Tuned absorbers
Bass frequencies are a relatively small segment of the human hearing range—but they can cause problems 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 of that absorption in order 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 that 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. One must 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 has the ability to 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.
The concept of diffusion
After covering a range of different materials that remove acoustic energy from a space, one must now consider materials that take the energy in a space and redistribute it. Firstly, one must more closely examine the concept of diffusion. In physics, a completely diffuse field is defined as one where energy travels equally in all directions (isotropic) and has uniform sound pressure.
‘Diffusers’ contribute to diffusion in a space through the redistribution of acoustic energy. The mechanics of how they do this can vary greatly—from reflective geometric shapes, slots, blocks, organic profiles, and mathematically calculated structures.
Geometric diffusers
The large geometric shapes are relatively straight forward. Barrels, pyramids, wedges, hemispheres, and basically any other set of large facets, faces, and curves can redirect or reflect sound. These shapes contribute to diffusion through spatial redirection. Flat plates and planes can even work in some environments, arranged in a way that breaks up parallel reflections in a room. These larger shapes are simple in their function—they just scatter energy. Here is how they work.
A room’s acoustic field is affected by every element within it—it is a system of materials and elements installed in a certain way. As sound travels through the space, some surfaces will absorb sound, while others will reflect it. If those surfaces are large and flat, the wave will stay mostly intact and continue as a cohesive and contiguous sound wave. That mirror-like reflection is referred to as ‘specular,’ which means it travels together in the same direction, and in phase—this is opposite of diffusion. By adding differing surfaces to the space, larger reflections brake up and redirect spatially, interacting with other surfaces which will absorb or reflect them and subdivide them even further. As sound then takes varying paths throughout the space, it will travel different distances—and with sound travelling at a constant speed, distance equals time. The shift in time balances out sound pressure, and the shift in direction breaks up the sound wave, making it more isotropic and hence more diffuse.
Mathematic diffusers
Another class of diffusers is the ‘mathematic diffuser.’ While the geometric diffusers are mostly large redirectors utilizing other elements in the space to develop diffusion over a wide range of frequencies, mathematic diffusers are more focused and efficient in their approach. Mathematic diffusers are optimized and tuned to affect different frequencies in different ways. One way is by using temporal offset. One must remember: distance equals time. A sound can cancel itself out by interacting with itself, offset by a one-half wavelength. Here is how it works.
Sound travels as waves of positive and negative pressure. The frequency of those oscillations is the frequency of the sound. The orientation of pressure is noted as its ‘phase.’ By inverting the phase, in effect flipping the time during which the positive and negative pressures are aligned, the negative pressure will “cancel out” the positive pressure. This is exactly what certain mathematic diffusers do. By forcing the wave to travel into a cavity that is a quarter of its wavelength, it then must come back out—travelling another quarter wavelength for a total of one-half wavelength—it exits the cavity 180 degrees out of phase with itself. This process is very frequency dependent, which is why one will see many diffusers that have arrays of wells or blocks at different heights or depths, which correspond to different frequencies.
Blocks and wells have evolved with more complex computing into other optimized shapes, which target frequency bands using a matrix of bicubic interpolation—which has the added benefits of enhanced spatial redirection and smoother frequency transitions. Some other variations use grating or targeted diffraction, which occurs as acoustic waves bend around certain shapes and expand outward. These shapes can be slots, rings, edges, perforations, corners, and peaks—and their orientation, size, and structure have a different impact on acoustic energy. The benefit is diffracted energy that disperses and dissipates over many different surfaces across a device. This lowers the reflection intensity as these diffracted “sources” are not specular in nature.
As noted above, mathematic diffusers are designed to affect different frequencies in different ways. Some wavelengths are too long for a certain size diffuser, limiting its impact on those frequencies. These diffusers are most often optimized for mid to high frequencies as low frequencies are difficult to diffuse. As with frictional absorbers, diffusers need to be gigantic to affect low frequencies. If a wavelength at 20 Hz is over 17 m (56 ft), the quarter-wavelength design would still need to be over 4.2 m (14 ft) deep. A diffuser would be impractical at that size.
