The noise level contributions from each segment of the flight path are derived from the NPD-data stored in the international ANP database. However it must be noted that these data have been normalised using average atmospheric attenuation rates defined in SAE AIR-1845. Those rates are averages of values determined during aircraft noise certification testing in Europe and the USA. The wide variation of atmospheric conditions (temperature and relative humidity) in those tests is shown in Figure D-1.

The curves overlaid on Figure D-1, calculated using an industry standard atmospheric attenuation model ARP 866A, illustrate that across the test conditions a substantial variation of high frequency (8 kHz) sound absorption would be expected (although the variation of overall absorption would be rather less).

Because the attenuation rates, given in Table D-1, are arithmetic averages, the complete set cannot be associated with a single reference atmosphere (i.e. with specific values of temperature and relative humidity). They can only thought of as properties of a purely notional atmosphere — referred to as the ‘AIR-1845 atmosphere’.

The attenuation coefficients in Table D-1 may be assumed valid over reasonable ranges of temperature and humidity. However, to check whether adjustments may be necessary, ARP-866A should be used to calculate average atmospheric absorption coefficients for the average airport temperature T and relative humidity RH. Where, from a comparison of these with those in Table D-1, it is judged that adjustment is required the following methodology should be used.

The ANP database provides the following NPD data for each power setting:

• maximum sound level versus slant distance, L_max(d)

• time integrated level versus distance for the reference airspeed, L_E(d), and

• unweighted reference sound spectrum at a slant distance of 305 m (1 000 ft), L_n,ref(d_ref) where n = frequency band (ranging from 1 to 24 for 1/3-octave bands with centre frequencies from 50 Hz to 10 kHz),

all data being normalised to the AIR-1845 atmosphere.

Adjustment of the NPD curves to user-specified conditions T and RH is performed in three steps:

1. 1.

   First the reference spectrum is corrected to remove the SAE AIR-1845 atmospheric attenuation α_n,ref:

   Ln(dref) = Ln,ref(dref) + αn,ref · dref

   (D-1)

   where L_n(d_ref) is the unattenuated spectrum at d_ref = 305 m and α_n,ref is the coefficient of atmospheric absorption for the frequency band n taken from Table D-1 (but expressed in dB/m).

2. 2.

   Next the corrected spectrum is adjusted to each of the 10 standard NPD distances d_i using attenuation rates for both (i) the SAE AIR-1845 atmosphere and (ii) the user-specified atmosphere (based on SAE ARP-866A).

   1. (i)

      For the SAE AIR-1845 atmosphere:

      Ln,ref(di) = Ln(dref) – 20,lg(di/dref) – αn,ref · di

      (D-2)

   2. (ii)

      For the user atmosphere:

      Ln,866A(T,RH,di) = Ln(dref) – 20,lg(di/dref) – αn,866A(T,RH) · di

      (D-3)

   where α_n,866A is the coefficient of atmospheric absorption for the frequency band n (expressed in dB/m) calculated using SAE ARP-866A with temperature T, and relative humidity RH.

3. 3.

   At each NPD distance d_i the two spectra are A-weighted and decibel-summed to determine the resulting A-weighted levels L_A,866A and L_A,ref — which are then subtracted arithmetically:

   $\mathrm{\Delta L}\left({\mathrm{T,RH,d}}_i\right)=L_{\mathrm{A,866A}}-L_{\mathrm{A,ref}}=10\times \lg \sum _{n=1}^{24}{10}^{\left(L_{\mathrm{n,866A}}\left({\mathrm{T,RH,d}}_i\right)-A_n\right)∕10}-10\times \lg \sum _{n=1}^{24}{10}^{\left(L_{\mathrm{n,ref}}\left(d_i\right)-A_n\right)∕10}$

   (D-4)

The increment ΔL is the difference between the NPDs in the user-specified atmosphere and the reference atmosphere. This is added to the ANP database NPD data value to derive the adjusted NPD data.

Applying ΔL to adjust both L_max and L_E NPDs effectively assumes that different atmospheric conditions affect the reference spectrum only and have no effect on the shape of the level-time-history This may be considered valid for typical propagation ranges and typical atmospheric conditions.