Auralex kits...??

[Eric_Desart]

Jeff,

Thanks for this latter post ....

My wording should have been less good.

Further the low frequent limits of measurement reverb rooms are (almost) known from their very existance, which is why they are and always were designed to cover a specific frequency range.
The distance from the walls, the radius of the microphones or positioning with non rotating mics, diffusion, are all designed and specified in function of that frequency range.

The standards themselves are build-up taking this in mind.

Hence I think you can go further back than half last century.

[lovecow]

Hence I think you can go further back than half last century.

Absolutely. I made the mistake of thinking strictly in terms of the ASTM and ISO standards, which haven't been around as long as the history leading up to them!

[Ethan Winer]

With all due respect, I think you're making this about you...and it's not about you.

Point taken. I'll try to work on that.

I do find it entertaining that my long-standing objections to standard tests are finally being recognized as valid. I remember well when one participant in this thread disagreed with me a few years ago and stated that standard C423 reverb room tests hold all the information that is needed to assess bass traps. Another of your friends used the expression "pant load" to describe my work. I think we now agree the points made in my article are valid.

And thanks for your other comments Jeff. I don't disagree with any of those either, though it was not clear in the Modex Plate PDF that the 5x4x3 room was a climate controlled lab. And thanks especially for posting the link to my article on the S&V site. I didn't realize it was online there, but I didn't want to post it on my own site as long as the December issue was still current. Hopefully my article will spur further discussion of the issues.

--Ethan

[Eric_Desart]

I do find it entertaining that my long-standing objections to standard tests are finally being recognized as valid. I remember well when one participant in this thread disagreed with me a few years ago and stated that standard C423 reverb room tests hold all the information that is needed to assess bass traps.

Since you clearly refer to me, I can only state that (as standard for you) this is a 10000 % complete misrepresentation of the facts.
Do you even believe yourself? This is a plain bunch of nonsense.

If you want some scientific honesty (or just honesty), then link to to the thread where your report is extensively discussed, rather than giving a false picture. This thread even established on YOUR initiative via Dan Nelson owner of Studiotips.

As always I still stand behind everything I said (for me words mean something).

Your current comment is about as misleading as your (indirect, direct?) suggestion in this very thread that the ,for ages and ages old, information related to low frequent limitations of reverb measurement rooms and related methods is somehow credit to you.

[lovecow]

I do find it entertaining that my long-standing objections to standard tests are finally being recognized as valid. I remember well when one participant in this thread disagreed with me a few years ago and stated that standard C423 reverb room tests hold all the information that is needed to assess bass traps. Another of your friends used the expression "pant load" to describe my work. I think we now agree the points made in my article are valid.

I think it's a misunderstanding on your part if you believe that people thought your objections were invalid. (As I no doubt may have contributed to your misunderstanding, I apologize.) Of course, your second sentence may indicate part of the reason for the misunderstanding: All the information needed to assess bass traps is contained in the reverb room standard test methods. It's simply a matter of how the standards are implemented. Obviously, you cannot take the standard verbatim and apply the methods to low frequencies, particularly when the standard suggests a frequency range that doesn't include the low frequency bands. However, RPG is using the standard methods with certain specific techniques included to account for modal behavior in the test room. (At least, that's how I understand it reading through the literature on their site.) It is simply important to ensure that the intent of the standard method is adhered to - which includes everything from type of measurement signal to size of room to temperature and humidity to documentation and reporting to mounting methods to...

Now, it is important to note that RPG - for one - appears to have utilized both reverb room tests and impedance tube tests to come up with their LF absorption coefficients. I think this is noteworthy and commendable. What testing with multiple methods does is it elevates the results an additional notch or three in terms of being able to rely on the data as objective, repeatable, and reproducible. But - IMHO - it may not be necessary in every circumstance. But that's purely my opinion based on my own experience.

[Ethan Winer]

Obviously, you cannot take the standard verbatim and apply the methods to low frequencies, particularly when the standard suggests a frequency range that doesn't include the low frequency bands.

My point exactly. What happens below 100 Hz is very important when assessing bass traps. My method of using ETF in a small room solves that problem. Maybe not perfectly, but it's more useful than a lab test at very low frequencies.

--Ethan

[lovecow]

My point exactly. What happens below 100 Hz is very important when assessing bass traps. My method of using ETF in a small room solves that problem. Maybe not perfectly, but it's more useful than a lab test at very low frequencies.

Perhaps within the contexts of whatever it is you're trying to show. But not if the goal is objective, repeatable, and reproducible low frequency absorption testing.

[Ethan Winer]

objective, repeatable, and reproducible low frequency absorption testing.

Jeff, have you ever tested the very same material three times in succession in an official lab and compared the results?

--Ethan

[lovecow]

Jeff, have you ever tested the very same material three times in succession in an official lab and compared the results?

No.

