ISO 9613-1:1993 pdf free download.Acoustics — Attenuation of sound during propagation outdoors — Part 1: Calculation of the absorption of sound by the atmosphere.
This part of ISO 9613 specifies an analytical method of calculating the attenuation of sound as a result of atmospheric absorption for a variety of meteorological conditions when the sound from any source propagates through the atmosphere outdoors.
For pure-tone sounds, attenuation due to atmospheric absorption Is specified in terms of an attenuation coefficient as a function of four variables: the frequency of the sound, and the temperature, humidity and pressure of the air. Computed attenuation coefficients are provided in tabular form for ranges of the variables commonly encountered in the prediction of outdoor sound propagation:
— frequency from 50 Hz to 10 kHz,
— temperature from —20 C to +50 C,
— relative humidity from 10 % to 100 %, and
— pressure of 101.325 kPa (one atmosphere).
Formulae are also provided for wider ranges suitable for particular uses, for example, at ultrasonic frequencies for acoustical scale modelling, and at lower pressures for propagation from high altitudes to the ground.
For wideband sounds analysed by fractional-octave band lifters (e.g. one-third-octave band filters), a method is specified for calculating the attenuation due to atmospheric absorption from that specified for pure-tone sounds at the midband frequencies. An alternative spectrum-mtegration method is described in annex D. The spectrum of the sound may be wide-band with no srgnifcant discrete-frequency components or it may be a combination of wideband end dIscrete frequency suunds.
This part of ISO 9613 applies to an atmosphere with uniform meteorological conditions. It may also be used to determine adjustments to be applied to measured sound pressure levels to account for differences between atmospheric absorption losses under different meteorological conditions. Extension of the method to inhomogeneous atmospheres is considered in annex C, in particular to meteorological conditions that vary with height above the ground.
This part of ISO 9613 accounts for the principal absorption mechanisms present in an atmosphere devoid of significant fog or atmospheric pollutants. The calculation of sound attenuation by mechanisms other than atmospheric absorption, such as refraction or ground reflection, is described in ISO 9613-2.
2 Normative references
The following standards contain provisions which. through reference in this text, constitute provisions of this part of ISO 9613. At the time of publication, the editions indicated were valid. All standards are subject to revision, and parties to agreements based on this part of ISO 9613 are encouraged to investigate the possibility of applying the most recent editions of the standards indicated below. Members of IEC and ISO maintain registers of currently valid International Standards,
ISO 2533:1975, Standard Atmosphoro.
ISO 266:1975, Acoustics — Preferred frequencies for measurements.
IEC 225:1966, Octave, halt-octave and third-octave band filters intended for the analysis of sounds and vibrations.
3 Symbols
f frequency of the sound, in hertz
fm midband frequency, in hertz
h molar concentration of water vapour, as a percentage
Pr reference ambient atmospheric pressure, in kilopasc&s
p initial sound pressure amplitude, in pascals
Pt sound pressure amplitude, in pascals
Po reference sound pressure amplitude (20 Pa) p8 ambient atmospheric pressure, in kilopascals
$ distance, in metres, through which the sound propagates
T ambient atmospheric temperature, in kelvins
T0 reference air temperature, in kelvins
a pure-tone sound attenuation coefficient, in decibels per metre, for atmospheric absorption
NOTE 1 For convenience, in this part of ISO 9613, the shortened term “attenuation coefficient” will be used for a in piece of the full description.
8L. attenuation due to atmospheric absorpton, in decibels
4 Reference atmospheric conditions
4.1 Composition
Atmosphenc absorption is sensitive to the composition of the air, particularly to the widely vaiying concentration of water vapour. For clean, dry air at sea level, the standard molar concentrations, or fractional volumes of the three principal, normally fixed, constituents of nitrogen, oxygen and carbon dioxide are:
0,780 84; 0,209 476; and 0,000 314, respectively (taken from ISO 2533). For dry air, other minor trace constituents, which have no significant influence on atmospheric absorption, make up the remaining fraction of 0,009 37. For atmospheric absorption calculations, the standard molar concentrations of the three principal constituents of dry air may be assumed to hold for altitudes up to at least 50 km above mean sea level. However, the molar concentration of water vapour, which has a major influence on atmospheric absorption, varies widely near the ground and by over two orders of magnitude from sea level to 10 km.
