Room modes, calculated versus measured

Posted: Fri Mar 18, 2005 4:33 pm

Jeff,

> I would like to reiterate that I think the book as a whole is quite good. <

Of course! I thought my humor was obvious enough that a smiley wasn't needed!

> (I am willing to give any author the benefit of the doubt having had my own share of "editing" nightmares...) <

No kidding. It's a real problem especially when editors do not send you a copy for approval, as happened when I wrote my article for Electronic Musician last year. In my opinion, every single edit made the text less clear than what I had submitted. I am not using this as an excuse for anything that can be perceived as a technical error in that article! Smile

But if anyone actually cares, the original version as I wrote it is on my company's web site, and the edited version is on EM's site.

--Ethan

Posted: Fri Mar 18, 2005 4:36 pm

Rod,

> any mode calcs based on room size are going to appear to be "wrong" when the room is subsequently tested. <

Agreed. Being off by a full 20 percent seems a little extreme to me, but there's no arguing with your basic premise.

> It surprises me that Mr Everest (being a very bright man) would be dissapointed by the inability to accurately predict reallity in this case for what he considered a "typical" bedroom. <

I put "disappointing" in quotes because that's exactly what he wrote.

--Ethan

Posted: Sat Mar 19, 2005 12:44 pm

Quote:

On page 332 he compares the predicted modes versus what was actually measured in a typical bedroom

Is this from the bedroom with:

asymmetrical doorway/entrance,
asymmetrical door/closet, and
sloped ceiling?

If so, together Rod's points about construction, the correleation is about as good as can be expected with ALL those deviations from the theoretical ideal model.

Waiting clarifiaction;
Andre

Posted: Sat Mar 19, 2005 2:17 pm

Andre,

> Is this from the bedroom with:
> asymmetrical doorway/entrance,
> asymmetrical door/closet, and
> sloped ceiling?

I didn't see anything about asymmetry, only the angled ceiling. But you are correct, this is a pretty poor room to use if the goal is to equate predicted versus measured modes. Though that still doesn't explain the disparity I measured in my perfect rectangle. Maybe if any other people here have a perfect rectangle room with no bass traps, they'll measure and report too. We'll call it the RO Room Mode Case Study Report. Smile

--Ethan

Posted: Sat Mar 19, 2005 4:49 pm

Ethan Winer wrote:

Though that still doesn't explain the disparity I measured in my perfect rectangle.

Ethan,

I've explained the disparity - it has to happen - believe it or not.

Rod

Posted: Sat Mar 19, 2005 6:11 pm

Rod,

> I've explained the disparity - it has to happen - believe it or not. <

Well, sure, of course I believe it. I mean, something has to account for the case that known math is off by 20 percent. I don't see how speaker and mike placement are a factor, because when I've measured in other places in the same room I get the exact same frequencies. Further, the frequencies of inherent resonances shouldn't vary with placement, only their relative strengths. That leaves the boundary properties. Can anyone here suggest other factors that could account for this difference?

So now we're left with two remaining questions:

1. If a perfect rectangle using standard sheet rock construction can be so far off from what's predicted, what value is there to performing a mode calculation in the first place? Yes, I am serious!

2. Since including non-axial modes gives a very large series of possible resonances, most of them very close together, how can one look at a response plot and say with certainty which modes are axial and which are not?

Thanks.

--Ethan

Posted: Sat Mar 19, 2005 7:19 pm

Ethan Winer wrote:

I don't see how speaker and mike placement are a factor, because when I've measured in other places in the same room I get the exact same frequencies. Further, the frequencies of inherent resonances shouldn't vary with placement, only their relative strengths. That leaves the boundary properties. Can anyone here suggest other factors that could account for this difference?

Ethan, figure that the lower the frequency - the greater the build-up within the space as you add mass and decrease the amount escaping......... seeing as the caculation assumes no energy escapes at all - the more isolated you make the room the closer you would come to the model.

