History lesson: improving acoustics with volume cavity resonators


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.

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