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by Anechoic
2170 days ago
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You can tell roughly how far away a sound source is by measuring the curvature of the wavefront, since sound radiates out in a sphere. The farther away you are, the lower the parallax between two sensors. That only works in the free field. In a small room (defined as dimensions being larger than ~1/6th to 100% of a wavelength) you no longer have waves, the entire room is pressurized. And as you mentioned, modal behavior is also an issue. If you're trying to measure the parallax of outdoor (free field) waves, that works in a truly free field, but once you have reflections (from, say, the ground) you have to account for that as well. |
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Further, that figure isn't very helpful for fluid acoustics. Within the equivalent distance you instead would get tons of nonlinear effects, because fluid flow dominates acoustic transmission. In addition to heavily changing transducer loading, things like vorticity also start to dominate. The net effect is that near field issues arise much earlier, at more like a third of a wavelength. Still, only the very lowest audible waves and quite small rooms create non-acoustic behavior.
> once you have reflections (from, say, the ground) you have to account for that as well.
Ish. Only for quite high frequency sounds which change very quickly. Otherwise reflections tend to mostly just overlap with the primary source. For low frequency waves the distance between the microphone and your ears is much smaller than the wavelength, so you don't need to worry about multiple waves very much.