As far as speaker and mic placement goes - it should effect the results........... I'm surprised to hear you say that it doesn't - the source relative to a surface woud have to effect things - as I move the source closer to a surface I would force higher frequencies to come into play - and the further away I got the more of an increase in lows........ each new distance from a surface would have a corresponding frequency which would either be in-sync or out of sync - with corresponding 1/4 waves. The only thing that I would expect to see as a constant would be boundry effect - which - assuming the source was strong enough to maintain the effect - would remain constant.

So any constant would be boundry effect - and everything else should be shifting as you move around the room.

How is anything else possible?

Think about it -

Quote:

So now we're left with two remaining questions:

1. If a perfect rectangle using standard sheet rock construction can be so far off from what's predicted, what value is there to performing a mode calculation in the first place? Yes, I am serious!

Personally, I'be been asking myself that question for quite a while now......... and yes I'm serious.

If you design rooms using reported acceptable ratios, and large enough volume - and still have to test and treat the room afterwards - what (other than our infinate fascination for acoustic mathematics) is the reason for bothering? I for the life of me cannot figure that one out.

It makes for interesting conversation - but not much else I can see. Not from a practical sense............

And most people making home studios do not have the luxury of begining with perfect ratios with lots of volume - so for them it makes no sense at all.

Regardless of what any model may predict for their room they still have to check and treat to make it work.

IF THE MATH WORKED PERFECTLY WE WOULD HAVE NO NEED FOR THE SOFWARE/HARDWARE REQUIRED FOR TESTING - AND THUS THERE WOULD BE NO MARKET FOR IT.

WHY BOTHER TESTING FOR THAT WHICH YOU KNOW EXISTS?

Quote:

2. Since including non-axial modes gives a very large series of possible resonances, most of them very close together, how can one look at a response plot and say with certainty which modes are axial and which are not?

Assuming you accept my answer for 1 above - it's a moot point,

I am however - wide open to more expert opinions than mine in this regards.

Sincerely,

Rod

Posted: Sat Mar 19, 2005 9:03 pm

Rod Gervais wrote:

Think about it -

Quote:

So now we're left with two remaining questions:

1. If a perfect rectangle using standard sheet rock construction can be so far off from what's predicted, what value is there to performing a mode calculation in the first place? Yes, I am serious!

Personally, I'be been asking myself that question for quite a while now......... and yes I'm serious.

I would agree.

I was thinking about this very notion while studio building last night.

I had a perfunctory look at a mode calc while designing my studio. But rooms i've measured with ETF before have never correlated to any mode calc results, so I never took much notice of them.

The only thing I can think of that might merit a look in a mode calculator is when you've a choice of room dimensions. The lucky few!

However that would only be useful if we were certain that a room that showed an even spacing of modes, actually had an even spacing of modes in real life.

So really, why bother? Everyone bungs in broadband absorption anyway ( mostly too little, but that's another story ), and if you do need extra troops n the way of tuned absorbers to ridthe final abberations, then you're gonna use the test results and not the calculated results aren't you?!! Smile

There ya go Ethan! Either rename your ModeCalc, to WhyBotherCalc, or take it off the market - hehe!

Quote:

WHY BOTHER TESTING FOR THAT WHICH YOU KNOW EXISTS?

...
Rod

ahem, erm, well.... to make sure you built it correctly??? Very Happy

Paul

Posted: Sat Mar 19, 2005 11:41 pm

Ethan Winer wrote:

Paul,

> I mean this sincerely in a non combatent way <

Okay, me too. But you have to let me make my points. And you have to read them and at least try to understand them! Please?

Yes of course Smile

As said my objective is to try and put a stop to the 'Feud'

Understanding each other by talking is a key part of that process. I'm pleased your starting to drop some of your combative barriers and open up to some personal chat. It's very important.

And when I say 'personal', I don't mean 'insulting personal', I mean we have to understand the human side of each other to make progress.

After all a lot of the issues fuelling the feud are misunderstandings.

Quote:

> You are only taking it as an insult because your still locked in defensive mode ... I love Eric to bits, but I do see he has built up an automatic negative response to you as well. <

I joined those two statements from opposite ends of your post to show that I'm not the only one who may be responding on automatic pilot. I could love Eric too - I really could! - if only he would act more civil toward me.

