Several things - first, don't try to filter frequencies to fix room problems. Fixing a time domain problem in the frequency domain is not helpful; and even in the frequency domain, filters may improve the frequency response in one specific location while actually increasing problematic ringing, and making the frequency response for the rest of the room worse. Unless you put your head in a vise, the filtered response is far less helpful than one might think.
On to why bass frequencies are trouble... When sound energy with a wavelength that corresponds mathematically to a room dimension reflects and bounces between two surfaces, a standing wave is created. This causes areas of constructive and destructive interference, which in practical terms means there will be areas with deep nodes, or nulls, where certain frequencies disappear, and other areas of antinodes, or peaks, where the bass at some frequencies will be far greater than what's recorded and intended to be reproduced.
As you add bass trapping, you stop these reflections from happening, which in turn prevents the resonance from building as they repeat between the surfaces, evening out the room response. Since the standing wave is diminished or eliminated, the nodes and antinodes are also diminished or eliminated, and ringing at modal frequencies is similarly improved.
The most common modes to treat are axial, where two opposite surfaces are involved. However, more complex paths including 4 and 6 surface interactions are also possible, causing oblique and tangential modes.
The other item to be concerned about is modal density. For high frequencies, there are plenty of reflections, spaced densely enough in time and frequency that they constitute ambience, and don't create such significant static peaks and nulls. At low frequencies in small rooms, the modes bunch up in some places, and leave others sparse, and end up causing the obtrusive nodes and antinodes.
As a room gets larger, the cutoff frequency gets lower. The is the frequency below which the modal density is not sufficient to be perceived as ambience rather than causing the detrimental interference. Small rooms have a high cutoff frequency, meaning there will be problems well up into the hundreds of Hertz. Large rooms, like concert halls, have a cutoff frequency so low that the modal behavior below it is no longer of concern. There are many other factors concerning large room acoustics that are not relevant here, so I'll leave it at that.
In practical response to your question, trapping the living daylights out of the room is fine as long as you include reflective and diffuse elements on top of enough of the trapping to create a well-balanced room.
Quote:
Originally Posted by The Studio RI  Ok, I've been sketching some designes to acoustically redesign my control room and I just had a few questions about how sound travels in a room.
My room is small, 13x9x7 and is rectangular. I have severe bass problems! I know how various sorts of filtering can cancel frequencies in the room and such, but my question would be... what does bass trapping really do to the bass waves? does it just try to suck up bass like a black hole? is that the desired result, to suck up as much bass as possible? If I make my room one big bass trap, with 6" traps over all the walls and clouded ceiling, would that give me the best results? |