Bruel & Kjaer 2250 Sound Level Meter

Methods have already been standardized for the description and assessment of noise from various sources and with respect to various effects on human subjects (risk of hearing damage, annoyance reactions, loudness, perceived noisiness, interference with speech communication). In all these cases, the bandwidth of the noise is either considered to lie within the conventional audio frequency limits from 20 Hz to 20,000 Hz or within a specified narrower band (for example, 45 Hz to 11,200 Hz in the case of perceived noisiness of aircraft). The band limits of 20 Hz and 20,000 Hz also define the frequency range for which the characteristics of sound level meters are fully specified (see IEC 651). In practice, some noises consist of, or contain components at, frequencies below 20 Hz. At present, there are no standardized methods for sound pressure measurements of these noises, nor for their description and assessment with respect to human response. Although research in this field is comparatively sparse, there is evidence of infrasonic effects which are potentially harmful or unpleasant to human subjects and some authorities may desire to extend their regulations or codes of practice governing noise emissions to cover sources of infrasound. For this reason, it is considered to be highly desirable to standardize measurement and description methods in order to facilitate the exchange of information and to avoid proliferation of incompatible procedures. Many types of human response can be distinguished and, correspondingly, different description methods are, in principle, appropriate. The method described in this International Standard corresponds to the direct perception of infrasound. At present, this is the only human response for which there is an ample research base. Some literature on annoyance from infrasound suggests that annoyance may be closely related to the direct perception. On that precondition, levels measured according to this International Standard would

This part of IEC 61672 gives electroacoustical performance specifications for three kinds of sound measuring instruments: • a time-weighting sound level meters that measure exponential-time-weighted, frequency-weighted sound levels; • an integrating-averaging sound level meters that measure time-averaged, frequency-weighted sound levels; and • an integrating sound level meters that measure frequency-weighted sound exposure levels. Sound level meters conforming to the requirements of this standard have a specified frequency response for sound incident on the microphone from one principal direction in an acoustic free field or successively from random directions. Sound level meters specified in this standard are intended to measure sounds generally in the range of human hearing. NOTE: The AU frequency weighting specified in IEC 61012 can be applied for measurements of A-weighted sound levels of audible sound in the presence of a source that contains spectral components at frequencies greater than 20 kHz. Two performance categories, class 1 and class 2, are specified in this standard. In general, specifications for class 1 and class 2 sound level meters have the same design goals and differ mainly in the acceptance limits and the range of operational temperature. Acceptance limits for class 2 are greater than, or equal to, those for class 1. This standard is applicable to a range of designs for sound level meters. A sound level meter may be a self-contained hand-held instrument with an attached microphone and a built-in display device. A sound level meter may be comprised of separate components in one or more enclosures and may be capable of displaying a variety of acoustical signal levels. Sound level meters may include extensive analogue or digital signal processing, separately or in combination, with multiple analogue and digital outputs. Sound level meters may include general-purpose computers, recorders, printers, and other

This standard conforms closely to the IEC Standard for Sound Level Meters, Publication 651, First Edition issued in 1979. This revision represents a significant improvement over ANSI S1.4-1971, particularly in its specifications relating to measurement of transient sound signals. It also permits the use of digital techniques and displays. The principal changes from ANSI S1.4-1971 are: inclusion of an optional impulse exponential-time averaging characteristic; inclusion of an optional peak characteristic; more rigorous definition of the dynamic characteristics for the Fast and Slow exponential-time-averaging; increase in the crest factor requirement to ten for type 1 instruments; specification of a type 0 laboratory instrument with generally smaller tolerance limits than those previously specified for type 1; and deletion of the type 3 survey instrument.