As for quality, I have every bit of faith that any in-depth small room measurements Auralex has performed are every bit as top-notch as those of Terry Montlick.
I didn't mean to imply that Terry took better measurements than Auralex.
I presume your company occasionally visits Studios to do testing. I don't think I've ever read of Auralex ever doing testing in the field. That doesn't mean it doesn't happen a thousand times a year of course.
It was my understanding that the Auralex Help Desk is a telephone based service that recommends placement and quantity of your products, and if any acoustic measurements are taken for them then they are taken by the customer.
Is it possible that since in your case the 'outer' wall is a lot farther away that the 41Hz could be a 2nd harmonic or even 3rd harmonic of that length?
Thanks for linking that PDF again. I thought I'd saved it and looked for it on my HD before making my original post, but couldn't find it.
BP:
> Is it possible that since in your case the 'outer' wall is a lot farther away that the 41Hz could be a 2nd harmonic or even 3rd harmonic of that length? <
Anything is possible. But we're seeing major modes here, not low-level artifacts. I have a lot of plots made from different locations in the same room, and they all show the same basic mode frequencies.
[ Anything is possible. But we're seeing major modes here, not low-level artifacts. I have a lot of plots made from different locations in the same room, and they all show the same basic mode frequencies.
Ethan,
This is exactly the same (although different) as modeling walls for sound isolation.
All of the math - all of the theories - all of the experience - and when it comes right down to it - the ONLY accuracy exists with the actual testing performed after construction.
I know it seems like this shouldn't be - but it is.
All of the mode calculations are based on a rom with a perfect boundry - nothing escapes - and they are also based with the sound emitting from the surface of the boundry.
Then we walk into a room - in most cases wallls against the building exterior are a totally different density and construction than the interior - some with brick - concrete - perhaps plywood (or exterior grade gyp) with stucco over the exterior of that.
Our mode calcs know nothing of this - nothing of the construction - nothing of the techniques involved in that construction.
We also do not produce the sound at the boundry - as it is assumed within the calculation.
We step inside - slide that speaker away from the wall - create a distand for the sound to travel that the calculator never took into account - a reflection off the wall behind that was never anticipitated by the mathematical equation.
And in the end - we are suprised to find a variance?
I (for one) would be surprised to find extreme accuracy -
By the way my friend - for what it's worth - I don't think (when we deal with sound) that we can look at it as percentages - I thought last night about Jeff's observation that it's only a 20% when viewed in relation to the nearest mode you expected - and would be about 6% veiwed in relation to another frequency.......
He's right - we can't view it in that respect - if it isn't consistent then it skews the results.
The long and short of it is this - modeling is good for leading us in a direction that will cause us the least problems down the road, but in the end - the only thing that's accurate is the final testing.
Sincerely,
Rod
_________________ Rod Gervais
Acoustics Moderator Sometimes - late at night..... when the wind whips
through the trees........ and the moon shines bright in my
face......... I think deep thoughts.......... and my head hurts.
Ethan Winer Respected Past Moderator
Joined: Mar 19, 2001
Posts: 3209
Location: New Milford, CT USA
> Jeff's observation that it's only a 20% when viewed in relation to the nearest mode you expected - and would be about 6% veiwed in relation to another frequency. <
In certain cases, such as Jeff's FFT window example, the bandwidth is in Hz and not a percentage, or fractional octave which is the same thing. But most things in audio are best expressed as a percentage. Especially with room modes, all of the data is percentages. Room ratios of Length, Width, and Height are related as percentages, mode bandwidth (Q) is a percentage, and these also change linearly as a percentage when considering the effect of temperature and humidity. So while Jeff is technically correct in that one instance, I maintain that percentages are more appropriate as applies to this discussion.
> the ONLY accuracy exists with the actual testing performed after construction. <
Yes, I'm fully aware of that. I've explained that to others many times myself!
But let's not forget one important reason for my original post. Someone claimed that a particular mode in my test room is non-axial because its frequency aligns with a tangential mode predicted by a mode calculator program. However, none of the other frequencies displayed by that calculator agree with what I measured! So why should I believe this particularly prominent mode is non-axial? All of this goes to the assertion by some folks here that non-axial modes are "extremely important" and, by extension, that I'm incompetent for not understanding this.
I'm simply trying to get to the bottom of this, and determine for once and for all how significant non-axial modes really are. So far, I haven't seen any evidence that non-axial modes are particularly important. And the deeper I look into this, the more convinced I am that the burden of proof is on those who dismiss me, my Acoustics FAQ, and my mode calculator program.
This is the real issue, as evidenced by insulting posts like THIS (7th post on the page) and THIS (last post on page) and THIS and THIS (6th post) that remain because insults are allowed as long as they invoke "science." There's also the insulting and demeaning post at the bottom of THIS page that remains even though not one bit of evidence is put forth. Nor are the "Well known and accredited acousticians who hold P.H.D.'s" identified.
