This Aerospace Recommended Practice (ARP) defines quantities that may be used to describe various attributes of the sound field in the interior of aircraft. For a particular aircraft, or for a specific situation in a particular aircraft, it may not be necessary to utilize all the quantities included here to provide an adequate description of an aircraft's interior acoustical environment.

The scope of this document includes frequency-band sound pressure levels, wideband sound pressure levels, frequency-weighted sound pressure levels, and quantities that may be calculated from band sound pressure levels. The primary frequency bandwidths are those of octaves and one-third octaves.

This ARP does not include recommendations for specific quantities to use in a given situation, nor does it include specific numerical values of any quantity for use in evaluating the acceptability of the acoustical environment in the interior of an aircraft.

Definitions in this document are compatible with procedures in SAE ARP1323A (Reference 1), ARP1964 (Reference 21, and the International Standard IS0 5129 (Reference 3) for measurement of aircraft interior noise. The definitions are also compatible with standards on acoustical terminology (References 4 and 5) and with standards on preferred abbreviations, quantity symbols, units , and unit symbols (References 6 and 7). If no abbreviation or quantity symbol is given, then, for the purposes of this document, none is recommended.

Quantities defined in this document may be used to describe the acoustical environment in the interior of any kind of aircraft including jet propelled and propeller-driven airplanes and rotorcraft. The quantities are applicable to measurements of aircraft interior noise where the sound pressure levels are relatively steady so that valid time-averaged measurements may be obtained in a reasonably short measurement period. If the sound pressure level is not relatively steady, then additional quantities other than those in this document may be needed for an adequate description of the acoustical environment. Examples of nonsteady sounds include those that occur during take off, climbout, descent, and landing.

Table 1 presents a summary of the terms defined along with recommended abbreviations, quantity symbols, units, and unit symbols. Abbreviations are most useful for column headings in tables and for printing by machines not able to print lowercase letters. Quantity symbols are recommended for use in equations.