the same shape.

All that to say, you can take your WinISD prediction and then find a transfer function that matches your in-room measurement. I should confirm this before mentioning it, but room gain is typically about 12dB/octave, but looks more like a shelf filter because eventually the walls start getting more transparent as you go lower in frequency. But knowing the shape that room gain should be helps you in determining the transfer function that matches the difference between your measurement and the WinISD model. When you get it right, this transfer function can be applied to alternate designs and yield predictable results.

The nice thing about sealed cabinets is they also roll off at 12dB/octave....so if you can find a driver that has an F3 with a Q of 0.707 where your room gain kicks in, then you should expect to measure a frequency response that extends down to DC - but it'll roll off earlier at the point where the room starts getting more leaky.

That said, I disagree with the notion that "flat after room gain" is the goal, but that's getting into the realm of psychoacoustics instead of the realm of understanding why things measure the way they do. Knowing why things behave the way they do makes it possible to design for a target dictated by psychoacoustics. Anyways, I might suggest that we can perceive room gain without there being any music playing and therefore we establish our perceived "zero" as starting with the room's natural "frequency response". My current position is to be flat after boundary gain, but before room gain...which is kinda hard to quantify without getting extreme with the measurements. I've actually come across a recent article that talks along those lines too...I wonder where I put that one.


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## burzel2 (Nov 24, 2008)

Mike, your forcing me to learn about aspects of this hobby that are clearly more than just a hobby to many people in this forum. It's great! I'm a mechanical engineer, I enjoy learning about this stuff. 

I had to read the first half of your post a few times, but the second half I understood. Transfer functions make sense to me and your explanation of


> perceived "zero" as starting with the room's natural "frequency response"


 makes perfect sense. Do you tune the simulation with equalization to represent the room's response? 

Also so can you explain the difference between boundary gain and room gain? Thanks.


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## DrWho (Sep 27, 2006)

I'm an electrical engineer myself and usually try to suppress the engineering topics because it ends up being more involved than people usually want to get (nothing wrong with that either)...so I enjoy an opportunity to talk engineering speak 



> Do you tune the simulation with equalization to represent the room's response?


Not sure what you're referring to here???



> Also so can you explain the difference between boundary gain and room gain? Thanks.


Here's how I understand it:

Measure the subwoofer in the middle of a field - this will be half-space. Now measure the subwoofer sitting beside a really big building (basically a big wall in the middle of a field). This gets you a 1/4 space measurement and ideally you'll see +3dB at every frequency relative to the 1/2 space measurement. Then measure with a big corner out in the middle of a field....so a 1/8 space measurement. You'll ideally see +3dB at every frequency compared to the 1/4 space or +6dB from the 1/2 space.

What's happening is the energy that would normally be travelling rearward is reflecting off the wall and being redirected forward. Therefore, the intensity (amount of energy per area) increases in front of the speaker. The intensity at a single point in space is what is being shown in a normal frequency response plot. If you integrate the intensity over all space, then you end up with the power response (the total energy being delivered by the speaker at each frequency).

The +3dB from halving the space the driver is firing into is true when you assume that the speaker has an omnidirectional polar pattern and that the reflections take zero time to occur. So the only time this is true is for a point source located on the vertex of the boundaries.

In the real world, the driver has a physical size and the cabinet requires that the driver sit away from the boundaries. This prevents the existence of a perfectly omni-directional polar pattern and the reflections are delayed by a small amount. This seems pretty simple, but I've always been surprised by just how different an outdoor 1/8 space looks from an outdoor 1/2 space measurement. I wish I had some electronic copies to share as it is really quite surprising. Anyways, the point is that the difference in behavior is not at all trivial (we're talking like +-3dB at least).

When you put the sub in the corner of your room, you're always going to see the same 1/8 space behavior, but it gets a bit more complicated because you have a ceiling, and extra walls to deal with. Some of the reflection paths will be long, and some of the reflection paths will be short. When they're short, our ears perceive it as part of the direct sound and so we hear the dips/peaks that it introduces. When they're long, our ears perceive it as part of the natural sound of the room - so the dips/peaks aren't necessarily perceived.

So is the ceiling a part of the boundary or room response? Well I think it will depend on the frequency in question. I think the real question should be, "is the delay long enough to be perceived as part of the natural sound of the room, or is the delay short enough to be perceived as part of the direct sound from the driver?" I think answering that question in context of a specific situation should likely reveal the best path to a solution.

And then another behavior to throw out there...

When you have reflections in the room, there is a good chance that many of those reflections will find their way back to the driver itself. Increasing the air pressure in front of the driver will improve the coupling of the driver. Likewise, decreasing the pressure in front of the driver will reduce its coupling. Horns basically work on the principal of providing more pressure in front of the driver so that more power gets transferred. In fact, it is this behavior that creates the 12dB/octave room gain that we observe - basically, the wavelengths are long enough such that the reflections are in phase with the driver - thereby increasing the amplitude in front of the driver and increasing its coupling. As you go lower, you get more and more in phase (since the phase rotates slower and the time of the reflections are fixed). Things can get real complicated real fast when you consider all the reflection paths that might improve or reduce the coupling of the driver. So in a case of a horn, I'd call those "reflections" as happening early and therefore being perceived as part of the direct sound. In the case of the 12dB/octave room gain of the room, that behavior is going to be delayed by quite a bit since the sound has gotta slosh around the room for a bit to capture the full effect. We also naturally hear the power response of the room with no music playing either, which just reinforces our perception of it being more of a "reverb" than a "direct sound".

All that said, I don't think there is any way to easily determine what would constitute an ideal target response for a given measurement in a particular location. I might suggest that it's easier to think of it in terms of "boundary gain" vs "room gain", but in reality it's going to be a continuum of both - and the fact that it is a continuum makes it real hard to draw lines. But really, it may not be necessary since there is only so much we can, or are willing, to do....in which case it is important to be aware of how we perceive the sound so that we don't make unintentional sacrifices.


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