Behavior: Basic Psychology of the User - Foundations for Designing User-Centered Systems - page 105

purity of the tone; is it made up of one single sine wave frequency or a broad
mixture of frequencies? A tuning fork has a relatively pure tone and therefore little
timbre. On the other hand, a piano or other musical instrument has timbre because
of the other frequencies present in its sounds. Different timbres are often assigned
different meanings by users, which may be important for your design.
Low tones of equal loudness appear to occupy more space and thus are said to
have more volume than high tones. On the other hand, high tones have a greater
density than low tones of equal loudness. The volume and density of tones are each
a joint function of intensity and frequency of the tones. However, they seem to be
as real as pitch and loudness which have simpler bases. In other words, listeners
have no difficulty making reliable judgments of volume or density of tones that
differ in frequency and intensity.
Hearing can be likened to carrying out a type of Fourier analysis of the auditory
stimulus, separating a complex wave into its sine wave components. There are
some situations where this analogy breaks down, such as when two stimuli of
approximately equal intensity and frequency are simultaneously presented to the
ear. Instead of hearing both tones, as a linear Fourier analysis would allow, a single
tone is heard which varies in loudness in a periodic manner. You may have heard
this when two people sing together or two instruments are played together. The
effect can be pleasant or unpleasant depending on the frequency of the beats.
The basis of beats is the following. If you have a pure tone of 256 Hz and another
of 257 Hz, each one would produce a steady pitch that would be difficult to dis-
tinguish from the other. When the two are played together the compressions and
rarefactions (expansions) of the air produced by the two tones will at some point be
in phase (synchronized) and the two tones will add together. However, because the
frequency of one is slightly greater than the other, they will get out of phase after a
while and their effects will cancel each other out. As this process repeats, they will
go in and out of phase as many times per second as the difference between the tones
in cycles per second. In this example, it would be once per second and so you will
hear one beat per second. This provides a very accurate way of measuring the
difference between two tones, far better than the ear could discriminate if the two
tones were presented separately. This fact is used to good effect by piano tuners.
They tune one note until it no longer beats with the standard tuning fork. Then the
other notes are tuned until their harmonics do not beat with the first note.
4.6.2 Measuring Sound
Sound intensity is normally measured using the deciBel scale. This is a relative
logarithmic scale where 10 decibels (dB) = 1 log unit ratio of energy, or a Bel
(name after Alexander Graham Bell). To give some practical examples, the
threshold of hearing is 0 dB, a whisper registers 20 dB, and normal conversation
registers between 50 and 70 dB.