No, I'm not on automatic pilot at all. I was in the Battle of SOS, but not now. I'm fed up with wasting negative energy on this feud.

I'm sure there will be a day when Eric is civil to you. It might not be tomorrow, or the next day, but it will come. Eric is uncivil to you IMO for two reasons. 1] He's on combative auto-pilot, and 2] He dislikes some of the things you've done.

We've all done things that are offensive. It's no great shakes at the end of the day if we recognise out mistakes.

Quote:

> Firstly, how do you define 'Expert' ? Is Eric an Expert? Is Jeff an Expert? Well compared to you and me, and just about everybody else on these forums, YES THEY ARE!! <

Please speak for yourself. Nobody is an expert in everything. Not me, not you, and not Eric.

I'm sorry, I don't undertstand your reply in regard to 'expert at everything'. I didn't mention ANYONE being an expert at 'everything'.

However I did read a slight defense of my suggestion that you were less of an expert than Eric and Jeff. Can I ask why being seen as less of an expert than Eric and Jeff matters?

Quote:

I see other shortcomings because of Eric's (self-admitted) lack of understanding about electronics and music. Acoustics and electronics are very similar, and a lot of the same principles apply. Likewise for music and musical instruments. So in that regard my extensive background in electronics and music theory, and knowing a lot about how string instruments work, helps me to understand quite a bit about acoustics.

I too have a background in music, instruments and electronics stretching back over 30 years. It also helped a great deal in understanding Acoustics. So yes, I know where you're coming from Smile

However I can't see how Eric's lack of understanding of electronics and music is a shortcoming to acoustics.

To me it's a no brainer.....

You and I have similar backgrounds. We both entered the House of Acoustics at roughly the same time. Myself in order to build a studio ( although I'm interested in it all now Smile

), and you I kindly presume to expand your academic and practical knowledge for both pure interest and improving your products.

Eric on the other hand has been in the House of Acoustics for decades.

Yeah, sure you might know a couple of things about acoustics than Eric. And that's great. But Eric's scope and wealth of knowledge comes from decades of being in that Acoustics House. And i know Eric would be the first one to admit that he's still got a lot to learn himself.

People don't expect you and I to be as knowledgable as Eric about Acoustics. And speaking for myself I Lose no pride in people knowing my place in the acoustical knowledge charts.

The notion someone being more knowledgable than you seems to trouble you. I find that a bit odd really?

My stating this has nothing to do with pride, and everything to do with my point that different people excel in different areas - even within the general umbrella of "acoustics."

Quote:

> Half the problems of this feud are caused by this 'Expert Issue' and the pride/ego that's fuelling it. <

Yes, but not only me!

Paul, I have less ego with this stuff than you think, and I have never hid the fact that I've been seriously into acoustics for only three years now.

Ethan, if that's really in your heart then your public perception is rather different.

I've been taking a close view of the personal side of this feud since it started. And not just you but everybody.

People on both sides, including myself, have done negative things btu really impartially speaking you have let your ego take control a lot more than anyone else.

The mention of 'expert' in my last post, and your subsequent reply was a mild example.

I would have thought someone of your knowledge an experience wouldn't need that kind of reassurance. Smile

Quote:

If we could keep the discussion to the facts, and not who's more of an expert, or who deserves to be taken more seriously, much of this would never have been an issue.

Ethan it's not about facts, it's about behaviour.

Keeping the discussion to the facts is the sole reason the feud has continued. the feud isn't about acoustics, it's about personalities

Quote:

> Look at your magazine article in EQ for example. It contained innacuracies. <

Such as?

This is a big problem with you and some of the others, claiming errors in my articles and FAQ, but when challenged never having an answer. Before you reply to anything else I've written in this post, please tell me what's wrong with any of my EQ magazine articles.

I'll reply to this in the future, as I'm getting too knackered now. It's not to allow me time to gether facts by the way. My reply won't contain any.

Quote:

> The same principles apply to your Modecalc. Why keep defending it? Why not simply improve it to contain Tangentials and Obliques? <

Because I'm still not convinced that adding them will make my program a better tool! In fact, I fear it will become less comprehensible to the very people who need it most.

