bass traps question-help

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my control room has a flat reverberation of about 0.4 seconds for frequencies 125Hz-4KHz i m thinking to put bass traps to avoid standing waves problems, would the bass traps cut some of the low frequency reflections? would that damage the flat reverberation responce? can someone help me with that? thanks
 
thanks ethan, i read your web site, nice actualy i ve been on it before, so...if i got it right... it would be better for a control room to have small reverberation time at low frequencies right? ...oh and another thing, axial mode shows that the room has problem at some frequencies below 240Hz, do you think i should drop the reverberation for 125Hz to..lets say 0.2 seconds and leave the rest frequencies at 0.4 seconds? thanks
 
The target time for a room's decay is determined by usage and the volume of the room. For the target in a mixing room to be .4 it would have to be pretty big.
 
i see...well my room is not a big one, has volume of about 34 cubic meters, do you think it would be better to shorten the decay time?
 
do you think i should drop the reverberation for 125Hz to..lets say 0.2 seconds and leave the rest frequencies at 0.4 seconds? thanks
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Only way to do that is Hemholtz resonators calculated for a specific mode or membrane absorbers calculated for a center frequency of an octave whose higher frequency is below 150 hz I believe. The room dimensions determine the lowest modal frequency which should make it easier.
However, you could use "Superchunks" of rigid fiberglass or rockwool in the corners of the room, but it will change your overall RT-60 as you are adding square footage of absorption which will also absorb mid/highs as well. Its like adding open windows. If you have one square foot of open window, you have 1 sabine of perfect absorption. If you add 4 1' square open windows, you've added 4 sabines of perfect absorption to the room. In the case of resistance absorbers, for every square foot of absorber face, you've added absorption at the coefficient/frequency band rating. An absorber with a coefficient of .9 at 4khz will absorb 90 percent of the incident sound entering the face. Add the proposed square footage(corner absorbers) to your existing absorption and I think you will understand. For frequencies above 500hz, thickness beyond 4" will not increase absorption, so corner absorbers can be considered about 4" thick in this regard. A Superchunk(or panel laid across the corner) of rigid fiberglass cut from 2'x4' panels and stacked in the corner, will have a face about 34" wide, from floor to ceiling. Thats approx. 96". 34x96=3264 sq inchs =22.6 square feet :eek:
Using superchunks in two corners adds up to about FOURTY FIVE square feet of absorber face!!! I believe that wll change your RT-60 at higher frequencies.
However, my disclaimer is in force here. I AM NO EXPERT! ;) Although I've never read anything about absorption of higher frequencies by corner bass traps, unless Ethan has reservations, I believe I am correct. If not. Well, I guess I'll eat humble pie for lunch. :D

PS. How did you determine your existing RT-60 and response curve?
 
> axial mode shows that the room has problem at some frequencies below 240Hz, do you think i should drop the reverberation for 125Hz to..lets say 0.2 seconds <

With small rooms I don't think in terms of reverb time so much as the negative effect of ringing at specific frequencies. The best first approach for all small rooms is to add as much broadband bass trapping as possible in as many corners as possible, and treat the first reflection points on the side walls and ceiling. Once that's done you can more easily assess whatever problems might remain.

--Ethan
 
hi rick, i got the RT60 with maths calculations, (sabines way) and then did axial mode analysis, its all theoretical though, i dont really know if the numbers are all corect
 
ITU recommendations for a control room of that volume are about .17 seconds
 
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