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Commission Directive (EU) 2015/996Show full title

Commission Directive (EU) 2015/996 of 19 May 2015 establishing common noise assessment methods according to Directive 2002/49/EC of the European Parliament and of the Council (Text with EEA relevance)

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E3CONSISTENCY OF MAXIMUM AND TIME INTEGRATED METRICS — THE SCALED DISTANCEU.K.

A consequence of using the simple dipole model to define the energy fraction is that it implies a specific theoretical difference ΔL between the event noise levels Lmax and LE . If the contour model is to be internally consistent, this needs to equal the difference of the values determined from the NPD curves. A problem is that the NPD data are derived from actual aircraft noise measurements — which do not necessarily accord with the simple theory. The theory therefore needs an added element of flexibility. But in principal the variables α1 and α2 are determined by geometry and aircraft speed — thus leaving no further degrees of freedom. A solution is provided by the concept of a scaled distance dλ as follows.

The exposure level LE,∞ as tabulated as a function of dp in the ANP database for a reference speed Vref, can be expressed as

(E-10)

where p 0 is a standard reference pressure and tref is a reference time (= 1 s for SEL). For the actual speed V it becomes

(E-11)

Similarly the maximum event level Lmax can be written

(E-12)

For the dipole source, using equations E-8, E-11 and E-12, noting that (from equations E-2 and E-8) , the difference ΔL can be written:

(E-13)

This can only be equated to the value of ΔL determined from the NPD data if the slant distance dp used to calculate the energy fraction is substituted by a scaled distance dλ given by

(E-14a)

or

with (E-14b)

Replacing dp by dλ in equation E-5 and using the definition q = Vτ from Figure E-1 the parameters α1 and α2 in equation E-9 can be written (putting q = q1 at the start-point and q – λ = q2 at the endpoint of a flight path segment of length λ) as

and (E-15)

Having to replace the slant actual distance by scaled distance diminishes the simplicity of the fourth-power 90 degree dipole model. But as it is effectively calibrated in situ using data derived from measurements, the energy fraction algorithm can be regarded as semi-empirical rather than a pure theoretical.

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