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Division Impedance adjustment of standard NPD data

United Kingdom

Original text. Later amendments are not shown.

The NPD data provided in the ANP database are normalised to specific atmospheric conditions (temperature of 25 °C and pressure of 101,325 kPa). Before applying the interpolation/extrapolation method previously described, an acoustic impedance adjustment shall be applied to these standard NPD data.

Acoustic impedance is related to the propagation of sound waves in an acoustic medium, and is defined as the product of the density of air and the speed of sound. For a given sound intensity (power per unit area) perceived at a specific distance from the source, the associated sound pressure (used to define SEL and LAmax metrics) depends on the acoustic impedance of the air at the measurement location. It is a function of temperature, atmospheric pressure (and indirectly altitude). There is therefore a need to adjust the standard NPD data of the ANP database to account for the actual temperature and pressure conditions at the receiver point, which are generally different from the normalised conditions of the ANP data.

The impedance adjustment to be applied to the standard NPD levels is expressed as follows:

Δ Impedance = 10 × lg ρ × c 409,81 (2.7.23)

where:

ΔImpedance

  • Impedance adjustment for the actual atmospheric conditions at the receiver point (dB)

    • ρ · c

  • Acoustic impedance (newton · seconds/m3) of the air at the receiver point (409,81 being the air impedance associated to the reference atmospheric conditions of the NPD data in the ANP database).

    • Impedance ρ · c is calculated as follows:

    (2.7.24)
    δ

  • p/po , the ratio of the ambient air pressure at the observer altitude to the standard air pressure at mean sea level: po = 101,325 kPa (or 1013,25 mb)

    • θ

  • (T + 273,15)/(T0 + 273,15) the ratio of the air temperature at the observer altitude to the standard air temperature at mean sea level: T0 = 15,0 °C

    • The acoustic impedance adjustment is usually less than a few tenths of one dB. In particular, it should be noted that under the standard atmospheric conditions (po = 101,325 kPa and T0 = 15,0 °C), the impedance adjustment is less than 0,1 dB (0,074 dB). However, when there is a significant variation in temperature and atmospheric pressure relative to the reference atmospheric conditions of the NPD data, the adjustment can be more substantial.

    Notes on Division 2.7.16

    Footnotes

    f1

    Although the notion of an infinitely long flight path is important to the definition of event sound exposure level LE , it has less relevance in the case of event maximum level Lmax which is governed by the noise emitted by the aircraft when at a particular position at or near its closest point of approach to the observer. For modelling purposes the NPD distance parameter is taken to be the minimum distance between the observer and segment.