| Presented at the 79th AES Convention |
| LISTENING ROOM - CORNER LOADED BASS TRAP |
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(...cont'd) Well, so far everything seems to be getting better. The traps are to be located in zones of maximum pressure fluctuations, the tri corners of the room. Unfortunately, few devices extract energy directly from pressure changes. The most common method for sound absorption is friction -- friction due to the air-motion part of the sound wave that is scrubbing through some micro-porous piece of material, usually fiberglass. Now, air-motion is very small in zones of pressure fluctuation, by definition. For example, at 100 dB, 100 Hz, it's on the order of 1/10 the diameter of 5 micron fiberglass fibers. Prospects for developing friction look poor unless we first transform energy. We'd like to convert the pressure fluctuations into substantial air motion, and then dissipate acoustic energy by friction against the air motion. A new device uses this approach with considerable success. It is tubular in form and is supplied in 3 foot sections, hence its generic name: TUBE TRAP. It is comprised basically of two distinct elements: an internal air chamber and a porous wall. The ends of the tube are sealed. The Tube Trap is, in fact, a sealed chamber with a resistive opening to its interior void. Its length is incidental and now functional to its operation. Air pressure fluctuations outside the tube impart motion to the air in the porous tube wall where friction operates.
If for example, the tube has a thin but highly resistive wall, the pressure drop would be very steep -- but the distance of the action would be too small for any real work to be done. Conversely, if the tube were simply full of loose fiberglass, the gradient would be too small, though the distance of the action would be large; again, the work would be minimal. The variables of wall material bulk flow resistance and the wall thickness, along with the air chamber volume can be manipulated to access any low frequency with optimal efficiency. |
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It
is interesting to note the pressure distribution associated with
the operation of the Tube Trap. When pressures outside the tube
are higher than those inside, a pressure gradient across the wall
of the tube results from friction as air is driven inwards through
the wall. The difference in pressure across the wall is the measure
of the force that is being transferred into frictional energy. The
thickness of the wall tells us the distance over which that force
is developed. Their combination tells us how much work is being
done. We like as much force to occur over a large distance to get
as much work out of each half cycle pressure fluctuation as possible.