# REW Measurements - with and without sound treatments - Not making sense



## kelton325 (Jan 29, 2012)

Hi everyone - I have been working to learn REW so I can optimize the sound treatments on my theater build in progress. I'm trying to interpret these graphs, however I'm confused that there doesn't seem to be much difference between treated and untreated. The treated measurements had OC703 laid against the wall for about 2/3 the length of the room, so about 48" high, and poly batting above that. The original plan was to use these two on the walls and cover with GOM fabric. The challenge I'm having is that there is not much difference in the REW measurements that I can see. But, when I take out the treatments there is audible echo in the room.

Relevant REW setup:

- RadioShack SPL meter
- Bohringer UCA202 USB Soundcard
- Calibrated card and mic, used Mic calibration file from this site

Questions I'm struggling with:
- I understand REW is best for sub frequencies, does the lack of differentiation at the high frequencies have any meaning then? Not sure if the use of the SPL meter influences that.

- Why am I not seeing much difference in high frequencies between the two if I can hear echo without the treatments but not with the treatment?

- Does anything appear overly out of whack indicating I may have a calibration issue?

- Waterfalls show a problem around 30 and 45 as far as I can tell, am I reading that right?

- SPLs show a problem around 110 without treatment - this was the only noticable difference I could see in the REW results between treated and non treated.

- I have a 7.1 speaker set up, and my Onkyo receiver has the soundcard right output feeding in. The Onkyo is set to all channel stereo, sound is only coming from the right side speakers. Is this OK?

- The SPL meter was pointed almost vertically, at about a 20-30 degree angle toward my front speakers. Is this OK?

Thanks for any feedback.

*Untreated*



















*Treated*


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## SAC (Dec 3, 2009)

Many issues jumbled and a single measurement assumed to provide universal insight!

The problem is not with REW, but with the assumptions.

You are lumping many issues into one and then assuming that a single measurement that is useful for one aspect of behavior is somehow capable of illuminating all other aspects of behavior.

And in particular, you are employing the wrong tool to evaluate specular behavior in a room. You are using the waterfall plot when the ETC response is the more appropriate tool to evaluate specular behavior that polys impact...

The measurement tools is not a substitution for learning and understanding the concepts underlying the various aspects of behavior and then using the tools to help ascertain the behavior of the system as an aide to determining various appropriate courses of action as well as the followup proof of performance functions to determine the degree of success that the various treatments (or other adjustments) may have affected.

Very simply, waterfall plots from 0 Hz-~250 Hz are useful for determining modal behavior - the resonant frequencies, their relative gain, spatial distribution within the space, etc..

Various options exist to moderate such behavior, ranging from room dimensions, to seating location, to porous bass traps to tuned resonant bass traps to the limited use of EQ (to moderate peaks below ~80 Hz).

Above the modal range (and the imaginary dividing point often referred to as the Schroeder critical frequency, Fc, is the specular range, where sound behaves as if it consists of focused rays.

This region above ~250 Hz is best imaged by the use of the ETC response., which shows the arrival times and gain of each direct and indirect energy at a measurement position; from which can be determined the distance of travel, specific vector pathways, points of boundary incidence, and the spatial and temporal qualities of the various specular energy arrivals. Only one source is driven at a time for each ETC response.

Typically, broadband (~250 Hz - ~5kHz+) absorption, diffusion, &/or redirection (reflection) is used surgically to variously control specular behavior. In other words, the goal is NOT to eliminate all reflections, but rather to identify and to surgically control only high gain destructive reflections! 

Indiscriminately applying adequate broadband absorption (4" thick ~3lb/ft^3 Fiberglas or 4 lb/ft^3 mineral wool panels spaced 4" from the wall) results in a dead room; or worse, applying less than adequate broadband absorption - meaning panels less than 4" thick panels spaced 4" from the wall - results in an overly dead room where the specular energy is not effectively absorbed - in other words, the indirect energy is effectively EQ'd and only the high frequency component is absorbed, leaving the low-mid portion to reflect which then colors the direct sound when it combines (superposes) at the listening position - further compromising issues. 

