BRUEL & KJAER 2209 - Introduction; Sound Level Meters

1. INTRODUCTION
1.1. SOUND LEVEL METERS
Sound Level Meters are required to measure noise of different levels,
spectra, and waveforms under widely varying conditions of sound source
distribution and reflections at the boundaries of the sound field. Usually the
purpose of these measurements, whether they are estimating hearing damage
risk, annoyance, acoustical insulation efficiency, acceptability of manu-
factured products or any other factor, is to collect data which will improve
the understanding of a problem and thereby help in solving it.
Obviously, an instrument giving readings which can be related to
subjective impressions of loudness would be desirable. Attempts have been
made in the past to design such an instrument, but the difficulties involved
in simulating the human hearing system cannot yet be overcome. The Inter-
national Electrotechnical
Commission (I EC), has decided that the most
practical solution is to standardize an apparatus by which sound pressure
can be measured
under closely defined conditions, so that the results ob-
tained by different users may be compared.
Two I EC publications , l:lave been issued to specify the desirable character-
istics
of sound measuring instruments. Publication 123 presents require-
ments for a Sound Level Meter, and these requirements were made more
stringent in Publication 179 for Precision Sound Level Meters. The Brliel &
Kjc;er Type 2209 is designed to the specifications of IEC Publication 179.
It is appropriate to mention some of the main requirements of the IEC
Standard:
The Precision Sound Level Meter must measure the free-field sound
pressure level with an accuracy of ± 1 dB under specified reference
conditions simulating normal use.
The microphone must have directional sensitivity within certain limits
which are specified. The indicating instrument must be of the square-
law type, and calibration of the meter scale and the error associated
with a range shift must be within specified tolerances.
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