Specifying sound absorption and diffusion for optimal acoustic spaces

The following is a summary of how absorbers and diffusers can function in a space.
• 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 generally 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.


Larger communal spaces

The label “larger communal spaces” covers a wide range and could be covered in its own article; therefore, this article will take a broad, generic approach towards them. Broadband absorbers can be used to reduce overall noise here. These noises could be people talking, coughing, laughing, eating, and other sounds of basic living. In spaces where many people congregate, a designer should look to reduce reverb time to help improve intelligibility for announcements, reduce the cacophony caused by numerous, simultaneous conversations (the “cocktail party effect”), and increase safety.

In reverberant spaces, people tend to speak louder in response to their own lack of clarity. These spaces become increasingly uncomfortable, and it becomes harder to hear safety announcements, alarms, warnings, and direction, and the space just seems awash in noise. Similar to an office space, designers should spread out the absorption to address the many sources and receivers of sound. Focus treatments on the speech range of frequencies, as these are the main sources of noise in these spaces. Diffusers should be used if the space has large flat surfaces—geometric diffusers generally work well in these environments. If there is a bit of a bass buildup, analyze it to add some tuned absorbers to the space.

Treating the larger problems first versus tuning the space

The acoustics toolkit is not only used to fix problems, but also to optimize the sound in the space. Use absorption for overall intensity and first reflections, tuned absorbers for specific bass problems or buildup, and diffusers to control specular reflections or to round out the uniformity of the sound field. Again, different spaces have different requirements—and different problems have different solutions at different frequencies. In a world where the reflections within a space define its feel and functionality, learning the strengths and limitations of the tools available is the key to picking the right tool for the job. Obtain the specific test data for specified materials and learn their performance profiles. Analyze the problems in the space and approach the problem with broad strokes, treat the larger problems first, then tune the space with targeted treatments for specific anomalies and, finally, tie it all together and smooth it out.


Jim DeGrandis is a research and development engineer (and chief science officer) at Acoustics First Corporation. He is a member of the Acoustical Society of America (ASA) and works with ASTM International on researching new acoustic testing methods. DeGrandis frequently lectures about acoustic phenomena, simulation, and architectural acoustic design.

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