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Calculating room modes for an irregular shape
Old 15th December 2010
  #1
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recordinghopkins's Avatar
Calculating room modes for an irregular shape

I understand the math to calculate room modes, but I am only sure of how to implement it with a square or rectangle room. Can someone help me understand how to calculate the modes for this room?

Thanks!
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Calculating room modes for an irregular shape-studio-g.jpg  
Old 15th December 2010
  #2
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Jens Eklund's Avatar
I use SoundEasy (Bodzio) that can handle BEM or FEM models. If the room has solid boundaries and not many openings, the precision of the prediction is very accurate, at least for the first orders of modes, up to approx. 150 Hz in this example:

https://www.gearslutz.com/board/5736162-post61.html

Sincerely Jens Eklund
Old 15th December 2010
  #3
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Ethan Winer's Avatar
 

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Quote:
Originally Posted by recordinghopkins View Post
Can someone help me understand how to calculate the modes for this room?
There's no need to calculate modes for an existing room. Room mode calculators give only an approximation at best, but it's better than nothing when designing a new room that's not yet built and thus can't be measured. If you already have a room, then all you need is to measure it! Here's my standard list of room measuring links:

Room EQ Wizard, Windows and Linux and Mac OSX 10.4+, Freeware
ETF, Windows, $150
FuzzMeasure, Mac, $150
Room Measuring Primer
Comparison of Ten Measuring Microphones

But even measuring is not strictly needed, since the treatment strategy is always the same in rooms this size: As many corner bass traps as possible, plus mid-high frequency absorption at reflection points and near performers and microphones. This short article explains the basics in plain English:

Acoustic Basics

--Ethan

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Old 15th December 2010
  #4
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I don’t agree.

A modal calculation can speed up the process of identifying each mode and this is needed if you don’t want to create a dead room (lots of porous material in the corners in addition to the first reflection treatment), and instead using, more or less, tuned traps to avoid absorption thus keeping the energy that hopefully will be diffused and return to the sweet spot.
Old 15th December 2010
  #5
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Ethan, with all due respect, I think that some of the information you are giving is subjective. If you want to get 90% there go ahead and put porous absorption everywhere and call it a day. However, if you want a better room, it is imperative to calculate predicted modal behavior and conduct extensive tests in order to know what to treat.

There are other options than just stuffing bass traps and absorption everywhere.

Again, I have a huge amount of respect for you and your work, but giving out answers like those is sort of a cop out in my opinion. I think you're better than that.

Sincerely,

Neil
Old 15th December 2010
  #6
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Jens Eklund's Avatar
+1 Neil.

/Jens


EDIT: And to further complicate matter:

https://www.gearslutz.com/board/5939703-post11.html
Old 15th December 2010
  #7
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Quote:
Originally Posted by Jens Eklund View Post
A modal calculation can speed up the process of identifying each mode
Quote:
Originally Posted by amishsixstringe View Post
Ethan, with all due respect, I think that some of the information you are giving is subjective.
LOL, but sorry guys, you're both wrong. First, this room is not a rectangle, so good luck calculating all the modes accurately. Second, measuring takes 5 minutes, plus a little prep the first time (only) to learn the software. Third, no mode calculator can predict modes accurately, even in a perfectly rectangular room, due to variations in wall compliance etc. Fourth, not all room acoustic problems at low frequencies are related to mode frequencies because of SBIR and LBIR which is why we always suggest broadband absorption at bass frequencies. So, as always, measuring trumps prediction every time.

I stand by my advice to put bass traps in all of the corners, because that will solve both the mode and SBIR problems regardless of what frequencies they occur at. And there is nothing subjective about that.

I also find this offensive and uncalled for:

Quote:
Originally Posted by amishsixstringe View Post
go ahead and put porous absorption everywhere and call it a day ... giving out answers like those is sort of a cop out in my opinion. I think you're better than that.
I never said to "put absorption everywhere," and in fact I was specific about what to put where. Had the OP included more detail in his pics I could have been even more specific about panel placement.

--Ethan

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Old 15th December 2010
  #8
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You think that it is not a fact that a modal calculation can aid in identification of a measured response? That's just silly. If a modal calculator says there will be a strong axial room height mode at f, and a measurement comes up with a value of the same, it seems to have aided in confirmation of that mode.

If said mode is 30Hz and rings for a second, I'm not going to to trust porous bass trapping in the corners to fix that problem.

You are right though, the OP did not issue enough information to recommend any specific treatment advice. But, you gave it to him anyhow with the usual 'as many corner traps as possible' advice, which is not always the greatest solution. That's like puting a band-aid on a wound that needs stitches.

