Table of Contents
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0. Introduction
Sound.
It
is generally known that sound is a transmission of energy through solid,
liquid or gaseous media in the form of
vibrati~ns.
These vibrations constitute
variations in pressure or position of the particles in the medium. Sound may
also be defined as the auditory sensation evoked when such vibrations,
normally in air, impinge upon the ear. As an auditory sensation sound is
limited to frequencies in the range from about 20 Hz to 20ooo Hz. *)
Pressure fluctuations outside this range will not generally produce the sensa-
tion of sound.
The Decibel Scale.
Acoustical instruments for
measuring pressure
variations
are
usually
calibrated in dB (decibel). A dB value is a measure of relative power, i.e.
so many dB above a reference power level:-
dB
=
10 log
_R_
Po
where Po is the reference and P is the actual power measured.
However,
the power transmitted by a sound wave is proportional to the square of the
pressure variations so that we have
2
dB
=
10 log
_E____,
=
20 log __E_
p o•
po
where po is the reference pressure and p is the root mean square value
of the pressure variations. When sound pressure is measured in dB re
o.0002 t-tbar with equal weight given to all frequencies it is termed sound
pressure level (SPL).
The logarithmic scale has been found very convenient because of the large
range of sound intensities that the human ear can
handle.
It
can detect
pressure variations as low as o.0002 .ubar and can also withstand levels
higher than 200 t-tbar. This is a ratio of more than 106 :
1
which on the
logarithmic scale is represented by 120 dB.
In Table 0.1 are given some commonly encountered sound pressure levels in
order to give a better appreciation of the dB scale.
*)
Hz (Hertz) is the international unit of frequency, equal to the number of vibrations
per second.
In the United Kingdom and U.S .A. the term c/s, or cps, (cycles per second) is used.
3
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