Tuned absorbers
Bass frequencies are a relatively small segment of the human hearing range—but they can cause problem due to how they behave. Low frequencies are the principal culprits in peaks and nulls caused by room modes or resonances which exist in the room. A common mistake designers make is to add too much broadband absorption to try to absorb low frequencies. As stated before, the low frequencies will remain, and the room frequency response will become bass heavy with no high-end frequencies. This is where tuned absorbers come into play.
Tuned absorbers such as slot (or slat) resonators almost always focus on bass frequencies, or a specific subset of those frequencies. Almost all of them limit the absorption in the high frequencies to some degree, but there are a few that retain some absorption to tune the absorber to a wider frequency range. There are several different methods for creating tuned absorbers—some have a more focused approach than others. Resonant traps (e.g. slot resonators or so-called Helmholtz resonators) absorb sound through a resonant cavity in the material with one or more openings. The size, depth, orientation, and dampening of the cavities and openings adjust their effective frequency range. Some are very narrow band, focusing on a specific frequency, while others have a wider range they can absorb.
Other tuned absorbers such as diaphragmatic low frequency absorbers or pistonic bass traps use a moving mass, either a limp mass or a membrane. These masses absorb energy by virtue of their ability to be moved. Remember the physics—when acoustic energy impacts an object, that object has a response to that energy. If a surface has very high impedance or resistance to sound passage, like a painted concrete slab, acoustic energy reflects off—travelling back into the low-impedance air from which it came. Now, by lowering the impedance a bit more—with a drywall surface for example—the high-energy, low frequencies do break the impedance threshold and impart some of their energy into the surface. But high frequencies will mostly still bounce off.
Just changing materials and their means of mounting will produce different impedance values. In the case of a limp mass, like a mass loaded vinyl or a suspended sheet of plywood, the impedance relates to the size, weight, thickness, and density of the material, as they affect the ability of sound to try to “move” the mass. Since these materials are free hanging, one can calculate the impedance around them as negligibly different. By introducing a sealed cavity behind that material, another force comes into play: the impedance and resonance of the cavity behind the membrane. Where the limp mass absorber is tunable via size and weight, the membrane absorber with a sealed cavity can further focus the tuning on a specific frequency or range.
Yet another way to tune absorbers is to create composite materials. A designer does this simply by combining two or more materials to create an assembly that has a different set of performance characteristics. By laminating a flexible, yet impermeable, layer on or within a broadband absorber, one can adjust its low-frequency response. Every time acoustic energy changes medium, the impedance mismatch at the boundary creates a conversion loss. This diminishes acoustic energy every time it needs to navigate through another material—creating laminates effectively tunes an absorber.
