A new prototype sound absorption technique cultivated through a partnership between the Seattle-based architecture firm NBBJ and acoustics researchers at the University of Washington demonstrates that the shape of empty bottles alleviates noise issues. If embedded within buildings or wood panels, the narrow necks and wide vacant cavities within bottles could double as design sound traps.

By tuning these forms to coincide with specified acoustical wavelengths—especially the lower frequencies of speech—ceiling or wall panels drilled with small holes can catch and neutralise noise. Tests of prototypes within NBBJ’s office analysed noise reductions of about 13 decibels, or approximately the equivalent of wearing noise-canceling headphones. Regarding the perception of noise in the space, that makes a 60% reduction.

Ryan Mullenix, a partner at NBBJ, points out that the human voice is a frequent source of stress. With individuals returning to the office, noise is once again a workplace problem. In today’s corporate environment, the human voice is ever present. Thanks to open floor plans and talkative co-workers, and reflecting hard surfaces, noise is difficult to circumvent at the office.

The problem is low frequency noise, says Tomás Méndez Echenagucia, an assistant professor at the University of Washington College of Built Environments. One method involves capturing the sound before it reflects across a room—which can be accomplished with the aid of sound absorptive bottles.

One can create a sound by blowing across a bottle top. The sound that emanates is determined by the resounding frequency of the cavity within the bottle, where the pressure of the blown air vibrates, resonates, and gets rereleased as sound. Yet sans the additional pressure of blown air—the sound of a human voice in a room—sound will penetrate the bottleneck, and the particular frequencies that resonate in the cavity will dissipate.

When sound energy flows before the cavity, a certain percentage of it penetrates that cavity and becomes a resonator, according to Méndez Echenagucia. The acoustic waves resound with the bottle’s interior and conflict with the differing shape of the bottleneck, dissipating the energy. The friction of airborne particles in the cavity, enhanced by resonance, changes acoustic energy to thermal energy. The sound morphs into heat and is not audible.

If we tune a sufficient number of these bottles to a specific frequency range and line them up side by side, they can rid of low frequency sound effectively, says Méndez Echenagucia.

To test this concept, NBBJ, Méndez Echenagucia, and University of Washington graduate student Vidhya Rajendran computationally evaluated the acoustics of three existing office spaces to determine the low frequency sounds that should be reduced and where sound absorption could prove most useful. They then tested arrays of sound-absorbing bottle-like shapes called Helmholtz resonators, built into small wood frames, with the small scale necks of bottles facing outward and the cavities concealed on the interior. The testing revealed an optimised variety of these resonator cavities to absorb the standard lower frequencies of human speech and milled them in the form of a sizable cross-laminated timber panel that can be posted on a wall or ceiling. The panels were posted in NBBJ offices, where measurements by Arup acoustic consultancy demonstrated that the panels reduced perceptible noise by 60%.

Mullenix says the prospects are strong for this soft material, culled from synthetic materials with an elevated carbon footprint. For locations such as health care settings, spongy acoustic panels are too tough to keep clean and secure.

The resonator-based approach provides an option that can be embedded in hard surfaces easier to clean. NBBJ and the University of Washington are testing their applications in walls and prefab conference rooms. They see the potential for resonators to be built into the building structures—perhaps even beyond walls to concrete floor slabs.

Mullenix said that NBBJ is now planning to integrate this sound absorption tactic in a number of projects, such as a soon to be built cross-laminated timber building in Seattle.

The possibilities are endless, according to Méndez Echenagucia, and could feature panels posted in noisy areas such as elevator lobbies, ceiling fixtures in lavatories and medical offices, or even resonators embedded in structural facades to alleviate noise before it enters. The ultimate goal, he says, is to ease and mitigate sound transmission for the good of the office buildings.

Source: Fast Company.Com

Image credit/[Photo: NBBJ + University of Washington]