Coincidence

Professional Definition

Coincidence is the phenomenon of resonant transmission of acoustic energy through a barrier, occurring when the projection of the wavelength of the sound wave incident on the barrier equals the wavelength of the free bending wave propagating in the material of that barrier.

This phenomenon occurs above the so-called critical frequency, which depends on the bending stiffness and the surface mass of the material. At the point of coincidence, the acoustic impedance of the barrier drops sharply, resulting in a significant decrease in acoustic insulation (by as much as 10-15 dB compared to the mass law).

Acoustics in Simple Words

To understand coincidence, imagine a surfer on the sea. For the surfer to ride the wave effectively, their speed must match the speed of the wave. If these two values coincide (coincidence occurs), the energy of the water easily carries the surfer.

A similar thing happens with sound and a wall. A sound wave strikes the barrier at a certain angle, attempting to bend it. Every material (glass, concrete, drywall) has its natural tendency to resonate (bending wave). If the wavelength of the sound in the air perfectly matches the wavelength that a given material 'likes,' the wall stops resisting. Instead of reflecting noise, it vibrates and 'lets' the sound pass through to the other side almost losslessly. As a result, for certain shrill or buzzing sounds, a thick wall behaves as if it were not there at all.

Conclusion

The coincidence effect is a natural enemy of acoustic insulation, causing characteristic 'holes' in the attenuation graphs of homogeneous barriers. To combat it, engineers use multilayer constructions or materials with high internal damping, which allows them to shift the critical frequency beyond the audible range or mitigate the depth of the insulation drop.