Acoustics in architecture21 July, 2022 shirin froozeh No Comment
Acoustics in architecture
Taken from the February 2020 issue of Physics World where it appeared under the title “Sound designs”. Members of the Institute of Physics can enjoy the full issue via the Physics World app.
The refurbishment of public buildings is often more complex than meets the eye. Anna Demming speaks to acousticians and architects about the acoustic considerations behind their designs for public spaces, and some of the tricks to tackle the conflicting demands on these venues
Complex space The new foyer and café at Bristol Old Vic has been acoustically designed so that small groups can enjoy intimate conversations (left). At the back of the foyer, the auditorium wall (right) has acoustic qualities that allow this area to be used as a performance space. (Courtesy: Fred Hawarth)
In the historic city centre of Bristol in the UK, down a cobbled street lined with mismatched buildings, is the oldest continuously running theatre in the English-speaking world – the Bristol Old Vic.
Built in 1766, and originally called the Theatre Royal, the building underwent a multi-million-pound refurbishment to mark its 250th anniversary. The work required detailed and careful design to ensure that the great Georgian auditorium – renovated in the first phase of the project – can serve the acoustic needs of a wide range of live theatre, music and dance.
Just as complex were the acoustic requirements of the rest of the building, which was renovated in phase two. This second stage included additional performance spaces and offices, as well as a foyer that accommodates a café bar and a further potential performance space. All these different functions have specific and often distinct acoustic requirements, which can be at odds with a host of other technical, cultural and aesthetic demands.
Someone who helps overcome these kinds of hurdles to achieve the ideal acoustic set-up is Bob Essert. Having studied both engineering and music, in 2002 he set up Sound Space Vision (SSV) – a London-based company of acousticians and architectural consultants.
One of SSV’s current projects is a £48.8m renovation of another Bristol auditorium: the city’s Colston Hall, which lies just down the road from Bristol Old Vic and is due to reopen in 2021. As an 1800-seat concert venue, the scale of Colston Hall offers plenty of space for the artists who have performed there since it first opened in 1867, from full-scale symphony orchestras to the Beatles. It has what is often described as a “shoebox” geometry – long with high ceilings that give plenty of space in front of the musicians for a rich sound around the audience, and less space for the sound to get lost behind the performance area (see Levitt Bernstein Architects’ rendering below). The shoebox design is a classic format that some say produces the best acoustics, with nine out of the world’s top 10 concert halls having this shape according to a 2016 survey by Business Insider.
While Essert says the biggest determinant of acoustics is scale, geometry comes second on his list of factors, followed by the materials used. “All three play a part,” he says. A vastness of length, height and general scale in a performance space is not, however, always desirable. Essert points to the hall at the Yehundi Menuhin School in Surrey, UK, as an example where SSV aimed for more compact dimensions that could seat 300 people in a space crafted specifically for solo and chamber performances. “The further away the boundaries of the room are from the listener and to a certain extent the performers, the weaker the sound is,” says Essert.
In simple terms you can think of sound waves attenuating and losing intensity as they travel across the dimensions of the room. As Essert emphasizes, how loud a performance sounds is a key factor for making the audience feel enveloped and immersed in the experience, and as a result designing specifically for solo performers means ideally designing a smaller space. So how can a solo be heard in a space designed to accommodate a full symphony orchestra, and give a feeling of intimacy in a hall that seats 1800?
Reflections on sound design
Ultimately the impact of a production on the audience is dominated by the artistry of the performers on stage. However, an effect that can help a performance to sound intimate and enveloping, even in a huge hall, is reflected sound. Because sound moves at a finite speed – 343 m/s in dry air at 20 °C – any reflections from the boundaries of the room will reach someone in the audience with a delay of several milliseconds compared with the sound that has travelled directly from the performers. You may not consciously hear the delay, but Essert points out that as the brain assembles audio input, this delay – and crucially, the amplitude and direction of arrival – affects the experience.
