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Division 2.7.19 Segment Event level correction terms

United Kingdom

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The NPD data define noise event levels as a function of distance perpendicularly beneath an idealised straight level path of infinite length along which the aircraft flies with steady power at a fixed reference speedf1. The event level interpolated from the NPD table for a specific power setting and slant distance is thus described as a baseline level. It applies to an infinite flight path and has to be corrected to account for the effects of (1) non-reference speed, (2) engine installation effects (lateral directivity), (3) lateral attenuation, (4) finite segment length and (5) longitudinal directivity behind start of roll on takeoff — see equations (2.7.25) and (2.7.26).

  1. The duration correction DV (Exposure levels LE only)
  2. Sound propagation geometry
  3. Engine installation correction ΔI
  4. Lateral attenuation Λ(β,ℓ) (infinite flight path)
  5. Finite segment lateral attenuation
  6. The finite segment correction ΔF (Exposure levels LE only)
  7. Specific Treatments of Ground-roll Segments, including the start-of-roll directivity function Δ SOR
  8. The start-of-roll directivity function Δ SOR
  9. Treatment of receivers located behind each takeoff and landing ground-roll segment

Annotations

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.