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EXPLANATORY NOTE ON SOUND MEASURING INSTRUMENTS
During the past decade, experience in Canada and elsewhere has shown that
accurate measurements of sound level are needed if industrial noise control
programmes are to be fully effective. To meet such needs, many new and improved
instruments have been developed in recent years, some of which are already in
general use. Unfortunately, the national and international standards that define
instrument performance have not kept pace with advances in technology and
design.
In an effort to address this situation, the Standards refer to a new
international standard (IEC 651 published in 1979) and two other standards,
which are in preparation and approaching finality.
In IEC Publication 651, the time weighting characteristics are designated S,
F and I ("slow", "fast" and "impulse").
Instruments that incorporate I time weighting (e.g. Type 1I and Type 2I Sound
Level Meters) have the wide dynamic range and high crest factor capability
needed for the accurate measurement of short-duration sounds. These features are
also desirable when measuring some of the highly complex steady-state sounds
that industrial machinery produces. Moreover, the detector performance of
instruments that do not include I time weighting is more closely defined in IEC
651 than in earlier standards such as CSA Z107.1-1973 and ANSI SI.4-1971. The
ANSI Standard now in preparation (the revision of ANSI S1.4-1971) conforms as
closely as possible to IEC Publication 651 (with respect to time- weighting, for
example) but deviates from 651 in the requirement for diffuse field calibration
rather than the free field method. It is understood that the Canadian Standards
Association has IEC Publication 651 under consideration for endorsement or
adoption and that a similar treatment of the revised ANSI Standard can be
anticipated in a year or two.
The standard for Integrating Sound Level Meters now in preparation in WG11 of
IEC TC29/SC29C is consistent with the relevant parts of IEC Publication 651 but
specifies additional characteristics necessary to measure the equivalent sound
pressure level LAeq of steady, intermittent, fluctuating and
impulsive sounds. It recognizes two degrees of pulse-handling capability
designated N and P. Category N instruments are not capable of measuring
impulsive sounds.
The Standards allow a wide latitude in the choice of measuring instruments
while requiring that the instruments comply with the precise standards of
performance defined in the new documents. In particular, the use of Type 2
instruments is accepted in keeping with current industrial practice. At the same
time, it is recognized that all measurements are subject to error and that
measurement differences may occasionally require resolution. (It is, for
example, conceivable that sound level measurements made with two properly
calibrated Type 2 instruments might differ by two decibels or more). In such
cases, section 5.7 of the Standards requires that reference measurements be made
with a precision instrument capable of handling complex or impulsive sounds
(i.e., a Type 1I SLM or Type 1(P) Integrating-Averaging SLM).
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