I am sorry if I offended you. It was not my intention. It is meant to be a healthy debate, not a targeted attack. There is more than one way to skin a cat, and it is sometimes important to bring that up. Not every problem has the same solution.

Neil
Old 15th December 2010
  #9
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After looking back at the original post I can't even see how we got here. The OP was looking for a way to calculate room modes of a non-rectangular room and for some reason we are assuming that he is calculating to apply treatment. It appears the room is already treated based on the image. Perhaps, he's already tested the room and knows of some its issues and wants to confirm them? But, these are all just guesses...wouldn't help you if they could.

Neil
Old 15th December 2010
  #10
JWL
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Predicting the modal behavior of irregular rooms is extremely complex. I think Ethan's point, which I agree with, is that the amount of work required to perform these calculations (which will be imperfect and theoretical anyway) is better spent measuring/testing the room anyway, which gives much more specific and useful information, and is a lot less work.
Old 15th December 2010
  #11
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My point is that it literally takes seconds to punch numbers into a mode calc, so what does it hurt? If the numbers line up, then it is obvious reinforcement of the theory and the field numbers.

Neil
Old 15th December 2010
  #12
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Allow me to jump in defense of Ethan. I think he does an awful lot for this community. Many, MANY people owe him much gratitude for his contributions.

That said, I do take issue with this portion of advise:

Quote:
Originally Posted by Ethan Winer View Post

But even measuring is not strictly needed, since the treatment strategy is always the same in rooms this size: As many corner bass traps as possible, plus mid-high frequency absorption at reflection points and near performers and microphones. This short article explains the basics in plain English:

Acoustic Basics

--Ethan

________________
The Acoustic Treatment Experts
This happens an awful lot around here. The heavy posters who supply a ton of people with a ton of info are often missing the point of the ? in order to post their FAQ's link. The OP is asking about rooms of irregular shape. Based on his question and the photo provided, one should deduce we are talking about his live room. "mid-high absorbers at reflection points" is irrelevant and confusing given the question. Furthermore, assuming "all rooms this size" require the same thing is kinda silly. This performance space is approaching 500 sq. ft.... and we have no insight as to the typical recording protocol. There are a multitude of options available to a owner of a live room that has a "color" opportunity, as opposed to a "control" environment.

Perhaps the OP is requesting modal calculations in hopes of adding to a boundary to qualm an overlap in modal support, but looking for a estimation of gain or performance handling before undertaking the task.




Ethan's "plain English" style is more than even some people are looking for or can handle. Without guys like Ethan around here, I'd love to get a chuckle out of SAC's response to the same question everyday.


I would simply ask the retailers to read a bit more carefully before posting the links, as this is not the first time I've found the response to be unrelated or barely vaguely to the question asked.
Old 15th December 2010
  #13
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Jens Eklund's Avatar
Quote:
Originally Posted by Ethan Winer View Post
LOL, but sorry guys, you're both wrong. First, this room is not a rectangle, so good luck calculating all the modes accurately.
Quote:
Originally Posted by jwl View Post
Predicting the modal behavior of irregular rooms is extremely complex. I think Ethan's point, which I agree with, is that the amount of work required to perform these calculations (which will be imperfect and theoretical anyway) is better spent measuring/testing the room anyway, which gives much more specific and useful information, and is a lot less work.

Quote:
Originally Posted by Jens Eklund View Post
If the room has solid boundaries and not many openings, the precision of the prediction is very accurate, at least for the first orders of modes, up to approx. 150 Hz in this example:

https://www.gearslutz.com/board/5736162-post61.html
What I forgot to mention but naturally important and absolutely necessary, is proper confirmation of the prediction (measurements).


/Jens
Old 15th December 2010
  #14
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Ethan Winer's Avatar
 

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Quote:
Originally Posted by amishsixstringe View Post
If the numbers line up, then it is obvious reinforcement of the theory and the field numbers.
And when they don't line up? heh

Seriously, have you ever tried this? I have, and I've seen modes as predicted differ from what was measured by almost 20 percent. I urge you to measure any "home size" rectangular room you have available, then post a waterfall (or plain FR) graph here along with the room dimensions. Yes, sometimes they align more or less, but often they do not align.

Quote:
If said mode is 30Hz and rings for a second, I'm not going to to trust porous bass trapping in the corners to fix that problem.
What frequency a mode occurs at, and what type of bass traps should be used, has no relation to calculating modes versus measuring.

--Ethan

________________
The Acoustic Treatment Experts
Old 15th December 2010
  #15
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Quote:
Originally Posted by Ethan Winer View Post
I stand by my advice to put bass traps in all of the corners, because that will solve both the mode and SBIR problems regardless of what frequencies they occur at. And there is nothing subjective about that.
?

