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I hope this is the most appropriate SE site to ask a question about psychoacoustics.

I don't quite understand critical bands. As far as I know, if two pure tones which frequencies are inside the same critical band are heard, a single tone is perceived. But what would be that tone's frequency?

According to the classical rule of thumb, the bandwidth would be the following:

  • 100 Hz if f<500 Hz
  • 20% of the tone frequency if f>500 Hz

But I really don't know how to apply this. If I have two tones with f1=800Hz and f2=950 Hz, what would be perceived?

Would the critical band be 800-0.2*800=640 Hz to 800+0.2*800=960 Hz and therefore a single tone would be perceived? Or is 0.2*800=160 the real bandwidth (so the band would be from 720 to 880 Hz)? And, if a single tone is perceived, what frequency would it be?

Thank you in advance.

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2 Answers

up vote 1 down vote accepted

I have been lucky enough to find a whole chapter about critical bands and masking in An Introduction to the Psychology of Hearing by Brian C.J. Moore in college library. The phenomenum I asked about in my question seems to be just one of the many consequences of critical bands. In fact, this is a way to experimentally find out the critical bandwith for each frequency.

Given two complex periodic sounds containing 12 sinusoidal components (partials), which are:

  1. fundamental frequency f0 and its harmonics: f2=2*f0, f3=3*f0, ..., f11=11*f0.
  2. fundamental frequency f0 and out of tune harmonics (i.e. not multiples of the fundamental frequency).

Subjetcts were requested to listen the following pure tones:

  • a tone which frequency was contained in the previously heard complex sound.
  • a tone which frequency was half-step between two of the harmonics contained in the previously heard complex sound (e.g. 3/2*f0)

Assuming that a partial is only heard when its nearest neighbour is at least a bandwidth away, the stated results for the bandwidth were found:

  • 100 Hz if f<500 Hz
  • 20% of the tone frequency if f>500 Hz

Being the bold sentence the one that really answers to my question.

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I'm not an expert in psychoacoustics, but in trying out examples such as an 800Hz tone and another (which I varied from 802Hz to 1khz) they were all distinguishable instantly, with no effort. At very close frequencies (ie 0 to about 2 Hz difference) beats were the major audible component, but above that, two pure tones are heard.

The only combinations which caused more interesting results were when one tone was a harmonic of the other - it was possible to 'lose' one in the other tone...

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