Commission Directive (EU) 2015/996Show full title

E1 GEOMETRY U.K.

The energy fraction algorithm is based on the sound radiation of a ‘fourth-power’ 90-degree dipole sound source. This has directional characteristics which approximate those of jet aircraft sound, at least in the angular region that most influences sound event levels beneath and to the side of the aircraft flight path.

Figure E-1

Geometry between flight path and observer location O

Figure E-1 illustrates the geometry of sound propagation between the flight path and the observer location O. The aircraft at P is flying in still uniform air with a constant speed on a straight, level flight path. Its closest point of approach to the observer is P_p . The parameters are:

d

distance from the observer to the aircraft

d_p

perpendicular distance from the observer to the flight path (slant distance)

q

distance from P to P_p = – V · τ

V

speed of the aircraft

t

time at which the aircraft is at point P

t_p

time at which the aircraft is located at the point of closest approach P_p

τ

flight time = time relative to time at P_p = t – t_p

ψ

angle between flight path and aircraft-observer vector

It should be noted that, since the flight time τ relative to the point of closest approach is negative when the aircraft is before the observer position (as shown in Figure E-1), the relative distance q to the point of closest approach becomes positive in that case. If the aircraft is ahead of the observer, q becomes negative.