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ANSI/ASA S3.4 2007 Edition, May 24, 2007
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American National Standard Procedure for the Computation of Loudness of Steady Sounds
Includes all amendments and changes through Reaffirmation Notice , June 15, 2012
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This standard specifies a procedure for calculating the monaural and binaural loudness experienced by listeners with normal hearing under the following conditions:

Listening conditions. The standard applies to three listening conditions: free field with a frontal incidence, diffuse field, or listening via headphones. Listening in a free field with a frontal incidence is assumed when the sound source is located at least one meter directly in front of the listener and there are no space boundaries or other surfaces that affect the sound field. The free-field condition can be achieved in an anechoic chamber or in open space. Listening in a diffuse field is assumed when the sound reaches the listener's ears from all directions with essentially the same acoustic power. The sound field in a room with hard reflective walls where the sound is reflected many times before being significantly absorbed approximates a diffuse field. Listening through headphones is assumed when the sound is delivered directly to the listener's ears through circumaural or supra-aural headphones, or insert earphones. For simplicity, the term headphones will be used hereafter to denote both headphones and earphones.

NOTE 1 The equal-loudness contour for narrow-band noise in a free field differs from that in the diffuse field, but the difference measured at 1/3-octave-band center frequencies varies from -2.0 to +4.3 decibels with the mean difference equal to only 0.8 decibels. These differences do not significantly affect calculations for broadband spectra. Nevertheless, since the loudness of a sound depends on the nature of the enclosure in which it is heard, comparative evaluations of different sources should be based on measurements all made in essentially similar enclosures or all made in a free sound field.

NOTE 2 Pressure levels in a diffuse field should be measured by means of an omnidirectional microphone located in the unobstructed sound field at the position corresponding to the center of the listener's head.

Spectrum. The computational procedure described in the standard applies to a variety of sounds including complex tones, noise bands, and mixtures of the two. The characteristics of the sounds are specified in terms of their spectra. The procedure applies to sounds with a frequency range that extends from 20 Hz to 18,000 Hz. However, the procedure may not give accurate estimates of loudness for sounds with strong components above 12,500 Hz, and perceived loudness for such sounds is likely to vary markedly across individuals.

Steady state. The procedure described in the standard applies to steady state sounds and should not be used for impulse sounds or intermittent sounds. Application to such sounds may lead to discrepancies between measured and calculated loudness levels. The magnitude of the discrepancy is related to the dynamic characteristics of the device used to determine the sound pressure levels.