We don’t even know the positions of the source and receiver so how can you say that porous in the corners will cure SBIR?
Old 15th December 2010
  #16
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Foot in mouth.

I now see as I look closer, the 29' dimension relates to the total length of the space... indicating a one room set up. Perhaps I too need to read more carefully.
Old 15th December 2010
  #17
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Jens Eklund's Avatar
Quote:
Originally Posted by Ethan Winer View Post
And when they don't line up? heh

Seriously, have you ever tried this? I have, and I've seen modes as predicted differ from what was measured by almost 20 percent. I urge you to measure any "home size" rectangular room you have available, then post a waterfall (or plain FR) graph here along with the room dimensions. Yes, sometimes they align more or less, but often they do not align.
I have measured a lot of rooms and I’m always surprised how well predictions fits real life. Naturally this will not be the case in a room with very lossy walls, lots of big openings and somewhat treated but the lowest modes usually fits more or less always and in the nonrectangular rooms it can be a hassle to try and manually trace the distribution of modes so a model is very useful, especially when it fits real life like a glove.
Old 15th December 2010
  #18
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Glenn Kuras's Avatar
recordinghopkins,
Trouble maker!!!!!!!!!!!!!! How dare you ask such a question. See what you have done? The acoustic world will never be the same. lol lol lol
Old 15th December 2010
  #19
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This is just silly. Why would any scientist knowingly skip a step? Sample size is important. Flip a penny 1 time. It landed on heads. That means that flipping pennies yields a result of "heads". Right? What's the first thing people do when a doctor tells him/her he/she has cancer? Get a second opinion.

Neil
Old 15th December 2010
  #20
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Ethan Winer's Avatar
 

Lightbulb

Quote:
Originally Posted by johndykstra View Post
The heavy posters who supply a ton of people with a ton of info are often missing the point of the ? in order to post their FAQ's link.
I post a lot of FAQ links, but I do that because many people don't do much research before asking questions. Not that I really blame them. I believe in my FAQs having written all of them myself based on years of experience, and knowing how to write in plain English with no math. If it were up to me I'd put a sticky at the top of this forum with links to all the non-sales-driven articles and videos on my company's site. heh

Quote:
"mid-high absorbers at reflection points" is irrelevant and confusing given the question.
Well, I did say to put mid-high frequency absorption "near performers and microphones."

Quote:
I'd love to get a chuckle out of SAC's response to the same question everyday.
LOL. Point.

--Ethan

________________
The Acoustic Treatment Experts
Old 15th December 2010
  #21
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Jens Eklund's Avatar
If i find the time, I can post a comparison between SoundEasy and a real room (nonrectangular). Both frequency and the location of the nodes (first 4 modes for the node locations). I measured the nodes as well since I wanted to know how accurate the model was.
Old 15th December 2010
  #22
JWL
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Quote:
Originally Posted by amishsixstringe View Post
My point is that it literally takes seconds to punch numbers into a mode calc, so what does it hurt? If the numbers line up, then it is obvious reinforcement of the theory and the field numbers.
If it were that simple -- as it is with rectangular rooms -- then I'd agree. Point is that it is not that simple with complex rooms.
Old 15th December 2010
  #23
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Jens Eklund's Avatar
Quote:
Originally Posted by jwl View Post
If it were that simple -- as it is with rectangular rooms -- then I'd agree. Point is that it is not that simple with complex rooms.
But if you have a modal calc that can handle a complex shape, it’s almost as simple.

... no, I don´t sell SoundEasy ... heh
Old 15th December 2010
  #24
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Exactly. You can do it very easily. You just have to pay for the software, which some people don't seem to be so eager to do since it doesn't model a fender twin or something.

Neil
Old 15th December 2010
  #25
SAC
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Traditional room mode calculators already function at a low level given their ideal assumptions, and function very poorly in simple irregularly shaped rooms, if at all in 'seriously' irregular shaped rooms.

And if the room is already built, they are akin to playing with a Ouija board. As any potential agreement is not almost arbitrary but only verified by empirical testing or measurement anyway.

One wonders what the benefit is, aside from keeping math and physics grad students off the streets and insuring that they will never have a social life, in using such an imprecise at best, and utterly erroneous at worst, exercise when one can simply measure and know the precise behavior in a very short period of time with minimal effort.

One is reminded of the account described by Shelby Foote in the Ken Burns' Civil War special where upon reporting to Gen. McClellans's army, Custer discovered the army stalled at a river because the engineer's weren't able to determine the depth of the river with the technology they had at hand. After listening to several hours of debate between the engineers regarding what the depth might be, Custer rode his horse out to the middle of the river and turned to the astonished officers of the staff and shouted, "That's how deep it is, Mr. General!"

