Division The duration correction DV (Exposure levels LE only)

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This correction^f1 accounts for a change in exposure levels if the actual segment groundspeed is different to the aircraft reference speed V_ref to which the basic NPD-data relate. Like engine power, speed varies along the segment (groundspeed varies from V₁ to V₂) and it is necessary to define an equivalent segment speed V_seg remembering that the segment is inclined to the ground; i.e.

V_seg = V/cosγ

(2.7.32)

where here V is an equivalent segment groundspeed (for information, see equation B-22 which expresses V in terms of calibrated airspeed, V_c and

γ = \tan^{- 1} (\frac{z_{2} - z_{1}}{s_{2} - s_{1}})

(2.7.33)

For airborne segments, V is taken to be the groundspeed at the closest point of approach S — interpolated between the segment end-point values assuming it varies linearly with time; i.e. if the observer is alongside the segment:

V = \sqrt{V_{2}^{1} + \frac{q}{λ} \times (V_{2}^{2} - V_{2}^{1})}

(2.7.34)

If the observer is behind or ahead of the segment, it is that at the nearest end point, V₁ or V₂ .

For runway segments (parts of the take-off or landing ground rolls for which γ = 0) V_seg is taken to be simply the average of the segment start and end speeds; i.e.

V_seg = (V₁ + V₂ )/2

(2.7.35)

In either case the additive duration correction is then

Δ_V = 10 · lg(V_ref/V_seg )

(2.7.36)

Annotations

Footnotes

f1: This is known as the duration correction because it makes allowance for the effects of aircraft speed on the duration of the sound event — implementing the simple assumption that, other things being equal, duration, and thus received event sound energy, is inversely proportional to source velocity.

Notes on Division 2.7.19

Footnotes

f1: NPD specifications require that the data be based on measurements of steady straight flight, not necessarily level; to create the necessary flight conditions, the test aircraft flight path can be inclined to the horizontal. However, as will be seen, inclined paths lead to computational difficulties and, when using the data for modelling, it is convenient to visualise the source paths as being both straight and level.