History lesson: improving acoustics with volume cavity resonators

Medieval acoustic jar embedded in the wall of the Church of the Chartreuse du Val de Benediction in France to control room acoustics and improve sound perception.
Photo courtesy Wikimedia Commons

Designing with ACMUs
For economic and operational reasons, rooms such as gymnasiums are typically built as big square boxes with exposed ceilings, painted concrete block walls, and hard-surfaced flooring. Athletic activities, band instruments, and cheering spectators contribute to the buildup of reflected noise and high reverberation times. Where activities may limit other acoustical surface treatments, ACMUs are an excellent solution to provide sound absorption (and with some system designs, diffusion) for improved acoustics. This is especially important when a gym space is multifunctional and utilized for activities where speech audibility is critical (e.g. rallies, wedding receptions, and sanctuary use).

It is recommended to seek the assistance of acoustical design professionals to formulate the best solution for improved acoustics. For simple noise reduction in hard-surfaced rooms, design professionals should consider the following.

Start with examining the nature of the noise issue. Determine the anticipated sound pressure strength and frequency intensity levels of the sound source. If designing enclosures for pumps, generators, or chillers, the manufacturers of the equipment should have sound data available. If designing walls surrounding gymnasiums, cafeterias, auditoriums, and worship centers, generalized charts can be accessed on the Internet.

Next, compare the noise source frequency levels to the noise reduction coefficient (NRC) levels of the absorptive product to determine its effectiveness at reducing the generated noise.
A broad frequency range is generally required for routine sound attenuation (e.g. voice, music, school, and workplace noise). Concentrated problem noises such as those produced by amplified music and mechanical equipment (e.g. pumps, transformers, and generators) generally have substantial low-frequency energy below the NRC frequency absorption of many acoustical products.

It is a mistake to simply assume a product with the highest NRC or SAA number will be the best absorptive material. As an example, even though an acoustical tile may attain a high NRC rating of 0.70 because it performs well from 250 to 2000 Hz, it could be inadequate at capturing low-frequency noise if it has little or no absorption at 250 Hz and below.

If noise source level measurements indicate a pronounced peak at a specific frequency or range of frequencies, the unit with the highest sound absorption coefficient at those frequencies is usually the best choice.

ACMU quantity
When determining how many ACMUs are required for a space it is best to use the services of an acoustical consultant to appraise the unique conditions involved with any particular project.

There are no absolute rules to determine coverage, but as simple as it sounds, the more ACMUs that are installed, the more sound will be absorbed. However, diminishing returns will be experienced as the coverage grows over a certain point. A basic rule of thumb is to calculate the cubic foot volume of the room and multiply this number by 3 percent to determine the square foot coverage of noise attenuating material needed for improved acoustics. For example, a 12 X 24 m (40 x 80 ft) room with a 6 m (20 ft) ceiling would require 178 m2 (1920 sf) of ACMUs.

ACMU placement
For rooms with variable noise sources (e.g. gymnasiums and cafeterias) or rooms with a central noise source (e.g. pump and generator rooms), it does not make much of a difference where the ACMUs are placed as long as they are spread out evenly around the room. For auditoriums and theaters, sound absorption is best when ACMUs are placed on the rear wall along with a combination of ACMUs having diffusion/absorption characteristics on the side walls.

Good architecture should incorporate the criteria of “firmness, commodity, and delight,” the fundamental criteria to creating a good building as taught by Vitruvius. Enhancing enjoyable occupation within hard-surfaced room enclosures should include the acoustical goals of sound absorption, decreased reverberation time, and improved human voice perception. Built-in-the-wall ACMU volume resonators have a long, successful history of achieving these goals within highly reverberating environments.

1 Refer to “Acoustics at the Intersection of Architecture and Music: The Caveau Phonocamptique of Noyon Cathedral” by Andrew Tallon, published in the Journal of the Society of Architectural Historians in September 2016.

Kerry VonDross is a masonry/acoustical expert with Kerion Consultancy in Green Lake, Wisconsin. Degreed in architecture, he has been awarded eight U.S. and Canadian patents relating to masonry and acoustical masonry products. VonDross can be reached at kerryvondross.kerion@gmail.com.


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