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Old 04-22-2012, 06:16 AM   #35
planetnine
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Location: Lincoln, UK
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Just double-checked these (I made up a spreadsheet from the mode equations and then sorted them in frequency order).

If I set my speed of sound to 344m/s, my list matches that of the axial frequencies of Bob Gould's online calculator:

http://www.bobgolds.com/Mode/RoomModes.htm

This will give you a list of Tangential and Oblique modes in addition to the Axial ones. Axial modes are the most significant in evening out a room (look at the "percentages" for relevance), many acoustic engineers ignore anything but Axial, but the lower value Tangential modes do have an effect in a room (especially if they coincide with another mode) and are worth noting for reference. The strongest mode for a room dimension will have a frequency whose wavelength is twice than of the room dimension. The next will be double this frequency, the next, three times, etc.

It is worth noting that these mode calculations are for dimensions between reflecting surfaces, and with numerous construction techniques might not be the surface you see. With lower frequencies, the sound energy may mostly pass through some surfaces and reflect off a more distant surface. This will obviously effect the wavelength of the LF modes; the most obvious example of this is a suspended ceiling (actually a great place to stuff dense trapping material if it can be done safely).

A rule of thumb to check these figures is 70Hz will have a wavelength of about 8 feet (about 6cm short of 2.5m), so for an 8-foot dimension the primary mode will be 16-foot wavelength or 35Hz. Add half again to the dimension (3/2 or 150%), multiply the wavelength by that, or the freq by 2/3 (inverse fraction or reciprocal) -so about 23Hz. Double the dimension, halve the freq (2/1 and 1/2) so for 16 feet it would be 70Hz (ish), and every other mode would be a integer multiple of this (x2, x3,x4, etc).

If you can hold this in your head, you'll know if these figure coming back are factors out...


Edit: It's the commas that screwed your figures up -I've duplicated it, put decimal points in there instead and they come out correctly.



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Last edited by planetnine; 04-22-2012 at 06:23 AM.
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