A sound level meter is an instrument
designed to respond to sound in approximately by the same way as the human ear and to give
objective, reproducible measurements of sound pressure level. There are many different
sound measuring systems available. Although different in detail, each system consists of a
microphone, a processing section and a read-out unit.
Figure A1 illustrates a functional
schematic diagram of the sound level meter.
Figure A1 Functional Schematic Diagram
of a Sound Level Meter
(b) Processing Section
(i) Weighting Networks
The signal may pass through a
weighting network and resulted in different weightings, say "A" "B"
"C' or "D". Sound level meters usually also have a linear or
"Lin." network. This does not weight the signal but enables the signal to pass
through without modification.
The frequency range of the sound
from 20 Hz to 20 kHz is divided into sections or bands by means of electronic filters
which reject all signal with frequencies outside the selected band. These bands usually
have a bandwidth of either one octave or one third octave. (See Figure A2) The above
process of dividing complex sound is termed frequency analysis and the
results are presented on a chart called a spectrogram.
Figure A2 Sound Filtering
(iii) Root mean square detector
After the signal has been weighted and
or divided into frequency bands the resultant signal is amplified, and the Root Mean
Square (RMS) value determined in an RMS detector. The RMS value is used because it is
directly related to the amount of energy in the sound being measured.
(c) Read-out Unit
The read-out unit displays the
sound level in dB, or some other derived unit such as dB(A) (which means that the measured
sound level has been A-weighted). The signal may also be available at output sockets, in
either AC or DC form, for connection to external instruments such as level or tape
recorders to provide a record and/or for further processing.
Sound level meters should be
calibrated in order to provide precise and accurate results. This is best done by placing
a portable acoustic calibrator, such as a sound level calibrator or a Piston
Phone, directly over the microphone (See Figure A3). It is good measurement
practice to calibrate sound level meters immediately before and after each measurement
Figure A3 Calibration of Sound Level