Hence the reason measurements are important in order to first determine the actual behavior that may require surgical treatment so that any subsequent treatment is appropriate. Once appropriate action is taken, the measurements then also allow for the verification that the treatment is adequate while allowing for any needed adjustment sufficient to optimize its effectiveness.

The limitation of using the frequency response and the frequency domain to analyze the interaction of energy that arrives with respect to time is that it is unable to discreetly display or provide usable information regarding the nature and behavior of the interacting component specular energy 'waves' that cause the anomalies. About all the frequency response provides is a destructive pattern that is created by the result of superposition (combining) of specular energy arrivals that results in spatial polar lobing within a space. And this polar lobing exhibit frequency dependent polar nulls relative to each measurement position that appear as "comb filtering" 'notches' in a frequency response. So., while the frequency response can tell you that such behavior is occurring, it is incapable of telling you anything actionable about the nature of the various specular energies that cause the anomalies. Hence the need to examine the specular behavior in the time domain with tools such as the Envelope Time Curve, or ETC, response.

A good introduction to such concepts at play in bounded spaces (both large and small acoustical spaces) is available in _Sound System Engineering_ by Davis and Patronis. 

If you would like more information or clarification with regard to these concepts, please ask. But know that there is a bit more than just taking a measurement and expecting a tool to interpret the results and to provide simple answers as to what you must do. The good news is that with a bit of knowledge and the use of the tools, that the actual behavior is able to be determined and that specific options are indeed able to be determined which can also be objectively evaluated in concert with the subjective evaluation for maximum effectiveness.

Also, perhaps most importantly...You need to take measurements first BEFORE applying treatment in order to determine the ACTUAL baseline behavior of the system. THEN you determine what behavior and make decisions in regards to appropriate treatment in order to achieve a desired acoustical response.

A pervasive simplistic and erroneous myth has dominated casual notions regarding room treatment for the past ~20 years based on a simplistic incomplete reading of articles describing various early research into room acoustics and treatment and the subsequent marketing of absorbent room treatment materials as complete solutions.

The result has been the all too common incomplete and erroneous assertion that has been widely promulgated on the web that 'all reflections are bad' leading to the all too common and utterly incorrect assumption (again based on the all too frequent INCORRECT assertion) that 'one cannot have too much absorption' that is simply _nonsense_ and risks giving legitimate treatment a bad name while also wasting allot of folks' money, time and energy only to achieve a result that is easily as bad if not worse than the original problem. 

Hopefully, with the appropriate use of measurements allowing for a more complete awareness of what is actually happening in the room, combined with a more judicious use of appropriate treatments, folks will truly enjoy the benefits of proper treatment in order to achieve an objective improvement in the subjective quality of their listening space.

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Questions I'm struggling with:
- I understand REW is best for sub frequencies, does the lack of differentiation at the high frequencies have any meaning then? Not sure if the use of the SPL meter influences that.

The waterfall from 0-~250 Hz is most useful for low frequency modal behavior. Above ~250 Hz the behavior is dominated by specular energy requiring different tools and a differential perspective (the ETC response). And for behavior above the modal range, you will want a calibrated omni-directional (small capsule) mic such as the Dayton EMM6 from Parts Express (~$49) and a dual channel phantom powered mic pre-amp such as the ART Dual USB Pre (available for ~$49). 

- Why am I not seeing much difference in high frequencies between the two if I can hear echo without the treatments but not with the treatment?

You are looking at the wrong tool. The waterfall shows modal distribution, standing waves and resonance. Specular energy behaves like focused rays - direct and indirect signals that behave similarly to billiard balls that arrive at differing times with differing amounts of energy (gain) at a given location. 

- Does anything appear overly out of whack indicating I may have a calibration issue?

Considering that you are looking at the wrong tool/perspective for insight into a given behavior, its hard to tell....

- Waterfalls show a problem around 30 and 45 as far as I can tell, am I reading that right?

You may have some modal resonance at those points. But I would recommend cleaning up the procedure a bit before one jumps to conclusions. See next question...

- SPLs show a problem around 110 without treatment - this was the only noticeable difference I could see in the REW results between treated and non treated.

