‘Acoustics’ in architecture is all about optimizing the sound environment. Two overarching categories used when discussing acoustics are soundproofing and acoustical treatment. While soundproofing is about reducing noise, acoustical treatment is about improving the quality of sound within a space. In this article, we will focus on the latter and take a look at several acoustical treatment strategies adopted in some of the world's greatest spaces for sound.
All building surfaces and materials have abilities to absorb, reflect or transmit sound. When sounds are reflected, they cause an increase in the overall echo and reverberation levels in a space. When treating rooms correctly, echo and reverberation is reduced. To treat rooms there are two methods available: sound absorption and diffusion.
The best acoustical treatment strategies combine these two techniques. Herzog & De Meuron’s Elbphilharmonie, developed in collaboration with acoustics expert Yasuhisa Toyota, is an excellent example of this. Its parametrically designed interior hall is clad with 10,000 unique fibre gypsum panels. When sound hits the panels, the uneven surface either absorbs or scatters sound waves. No two panels absorb or scatter sound waves alike, but rather create a balanced reverberation across the entire auditorium. The technique has been used for centuries, most notably in Vienna’s famed Musikverein.
Sound absorption refers to the incident sound that strikes a material that is not reflected back. As a rule of thumb, fibrous materials typically have better absorption properties, while denser materials are less absorbent. The acoustic absorption of a material varies by frequency and there are tables that list the absorption coefficients of materials when exposed to certain frequencies (Herz). To standardize, the label NRC refers to the "Noise Reduction Coefficient" and is a standard classification that corresponds to the arithmetic mean of the absorption coefficients for the frequencies 250, 500, 1000 and 2000 Hz.
On the low end of noise reduction coefficients are materials such as exposed concrete (0.03), glass (0.03) and metal (0.035). Meanwhile, more absorptive materials are glass wool (0.68), rockwool (0.72), acoustic foam (0.052) and wood fibre acoustic panel (0.57).
Aside from these standard materials, there are numerous innovative hybrid materials. One example is featured in Concert Hall Jordanek in Torum, by Fernando Menis. To create its cave-like interior finish, the architect experimented with a combination of mixed concrete with other aggregates in a technique he calls picado. A volcanic reddish stone from China is mixed with the concrete for sound absorption, while red brick is mixed in where diffusion is required.
Perforations play a key role in absorbing sound, working as resonators. The higher the perforation percentage, the greater the sound absorption. Perforations are possible on all wood, metal and painted finishes with standard perforations in straight, diagonal or staggered design, providing a Noise Reduction Coefficient from .70 to .95.
Perforation is a highly effective strategy for creating a range of acoustic effects. For the National Sawdust project Bureau V together with Arup’s acoustics team devised a custom skin made of perforated metal and fabric composite panels that is supported by an intricate network of recessed channels. This skin system remains visually translucent yet acoustically transparent, enabling sound to travel through it freely, while generating a wrap-around sculptural enclosure. The result is seamless experience for a wide repertoire of performance. Bureau V wrote a series of custom software programs to help navigate and optimize this complex, three-dimensional interior membrane.
Diffusion is the method of spreading sound energy with a diffuser to improve sound in a space. By spreading reflecting sound energy around a room, diffusion helps to reduce the harmful effects of strong echo and reverberation. Typical examples of a sound diffusor are curved or multi-faceted panels. Providing relief on a surface, these types of panels have the advantage of uniformly spreading flat-wall reflections that would otherwise be combined with original sound waves to create destructive interference. In a concert hall, for example, diffusion panels are used to enhance the richness of sound and help create a sense of spaciousness.
One of the most famed and classic examples of multi-faceted panels can be found in UNStudio’s Agora Theatre in Lelystad, which has become an icon not only for its bold use of colour but also for its folding acoustic surfaces.
Diffusing sound is also commonly achieved with bars and clouds. The Jazz Campus by Buol and Zund can be seen as an excellent example of this. During the process, the architects learned there was no such thing as the perfect rehearsal room, as musicians prefer a range of different spatial and acoustic qualities ranging from the feeling of a garage to a church. The project features over 50 different rehearsal rooms and a range of strategies including wood panelling and a flying acoustic sky.
A complex science, acoustics for auditorium, theatres and cinemas rely on the experience of an acoustics expert. But architects have shown a basic understanding of principles key in generating some of the world’s most inventive interior spaces.