The coincidence frequency occurs when the "projection" of the wavenumber (or the wavelenght) in air on the wall plane (■ in reactive document above) corresponds to the bending wavenumber (or wavelength) of the wall (■) :

The sound transmission loss for diffuse sound field (usually reduced to transmission loss) is the result of the sound transmission loss for all oblique incidences in the range $\theta = [0$   $\pi/2]$ (the wall is considered as isotropic here and only one angle is of interest : $\theta$).

The critical frequency is the frequency of the lowest coincidence effect (i.e. for $\theta = \pi/2$).

For non-isotropic wall, the critical frequency spans on a range of frequencies and is sometimes called the critical valley to illustrate this effect.

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Luc Jaouen (@ljaouen), ISSN 2606-4138.
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