Sound Transmission Loss of a single partition wall
(the coincidence frequency)
Plane waves, oblique incidence : °
• thickness mm,
• mass density kg.m-3,
• Young's modulus GPa,
• Poisson's ratio ,
• loss factor
The coincidence frequency is estimated at : Hz (critical at : Hz).
Wavelengths at Hz
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.