ASA - ANSI/ASA S1.26
American National Standard Methods for Calculation of the Absorption of Sound by the Atmosphere
|Publication Date:||1 January 2014|
This Standard specifies an analytical method to calculate the attenuation of sound as a result of atmospheric absorption for a variety of meteorological conditions when the sound from any moderateamplitude source propagates through the atmosphere. The calculation method of the Standard applies for molar concentrations of water vapor in the atmosphere from less than 0.005 percent to greater than 5 percent and for ratios of the frequency of the sound to the atmospheric pressure from as low as 4 × 10-4 Hz/Pa (40 Hz per atmosphere) to as great as 10 Hz/Pa (1 MHz per atmosphere).
For pure-tone sounds, attenuation, using descriptor "decibels," owing to atmospheric absorption is specified by formulae in terms of an attenuation coefficient, in decibels per unit sound-propagation distance, as an analytical function of four variables: the frequency of the sound, and the temperature, humidity, and pressure of the atmosphere. Computed attenuation coefficients are provided in tabular form for ranges of the variables commonly encountered in prediction of outdoor sound propagation.
For wideband sounds analyzed by fractional-octave band filters (e.g., one-third octave band filters), an approximate method is provided for calculating the attenuation owing to atmospheric absorption from that specified for pure-tone sounds at the midband frequencies. The spectrum of the sound can be wideband with no significant discrete-frequency components or it may be a combination of wideband and discrete frequency sounds.
NOTE The approximate method will not necessarily give the same results as the primary analytical method.
This Standard applies to an atmosphere with uniform meteorological conditions and to a stratified atmosphere in which the meteorological conditions may be considered to be uniform within layers. The procedures described in the Standard may 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. The calculation method may also be applied to assess the contribution of atmospheric absorption to the decay of sound pressure level in a reverberant sound field often found in rooms.
This Standard accounts for the principal absorption mechanisms present in an atmosphere devoid of significant fog or atmospheric pollutants. It does not cover sound attenuation by mechanisms other than atmospheric absorption such as wave divergence, refraction, scattering by turbulence, ground reflection, or non-linear propagation effects.