Specifying sound absorption and diffusion for optimal acoustic spaces

This church uses fibreglass absorbers around the room to lower the overall intensity of the sound and cut down the specular reflections from the large amount of wall space. Large barrel diffusers were also added which not only further break up the specular reflections, but also function as bass traps due to their large resonant cavity filled with fibreglass batting.

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 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 fibreglass 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, such as 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 office 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 what would be used 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 theatres, 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, the designer should calculate or measure the room modes, and select the tuned treatments accordingly.

Finally, all 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.

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