Soft furnishings as opposed to hard walls will dampen these reflections as demonstrated back in 1895 by US physicist Wallace Clement Sabine, who is widely acknowledged as the founder of architectural acoustics. During an assignment to improve the acoustics of the Fogg lecture hall at Harvard University, he armed himself with an organ pipe and a stopwatch and embarked on a series of experiments, determining by ear how long a sound took to decay as he, for example, changed the number of cushions in the room. Sabine soon established that it was the area of cushions (or any absorbing material) that was linearly related to reverberation time.
The advent of the oscilloscope in the 1960s moved acoustics technology up a gear, making it possible to directly image sound input and analyse the delays from these reflections. Researchers then began to find out more about the role of the direction of sound. For instance, reflections from the sides can make audiences feel more immersed in the experience, just by being surrounded by the sound.
An appreciation of the role of reflections drew attention to the way sound is fed from one surface to another, and affected the design of performance spaces. The basic shoebox geometry is still popular with architects as it has been since the construction of medieval churches, effectively the concert halls of their day. But in the early 1980s – following research in the 1960s and 1970s by Michael Barron and Harold Marshall in the UK and research groups in Göttingen and Berlin – Essert and other acousticians began shaping geometries to guide sound. By engineering the direction in which they reflected the sound, they could bring more sound in from the side. Examples of this architecture include Christchurch Town Hall in New Zealand, the Royal Concert Hall in Nottingham, UK, and the Meyerson Symphony Center in Dallas, US.
Levels of sound
Colston Hall has already seen several renovations and reconstructions (figure 1), the most recent being in 1951 led by Philip Hope Bagenal, the UK’s most prolific concert-hall acoustician of that period. The 1936 renovation had been focused towards cinema – which was then the market-leading use for halls of that nature – resulting in an emphasis on sight lines, audience capacity and cinema sound. But, having survived the Blitz, the concert hall fell victim to a fire started by a cigarette in 1945, and in the 1951 rebuild, Bagenal and architect J Nelson Meredith restored the interior to prioritize classical music performances. Most notably, Bagenal and other acousticians in the UK back then felt that British concert halls lacked definition. The British musical life and taste had been coloured by the sound of town halls around the country, explains Essert – “tall, flat floor spaces that produced a muddy sound”.
Bagenal endorsed a stepped rectangular plan for Colston Hall and introduced materials that would absorb bass “to avoid boom”. In particular, he added a canopy over the stage to project the clarity of string instruments. Although the oscilloscope was not yet established in 1951 so not available to aid design, it had been realized that canopies can reflect sound back to musicians so they can hear themselves.
One of the issues now being addressed by SSV’s renovations at Colston Hall is a literal shortcoming of this canopy. Following extensions to the stage to accommodate larger orchestras, the canopy no longer covers the string section who sit at the front of the stage. In addition, it also turns up at the leading edge, directing the sound out to the audience and making it even harder for the string musicians to hear themselves. Among the renovations SSV is helping implement will be an extended and reshaped canopy with more rigging in it to meet more extensive technical requirements.
Not all reflections are helpful either. The balconies at Colston Hall previously extended over 14 rows of the auditorium, creating a “dead zone” for hundreds of seats: multiple reflections from the bottom of the balcony attenuated a lot of the sound, leaving it dry and weak by the time it reached the seats at the back of the tier under the balcony. The renovation project will include dividing the balcony from one deep structure into two shallower ones so that there are no seats so deep under one low ceiling.
Back at the Bristol Old Vic, reflections again came in handy to meet the multipurpose needs of the new foyer. It has been cleverly designed so that people can enjoy a quiet conversation over a coffee without being deafened by the sound of everyone else’s chatter. However, with a pressure to maximize revenue from the building, the same space also needs to provide a more vibrant atmosphere and is even designed to accommodate gigs, where audiences do want to be immersed in sound. Vangelis Koufoudakis – an acoustician from the design firm Charcoalblue who worked on the Bristol Old Vic refurbishment – admits that trying to meet multipurpose requirements like this can be problematic. “You can end up with something like a sofa bed – it’s not a great sofa and it’s not a great bed.” Fortunately, architects and acousticians on the project were able to “dig out” a unique solution 250 years in the making.