[Ethan Winer]

Jeff,

I've seen negative numbers reported at low frequencies, and I'm pretty sure a second test five minutes later would give different results. Perhaps very different results. I assume the main reason for run-to-run variation is the moving microphone. The very movement that helps at mid and high frequencies makes for unreliable results below a few hundred Hz. Versus ETF tests I've done that are pretty much the same when you run them all in succession.

--Ethan

[lovecow]

I assume the main reason for run-to-run variation is the moving microphone. The very movement that helps at mid and high frequencies makes for unreliable results below a few hundred Hz.

In accordance with the standardized test methods, multiple stationary microphone positions can be used in lieu of a rotating mic. As I understand it, this is (one reason) why RPG has been able to get reliable data. And it makes sense; careful selection of microphone positions with respect to room mode pressure distribution patterns would overcome (m)any anomalies that arise as a result of a moving microphone.

Versus ETF tests I've done that are pretty much the same when you run them all in succession.

Of course.

[Ethan Winer]

multiple stationary microphone positions can be used in lieu of a rotating mic.

That makes sense to me.

[Eric_Desart]

I've seen negative numbers reported at low frequencies, and I'm pretty sure a second test five minutes later would give different results. Perhaps very different results. I assume the main reason for run-to-run variation is the moving microphone.

The reason for these negative numbers lies in the fact that there is much too little absorption present (< 1m2 added absorption, not physical material).
It's some slogan you mentioned a lot anywhere already to substanciate whatever. It's just a logical physical limitation.
You noticed these negative numbers in these microtraps where they hardly show absorption in the lows.
You compare an empty room with a certain soundfield versus a room where hardly any absorption is added, but whatever added absorption the soundfield will still, even when it's a bit, an altered soundfield.
This falls outside ANY measurement accuracy, hence these values have no sense, and point to physical limitations, not a weak method.

In a reverb room it's of no importance that you add a number of m2 or sft physical material, you have to add a minimum quantity of additional absorption.

There is NO way you can prevent such tolerances with whatever test method.
And it's not even important, since it only occurs in the range where the added absorption material hardly makes a difference with the empty room, hence having very little absorption.
Normally these bands will never be shown in whatever official report.
Since measurements are compared which aren't done at the same time (first empty room, later room with sample), one does assume in the calculation method that the part empty room comprised in the room with sample is still exactly the same as that first measurement.
Since you almost added no other additional absorption, by definition these tolerances will influence that result.
That empty room part in that measurement with sample is too dominant in these numbers. Only a minor mini deviation can result in such seemingly stupid numbers.

• As an analogy (poetic license), that's like trying to measure the weigh of a 2 pound package of salt by measuring a truck on a weighbridge, then putting that package of salt on that truck and weighing the truck again on that same weighbridge, hoping the difference represents the weight of that package of salt. It's very well possible that that package of salt calculated as such results in a zero or negative weight also.
  No matter what this calculation sais: you measure tolerances of whatever phenomenon (-na), not or hardly the weight of that package of salt.
  And it is clear the lower the original weight of that package of salt the larger that mistake expressed in percent, or relative absolute, can become.
  But that doesn't matter, the outcome is senseless and recognized as such.
  Back to reverb room measurements it will show that effect only in a range where there is hardly any or little absorption to speak of. Hence people don't miss out on useful but lacking data.

Hence that argument is no argument to accept or reject whatever method for practical applications. I think you use it because, without whatever context, it sounds impressive for layman.

The only manner one possibly could minimize this error is in an impedance tube (is very controlled field), where then other limitations occur.
E.g. : small measurement sample (results and depends on properties of small pieces rather than being averaged over more material), sculpted profiled boards = how to measure them (how do you cut e.g. a wedged board?), how to simulate random incidence, how to simulate corner behavior, how to make sure a membrane behaves similar as that same membrane in real live applications?
In fact without these limiting side effects impedance tube measurements are the most accurate method to compare materials.
And you can go accurately down to 20 Hz if you have the right sized and well designed tube (is huge).
I don't see very well how corner absorbtion behavior should be simulated in an impedance tube.

Often people look for absorption numbers of different tiles, not understanding that this has hardly any importance in real live situations, and that this is hardly to measure with room methods. These differences with the room itself are that small that you measure anything except the absorption value of these tiles (no matter which method). And it doesn't matter that you use 72 sft or not.
The added (+ or -) absorption (difference with room and/or room boundaries) is what counts.

If you look at your own density report based on these levels (which were relative, what is wrong about it) you see that there are measurements giving the impression of also showing negative absorption with added absorption. And that's not even for low absorptions.

You can't conclude whatever details (only the obvious) of that density report. You don't know how the exact numbers should look like, since you just have pictures based on a RELATIVE y scale, showing some obvious things and leaving lots of other questionmarks.