4.2 Atmospheric pressure and temperature
For the purposes of this part of ISO 9613, the reference ambient atmospheric pressure, Pr. is that of the International Standard Atmosphere at mean sea level. namely 101,325 kPa. The reference air temperature, T1,. is 293,15 K (20 C), i.e. the temperature at which the most reliable data supporting this part of ISO 9613 were obtained,
5 Attenuation coefficients due to
atmospheric absorption for pure-tone sounds
5.1 BasIc expression for attenuation
As a pure-tone sound propagates through the atmosphere over a distance s, the sound pressure amplitude pt decreases exponentially as a result of the atmospheric absorption effects covered by this part of ISO 9613 from its initial value in accordance with the decay formula for plane sound waves in free space
— p1 exp( — 0,115 1 s)
NOTE 2 The term exp( —0,115 1 a) represents the base
e of Nepenan logarithms raised to the exponent indicated
by the argument in parentheses end the constant
0,115 1 — lj[10 19(e2)].
5.2 Attenuation of sound pressure levels
The attenuation due to atmospheric absorption 5L.(J), in decibels, in the sound pressure level of a pure tone with frequency f, from the initial level at .1 — 0 to the level at distance s. is given by
8L(j) = 10 Ig(p2Ip12) dB = as
6 Calculation procedure for pure-tone
attenuation coefficients
6.1 Variables
The acoustic and atmospheric variables, i.e. frequency of the sound, ambient atmospheric temperature, molar concentration of water vapour and ambient atmospheric pressure, are listed in clause 3, together with their symbols and units.
NOTES
3 For a specific sample of moist air, the molar concentration of water vapour is the ratio (expressed as a percentage) of the number of kilomoles (I.e. the number of kilogram molecular weights) of water vapour to the sum of the number of kilomoles of dry air and water vapour. By Avogadro’s law, the molar concentration of water vapour is also the ratio of the partial p’essure of water vapour to the atmospheric pressure.
6.3 Computation of the att.nuation co.ffici.nt
Equations (3) to (5) are all that is needed to calculate the pure-tone attenuation coefficient for atmospheric absorption for selected values of the variables. Although air temperature and air pressure data may not be supplied in the units of measure given in clause 3. conversion factors are readily available to convert the given unit to kelvins or kilopascals respectively. Humidity data, on the other hand, are rarely supplied in terms of molar concentration of water vapour. Annex B provides information on conversion of humidity data that are supplied in terms of relative humidity, dewpoint and other measures, to corresponding values of molar concentration.
The means by which a real inhomogerieous atmosphere may be approximated by the uniform atmosphere assumed in the formulae of 6.2 are discussed in annex C.
6.4 Tabular valu.s of the attenuation coefficient
For selected values of T, I, and fat a pressure of one standard atmosphere (101,325 kPa), table 1 lists pure-tone attenuation coefficients for atmospheric absorption calculated by use of equations (3) to (5), but using the unit “decibels per kilometre” for convenience in applications to sound propagation outdoors over path lengths of the order of a few kilometres. Tabular values are presented in scientific notation to preserve accuracy at low frequencies. Users of table 1 should not interpolate between the entries, or extrapolate beyond the table range, but should use equations (3) to (5) to calculate the specific pure-tone attenuation coefficients for desired conditions.