In the world of acoustics, we love irregular shapes because they stop sound focusing or other unwanted acoustic artefacts
In the case of the foyer, the architects were keen to provide an open space that connected the theatre to the street and city beyond. Most of the walls of the café-bar area are sound-absorbing. Irregular angles as opposed to parallel walls avoid strange resonances and the room makes liberal use of wood wool – recycled timber and wood filings that absorb sound and convert it to heat. The ceiling of the foyer is a structural diagonal grid formed by glued laminated timber – “glulam” beams. The diagonals form irregular angles that trace back to historical room geometries in the rest of the building. “In the world of acoustics, we love irregular shapes because they stop sound focusing or other unwanted acoustic artefacts,” says Koufoudakis. As a result of these and other acoustic tricks of the trade, the vast open-plan foyer – which you might expect to sound clanging and echoey – provides the perfect acoustics for a quiet tête-à-tête. How then to allow for a more vibrant atmosphere in the same space at different times?
By unearthing the building’s original stone wall to the Georgian auditorium at the far end of the café-bar area, the project team was able to exploit it as an acoustically reflecting backdrop for a performance space directly in front. The wall itself is broken and pockmarked from the passage of time, which means that it reflects a diffused sound with no strange high-frequency resonances. “It’s an amazing architectural surface that reveals the historic scars of the theatre,” says Tom Gibson of Haworth Tompkins and the project architect for phase two of the refurbishment. The thermal mass of the rugged masonry surface also helps regulate the temperature in the café bar.
The foyer benefits too from another architectural quirk that turned out to be a blessing in disguise. Various add-ons and renovations over the centuries since the theatre was first constructed have led to different ground levels. The project team did not want to disturb the 1970s basement slab or foundations as this could have been expensive and an archaeological risk. “Basically, the old city wall used to run through the foyer and we were worried we might find some historic skeletons,” says Gibson. One of the design challenges was therefore to resolve the difference between the historic floor levels, 1970s floor levels and the newly proposed levels. The solution has been to ramp the new foyer down to street level to provide universal access for the first time in the theatre’s history, while the upper ground floor level creates a convenient raised stage area in front of the original auditorium wall.
The architects have also been able to exploit the various ground levels throughout the site to ventilate the venue’s studio theatre. This relatively small room was moved from the basement and ground floor in front of the auditorium to the basement and ground floor in the Coopers’ Hall section, an adjacent building that served as the theatre’s entrance in the 1970s design (figure 2). The move led to a non-compliant head height in the basement directly under the foyer next to the street and created space constraints that made it difficult to install traditional mechanical ventilators, which need a lot of room. “There was in any case an intent from the project team to naturally ventilate the new studio theatre to save energy and associated costs,” adds Gibson. The basement spaces (with non-compliant head height once the new foyer ground floor level was designed) provided an opportunity to build in a new natural ventilation “labyrinth”. It draws in air from the roof of the foyer through a masonry maze, which chills and quietens the noisy outside air. The result: cool air enters the studio theatre with minimal acoustic disturbance.
In fine shape
Not all architectural statements come from a fortunate alignment of pragmatic technical requirements, however. The Berliner Philharmonie in Germany is widely considered a milestone in the history of concert-hall design, and made a landmark departure from the basic shoebox geometry that had dominated for so long. It was constructed between 1960 and 1963 to replace the former home of the Berlin Philharmonic orchestra, which had been bombed in the Second World War. “People always gather in circles when listening to music informally,” said the architect Hans Scharoun, an observation that led him to design the concert hall with the audience seated around the orchestra on the slopes of a large bowl, like vineyard terraces. This bold design inspired a number of architects who also wanted to make “a statement building” and the vineyard geometry has been widely adopted over the past 15 years.