On the other hand there was the defense department's weather prediction algorithm at Lawrence Berkeley 10 years ago (when we were dealing with it and others) on their massively parallel SP complex that could, upon input of all of the myriad variables, quite accurately predict the next days weather. The program took two days to run to completion. But it was uncannily accurate. :-S

Constructing abstract processes for real phenomena, which by virtue of the ideal and simplistic assumptions are simply guesstimates, can be fun. And sometimes in the absence of empirical tools they can provide at least a semblance of what conditions one might be subsequently confronted. But sometimes, when faced with real complex constructs, it is more prudent to simply cut to the chase and determine not what is predicted, but rather precisely that which is actual based upon all of the real world variables.
Old 15th December 2010
  #26
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Why not just for fun have Jens put the number in his calculator and have the OP measure the room's response with a loudspeaker and omni mic across the longest dimension of the room corner to corner. Then we can see the results too? I'm just saying if you have the tool, and its simple, there's no harm in looking at it.

Neil
Old 15th December 2010
  #27
SAC
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It may not hurt, but does it help?

And the distribution is what?

Its nice to know that so many variable that we (or at least some) are aware modify such behavior, but which are utterly and completely ignored in modal calculators (as all bounded spaces are amazingly indeed ideal rectangular polygons featuring perfect boundaries with a constant acoustical impedance that just coincidentally happen to be 100% reflective...) are inconsequential.

But why does one even need a modal calculator? Just measure the longest boundary dimension (and ignore the diagonal despite it determining the lowest mode and just worry about the base modal energy...right?) . And its not like single valued algebra is that complex. A simple calculator will do.

And after all is said and done, you have a number. No distribution nor anything else of practical value as we are just going to stuff porous traps into the corners anyway...

In a real rooms, measurements trump calculations based upon ideal assumptions in EVERY case, be it for modeling or treatment. And for someone familiar with what they are doing, measurements are often as quick or quicker.

The irony is that this debate, like McClellan and his engineers, takes longer than the measurement and in the end you are still stuck with speculation.

It must be fun to watch as some of you attempt to determine one's weight, not by using a scale or even a water displacement method, but by attempting to calculate the precise volume geometrically. And 3 days later you manage to derive a figure within 25 pounds of the actual weight! Amazing.

This entire discussion ignores the fundamental limitations of using ideal models to describe complex variable realities. And here we have folks asserting that utterly simplistic room mode calculators can determine pressure variability within irregular and coupled spaces.

We're better than that...
Old 16th December 2010
  #28
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Quote:
Originally Posted by Glenn Kuras View Post
recordinghopkins,
Trouble maker!!!!!!!!!!!!!! How dare you ask such a question. See what you have done? The acoustic world will never be the same. lol lol lol
geez! a bit of a firestorm here. Thanks for the responses! Lots to read, will reply shortly.
Old 16th December 2010
  #29
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recordinghopkins's Avatar
Ok, first, some more information. I have attached a couple more views of the room to clear up some of the confusion of it's configuration. Also, a picture of my control area, and the nook opposite the couch. There is some treatment on the ceiling above control that you can't see, but you can see my homemade cloud over the drums.

The reason that I posted my question is indeed to verify the results of my testing, which shows a nearly 10dB dip around 60Hz at the mix position, see attached. I have about as much bass trapping that I can fit in because of the limitations of door and window frames that go almost all the way to the corner. This is an all-in-one room, and while I am quite pleased with the way it is sounding, I have plenty of absorption, and I don't really want to kill any more of the air of the room. I know, I know, I am already using pegboard on the face of many of these panels you see, and I know there are ways to use absorption while minimizing the upper mid/high frequency trapping.

I am contemplating building some resonators to correct the dip at ~60Hz, and before I go getting myself into another DIY project, I want to better understand the behavior of low freq energy in this ~400 sq. foot room. Like so many others, I am trying to make the best of the space I have available and don't have the luxury of working in a purpose built space. So far, this room is working really well, I'm just in the final tweaking phase and will be adding some diffusion on the walls opposite the mix position (29' dimension) and over the couch, across from where I track drums (17' dimension). Resonator placement tbd.

Thanks guys. I am still trying to make sense of some of your responses....
Old 16th December 2010
  #30
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pics failed

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Calculating room modes for an irregular shape-sg1.jpg   Calculating room modes for an irregular shape-sg2.jpg   Calculating room modes for an irregular shape-sg3.jpg   Calculating room modes for an irregular shape-control.jpg   Calculating room modes for an irregular shape-drum-nook.jpg  

Calculating room modes for an irregular shape-frequency-response-studio-g.jpg  
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