Division 2.7.15 Single event metrics

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The sound generated by an aircraft movement at the observer location is expressed as a ‘single event sound (or noise) level’, a quantity which is an indicator of its impact on people. The received sound is measured in noise terms using a basic decibel scale L(t) which applies a frequency weighting (or filter) to mimic a characteristic of human hearing. The scale of most importance in aircraft noise contour modelling is A-weighted sound level, L_A .

The metric most commonly used to encapsulate entire events is ‘single event sound (or noise) exposure levels’, L_E , which account for all (or most of) the sound energy in the events. Making provisions for the time integration that this involves gives rise to the main complexities of segmentation (or simulation) modelling. Simpler to model is an alternative metric L_max which is the maximum instantaneous level occurring during the event; however it is L_E which is the basic building block of most modern aircraft noise indices and practical models can in future be expected to embody both L_max and L_E . Either metric can be measured on different scales of noise; in this document only A-weighted sound level is considered. Symbolically, the scale is usually indicated by extending the metric suffix, i.e. L_AE , L_Amax .

The single event sound (or noise) exposure level is expressed exactly as

L_{E} = 10 \times \lg (\frac{1}{t_{0}} \int_{t_{1}}^{t_{2}} 10^{L (t) ∕ 10} dt)

(2.7.17)

where t ₀ denotes a reference time. The integration interval [t₁,t₂] is chosen to ensure that (nearly) all significant sound in the event is encompassed. Very often, the limits t₁ and t₂ are chosen to span the period for which the level L(t) is within 10 dB of L_max . This period is known as the ‘10-dB down’ time. Sound (noise) exposure levels tabulated in the ANP database are 10-dB down values^f1.

For aircraft noise contour modelling, the main application of equation (2.7.17) is the standard metric Sound Exposure Level L_AE (acronym SEL):

L_{AE} = 10 \times \lg (\frac{1}{t_{0}} \int_{t_{1}}^{t_{2}} 10^{L_{A} (t) ∕ 10} dt)

with t₀ = 1 second

(2.7.18)

The exposure level equations above can be used to determine event levels when the entire time history of L(t) is known. Within the recommended noise modelling methodology such time histories are not defined; event exposure levels are calculated by summing segment values, partial event levels each of which defines the contribution from a single, finite segment of the flight path.

Annotations

Footnotes

f1: 10dB-down L_E may be up to 0,5 dB lower than L_E evaluated over a longer duration. However, except at short slant distances where event levels are high, extraneous ambient noise often makes longer measurement intervals impractical and 10-dB down values are the norm. As studies of the effects of noise (used to ‘calibrate’ the noise contours) also tend to rely on 10-dB down values, the ANP tabulations are considered to be entirely appropriate.