Effect of Sound Level on the Perception of Pitch

The level of sound affects the perception of pitch. For low frequencies, the pitch goes down as the level of sound is increased. At high frequencies, the reverse takes place—the pitch increases with sound level. The following is an experiment within the reach of many readers that was suggested by Harvey Fletcher. Two audio oscillators are required, as well as a frequency counter. One oscillator is fed to the input of one channel of a high-fidelity system, the other oscillator to the other channel.
After the oscillators have warmed up and stabilized, adjust the frequency of the left channel oscillator to 168 Hz and that of the right channel to 318 Hz. At low level these two tones are quite discordant. Increase the level until the pitches of the 168-Hz and 318-Hz tones decrease to the 150-Hz–300-Hz octave relationship, which gives a pleasant sound. This illustrates the decrease of pitch at the lower frequencies. An interesting follow-up would be to devise a similar test to show that the pitch of higher frequency tones increases with sound level.