ISO 7196:1995 Acoustics — Frequency-weighting characteristic for infrasound measurements
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 reflect the annoyance as well as the direct perception.
The perception of infrasound, although apparently achieved through the auditory mechanism, differs in some respects from that usually understood by hearing. The normal threshold of perception is considerably higher than at audio frequencies (about 100 dB relative to 20 μРа at 10 Hz), whilst toleration for high levels is not raised correspondingly, that is, the dynamic range is smaller and the rate of growth of sensation with sound pressure level is much more rapid. In the frequency range 1 Hz to 20 Hz, sounds that are just perceptible to an average listener will yield weighted sound pressure levels close to 100 dB when measured in accordance with this International Standard. A very loud noise will yield a weighted level in the order of 120 dB, only 20 dB above. Weighted sound pressure levels which fall below about 90 dB will not normally be significant for human perception.
This International Standard specifies a frequency-weighting characteristic, designated G, for the determination of weighted sound pressure levels of sound or noise whose spectrum lies partly or wholly within the frequency band from 1 Hz to 20 Hz.
1. In the case of wide-band noises whose spectrum embraces both the infrasonic and audio-frequency ranges, use of this International Standard to determine the infrasonic weighted sound pressure level is supplementary to the description of the same noise by methods already standardized which cover the audio-frequency range only, for example the description in terms of Α-weighted sound pressure level. The numerical value of the results will, in general, differ from the G-weighted result. No significance should be attached to this difference since the respective results relate to different parts of the noise spectrum with little overlap.
2. The G-weighting can also be used in connection with the determination of sound power levels and sound intensity levels. When measured with the G-curve, these are symbolized respectively as LWG and LIG.
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