Sound Waves:
Variations in air pressure
against the ear
drum, and the subsequent physical and neurological processing and
interpretation, give rise to the experience called "sound".
Most sound that people recognize as "musical"
is dominated by
periodic or regular vibrations rather than non-periodic ones (called a
definite pitch), and we refer to the transmission mechanism as a
"sound wave". In a very simple case, the sound of a sine
wave, which is considered to be the most basic model of a sound
waveform, causes the air pressure to increase and decrease in a regular
fashion, and is heard as a very "pure" tone. Pure tones can be produced by
tuning forks or
whistling. The rate at which the air pressure varies governs the
frequency of the tone, which is also measured in oscillations per
second, or
hertz. Frequency is a primary determinate of the perceived
pitch.Whenever two different pitches are played at the same time, their sound
waves interact with each other — the highs and lows in the air pressure
reinforce each other to produce a different sound wave. As a result, any
given sound wave which is more complicated than a sine wave can,
nonetheless, be modelled by many different sine waves of the appropriate
frequencies and amplitudes (a
frequency spectrum). In humans the
hearing apparatus (composed of the ears and brain) can
isolate these tones and hear them distinctly. When two or more tones are
played at once, a single variation of air pressure at the ear "contains"
the pitches of each, and the ear and/or brain isolate and decode them into
distinct tones.
When the original sound sources are perfectly periodic, the note consists
of several related sine waves (which mathematically add to each other)
called the
fundamental and the harmonics, partials,
or
overtones. The sounds have harmonic
frequency spectra. The lowest frequency present is the fundamental,
and is the frequency at which the entire wave vibrates. The overtones
vibrate faster than the fundamental, but must vibrate at integer multiples
of the fundamental frequency in order for the total wave to be exactly the
same each cycle. Real instruments are close to periodic, but the
frequencies of the overtones are slightly imperfect, so the shape of the
wave changes slightly over time.
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Sound Waves
