Critical Frequencies Measurement:  

f_{c} = Coincidence Frequency [Hz]
c = Speed of sound in air [ms^{1}]
d = Thickness of panel [m]
ρ = Density of panel [kgm^{3}]
Y = Young\'s modulus of panel [Nm^{2}]
Example:
Calculate the coincidence frequency or critical frequencies for the given details.
Speed of sound, c = 25 ms^{1}
Thickness of panel, d = 20 m
Density of panel, ρ = 15 kgm^{3}
Young\'s modulus, Y = 10 Nm^{2}
Solution:
Apply Formula:
f_{c} = [ sqrt(3)c^{2}/pi*d ] * [ sqrt(ρ/Y) ]
f_{c} = [sqrt (3)25 2 / 3.14*20] * [sqrt (15/10)]
f_{c} = [sqrt 3* 625 / 62.8] * [sqrt(1.5)]
f_{c} = [1.73*625/62.8] * [1.22]
f_{c} = [1081.25/62.8] * 1.22
f_{c} = 17.217*1.22
f_{c} = 21.1118 Hz
Coincidence frequency, f_{c} = 21.1118 Hz