In the discussion of critical bands, Dr. Russell includes this
figure as an illustration of how two pure sine tones
(single-frequency tones) interact. Outside the critical band, the
tones are heard distinctly as they excite different parts of the
basilar membrane in the cochlea. If the frequency of one of the
tones is swept through the frequency of the other, we hear first a
"roughness" then "beating." As the tone continues higher than the
stationary tone, we go through another beating region and roughness,
then hear two distinct tones once again.
Hear the aural demonstration in this WAV
file (25 seconds, 500 KB).
The spectrograph of this WAV file shows the two single-frequency
tones as they cross around 12.5 s. The wavering that you hear
just before and just after this is called beating, and arises from
alternating constructive and destructive interference to give us high
and low amplitude (loud and soft sound). The roughness that you
may hear is more subtle, and different people may respond
differently.
Copyright Daniel O. Ludwigsen, 2005
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