Specifying sound absorption and diffusion for optimal acoustic spaces

Diffusion

After covering a range of different materials that remove acoustic energy from a space, 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 straightforward. 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 break 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, therefore, 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. 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.