Specifying sound absorption and diffusion for optimal acoustic spaces

In smaller spaces, primary acoustic problems are caused by smaller dimensions and parallel surfaces.

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 more than 17 m (56 ft), the quarter-wavelength design would still need to be more than 4.3 m (14 ft) deep. A diffuser would be impractical at that size.

Where diffusers come into play

Imagine a room with four parallel walls, a parallel ceiling and floor, and no other materials. Any sound energy will remain mostly intact as it bounces off each of these surfaces. These intact reflections are essentially echoes—continuing to the next surface and reflecting off again. While an echo can enhance one’s experience at, say, the Grand Canyon, it will be problematic inside a room. A room would not be the size of the Grand Canyon, so there is little delay in echo. Echoes in this hypothetical room play on top of the next source sound and echo from other surfaces in regular, but rapid, succession—think hundreds of times a second. These overlapping echoes create artifacts or unwanted sound or noise through wave interference, which manifest themselves as flutter, ringing, comb filtering, and other unpleasant anomalies.

Diffusers break those contiguous waves down into thousands of lower intensity reflections, many of them travelling different paths, and with various degrees of phase shift. The large, intact echoes disappear as the energy is dispersed throughout the space uniformly, removing the harsh artifacts. Again, like absorbers, there are geometric diffusers and mathematic tuned diffusers. Breaking up large, flat surface reflections can be accomplished with those geometric diffusers and for more refined control of specific frequency ranges one can turn to the mathematic diffusers. Both types are designed with different diffusion patterns, so directing the energy where it needs to go is as simple as choosing the right diffuser and installing it in the right place.

The following is a list of several examples of the different treatment options and related considerations designers commonly consider when assessing acoustical issues in certain spaces.

Offices or small rooms

In smaller spaces, primary acoustic problems are caused by smaller dimensions and parallel surfaces. Those flat surfaces near sound sources and the listener are of immediate concern. A designer’s primary objectives should be to increase comfort, intelligibility, and clarity. Just imagine a conference call, speakerphone, zoom, or other type of auditory information exchange. Those “small room problems” will be amplified and retransmitted in such a space. Secondary concerns are the energy build-up and tuning the space to sound less like a little box. For reference, assume this room is not a critical listening space (i.e. a recording studio or meeting room, and is more like a home office or living space).

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