Page 88 - The Master Handbook Of Acoustics

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THE EAR AND THE PERCEPTION OF SOUND

detected by the human ear. As changes in intensity and pitch are the
very stuff of communication, it would be interesting to know how
many combinations are possible. Offhand, it might seem that there
would be 280 × 1,400 = 392,000 combinations detectable by the ear.
This is overly optimistic because the tests were conducted by compar-
ing two simple, single-frequency sounds in rapid succession and bears
little resemblance to the complexities of commonly heard sounds.
More realistic experiments show that the ear can detect only about 7
degrees of loudness and 7 degrees of pitch or only 49 pitch-loudness
combinations. This is not too far from the number of phonemes (the
smallest unit in a language that distinguishes one utterance from
another) which can be detected in a language.

An Experiment
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 frequen-
cies, 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 chan-
nel. After the oscillators have warmed up and stabilized, adjust the fre-
quency 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 pleas-
ant 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.

Timbre vs. Spectrum

Timbre has to do with our perception of complex sounds. The word is
applied chiefly to the sound of various musical instruments. A flute
and oboe sound different even though they are both playing A. The
tone of each instrument has its own timbre. Timbre is determined by
the number and relative strengths of the instrument’s partials. Tonal
quality comes close to being a synonym for timbre.