You treat modal issues by seating location, large thick porous bass traps (think corner traps), tuned resonant bass traps, and resonant peaks to a smaller degree below ~80 Hz, with PEQ. Polys generally are the wrong tool to address modal issues. And they have limited use with specular reflections. Instead quadratic based diffusors are optimal (QRDs and PRDs). Thus, the biggest return is going to be affected by using the right tool for the job, both analytically and in terms of treatment.

- I have a 7.1 speaker set up, and my Onkyo receiver has the soundcard right output feeding in. The Onkyo is set to all channel stereo, sound is only coming from the right side speakers. Is this OK? 

You drive one speaker at a time in order to properly generate an ETC.

- The SPL meter was pointed almost vertically, at about a 20-30 degree angle toward my front speakers. Is this OK?

In a word: no, it's not OK. An SPL meter has very limited low frequency application for generating low frequency waterfalls, where for higher frequency specular energy and ETC responses a calibrated small capsule omni-directional mic such as the Dayton EMM6 is most definitely preferred.


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## bpape (Sep 14, 2006)

A lot of good information in what SAC said. That said, my very quick response to the OP in more layman's terms would be.....

The SPL meter will be fine for what you're looking to do. This is not a professional studio. Yes - a dedicated measurement mic would be preferable but for most home gamers, what you're looking for is relative change and general information as to where problems lie. Whether the null you have is 30db down or 20db down really doesn't matter, you still know you have a null. You'll get pretty close from a frequency response standpoint (close enough) once you use the correction plug-in for the meter. If you want to go farther and be more accurate, by all means, get a better mic/pre.

When looking at the lower end, set your scale to look at say 20-300 or a bit higher. That will give you a lot more insight and clarity into what's really going on in terms of modal problems and still stay in the range of identifying where to possibly look for boundary reflection related aberrations. Still take the measurements up to at least a few kHz though. 

Leave things at no smoothing in the frequency response until you get a better feel for what's going on. Once you get close, you can look at a 1/24 or even 1/12th to see a more realistic picture of what you're likely to actually hear. As you get higher in frequency, the odds of a steep, narrow null actually falling on a note diminish considerably. Figure that there are 13 1/2 steps to an octave. If starting at 40Hz, the octave falls at 80. So, there are 13 discreet 'notes' in that 40hz range. Chances are pretty good it could be problematic. At 500Hz, the octave is at 1kHz. So now, there are the same 13 notes but spread over a 500Hz range. 

Your time in the waterfalls is a bit long in time (depth of chart). You're really interested in the first 30-40db of fall on the left scale too. Right now, the window is showing approx 55db of range from the highest peak to the floor.

Moving out of nulls is absolutely preferable if possible from a frequency response standpoint, though you'll still want to address them in terms of decay. I would agree that trying to address low frequencies with poly's isn't terribly practical in most cases.

Bryan


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## kelton325 (Jan 29, 2012)

I appreciate the detailed and informative responses, thank you both SAC and Bryan. One mistake I was making was not including the higher frequencies in the REW test, however it sounds like it's not as suited to those, especially with an SPL vs. a mic. My needs are fairly simple, I'm trying to seek some objective data that proves what I'm hearing, and what I'm hearing is that the treatments I selected do sound better and eliminate echoes. I'm hoping for a little validation before I spend the money on any more OC and also go to the time and effort of covering it all in fabric. 

I re-ran, this time going up to 20k. I can see improvement in the 200hz to 1.5khz range and around 6-7 khz, but not sure how significant it is. I will say that when I have the OC up to ear level and the poly batting above that, it definitely does not echo in the room. Without treatment it echos badly while talking, especially my 4 year old's voice (obviously a higher pitch). I watched most of Dances with Wolves extended edition Blu-Ray this afternoon, chosen because it has lots of dialog but also some loud scenes (buffalo herd, battles, etc.). It sounded pretty good with the treatments, with clearly audible subtle sounds like fires crackling, twigs breaking, crickets chirping, etc.

I've included a shot of the treated room for a visual of what I did. There is 1" OC on the walls with what is supposed to be 1" poly batting, however I doubled it over because I don't think it will match up to the OC when the fabric goes on unless it's doubled up. You can see a bit of the screen wall treatment, which is 2" OC.