While I'm at it, I'd also like to respond to this:
> Ethan, I measure for 25 years. You have ETF as your first measurement tool for how long? <
I have been measuring rooms for just as long as you. Way back in 1982 R-e/p magazine published THIS article with schematics for a spectrum analyzer and digital noise source I designed.
By the way, I still have that analyzer, and it still works. I used it just last year to measure the response in my home theater.
I don't have too much time to dwell on things here. But I will stir the pot a little...
• Your mode calculator doesn't predict anything except axial modes. Personally, I wouldn't recommend the use of such a thing since it has the potential to omit important information. That's all people take issue with, IMO. (Personal conflicts aside, please, for the moment. ) If you're going to take the time to predict modes, it might as well be all of them. Your attempts to justify this with speculation that "non-axials" aren't important can appear to some to be dodging the issue. Why don't you simply go back, program in the tangentials and obliques, and give the user the ability to pick and choose what they want to look at? That's what my spreadsheet does. I've never had anyone complain about it...
• Bear in mind that it's not about which types of mode are more important. It's about which types of modes are creating the problems in a given room. There is nothing universal that can be said about tangentials versus axials. Certain modes will dominate in certain rooms and that's about all you can assume. If it helps, think of it this way: Two floor toms of idential diameter and height are evaluated by banging on them. Unless they are tuned identically and made of EXACTLY the same materials, they will sound different. The volume of air in each drum is the same. Just like the volume of air in two rooms can be the same (even with different dimensions, but that's, er, tangential to this point I'm getting to...which is). If two drums of otherwise identical geometry can sound totally different, so can two rooms. The {2,0,0} mode may be overwhelming in one room. An identically sized room may have a huge problem with the {3,0,1} mode. Nothing you study or read or calculate will be able to tell you in advance how any given room will behave.
_________________ ---lovecow---
"You've got to be very careful if you don't know where you're going, because you might not get there." - Yogi Berra
Ethan Winer Respected Past Moderator
Joined: Mar 19, 2001
Posts: 3209
Location: New Milford, CT USA
> it has the potential to omit important information. <
But this is exactly what I'm trying to determine. I know that in an ivory tower non-axial modes are "very important." But programs written in those ivory towers seem unable to predict even plain axial modes in a typical sheet rock room. So why should I believe that non-axials matter enough to be considered?
> Why don't you simply go back, program in the tangentials and obliques, and give the user the ability to pick and choose what they want to look at? <
I can think of two reasons:
1) If non-axials really don't contribute much, then why even bother? That just clutters up the display with irrelevant information while obscuring the axials which I know for a fact matter a lot.
2) I can't speak for users of your spreadsheet, but a lot of the people that read my FAQ are learning for the first time that there even is such a thing as modes. So then I'd need to explain when they should "choose" to consider non-axials, even though I'm not convinced myself when or even if they should be included. This is why I'm trying to get to the bottom of this myself, so I can better advise others.
> There is nothing universal that can be said about tangentials versus axials. Certain modes will dominate in certain rooms and that's about all you can assume. <
This is exactly why I asked people to submit examples of measured versus predicted room modes! Jeff, I want to believe you. I really do. But as a born skeptic I need proof, dammit!
So far you are the only person here who has offered up anything concrete! One guy says he has some useful data, but won't share it because I once refused to hand over my proprietary information to a stranger in email. And now he's afraid I might learn something useful that - heaven forbid - might help me create a better bass trap product. Another guy claims he's been measuring rooms for 25 years, yet so far he's been unable to offer anything either.
> Two floor toms of idential diameter and height are evaluated by banging on them. Unless they are tuned identically and made of EXACTLY the same materials, they will sound different. <
That's a good analogy, and it sort of makes sense. I don't think normal rectangular one-layer sheet rock rooms will vary as much as a floor tom can be tuned, but I get your point. Again, I'm not disagreeing with you - just asking folks here to show me some concrete examples of non-axials showing up prominently in a room measurement. I'm an empirical kind of guy, and that's the kind of proof I find acceptable.
I've been insulted by a lot of people who took pot shots at me from the side lines over this, yet not one of them has stepped up to the plate with some hard data either.
Hey, the next time I come across a room with an obvious tangential issue, I'll post something. I'm sure there are many in the "archives." But I honestly never gave it much thought. I might be oversimplifying a bit, but a mode is a mode is a mode. Sometimes they behave, sometimes they don't. (Hmm...Perhaps there's a mode=NYC cabbie analogy in there somewhere...) The main place they all differ is not in amplitude, but in decay. Axials do take the longest to decay, in most cases. You could certainly draw some sort of "importance" conclusions from that, but the reality is - for me, anyway - that I find tangentials (not so much the obliques - they're much higher and tend to decay much quicker) to rear their ugliness enough not to omit them. My $0.02.
The only other thing I can think to offer is a repeat of avare's citation above; it shows one example of what you seek.