> I thought you were a programmer? <

Yes, I am absolutely an expert programmer, proven by my best-selling book (from years ago) that's now on my site as a download. Being able to program non-axials is not the issue at all.

> It almost seems you're challenging the importance of non-axial modes simply to defend your Mode-Calc? <

No, I'm challenging the importance of non-axials because I'm not convinced they're as important as some people say they are. I have already stated here that I don't doubt they account for something. But if they're a problem only sometimes, and even then contribute only a little, why bother? More to the point, how do you explain to a newbie how much weight to give the non-axials?

Suppose someone is designing a basement home theater and they enter their proposed dimensions. Now, when they look at all the modes it might look like the distribution is good when it's really not because the axials, which are far more important, are too widely spaced. Maybe a different type of display could be devised, where axials are shown more darkly, and the others progressively less dark to better show their true importance. Again, I already said I'll consider that. But I need to be convinced they matter enough to include at all. Wes Lachot said here "axial modes are by far the most important modes," and I quoted something similar from Everest. These guys are experts too, more so than you or I. You can't ignore this, or say it's only Ethan who dismisses non-axials!

Thanks in advance for addressing the points I've made here, and not injecting ego or pride into the discussion.

--Ethan

I'll reply to the rest tomorrow hopefully. I got distracted by some music, and came back to finish the post, but I'm too drained now. It's nearly 6am here.

laters

Paul

Posted: Sun Mar 20, 2005 4:36 am

Paul,

Can you please stop discussing Ethan versus Eric and vice versa.
Maybe you feel good with it, but don't act as a third party.

I defend acoustic topics, and try to set wrong things straight.
And do not concentrate all that happened specific to me, this concerns lots of people.

What Ethan thinks, finds or believes doesn't matter to me. It only matters when it relates to the interest of others in function of acoustics, scientific and other integrity, etc.

And I don't say a thing here: just ask to stop discussing Ethan versus me and vice versa as a third party on the side-line. Do that in private if you like.

I'm not more or less clever than I am.
And if Ethan feels better to look cleverer by showing, or trying to, how less clever I am, than that's OK for me, as long it's not at the expense of the content or whatever [direct or indirect] in function of others.

Only his urge to do so tells something in itself.

BTW (not to Paul): I don't ""admit"" I don't know electro, I just tell it as a matter of fact or info, as I normally do if I don't know whatever.

That's what my profile says:

Quote:

Interests:
Physics - Nature - Studio acoustics - Enjoying people, truth & genuineness. Ignorant: Electro

And that's what drives me. I just fight/stand for the things I believe in. And show myself.

I know you mean well,

Posted: Sun Mar 20, 2005 9:03 am

Rod,

We're not really in disagreement here, I'm just thinking out loud. I do that a lot. Wink

> As far as speaker and mic placement goes - it should effect the results ... I'm surprised to hear you say that it doesn't <

I didn't say it doesn't. I said, "I don't see how..." Which is very different.

Obviously boundary interference can affect measured results, but as Bob pointed out earlier (Page 6), we can distinguish modal response from boundary interference by looking at the decay times. When I look at the modes measured at five places around the room all the frequencies stay the same.

> Personally, I'be been asking myself that question for quite a while now......... and yes I'm serious. <

I thought more about the practical implications of this last night. Suppose someone is building a studio in a basement. The floor is concrete, two walls will be concrete, they'll be building two walls out of sheet rock - maybe one side of the studs or both, and maybe one layer or two - and the floor above is open joists (or maybe not) with some floor surface above that.

Following conventional acoustic advice they consult a mode calculator to find the best place to put the two new walls. Oops. The floor to ceiling mode ends up being off by [who knows how much] and so do the other two wall-wall combinations. So now their perfect ratio is off by [who knows how much] in [who knows which direction].

Wow.

--Ethan

Posted: Sun Mar 20, 2005 9:13 am

Paul,

> I had a perfunctory look at a mode calc while designing my studio. But rooms i've measured with ETF before have never correlated to any mode calc results, so I never took much notice of them. <

Another one chimes in with a disparity. Paul, I really think we're onto something here.

