BRUEL & KJAER 2204 User Manual

1. General
Vibration and Sound.
It is generally known that sound is a transmission of energy through solid,
liquid br gaseous media in the form of vibrations. These vibrations con·
stitute 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
sensation of sound .
The Decibel Seale.
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:-
p
dB= 10 log -
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
p2 p
dB= 10log
2
= 20 log -
Po 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 ,ubar with equal weight given to all frequencies it is termed
sound pressure level.
fhe 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 pbar and can also withstand levels
higher than 200 pbar. This is a ratio of more than 106: 1 which on the
logarithmic scale is represented by 120 dB .
In Table 1.1 are given some commonly encountered sound pressure levels in
order to give a better appreciation of the dB scale.
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BRUEL & KJAER 2204 Specifications

General

Brand	BRUEL & KJAER
Model	2204
Category	Measuring Instruments
Language	English