And what relates to these 3 measurements in row you asked for/suggested.
I assume you refer to the 3 MiniTraps measurements you did at IBM (spread over some years).

• The 1st was very different from the 2nd and 3rd, but the main reason was that you altered the membrane of these MiniTraps significantly, changed the combination of the wool and so on. Hence, nothwithstanding a layman shouldn't notice a visual difference and the name for both was MiniTraps, you were measuring acoustically different products.
  
  The 2nd and the 3rd (with > year difference) were, as per your own words very alike, giving you a reason not to enter your most recent measurements on your site. You yourself said the difference was that minor it wasn't worth to bother.
  Hence what people see on your site are your second (2004?), not your most recent measurements (2005/2006?).

There are problems with measuring bass traps in reverb rooms and certainly corner absorption, but I want to put these suggestive comments (unclear what they should substanciate or not) into context.

[Ethan Winer]

that's like trying to measure the weigh of a 2 pound package of salt by measuring a truck on a weighbridge

Yes, exactly. A truck scale is not suitable for a 2 pound bag of salt, just as a reverb room is not suitable for measuring bass traps at 30 Hz.

I think you use it because, without whatever context, it sounds impressive for layman.

You are much more successful when you don't try to psychoanalyze me.

You noticed these negative numbers in these microtraps

No, I've seen negative numbers with MiniTraps and MondoTraps, even when testing 64 square feet. In an IBM test from 2004, MondoTraps flat on the floor with a 4-inch air gap measured 0.87 sabins at 40 Hz, 1.02 sabins at 31.5 Hz, and -0.13 at 25 Hz. Interestingly (is that a word?) when we tested our cardboard encased SoffitTraps in 2005 we saw negative values at high frequencies where the reverb room method should be more reliable. When placed in the standard C423 position on the floor they measured -0.25 at 8,000 Hz and -2.28 at 10,000 Hz.

And you can go accurately down to 20 Hz if you have the right sized and well designed tube (is huge).

Yes, is huge, and thus expensive.

I don't see very well how corner absorbtion behavior should be simulated in an impedance tube.

Agreed, that too.

If you look at your own density report based on these levels (which were relative, what is wrong about it)

Why do you believe relative is wrong?

--Ethan

[Eric_Desart]

that's like trying to measure the weigh of a 2 pound package of salt by measuring a truck on a weighbridge

Yes, exactly. A truck scale is not suitable for a 2 pound bag of salt, just as a reverb room is not suitable for measuring bass traps at 30 Hz.

I didn't use this argument in function of frequency, but in function of too small difference between empty room (existing absorption) and room with sample (existing + added absorption).

I think you use it because, without whatever context, it sounds impressive for layman.

You are much more successful when you don't try to psychoanalyze me.

It's the easiest way to understand your arguments.

You noticed these negative numbers in these microtraps

No, I've seen negative numbers with MiniTraps and MondoTraps, even when testing 64 square feet. In an IBM test from 2004, MondoTraps flat on the floor with a 4-inch air gap measured 0.87 sabins at 40 Hz, 1.02 sabins at 31.5 Hz, and -0.13 at 25 Hz. Interestingly (is that a word?) when we tested our cardboard encased SoffitTraps in 2005 we saw negative values at high frequencies where the reverb room method should be more reliable. When placed in the standard C423 position on the floor they measured -0.25 at 8,000 Hz and -2.28 at 10,000 Hz.

Again I was not discussing these very low frequencies for which such a room and related standards are not designed. Hence you get a combined problem.
I have to check these SoffitTraps measurements (not on your site anymore is it?), but at a first glance it just confirms what I said in function of adding enough absorption. Certainly in the highs a room has a lot of air absorption already, which is why officially the range is limited to ca 5000 Hz. With low added absorption the significant lower high frequent RT of a reverb room becomes as explicit a problem than at low frequencies.
Therefore a reverb room has not only a lower but also an upper cutoff.
Often with the very high absorption values at the high frequencies of standard absorption material these limitations won't or only little show, but with a resonant absorber (as your soffits) very reflective in these highs (little absorption) you can easily be confronted with these same limitations related to my truck/salt analogy.

If you look at your own density report based on these levels (which were relative, what is wrong about it)

Why do you believe relative is wrong?

I answered that in great length already on many spots, and you just act if my comment is new (provocation). I invited you to link to the related threads, which you know very well. I don't start a fight with you here.
You seem to remember (as shown in this thread) things I never said, but forget the things a repeated over and over.
You hear what you want to hear anyhow, and ignore the rest.

Further I saw at Gearlutz, that you intended to do these tests again (must be a reason for that suggestion).

BTW it is not by repeating that a reverb room has its (for ages known and described) limitations that it proves that another method is better (and transportable between measurement rooms) for these purposes.
Therefore one has to investigate, test and substanciate it.
http://en.wikipedia.org/wiki/Scientific_method