Untreated Waterfall









Treated waterfall









1" OC on the walls









2" OC on the back walls (ran out of 1")


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## SAC (Dec 3, 2009)

The problem is_ not _that you are not including the mid and high frequencies in the frequency response and waterfall.

Rather the problem is that the frequency domain and the various frequency responses and waterfalls are incapable of providing much causal and hence actionable information in the specular domain above ~250 Hz. You are using the wrong tool.

All these frequency domain tools show is some combination of the speaker response and of the interaction (properly called "superposition") of the direct (featuring whatever frequency response the speaker may exhibit...) with various indirect arriving energies that constitute the speaker-room interaction.

In other words, you have a comb filtering lab that provides an indication that the superposition of the direct and indirect reflections have created an environment populated by extensive polar lobing - that, based on the particular measurement position, happens to be in a number of the frequency dependent cancellation nulls - hence the frequency response pattern often referred to as comb filtering for its appearance. but note; there are no such 'things' as 'comb filters'. They are simply a measurement pattern. What exists in reality are the 3space areas of polar lobing in which you have a combination of spatially distributed energy exhibiting a pressure above the ambient air pressure (the lobes) as well as areas of sound cancellation and pressure at the ambient sound pressure where there is no sound (the null areas between the lobes) corresponding to specific frequencies at the particular spot at which the mic is located.

While the frequency response and waterfall can indicate that some combination of energies have interacted to produce some combination of peaks and nulls, it fails utterly in providing any information about the constituent energies that interact to produce such summed behavior.

In fact, I can tell you exactly what the frequency response will tell you without having to bother making a frequency response measurement: Namely, that the direct and indirect energies in a bounded space, or an open space with multiple non-co-located and non-coherent energy sources will interact via superposition to create a spatial pattern that appears as "comb filtering" in a frequency response and waterfall response.

Thus, the answer is NOT simply to take more elaborate frequency response plots or waterfalls!

Instead you need measurements that are able to discreetly display and provide specific behavioral information about the various discrete signals that superpose. And you need specific details about each signal that allows you to isolate and analyze these signals. And having such information, to be able to variously address each signal and to then determine the effect this has on the interaction of the myriad discrete signals.

In order to do this we must change our point of reference from the frequency domain to the time domain, where we examine the behavior of energy with respect to time.
And the specific tool that enables this is the envelope time curve, or ETC, response.
And as we have noted earlier, one needs a calibrated omni-directional microphone capable of accurately capturing the specular energy above 250 Hz.

Thus, the immediate take away is that the various measurement tools provide insight into various aspects of acoustic behavior. Also, that a 'new' approach is required to observe, identify, analyze the nature of the contributing factors sufficient to affect treatment and to observe the specific effects of such treatment. Namely, the ETC response.

So, to summarize, if you want to evaluate the comb filtering above the modal frequencies, you need a set up capable of adequately and accurately capturing the specular behavior via the ETC response. (Not simply make more elaborate frequency domain measurements...) 

And you need to properly employ the ETC response and what it can tell you about the specific direct and indirect energies - including sufficient information to absorb, redirect, or diffuse such energies and the result this has on the direct energy in a manner sufficient to treat and to avoid such destructive behavior and to optimize the response - factors which directly determine such psycho-acoustical characteristics of localization, imaging, intelligibility, colorization, spaciousness and liveness.


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## bpape (Sep 14, 2006)

Looking at the pictures and your description, the result is not surprising. You have a lot of high frequency only absorption in the room and essentially nothing that's really going to do much below a few hundred Hz. 

The boundary interactions from the speakers (and double the issues since you're corner loading your speakers) is going to require significantly thicker control on the side walls at the reflection zone at a minimum. 

The room is still 'echoing' even with the treatments, just at lower frequencies where it doesn't sound like the typical echo you're used to and more sensitive too. The bass is ringing out of control as shown by the waterfalls. Regardless of the accuracy of the measurement setup, the relative change (or lack thereof) is still going to be obvious. 

You also likely have significant cancellations off of the rear wall which again will require much thicker treatment - though it can be faced (unlike the reflection zone) to minimize mid and high frequency absorption.

What SAC is referring to is to be able to identify where in space and time destructive interference between direct and reflected waves. Once you can identify that based on the frequency in question, you can address it accordingly.