_________________ ---lovecow---
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z60611 Recording Org Pro Audio Forums
Joined: Dec 29, 2003
Posts: 820
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I find tangentials (not so much the obliques - they're much higher
I'm not fond of adjectives like 'much higher' because they mean different things to different people at different times in different conversations, so I looked up an example. In a room 12.46'x11.42'x7.9':
Lowest Axial: 45.3 hz
Lowest Tangential: 67.1 hz
Lowest Oblique: 98.1 hz
Ethan Winer Respected Past Moderator
Joined: Mar 19, 2001
Posts: 3209
Location: New Milford, CT USA
> the next time I come across a room with an obvious tangential issue, I'll post something. <
That would be great. But please understand, I'm not looking for a rare example where a non-axial mode turns out to be a problem. Rather, I'm more interested in learning how often non-axials actually are a problem. And when they are a problem, how big a problem are they really?
> a mode is a mode is a mode ... The main place they all differ is not in amplitude, but in decay. <
It seems to me there are only two damaging things any mode can do: 1) increase the amplitude in the room at its resonant frequency, and 2) cause that frequency to decay longer than it should and for longer than other non-modal frequencies. So what I'm looking for is something specific that shows the typical increase in level for an axial versus non-axial mode, and also the typical decay times for one versus the other.
I could accept without much argument that non-axials can contribute up to, let's say, 20 percent of the "damage" that an axial contributes. What I question is people's continually repeating that non-axials are "very important." That has been stated over and over again by Eric and his friends.
For example, in THIS thread at SoundOnSound Paul Woodlock claimed:
Quote:
"Ethan's room mode calc only shows AXIAL room modes. It completely ignores tangential and Oblique modes which have a profound effect on the rooms acostic performance."
Profound? How profound? Please prove it with scientific data.
And in THIS mini-article at StudioTips Eric Desart uses the largest, boldest, red printing he can to make sure everyone knows to avoid crappy mode calculators like mine because, as he says:
Quote:
"NEVER use a Room Modes Calculator which DOES NOT take the Tangential and Oblique modes into account."
This is what I'm asking for proof of.
> The only other thing I can think to offer is a repeat of avare's citation above; it shows one example of what you seek. <
I don't have that book, so how about the Reader's Digest version? Does it even show what I'm specifically asking about? Would it be too much to ask you or Andre to post a scan of the single most relevant graph? Note that if this book presents data for 12 rooms and only one shows non-axials contributing much, I'd expect that to be stated too.
I'm willing to accept that tangentials are important simply by combining
a) Einstein's version of the limits of Occam's Razor: "Things should be as simple as possible, but no simpler"
b) Rayleigh's 1869 mode equasion is more complex than simply taking room lengths, divide by two, and integer multiples thereafter to get axial wavelengths. i.e. Tangentials have been accepted for 136 years.
If Figures 10.4 and 10.5 on pages 290 and 291 of "Acoustics" by Leo L Beranek are the one's Avare is thinking of, then it looks to me as if the first room test has the first three modes of almost equal strength the third mode of which is tangential, and the second room the tangentials and obliques are the strongest modes in the room, outweighing the first two axials by 13db.
The first test was done with the microphone in one corner and the speaker in the other. The second test was done with the speaker in the middle of the room.
The test room, if I may use a pair of adjectives after my post to Jeff above, is a very small room.
Ethan Winer Respected Past Moderator
Joined: Mar 19, 2001
Posts: 3209
Location: New Milford, CT USA
> I'm willing to accept that tangentials are important simply by combining <
Sorry, but that's just hearsay and "arguing from authority." I need proof in the form of hard data.
> If Figures 10.4 and 10.5 on pages 290 and 291 of "Acoustics" by Leo L Beranek <
I just searched the Barnes & Noble site and apparently this book is out of print, and even used copies are not available. So all the more reason to ask you guys to post a relevant page or three.
> it looks to me as if the first room test has the first three modes of almost equal strength the third mode of which is tangential <
How many rooms total are shown, and of those, how many show tangentials as being prominent? This is the crux of it. More to the point, a graph of the raw LF response doesn't tell nearly as much as a plot of the decay at each mode frequency. Peaks and nulls happen at all frequencies, depending on position, so measuring a peak does not necessarily indicate a mode.
Also, if tangentials can be shown to be important, but not obliques, then why waste time and graph "real estate" with obliques?
> The first test was done with the microphone in one corner and the speaker in the other. <
Yeah, that's what Jeff says he does, which makes sense.
> The second test was done with the speaker in the middle of the room. <
I'd rather see something that shows how important non-axials are with normal speaker and microphone (listener) placements, but I'll gladly look at anything y'all have.
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) If you're going to take the time to predict modes, it might as well be all of them. Your attempts to justify this with speculation that "non-axials" aren't important can appear to some to be dodging the issue. Why don't you simply go back, program in the tangentials and obliques, and give the user the ability to pick and choose what they want to look at? That's what my spreadsheet does. I've never had anyone complain about it...
Another guy claims he's been measuring rooms for 25 years, yet so far he's been unable to offer anything either.
