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 filters (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-integration method is described in annex D. The spectrum of the sound may be wideband with no significant discrete-frequency components or it may be a combination of wideband and discrete frequency sounds. 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.