History lesson: improving acoustics with volume cavity resonators

September 5, 2018

Slot-type, divided cavity resonators are used to control problem noise reflecting off water surfaces in aquariums, natatoriums, and indoor pools.             Photo courtesy SoundSeal

by Kerry VonDross

Providing a good acoustical environment is critical to the well-being of people occupying interior spaces; especially for rooms having hard-surfaced, highly reflective enclosures.

Volume resonators—acoustical devices comprising a hollow cavity connected to the atmosphere via an aperture—are one means of achieving suitable environments for human occupancy and superior audibility for social interaction.

Origins of cavity resonators
The Vitruvian Man by Leonardo da Vinci (c. 1492) is one of the world’s most iconic images. It depicts the human body inscribed within the fundamental geometric patterns of the circle and the square. Da Vinci’s illustration of the ideal human body is based on concepts of geometry and human proportions as developed by the Roman architect and engineer Marcus Vitruvius Pollio (1st century BC) in his 10-volume work, De Architectura. Vitruvius’ texts, also known as the Ten Books on Architecture, are the earliest known treatise on architectural theory. He wrote guiding principles about the design of Roman structures, their materials and building methods, as well as precepts concerning the aesthetic and practical effects of architecture on the lives of citizens.

In his Book V, Vitruvius describes the proper arrangement and construction principles of various public places such as Roman forums and basilicas; emphasizing their design should follow the classic principles of symmetry and harmony and adopt proportions that imitate nature. Vitruvius also provides design strategies for building a proper community theater including site and orientation, foundation, enclosures, seating, and acoustics.

Sympathetic to the importance of good sound quality in a theatrical venue, Vitruvius provided design insights toward improving acoustic intelligibility. He wrote on the science of sound transmission through air movement, how stage voices best reach the audience, reflective vocal sounds and echoes, and on “methods increasing the power of the voice in theaters through the application of harmonics.”

Due to the provenance of Vitruvius’ writings, he is often regarded as the father of architectural acoustics. He is most noted for his examination of using echéa (literally: echoers) as instruments for improving the sound in theaters. These “sounding vessels” were bronze urns. Their size and number were proportionate to the volume of the amphitheater, distributed in tiers with equal space between them, placed free-standing in hollowed arched niches, and supported on wedges to minimize damping. Uttered stage voices, upon entering the cavities of these sounding vessels, were to cause it to resonate in unison with the voice frequencies and produce an increased harmonious note, thereby increasing the clearness of voice sounds for the audience.

There is debate among acousticians whether the echéa were sympathetic resonators that increased power, or instead, acted as sound absorbers that reduced reverberation and improved sound quality. However, both resonance and absorption may have taken place in the ancient theaters. The lively, free-perched echéa may well have acted in a reverberatory function as harmonic amplifiers. Additionally, the rigid stone masonry, arched-opening niches constructed to house the sounding vessels performed as built-in bass traps, dampening low-frequency resonances and improving the listening environment.



Leonardo da Vinci’s pen and ink drawing of The Vitruvian Man.
Photo courtesy Wikimedia Commons

Acoustic pots in medieval churches
The difficulty of sound intelligibility in rooms having hard-surfaced walls was evidently understood by church and cathedral builders in medieval times. Their religious structures were ultimately purposed for hearing spoken words and song of the human voice, but these majestic edifices were constructed with solid, permanent, stone and brick masonry—materials that produce strong, clear reflections, but are disastrous for interior room acoustics.


Pulpit voices emanating from the apse and liturgical song from the choir hall reverberated through the church before reaching the oblong naves where the listening laity was seated. These spaces were constructed with semi-circular walls, arched, vaulted, and domed ceilings, which exasperated audibility by prolonging sound resonance.

The idea of utilizing clay, acoustic pots—volume resonators—to control the room acoustics and improve sound perception was employed in church architecture from the 10th to the 16th centuries.
Urns and earthenware pots were placed within the buildings’ structural matrix. Instead of exploiting free-standing vibratory bronze urns, clay jars and pots of various sizes were implanted into the stone and brick masonry walls and vaults of more than 300 churches throughout Europe, with the vast majority located in France.

Again, there is debate amongst archeologists and acousticians regarding the ancient intention of these acoustic pots, whether they were for amplification of the voice or to dampen sound vibrations. However, with recent understanding of room acoustics and modern acoustical testing equipment, the evidence leans toward the point of view that acoustic pots provided absorption, decreased resonance time, and improved human voice perception within these highly reverberating environments.

The church builders had a practical knowledge of utilizing earthen pots effectively as volume resonators to control room acoustics. Not just one size, but various-sized pots and jars were utilized together to broaden resonant frequency absorption. Dust, potsherds, straw, and peat were found placed in the wall pots to baffle the resonator, thereby expanding the effectiveness of absorption at greater frequencies. The clay pots were made with their volume tuned towards low-frequency absorption—a sound control difficult to capture by other available means.

As explained by medieval architecture expert Andrew Tallon, “In the past, furniture, such as tapestries, wooden panels, or paintings, was present in large quantities in churches. It absorbed some of the reverberation. Notice the pots are tuned at low frequencies for which absorption by furniture is less efficient.”1

Built-in-the-structure volume resonators became a noise abatement tool for improving sound quality in hard-surfaced rooms (natural echo chambers amplifying generated noise and garbling sound perceptibility).


