Test level
	B0 = 20 log(q) + 76.4 dB (ref 20 µPa)
fN =	(U∞ / L)    [ (N - 0.25) / M + 0.57 / √((1 + M2 γ-1) / 2) ]   Hz Equation (1)
For Equation (1):
	- For cavities that have a length/depth (L/D) ≥2
	- For applications where 0.4 ≤M≤1.5
For Equation (2)
	- For cavities that have a length/depth (L/D) <2
fN =	((2N - 1) / 4) * (c / h)   Equation (2)
Where
c =	speed of sound
h =	cavity depth
fN =	Resonance frequency for the Nth mode (where N=1, 2, 3, ) up to 500 Hz (where f1>500 Hz use only this mode)
N =	Mode number
L =	Length/radius of opening exposed to air stream (m).
M =	Mach number
U∞ =	Free-stream velocity
γ =	Ratio of specific heats of gas (1.4 for air)
Test duration:	T=1 hour per resonance frequency