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 sound field boundaries. Usually the purpose of these
measurements, whether they are estimating hearing damage
risk, annoyance, acoustical insulation efficiency,
acceptability of manufactured products or any other factor,
is to collect data which will improve our understanding of
the problem and also help in solving it.
Obviously an instrument giving readings that can be related
to subjective impressions of loudness would be desirable.
Attempts have been made in the past to design such an
instrument, but in view of the difficulties involved in
simulating the Human
hearing system for all types of noise,
the International Electrotechnical Commission (IEG), has
decided that the most practical solution is simply to
standardize an apparatus by which sound pressure can be
measured under closely defined conditions, so that results
obtained by different users can be compared.
The Type 2203
Sound Level Meter
is an instrument with a
practical combination of characteristics that will achieve a
high degree of stability and accuracy. The accuracy and
validity of the results are, however, determined by the
manner of use, which must be chosen to suit the situation.
In particular, care must be taken so that the presence of
the observer does not invalidate the calibration. The
instrument is not intended for measuring sounds of very
short duration or discontinuous sounds.
Some of the main requirements in the IEG specification are
quoted here: A precision sound Level Meter
shall include at
least one of the three weighting networks called A, B or C,
and should cover the frequency range 20 to 20ooo Hz (c/s)
within certain tolerances. (Table 5.6).
The microphone shall be of the omnidirectional pressure
type. Permissible tolerances on the variation of sensitivity
with angle are given in Table 2.1, and it is suggested that
the diffuse sound field sensitivity (i.e. the
root-mean-square of the sensitivities for all orientations)
should by some means be brought within the tolerances
already mentioned for the specified incidence.
A square law indication instrument is specified, i.e. it
must be capable of correctly summing two pure tones
according to the root-mean-square law.
The Precision Sound Level Meter
is a highly
accurate instrument designed for outdoor use as well as for
precise laboratory measurements. It is easily portable,
battery driven and completely self-contained for ordinary
sound level and vibration measurements. Used in conjunction
with a suitable filter set e. g. the B & K Octave Filter Set
the instrument becomes a handy and easily
operated frequency Analyzer
There are no requirements stated in the IEG Draft
Specification regarding dynamic range, but the B & K Type
covers the range 22 to 134 dB (or 40 to 148 dB using a
l/27/ microphone) and as will be seen from Table 1.1 this
covers most sound levels which need to be measured. All
three weighting networks (A, B and G) are included in the
instrument as well as a linear characteristic and means for
connecting external filter circuits for further shaping of
the frequency characteristic if necessary.
From instrument serial No. 82939 the Sotind Level Meter
complies with the American Standard for General Purpose
Sound Level Meters, ASA S 1.4-1961.