Originally Posted by tuxedocivic
Ya, I don't actually think velocity changes, just if it were to change I'd think at the peaks. Or at least acceleration would be 0 at those points. I'd tend to think velocity doesn't change at all. It's like a pulse in the air (or any medium). In my field we measure the shear wave velocity of soil and rock. We initiate a pulse that travels through the medium and that velocity is measurable. It doesn't change either. The same thing happens in air. The speaker piston creates an out/in pulse (over and over to create frequency) which travels through the air medium at a constant 340m/s (depending on air temperature and atmospheric pressure, but pretty darn close to 340m/s). When it reflects off a wall and strikes a bass trap, still constant velocity.
Now, it is my understanding that certain trap materials can change the speed of sound, but I don't know much about it.
I'm hearing confusion of particle velocity with wave packet (or pulse) velocity.
Particles engulfed by a wave packet vibrate with real particle velocites, but they go nowhere, just as your rock's don't move. The wave packet, or pulse, does move, engulfing new particles as it does so.
Velocity of a wave packet is dependent on the medium; speed of sound changes in water, solids, etc., but independent of the packet's amplitude.
Particle velocities in the region engulfed by the wave packet will depend on the amplitude of the wave, as well as the medium.
Resistive sonic absorbers use the air particle velocity, which depends on amplitude, to absorb sound. Amplitude of reflected waves varies with location, so resistive absorbers are most sensitive where particle velocity is maximized...
Energy can be removed at a given wavelength without locating the absorber at maximum particle velocity, the 1/4 wave point. At 1/3 this distance (1/12-wave), you still have 50% of max. particle velocity; at 1/10, 30%. This is why wall-mounted resistive absorbers go as low as they do.
Put another way, for the fundamental room mode, an absorber placed at room center (1/4 wave point) gives maximum absorbance
- 1/3 of the way to the center = 50% of the maximum
- 1/5 of the way to the center = 30%
Note that "1/5 of the way to the center" is 10% of the room dimension. In a 10' room, only 12" from the wall air particle velocity is 30% of maximum for the longest possible wavelength resonance. It's how the sine function operates...
But you still get a broad band absorber, and need to consider it's affect over the full freqeucny range.