Now, if you're not willing to go that thick (or can't due to space constraints) at the reflections to try to address these problems, then you should at a minimum look to bring the decay times more into line with the targets for a room of your volume and usage. While it won't help with frequency response, it will help minimize boominess and help some with dialog clarity.

Bryan

Bryan


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## kelton325 (Jan 29, 2012)

1" is the absolute max I can go, I'm already pushing it given a 3 seat row in a 10' 7" room width.

I definitely get that I need more control of the lows, the main question is whether going through the pain of installing the OC and poly and covering with fabric is worth it versus just regular painted walls for mids/highs. It seems I may not have the right mic or be using the right graphs to get a perfect answer, however perfect went out the window a long time when this very small room got the nod for a theater build.

Based on my very uneducated ears, it sounds better and not too dead set up as pictured. I was hoping for some validation quantitatively with REW but might not be able to achieve that without investing more time and money to get better analysis.


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## bpape (Sep 14, 2006)

Skip the batting. It's just more HF absorption you don't want or need. 

Get some additional thickness on the rear wall where it's not impacting the width of the room.

Bryan


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## monomer (Dec 3, 2006)

SAC said:


> ...a dual channel phantom powered mic pre-amp such as the ART Dual USB Pre (available for ~$49)...


Okay this is like about the fourth time I've seen this price quoted but I cannot find it anywhere. Most places want $79 or more but I have located one single place that will sell it for $69 but that's it... there are a couple 'used' source for ~$65 but I don't trust used electronics. So please tell me where I can find an ART Dual USB Pre for $49. I'm looking to get one and would appreciate the tip.


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## monomer (Dec 3, 2006)

So apparently the ~$49 quote was bogus...
however it no longer matters, I was able to find a used one in a pawn shop for $26... not bad I think... I don't like the risk associated with used electronics but at that price I'm willing to give it a go.
Got the Emm-6 for $62 (that includes shipping) and bought a $10 XLR cable. Don't know what I'm going to do for a mic stand at this point though. So far, its cost me $98... a far cry from what a RS SPL meter costs (it's what I've been using for the last 6 years with REW)... I guess its about time I take it to the next level.

Most places have the ART USB dual pre for $99 plus shipping though it can be found for $79 + ship rather easily too... also most XLR cables cost more (some a lot more) than what I paid for mine and if you include in a decent mic stand (as oppose to the camera tri-pod I used with the RS meter), one could easily spend ~$200 going the EMM-6 mic route as opposed to a $25-35 Radio Shack sound-level meter. I sure hope the extra expense is going to prove to be worth it. Now to read up more on using ETCs


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## SAC (Dec 3, 2009)

monomer said:


> So apparently the ~$49 quote was bogus...
> however it no longer matters, I was able to find a used one in a pawn shop for $26... not bad I think... I don't like the risk associated with used electronics but at that price I'm willing to give it a go.
> Got the Emm-6 for $62 (that includes shipping) and bought a $10 XLR cable. Don't know what I'm going to do for a mic stand at this point though. So far, its cost me $98... a far cry from what a RS SPL meter costs (it's what I've been using for the last 6 years with REW)... I guess its about time I take it to the next level.
> 
> Most places have the ART USB dual pre for $99 plus shipping though it can be found for $79 + ship rather easily too... also most XLR cables cost more (some a lot more) than what I paid for mine and if you include in a decent mic stand (as oppose to the camera tri-pod I used with the RS meter), one could easily spend ~$200 going the EMM-6 mic route as opposed to a $25-35 Radio Shack sound-level meter. I sure hope the extra expense is going to prove to be worth it. Now to read up more on using ETCs


The quote was NOT bogus! 
It was available from quite a few dealers for $49, many of whom now have it for $69. (currently 2 places have it for $69 and free shipping at B&H Photo and ProAudioStar...)

XLR & TRS Cables are a commodity at MonoPrice.com.

Just like the Dayton EMM6 was available for $39.

And anyone spending "~$200 going the EMM-6 mic route" needs to find a kid who can use Google.

The prices were indeed legitimate. That is why we reported and recommended it. NOTHING was "bogus". 
But hey, you snooze, you lose.


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