NOTES
5 For convenience, the frequencies shown in table 1 are the preferred frequencies for one-third-octave bend filters (see ISO 266 and IEC 225). However, the attenueton coefficients in table 1 were calculated for the exact midband frequencies j,,, in hertz, using the general exp(ession according to the base 10 system
fm(1 000) (103b10)*
where 1 000 Hz is the exact reference frequency and b is a rational fraction that serves as the bandwidth designator for any fractional-octave band filter (e.g. with b — 113 for one-third-octave band filters, end so on for other bandwidths). For table 1, index k i.s an integer from —13 to + 10. corresponding to preferred frequencies from 50 Hz to 10 kHz. For exact ultrasonic frequencies at one-third- octave-bend intervals from 10 kHz to 1 MHz, equation (6) may be used with k ranging from +10 to + 30.
6 Relative humidities given as column headings in table 1 are with respect to saturation over a surface of hquid water at all temperatures; see annex B. The saturated vapour
atmospheric pressure: less than 200 kPa (2 atm)
frequency-to-pressure ratio: 4 x 1 0 HzIPa to 10 HzIPa
7.3 Accuracy of ± 50 %
The accuracy of the calculated pure-tone attenuation coefficients due to atmospheric absorption is estimated to be + 50 % for variables within the following ranges, which include environmental conditions encountered at altitudes up to 10 kin:
molar concentration of water vapour: less than
0,005 %
air temperature: greater than 200 K (—73 C)
atmospheric pressure less than 200 kPa (2 atm)
frequency-to-pressure ratio: 4 x i0 HzIPa to 10 HziPe
8 Calculation of attenuation by
atmospheric absorption for wideband
sound analysed by fractional-octave-band filters
8.1 DescriptIon of the general problem and calculation methods
8.1.1 Previous clauses of this part of iSO 9613 have considered the effects of atmospheric absorption on the reduction in the level of a pure tone during propagation through the atmosphere. In practice, however, the spectrum of most sounds covers a wide range of frequencies, and spectral analysis is normally performed by fractional-octave-band filters that yield sound pressure levels in frequency bands.
losses as appropriate for the reference distances, and apply the standard A-frequency weightings to the band sound pressure levels at the prediction distance.
NOTE 8 As me length of the sound propagation path increases above the limiting values described in 8.2.2. the errors in caiculating the band-level attenuation Mt by the method described in 8.2.1 increase also, and often rapidly. However, even when this error in sound pressure level for individual frequency bands becomes large. It may still be practical to use the method given in 8.2.1 for wideband sound because the error in the calculation of A-frequancyweighted sound pressure level, obtained by combining the band levels, is often very much smaller, The reason is that the attenuation due to atmospheric absorption, and hence the filter errors described in 8.1.2, wIll be large only in the heavily attenuated bands that may not contribute substantia y to the A-frequency-weighted 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 up of a wideband component plus one or more pure-tone components, the following procedure should be used to calculate the attenuation ot 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 source is moving, attenuation coefficients should be calculated for the Doppler- shifted frequencies of the pure-tone components or the midband frequencies of the wideband component, as described in 8.2.3.
Step 1: Separate the measured spectrum, on the basis of time-mean-square sound pressures, into pure- tone and wideband components. For pure-tone components, the frequency of the tone may be determined by spectrum analysis with a narrow-band filter, by prior knowledge of the source of the tones. or by a defined protocol for estimating the presence and level of a tone based solely on relative changes in the level of adjacent fractional-octave-band sound pressure levels. For the latter case, the frequency of the tone may be assumed to be the exact midband frequency of the filter band. However, if the pure tone approximation method given in 8.2 is used for the wideband element, and if the frequency of the tone Is also assumed to be the exact midband frequency of the fitter band, then the procedure of separating the spectral components is not necessary because the same pure-tone attenuation would apply to both the wideband and discrete-frequency components.
Step 2: Calculate the attenuation over the specified path length for each spectral component separately, employing the methods specified in 5.2 and 6.3 for the pure-tone components, and in 8.2 for the wide- band component.