Division Reference conditions for NPD data
United Kingdom
101,325 kPa (1 013,25 mb)
Attenuation rates listed in Table D-1 of Appendix D
None
Less than 8 m/s (15 knots)
160 knots
Flat, soft ground free of large structures or other reflecting objects within several kilometres of aircraft ground tracks.
Standardised aircraft sound measurements are made 1,2 m above the ground surface. However no special account of this is necessary as, for modelling purposes, it may be assumed that event levels are relatively insensitive to receiver heightf1.
Comparisons of estimated and measured airport noise levels indicate that the NPD data can be assumed applicable when the near surface average conditions lie within the following envelope:
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Air temperature less than 30 °C
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Product of air temperature (°C), and relative humidity, (percent) greater than 500
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Wind speed less than 8 metres per second (15 knots).
This envelope is believed to encompass conditions encountered at most of the world's major airports. Appendix D provides a method for converting NPD data to average local conditions which fall outside it, but, in extreme cases, it is suggested that the relevant aeroplane manufacturers be consulted.
Annotations
Footnotes
- f1
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Calculated levels at 4 m or higher are sometimes requested. Comparison of measurements at 1,2 m and 10 m and theoretical calculation of ground effects show that variations of the A-weighted sound exposure level are relatively insensitive to receiver height. The variations are in general smaller than one decibel, except if the maximum angle of sound incidence is below 10° and if the A-weighted spectrum at the receiver has its maximum in the range of 200 to 500 Hz. Such low frequency dominated spectra may occur e.g. at long distances for low-bypass ratio engines and for propeller engines with discrete low frequency tones.