[mluckham]

I would guess infrared, but with acoustic the problems are pretty much the same because the frequencies are still quite high ... the frequency ranges involved, reflected signals, and interference from other IR sources. I would go with infrared though, can you imagine someone bringing their dog and the room full of conflicting acoustic sources?

With infrared, you have the wavelength of the light ... different infrared sources and detectors operate at different wavelengths, by means of filters/etc.

IR remotes don't just turn on the IR beam, they modulate it with a chosen frequency (say, 10 KHz). The detector circuitry has it's own 10 KHz reference frequency, and rejects IR signals with different modulation frequency.