However, the vineyard geometry has been less popular with acousticians. When the audience is spread out so far in such a wide room, the sound intensity and the subjective intensity of the music are reduced for all. As a result, extending the surround form to a 2000-seat hall with no balconies reduces the intensity and immersion in sound that was intended by a musical composers. And because the audience encircles the stage, people sitting behind the orchestra will hear things differently to those in front, and instruments such as the trombone may sound bright on axis but quieter elsewhere. “You may be effectively getting a French horn concerto because you’re only two feet away from them,” says Essert.
That’s why Essert feels the shoebox-like geometry is getting a revival. There has also been interest in the psychoacoustics of tall narrow concert halls to stop audiences from feeling “boxed in”. The new ceiling at Colston Hall , for example, will have a slight pitch at the sides, mitigating the negative focusing effects of the previously concave ceiling. Convex curves spread the sound in a helpful way and deviate from a pure cuboid, feeling less “boxy”.
Another challenge in venues like Colston Hall is to cater for amplified and non-amplified music in the same space. While acoustics optimized for an orchestra will ideally enrich the sound, designs for amplified music aim for clarity of sound with little reverberation so that what the audience hears is almost exactly what is coming from the speakers. Digital engineering can adjust levels for an amplified performance in an idealized neutral space to a degree, but it cannot fully replace what a room with richer acoustics would do for a live classical performance. Working with constraints of building budgets, retractable panels made from glass-fibre board or even just curtains may be incorporated to absorb reverberation for amplified music and introduce some acoustic versatility.
One of SSV’s projects that took these versatility requirements to a new level was the Xiqu Centre in Hong Kong, where the space has to cater not only for amplified and non-amplified Western music but various traditions of Chinese operas from Beijing, Shanghai, Guangdong and Hong Kong as well. Optimizing this concert venue meant providing the means to balance the sound of the singers with respect to the orchestra, and to emulate the open-air acoustics these traditions were fostered on. The room’s finishes and the audio system in the Xiqu Centre were developed hand in hand.
The situation gets further complicated, however, as acousticians are no longer catering for audiences expecting great live orchestral sound. Today’s concert-goers instead expect to hear something that sounds like what they hear on their sound systems at home. The problem is that these recordings are generated by engineers who locate microphones at carefully identified positions around the hall or recording studio and then electronically mix the levels and add channels so that you can hear the clarity of the solo and have the resonance of the room at the same time. “You can’t actually get that sound,” says Essert. “But our ears have been attuned to it.” One approach to deliver simultaneous clarity, resonance and envelopment with architecture is to build a room within a room.
The idea emerged during Essert’s projects with Russell Johnson from Artec Consultants in New York, where he found himself repeatedly faced with the problem of devising multipurpose design solutions. In the 1980s Artec introduced a “reverberation chamber” to certain concert halls, such as the Meyerson Symphony Center in Dallas, US, and Symphony Hall in Birmingham, UK. Essentially this couples the inner concert hall that the audience sees to a secondary space, often using concrete doors on heavy pivots. That secondary space will usually have a volume of another several thousand cubic metres and can be a “hard” or “soft” space depending on the use of curtains. This allows it to act as a net absorber or net reverberation generator, but the initial time decay of the room – the first 10–20 dB decay of sound after it arrives – is generated by the geometry of the inner room. The idea was developed further by Artec in Singapore, Los Angeles, Reykjavik and Budapest, and also influenced the design team working on the Paris Philharmonie. Essert used the same principles on the Sage Gateshead in the UK, partially coupling the main space with another above a movable ceiling.
While acoustic design is based on the physics of sound, it hinges on a legion of other structural and technical considerations that multiply as venues take on additional functions to assist their revenue streams. And when it comes to renovating historic venues, engineering solutions must not only be sensitive to the building’s history, but also comply with planning constraints and meet the wide-ranging expectations of audiences. Pulling off that tricky combination is no mean feat. But by ensuring that all contributing factors come together – room geometry, sight lines, comfort, architectural features, building materials and so on – architects and acousticians can provide an experience that, be it rap or rhapsody, coffee or cabaret, leaves every visiting artist, customer and audience member content.