treating a theoretical room with axial modes at 57Hz and 86Hz

[samtastic]

hi all,
im treating a theoretical room. I've calculated the axial modes below the fifth harmonic to be 28.7Hz, 43.1Hz, 57.4Hz, 86.1Hz, 114.8Hz, 129.2Hz, 143.5Hz, 172.3Hz, 215.3Hz, 229.7Hz and 287.1Hz

In these I've noticed an obious pattern between 43.1Hz, 86.1Hz and 172.3Hz

and 57.4Hz, 114.8Hz and 229.7Hz

and 143.5Hz and 287.1Hz

These are quite clearly going up in series. So if I were to treat a room for one of the lower frequencies in the series, would it help iron out the whole series?

if so, my thoughts are:

would it be possible to use a pair of bass traps with a narrow Q, tuned for 43.1Hz or 86.1Hz to help with one pattern of axial modes, and another pair tuned at either 57.4Hz or 114.8Hz to help with another?

Something like the tunable Vari-bass from Vicoustic.

Would it be a bad idea to have 4 bass traps in a room, each pair tuned to different frequencies? This wouldn't be symmetrical.


ideally I'm trying to target these different series of axial modes, without affecting the RT60 at 1000Hz (something which I've already treated the room for, to get the right results - I've factored four bass traps into my RT60 calculation already).

Or would I be better off going for more traditional bass traps, that target a wider bandwidth of frequencies?


As i said, this is all theoretical. In reality, I'm told room mode calculators are best for building new rooms. And for working on existing rooms, you're best bet is to measure what it actually happening. However this room doesn't actually exist - it's simply an exercise in putting acoustic calculations into practice : )

sam

[Ethan Winer]

Originally Posted by samtastic

if I were to treat a room for one of the lower frequencies in the series, would it help iron out the whole series?

No, it doesn't work that way. Tuned traps have a single center frequency which does not affect the multiples. Porous traps are more broadband, which is why they're preferred most of the time.

I'm told room mode calculators are best for building new rooms. And for working on existing rooms, you're best bet is to measure what it actually happening.

Yes, and for good reason. But for a theory exercise you can do whatever you want.

--Ethan

________________
The Acoustic Treatment Experts

[SAC]

A few comments.

What RT60 times are you referring to? A small acoustical space does not have a reverberant soundfield at any frequencies that we care about. Reverberant sound fields and such indicators as RT60s, 30s, etc., are a characteristic of large acoustical spaces. (Refer to Sound System Engineering - ch. Small Room Acoustics, by Davis & Davis - 2nd Ed., or Davis & Patronis, 3rd Ed.)

And you speak of using tuned/resonant LF traps but are worried about symmetry....

These are not simply distributed symmetrically about the room, but must be placed at specific locations corresponding to high pressure regions corresponding to the location of the particular modes. Symmetry is not really the issue, as the distribution of modes in a real space may not follow symmetry. So we treat as necessary.

You may (read: will probably) require more than one located at the various modal points about the room for that that particular anomaly. Again the positioning will ideally be determined by measurement, as well as the fine tuning regarding their location as in the process of employing them. And as they become a part of the system and have a small impact on the behavior of the system, you will want to remeasure when installing and fine tune their location accordingly for optimal effectiveness (as well as possibly modifying their effective center frequency which can often be done via modification of their internal volume).

An aspect that can moderate this process a bit is the use of a multi-layered perforated trap offering a broader (lower) Q that may eliminate the need for much of the iterative tuning, while positioning them is relatively easy.

But of course, these ideas and what methods and designs are optimal are almost all subject to modification based upon the amount of gain reduction required and the effective 'width' of the mode.

But, whatever method you chose to employ, get away from the notion of reverberant sound fields and RTxxs in a small acoustical space. Instead you may/will have persistence at the modal frequencies that you will address with LF traps that are adequately determined via the use of waterfall or cumulative spectral analysis plots.

[PaulP]

Originally Posted by samtastic

Something like the tunable Vari-bass from Vicoustic.

Vicoustic makes some interesting products. Unfortunately there is no test
data (that I can find) on the Vari-bass. Test results for most of their products
can be found in this document.

Paul P

[samtastic]

to SAC: the particular RT60 time I have to be aware of is at 1000Hz. The goal is to have it at around 0.5sec post-treatment. Thanks for the tips on placement of traps - that makes sense.

and thanks Ethan for pointing out that modes can't be simply treated in series - what a bummer.

and thanks for the pdf on Vicoustic's products : )

[DanDan]

Hi Sam, welcome.
In small rooms, the modes are so strong and separated from each other that they stick out. There are not enough of them to form a properly diffuse sound field. They ring on individually and separately as you will see in Waterfall plots. RT60 is the classical measure of reverberant decay in a diffuse field. Since small rooms can't do diffuse, thus many of us prefer to use the term Room Decay to illustrate the difference. There is no law about this :-) but I reckon it is good to be reminded that there really is no proper reverb in these small spaces.

In these I've noticed an obious pattern between

43.1Hz, 86.1Hz and 172.3Hz

and 57.4Hz, 114.8Hz and 229.7Hz

and 143.5Hz and 287.1Hz

Yes, the lowest mode 'fits' exactly between two walls. This mode has of course the longest wavelength. At twice the frequency, half the wavelength, so two of them will 'fit' exactly. x3 x4 and so on.

Here an interesting article by Wes Lachot Wes Lachot Design || Studio Design and Acoustic Consulting

I find it useful to become familiar with where modes actually hang out. Where the hot spots and nulls are. This calculator shows them nicely
hunecke.de | Room Eigenmodes Calculator
I also find it useful to make these things real now and again.
e.g. Play a sine wave at the lowest axial mode, into a real room. Walk along the path of the mode. You will be amazed at how loud it gets at the peaks, and even more so at the uncanny almost silence in the nulls.

DD