Calculating Room Modes (from which surface do I measure?

Posted: Mon Feb 24, 2003 9:22 am

Ethan,
I am trying to design my room to minimize the effects of room modes (attempting to spread out the modes as evenly as possible to avoid stack ups of several modes near the same frequency). I plan to implement the bass trap design that you have written about.

My question is, when calculating the room modes from which surface should I take distance measurements - from the surface of the bass trap membrane, or the surface of the original wall on which the bass trap is mounted?

For example, currently I plan to build the room 20'-4" long, 14'-3" wide, and 8'-7" high, which results in modes that I think are fairly well distributed. Only 3 frequencies (197 Hz, 198 Hz, and 277 Hz) have modes within 5% of each other.
Do I need to make the length 20'-4" from the original surface of the front wall to the surface of the rear wall, or from the membrane surface of the bass trap mounted on the front wall to the membrane surface of a bass trap mounted on the rear wall?

Thank you very much for your help!
Scott Hill

Posted: Mon Feb 24, 2003 12:00 pm

Scott,

> when calculating the room modes from which surface should I take distance measurements <

Good question. I'll guess you should use the empty room dimensions, since the traps will probably not be lined up on every inch of every wall. But I suppose you could split the difference and make the room two inches larger in all dimensions.

--Ethan

Posted: Mon Feb 24, 2003 6:03 pm

The room modes should be figured from an empty room in 3-D. This should show you the problems. Then treat the room with bass traps, absorption and diffusion.

Posted: Tue Feb 25, 2003 3:17 am

Thanks to both of you for your replies.
Scott

Posted: Sun Mar 09, 2003 11:05 am

Scott,

Use the outer, hard surface dimensions since those are the true dimensions as far as the bass is concerned. And modal ratios are all about bass.
I checked your ratios and they are very good indeed. The length of 20'-4" corresponds to a frequency of 27.78Hz, or the lowest 'A' on the piano. The width of 14'-3" corresponds to 'Eb' above that 'A', and the height of 8'-7" produces a 'C' above that. These three keys have a nice symmetry with respect to each other (hence the mathematical elegance of the modal distribution), and are 3 jumps away from each other on the Circle of Fifths. That's nearly as good as it gets, and nearly in tune, to boot.
I hope everyone will pardon my speaking in terms of music theory, but we are all in it for the music, right?
Cheers,

Wes

Posted: Mon Mar 10, 2003 8:38 am

Wes,
Thanks for your comments. I worked for a long time to optimize those room dimensions within the space I have. Now I just have to get the actual room to approximate as closely as possible the theory!
Scott

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