Getting to Grips with Aircraft Noise

Flight Operations Support & Line Assistance
Getting to grips with aircraft noise
9 - A BIT OF THEORY
The sound pressure amplitude varies inversely with the distance from the source.
Nonetheless, the practical way to measure the wave amplitude is to use the effective
acoustic pressure p e , which is the root-mean-square of the instantaneous sound
pressure variations over one period. This leads to the following relationship:
Pmax
pe = (9.2.1-2)
r 2
9.2.2. SOUND INTENSITY
It is the energy per unit of time per unit of area transmitted by a sound wave. The
instantaneous sound intensity at a distance r from the source is equal to :
p.
dA dr
I ( r)
= .
(9.2.2-1)
dA dt
Where: p.dA is the pressure force on the element of surface dA
dr/dt = vr is the speed of the molecule in the r direction
Thus:
I ( r)
= pv
(9.2.2-2)
r
59