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, with 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 frequency range.  

Absorption Value

In a tuned resonant sound absorber, the sound absorption reaches the maximum value at the resonant frequency, Fr, falling off to lower values at higher and lower frequencies. We can control this maximum absorption value by choosing the sound-absorbent material with which the air space is filled. Usually, the material would be a kind of porous blanket or board made of glass or mineral fiber. The maximum absorption value depends only on that material's flow resistance. Not on any of the physical dimensions of the sound absorptive treatment (such as the depth of airspace, perforation diameter, percent open area, etc.)  

Bass Frequencies

Compared to porous absorption, which offers excellent broadband coverage but often lacks 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. This is unlike porous traps, which work better when spaced off the wall. Resonant absorbers (sometimes called "pressure absorbers") work best 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:

1. Helmholtz Resonators – Absorb bass frequencies through a small port in an air-tight cavity.
2. Diaphragmatic Absorbers – Neutralize bass frequencies with a vibrating panel or membrane.

Source Texts

• Sound Absorption: Designing Buildings Wiki
• The Ultimate Guide to Bass Traps for Home Recordings: E-Home Recording Studio