I also agree with the notion that regardless of what you end up with, or are stuck with, the solution is as much bass trapping as you can possibly manage. You wouldn't believe how much heat I've taken over the past few years for stating that exact position. Mark's ETF graph you posted was absolutely fabulous, because it proves that point beyond dispute to the ignorant nay-sayers.

--Ethan

Posted: Sun Mar 20, 2005 10:02 am

Paul,

> No, I'm not on automatic pilot at all. <

I wasn't referring to you.

> Can I ask why being seen as less of an expert than Eric and Jeff matters? <

I am fully aware of how much more Jeff and Eric know about some aspects of acoustics. Look, I'm not an idiot. And I have acknowledged my lack of math skills many times. But I do know an awful lot about this stuff, including a lot of things they don't. I'm tired of arguing about who knows more about which subset of what subject. The content on my Articles and other web pages on my personal site stands on its own. I'm the one with two books and nearly 100 published articles. (Have you seen all the recent activity in the audiophile forums about my article that just came out in Skeptic magazine?)

> I can't see how Eric's lack of understanding of electronics and music is a shortcoming to acoustics. <

Oy vey.

Knowing how inductors and capacitors work gives a much deeper understanding of wood panel membrane traps.

Wes Lachot loves to point out the relation between mode spacing and musical intervals.

My understanding of electronics is why I know a standing wave is not a mode, even though the non-electronics guys don't get it.

I learned a lot about comb filtering and what causes it when I built bucket brigade delay lines and pitch shifters and a stereo phaser effect - more than 25 years ago.

Designing and playing around with parametric equalizers will teach you, in a day or two, more about resonance than a year's worth of reading about it in college text books.

And so on.

> The notion someone being more knowledgable than you seems to trouble you. <

Hell no! This is your best argument? Look at some of my technical articles and you'll see frequent reference to people who taught and guided me. I know exactly what I know and what I don't know. It's obvious you don't because you've never taken the time to find out.

> I'll reply to this in the future, as I'm getting too knackered now. It's not to allow me time to gether facts by the way. My reply won't contain any. <

If you have no facts, then please don't bother. And if you have no facts, please stop stating that my articles and Acoustics FAQ are wrong. If anyone owes anyone an apology here, it's you to me for making these claims over and over and then failing to back them up. (In fairness, you're not the only one who's done this.)

In this case I agree completely with Eric's post above. Please stop making this about personality defects, or who knows more than whom, or attitude, or ego, or anything else not related to the science of acoustics. I'm not going to change, and neither are you, so let's move on to what really matters. Deal? Thanks.

--Ethan

Posted: Sun Mar 20, 2005 1:10 pm

Quote:

1. If a perfect rectangle using standard sheet rock construction can be so far off from what's predicted, what value is there to performing a mode calculation in the first place? Yes, I am serious

The plots you made were of room response/deacy rates, NOT modes exclusively. Resonant behaviour from:

doors,
spaqces adjacent to doors (hallways, etc),
windows,
HVAC ducts,
ceilings, and
cable raceways/ducts,

to name a few.

The most blatent indication of avoiding the construction factors is your "standard sheetrock construction" phrase. NRC's IR 761 has over 350 different drywall wall constructions! Granted it goes into double layers, RC etc, but that still leaves so mauch to vary. Just some of these variables are:

board thickness,
board quantity per side,
stud spacing,
stud metal or wood,
metal stud metal thickness,
presence or absence of insualtion in cavity,
type insulation in cavity, and
thickness of insulation in cavity.

That does not even start to get into more more expensive but standard (remeber IR 761 was driven by the constuction industry, not the studio world) designs as double stud, staggered stud, and thicker studs.

Add to that above if "standard" constuction techniques were used, then the acoustic performance of the walls are not even close to what they wouod be with tight tolerances.

There is your 20 Hz discrepancy

Andre

Posted: Sun Mar 20, 2005 1:26 pm

Andre,

> There is your 20 Hz discrepancy <

That's all great stuff for sure, and not unlike what Rod said.

Now all we need is an explanation for why one type of wall versus another would shift the modes, and by what mechanism, and why they shift up rather than down.

Any ideas?

--Ethan

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