THE WEBSITE OF the Acoustic Ecology Institute is always worth checking up on, and recently there appeared a couple of interesting links about singing sands. There’s this scholarly take on the phenomenon, and a more artistic approach. Both have sound recordings you can listen to of the eerie humming and droning sometimes made by sand dunes.
There aren’t nearly as many auditory illusions as optical ones. Perhaps that’s because visual experiments are easier to devise and implement than auditory ones, or because the brain devotes more computational clout to vision than to hearing. Anyway, this page features Risset tones, an auditory equivalent to the famous Escher drawing of the never-ending staircase.
When parrots mimic a human voice, the underlying acoustics are similar to sine-wave speech. Matt Davis of the Medical Research Council describes sine-wave speech as a form of abrupt perceptual ‘pop out’, where successful perception of an impoverished signal depends on top-down knowledge. It’s like those 3D posters that were popular some years ago, and for which you had to go cross-eyed to enjoy properly. It helped once you knew what the hidden object was, and you either got it or you didn’t.