Noise
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The basics of sound
Sound is a wave motion that carries energy from one point to another. For the
purposes of this introduction, the medium through which the sound wave travels
will be air, although sound can also travel through solids and liquids. The "wave" itself
consists of small pressure fluctuations in the air about the ambient (atmospheric)
pressure. At some points along the sound wave, the air pressure is slightly
above the ambient level (the air is compressed), and at others it
is below (the air is rarefied). These compressions and rarefactions
are generated by the source of the sound wave, usually a vibrating object
such as a violin string, a loudspeaker, or a motor in a machine. When the
pressure fluctuations in the wave reach the ear, the eardrum vibrates in
direct response, and the pressure fluctuations are
heard as sound.
The amplitude of the pressure fluctuations in the sound wave (how
far they are above and below the ambient pressure) determines how loud the
sound is, while the frequency of the pressure fluctuations (how rapidly
they change from above to below the ambient pressure) determines its pitch.
Amplitude
Humans are sensitive to a remarkably wide range of sound pressure amplitudes
(about 1 million to one). At the highamplitude end is the socalled threshold
of discomfort, at which sound begins to cause a tickling or painful sensation
in the ear, and at the lowamplitude end is the threshold of audibility, at
which a sound is just detectable in complete quiet. The unit of sound pressure
is the pascal (Pa), where one pascal is equal to one newton per square meter
(N/m2). Because the instantaneous value of the pressure in a sound
wave changes rapidly with time, the amplitude is usually characterized by the root-mean-square,
or rms, value of the pressure, a value based on time averaging. The
energy, or power, in a sound wave is
proportional to the square of the pressure
amplitude and is thus proportional to the meansquare value of
pressure.
In order to express the wide range of sound pressure amplitudes in a convenient
form, a logarithmic metric has been universally adopted. This metric is the decibel,
which expresses the level of a given sound in terms of the ratio, on a logarithmic
basis, of the meansquare pressure of the sound
to a reference meansquare pressure. In equation
form, this becomes
Sound Pressure Level = 10 log10  decibels,
where p is the rms pressure of the sound under consideration and  is
the standardized reference pressure, equal to
2 X 10-5 N/m2 = 20μ Pa.
Note that the symbol for the decibel is dB and that the word level,
as in sound pressure level, is usually reserved for use with decibels.
On the decibel scale, the range of sound pressure levels from discomfort to
audibility is about 120 dB.
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