What are resonant absorbers?
The resonant absorbers are frequency-dependent because of the desired resonance of the material at a particular wavelength. Different types of resonant absorbers are the Salisbury screen, the Jaumann absorber, the Dallenbach layer, crossed grating absorbers, and circuit analog (CA) absorbers. Resonance absorbents consist of a mechanical or acoustic oscillation system, such as membrane absorbers, where there is a solid plate with a tight air space behind. Absorption reaches its maximum at the resonance frequency. Sound engineers fill the cavity with a porous material, to broaden the absorption over the range of frequency.
In a tuned resonant sound absorber, the sound absorption reaches the maximum value, max, at the resonant frequency, Fr, falling off to lower values at higher and lower frequencies. We can control this maximum value of absorption by the choice of the sound absorptive material with which the air space is filled. Usually, the material would be a kind of porous blanket or board, made of glass fiber or mineral fiber.
The maximum value of absorption depends only on the flow resistance of that material. Not on any of the physical dimensions of the sound absorptive treatment (such as the depth of airspace, perforation diameter, percent open area, etc.)
Compared to porous absorption, which offers great broadband coverage but often lacks in bass absorption, Resonant absorbers (aka tuned traps, aka narrowband absorbers) essentially do the opposite. They zero in on specific problems with bass frequencies while ignoring everything in the mid/upper range. And unlike porous traps which work better when spaced off the wall. Resonant absorbers (sometimes called “pressure absorbers”) work best up against the wall. Here the sound waves collide because that’s where the pressure is highest. And this is good news because it means they occupy far less space in the room.
The two standard types to know are:
- Helmholtz Resonators – Absorb bass frequencies through a small port in an air-tight cavity.
- Diaphragmatic Absorbers – Neutralize bass frequencies with a vibrating panel or membrane.