Obverse and reverse side of the Pritzker Architecture Prize bronze medal, inscribed with Marcus Vitruvius Pollio’s principles, “firmness, commodity, and delight.”
Photos © 2018 The Hyatt Foundation

Good architecture includes acoustic design
The annual Pritzker Architecture Prize honors living architects for their significant contributions to humanity and the built environment through the art of architecture. Along with a monetary prize the laureates are honored with a distinctive bronze medallion. Transcribed on the reverse side of the medal are three words, “firmness, commodity, and delight,” which recall the fundamental criteria to creating a good building as taught by Vitruvius.


Contemporary designers are tasked with satisfying Vitruvius’ ancient triad of architectural principles to produce modern well-built structures that are solid, useful in function, and are pleasing to the occupants. Yet, the ancient problem remains; selection of robust and durable materials for optimum strength and lasting shelter to meet criteria for solid design results in building spaces with dreadful acoustics.

Experiencing delightful architecture entails more than mere aesthetics. Along with visual appreciation of design, thermal comfort and acoustical well-being are other design concerns directly affecting human satisfaction of the built environment.

Hard-surfaced rooms promote noise
Controlling unwanted noise and providing a good acoustical environment are essential design concerns that should be addressed to provide maximum enjoyment of interior spaces. Rooms constructed with flat, hard-surfaced, highly reflective enclosures are prone to encounter noise problems (i.e. reflected sound, flutter echo, and standing wave resonant frequencies). Even at low levels, noise annoyances affect the acuity of wanted sound such as music or speech and cause confusion and anxiety. Hard-surfaced environments maintaining prolonged mid-power (e.g. cafeterias) or even brief durations of high-power noise levels (e.g. pump/generator rooms) may lead to human health issues.

Noise-induced hearing loss may be caused by prolonged or repeated exposure to noise at or above 85 decibels (dB). The cacophony of noisy school cafeterias averages 85 dB but may reach ratings over 100 dB. While the exposure time needs to be measured in hours for hearing loss to develop, even short durations at these noise levels can lead to emotional stress, fatigue, and headaches. The louder the noise the shorter the amount of time it takes for hearing loss to occur. This is a concern for occupational noise exposure where noise levels can reach up to 120 dB. It only takes 15 minutes of hearing 115 dB noise before ears get damaged.

The major acoustical problem with hard-surfaced rooms is reflected noise causing high reverberation times. Sound from a source bounces off the walls and reaches the listener in a delayed, rambling mode. This appears as muddled and distorted noise, resulting in confusion and anxiety. A good example of this is at a busy, chic, night club with metal, tile, and hard plaster walls and ceilings. Conversational speech builds up and gets louder and louder as it reflects off these flat surfaces until there becomes no coherent sound except from the people yelling at each other.

Problem noise within a room or enclosed space is primarily addressed by means of noise reduction through sound energy absorption. Various acoustical materials attempt to dampen sound annoyances, but thin, add-on, acoustical treatments typically do not have the depth of absorption required to capture noise at the low-end of the audible frequency bandwidth. A good means to achieve sound absorption at all frequencies is with cavity resonators.

Acoustical concrete masonry units (ACMUs) improve speech audibility and can be employed in multifunctional spaces like this West Philadelphia High School gymnasium where 2017 Democratic presidential nominee Hillary Clinton held a rally.
Image courtesy The Daily Pennsylvanian

Acoustical CMUs
One of the most versatile and durable materials for constructing permanent, “firm” buildings is modular unit masonry. Clay brick and concrete masonry units (CMUs) are utilized in constructing a multitude of versatile public and private building types because of their loadbearing characteristics and ability to provide lateral structural stability, and because these materials are tough and hard wearing, require minimal maintenance, and possess fire-resistant properties.

Structural acoustical concrete masonry units (ACMUs) were developed as an acoustically responsive design solution to control adverse noise propagated within masonry enclosures. ACMUs are modern built-in-the-wall volume resonators. They are available in two basic configurations: slot-type, divided cavity resonators and stacking volume resonators.

Both resonator types consist of a rigid structure (masonry face shell) encompassing a volume of air (the unit core) connected to the interior room air via a narrow aperture.

As noise flows between the coupled atmospheres through the aperture, sound energy is compressed, expands and resonates within the core space, depleting a significant amount of energy from the sound wave and resulting in noise absorption.

Alterations to the aperture opening and core space configurations have been developed over time to add impedance and improve absorption across broader Hz frequencies. Similar to the effect of adding peat or straw into medieval acoustic pots, ACMUs have fibrous filler inserts placed in their cavities to improve mid- and upper-range frequency noise absorption.

ACMUs tested according to ASTM C423, Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method, yielded sound absorption average (SAA) ratings of 0.70 to 0.85. Stacking volume resonators attained 100 percent average absorption efficiency at the 100-125-160-200 Hz frequency bandwidth. This low-frequency absorption is invaluable in supplying sound control that cannot be captured by carpets, surface treatments, acoustical tile, and related products.

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[6].


  1. [Image]: https://www.constructionspecifier.com/wp-content/uploads/2018/09/SoundBlox_DolphinPhoto.jpg
  2. [Image]: https://www.constructionspecifier.com/wp-content/uploads/2018/09/Da_Vinci_Vitruve_Luc_Viatour-1.jpg
  3. [Image]: https://www.constructionspecifier.com/wp-content/uploads/2018/09/Untitled-1.jpg
  4. [Image]: https://www.constructionspecifier.com/wp-content/uploads/2018/09/Gym_ACMUs.jpg
  5. [Image]: https://www.constructionspecifier.com/wp-content/uploads/2018/09/Chartreuse_du_Val_de_Bénédiction_Vase_acoustique.jpg
  6. kerryvondross.kerion@gmail.com: mailto:kerryvondross.kerion@gmail.com

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