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The
sounds produced by musical instruments are not pure sounds but
composite sounds. A composite sound is the result of the superimposing
of several waves.
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One is the fundamental note, which has the same frequency
as the vibration of the string and determines the pitch of
the sound perceived.
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The others are harmonic and determine the timbre of the instrument.
Two different instruments, such as a flute and a violin, playing
the same note have a different timbre and therefore a different
spectrum.
This
is the sound intensity with respect to the frequency for two
composite sounds of a like pitch but of a different timbre.
A
sound at A pitch contains approximately 40 audible harmonics
whose respective intensities can vary! It is easy to understand,
then, the immense range of possibilities in the variations of
timbre. Such variations allow one to individualise not only
musical instruments, but the human voice as well.
The
superimposing of various pure or composite sounds is a chord.
A chord may be consonant or dissonant according to whether or
not it is pleasing to the ear - a rather subjective criterion!
The
spectrum represents a three-note chord ~ G3, B3, D4 and its
harmonics.
If
there are too many fundamentals in a chord - for instance, when
one bangs simultaneously on all the keys of a piano - the ear
perceives noise rather than a chord. Noise is defined as any
wave which has no periodic element (i.e. no repetitive cycle)
and is therefore composed of an infinite number of pure sounds.
An explosion is a good example!
Non-periodic
curves of different sounds.
Let
us conclude by pointing out that the manner in which sounds
and harmonies are perceived vary from one person to another
and according to time periods, cultures, countries and civilisations.
This also explains why classical harmonies are not always appreciated.
For
more information on sound in general, we highly recommend The
Soundry (ThinkQuest 1998) site.
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