# ISO 9613-1:1993 pdf download – Acoustics – – Attenuation of sound during propagation outdoors一 Part 1: Calculation of the absorption of sound by the atmosphere

ISO 9613-1:1993 pdf download – Acoustics – – Attenuation of sound during propagation outdoors一 Part 1: Calculation of the absorption of sound by the atmosphere.

82 Pur.-tone method to approximate band-i.v.l attenuation

82.1 For each tractional-octave band of interest and specified uniform meteoi-ologic conditions along the sound propagation path. calculate the attenuation coefficient resulting from atmospheric absorption for the exact midbend frequency (as determined from equetion(6)], using the procedure for pure tones described in clause 6. The band-level attenuation for each frequency band, in decibeb. is then the product of the attenuation coefficient for the midband frequency and the path length, as in equation (2) for pure tones. Non-uniform meteorological conditions may occur along long sound paths, as discussed in annex C.

•.22 The error in band-level attenuation introduced by this pure-tone method of calculation is estimated to not exceed ± 0.5 dB provided that:

a) the bendpas fitters comply with the Class 1 or Class 0 tolerance limits of IEC 225;

b) for one-third-octave-band filters, the product of the source-receiver path length, in kilometres. and the square of the midband frequency, in kilohertz. does not exceed 6 km’ktlz2, nor does the path length exceed 6 km (at any midbend frequency);

C) for octave-band titters, the product of the sourcereceiver path length, in kilometres, and the square of the midband frequency, in kilohertz. does not exceed 3 km’kHz2, nor does the path length exceed 3 km (at any mtdband frequency).

82.3 The method described in 8,2.1 ii applicable to the calculation of bend-level attenuation of the sound produced by stationary or moving sound Sources. if the sound source moves during the period of interest. the attenuation from atmospheric absorption will vary with time because the effectrve frequency (or effecthe wavelength) vanes with time owing to the Doppler effect. This effect should be taken into account by calculating the attenuation coefficient for the DOppler-shifted frequency applicable to the sound- emission angle for each tIme of interest.

8,3 Calculation of atmosph.ric-abso.’ption attenuation for A-weighted sound pressure levels

Because the effects of atmospheric absorption are very frequency dependent, the recommended procedure for predicting the influence of atmospheric absorption on A-weighted sound pressure levels, as described by en example in annex E, is first to determine the band-level attenuations applicable to the atmospheric conditions. Apply the calculated band-level attenuatons to the band sound pressure levels d.lermined at a reference distance. Account for other losses as appropriate for the reference distances, and apply the standard A-frequency weightings to the bend sound pressure levels at the prediction distance.

NOTE 8 As me length of the Sound propagation path – creases ove the limiting values described in 822. the errors in CaIC*iatlflg lb. hanc$4rnvel stienuttlon M. by the method described wi 82.1 increase iso, and often rapdy. However, even when this e,yor fri soLN’id pressure level for andividuel frequency bends becomes large, It may sill be practical to use the method even In 82.1 for wid.b.nd Soixid because the error in the datIon of A-franuimncy. weiited sound presse lyel, obtained by combining the bend Ievls, is often very much smaler. The reason is that the attenuation due to atrnoherlc absorption. and hence the filler errors described in 8.1.2. wil be Iwge ody In the heavily attenuated bands that may not oontrtute Substanteay to the A4r.quency’weeghted Sound pressure level.

Annex E provides a worked example of the calculation of atmospheric-absorption attenuation for A- weighted sound pressure levels.

8.4 CombIned wideband and pure-tone sounds

For sound signals made uc of a widebend component plus one or more pure-tone components, the following procedure should be used to calculate the attenuation of tractional-octave-band sound pressure levels as a result of atmospheric absorption, The procedure is applicable to sound produced by stationery or moving sources. If the sotce is moving, attenuation coefficients should be calculated for the DOppler- shifted frequencies of the pure-tone components or the mbend frequencies of the wideband component, as described in 8.2.3.

Step I: Separate the measured spectrum, on the bests of time-mean-square sound pressures, into pure- tone end wideband components. For pure-tone components, the frequency of th. tone may be determined by spectrum analysis with a narrow-band filler, by prior knowledge of the source of the tones. or by a defined protocol for estimating the presence and level of a torte based solely on relative changes in the level of adlacent fractional-octave-band sound pressure levels, For the lattar cat., th. frequency of the tone may be assumed to be the exact midband frequency of the filter bend. However, if the pure tone approximation method given in 8.2 is used for the widebend element, end if the frequency of the tone is also assumed to be the exact m.dband frequency of the filter bend, then the procedure of separating the spectral components is not nocescary because the same pure-tone attenuation would apply to both the widebarid and discrete-frequency components.