Division 2.7.16 Determination of event levels from NPD-data

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The principal source of aircraft noise data is the international Aircraft Noise and Performance (ANP) database. This tabulates L_max and L_E as functions of propagation distance d — for specific aircraft types, variants, flight configurations (approach, departure, flap settings), and power settings P. They relate to steady flight at specific reference speeds V_ref along a notionally infinite, straight flight path^f1.

How values of the independent variables P and d are specified is described later. In a single look-up, with input values P and d, the output values required are the baseline levels L_max(P,d) and/or L_E∞(P,d) (applicable to an infinite flight path). Unless values happen to be tabulated for P and/or d exactly, it will generally be necessary to estimate the required event noise level(s) by interpolation. A linear interpolation is used between tabulated power-settings, whereas a logarithmic interpolation is used between tabulated distances (see Figure 2.7.i).

If P_i and P_{i + 1} are engine power values for which noise level versus distance data are tabulated, the noise level L(P) at a given distance for intermediate power P, between P_i and P_{i + 1} , is given by:

L (P) = L (P_{i}) + \frac{L (P_{i + 1}) - L (P_{i})}{P_{i + 1} - P_{i}} \times (P - P_{i})

(2.7.19)

If, at any power setting, d_i and d_{i + 1} are distances for which noise data are tabulated, the noise level L(d) for an intermediate distance d, between d_i and d_{i + 1} is given by

L (d) = L (d_{i}) + \frac{L (d_{i + 1}) - L (d_{i})}{\lg d_{i + 1} - \lg d_{i}} \times (\lg d - \lg d_{i})

(2.7.20)

By using equations (2.7.19) and (2.7.20), a noise level L(P,d) can be obtained for any power setting P and any distance d that is within the envelope of the NPD data base.

For distances d that lie outside the NPD envelope, equation (2.7.20) is used to extrapolate from the last two values, i.e. inwards from L(d₁) and L(d₂) or outwards from L(d_{I – 1}) and L(d_I) where I is the total number of NPD points on the curve. Thus

Inwards:	$L (d) = L (d_{2}) + \frac{L (d_{1}) - L (d_{2})}{\lg d_{2} - \lg d_{1}} \times (\lg d_{2} - \lg d)$	(2.7.21)
Outwards:	$L (d) = L (d_{I - 1}) - \frac{L (d_{I - 1}) - L (d_{I})}{\lg d_{I} - \lg d_{I - 1}} \times (\lg d - \lg d_{I - 1})$	(2.7.22)

As, at short distances d, noise levels increase very rapidly with decreasing propagation distance, it is recommended that a lower limit of 30 m be imposed on d, i.e. d = max(d, 30 m).

Impedance adjustment of standard NPD data

Annotations

Footnotes

f1: Although the notion of an infinitely long flight path is important to the definition of event sound exposure level L_E , it has less relevance in the case of event maximum level L_max 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.