Where diffusers come into play
Imagine a room that has 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, as 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.
Putting it all together
Here is a quick summary before moving into some treatment cases in familiar spaces.
∞ Absorbers are generally broadband or tuned.
∞ Broadband covers a wide range of frequencies and generally gets better the thicker it is, but there are practical limits.
∞ Tuned absorbers are usually implemented to improve bass absorption performance.
∞ “Too much,” “not enough,” and “wrong type” of absorption can be bad.
∞ Diffusers have several different types and configurations.
∞ Geometric diffusers are used to break up large, flat, surface reflections.
∞ Mathematic diffusers are more frequency specific.
∞ Diffusers have different reflection patterns.
∞ Diffusers are generally limited to mid and high frequencies, again due to practical limitations.
The following are several examples of different treatment options and related considerations designers commonly consider to assess acoustical issues in certain space.
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 image 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).
Broadband absorption should be immediately implemented to tame the parallel surface reflections which are the prime contributors to the flutter, ringing, and other artifacts in the space. The bonus is that by deploying broadband absorption, it will also reduce the overall buildup of energy in the room. These treatments do not need to be thick (e.g. up to 51 mm [2 in.] thick fiberglass or foam panels), but should be dispersed relatively evenly throughout the space, as sound could be generated from different sources, or received in different locations. Carpets or area rugs can also be effective to help control reflections between the ceiling and floor.
These steps will address the main issues of the space and can be implemented using a variety of ceiling and wall treatments for the desired effect. Diffusion may be attained through other furnishings in the space—bookshelves, tables, chairs, or by adding some mathematic diffusers to assist in the development.
Critical listening room
A critical listening environment, like a recording/broadcast studio or a mixing/mastering room, requires a very different approach than the previous example. Begin by analyzing the differences between a critical listening space and a normal office space. Assume the spaces are roughly the same size and construction.
The critical listening space is more deliberate in its function than other rooms, such as offices spaces. This room is laid out with a purpose. A user may still be at a desk, but now they are mixing music in this space—so the speakers are the primary sources of sound in this environment, and the listening location(s) will be generally static. When treating a room like this, start by determining the locations of the sound sources and the listener and address any issues that would interfere with the sound travelling to the listener’s ears.
First reflection points are a primary concern as they will smear the source content as it interacts with those secondary sound paths due to the reflection points. One should map those locations and treat them as appropriate with broadband absorbers which are thicker than in the previous example due to the wider frequency range and higher sound pressure levels experienced in these environments. Depending on the space and speaker configuration, a designer will examine the first reflections on the side walls between the speakers and the listener, on the ceiling in the first reflection zone, and on the front wall behind the speakers and on the rear wall. While this sounds similar to the previous example, the designer must be more deliberate with the placement of these thicker absorbers, as their function is less about broad control of parallel reflections and more about making sure a clear source sound gets to the listeners ears.
Second, bass control is a priority in a critical listening space. Music rooms, movie theaters, and other critical listening spaces can have a great deal of bass content. Where it was an option in the previous example, controlling the bass here will be essential. The room will likely have tuned traps in the corners as well as some composite tuned absorbers for areas outside of the first reflection points. To get an idea of the correct frequency ranges, a designer should calculate or measure the room modes, and select the tuned treatments accordingly.
Finally, all of this absorption may have cleaned up the modes and reflections, but it may also have left the space a little “dry” sounding. This is a common problem that occurs is these critical listening spaces. One should add diffusers to the back wall, rear upper side walls, and rear ceiling to redistribute some of the remaining energy back into the room. While it may seem a bit counter intuitive, having that energy back in the room is a good thing, and balances the sound field while reducing listener fatigue.
