The difference in frequency between pure tones of equal strength and of which the difference in pitch is just perceptible is defined by Knudsonís law as approximately:
If we apply this law to see how many tones whose frequency difference is audible occur in the audible frequency range of 20Hz to 20000Hz.
If there are z tones
substituting Δf/f and taking the log gives
this can then be rearranged to give
There are 2000 tones whose frequency difference is audible in the audible frequency range.