We Made It: Concert hall acoustics

Glasgow has a brand new concert hall, and the Royal Scottish National Orchestra has a brand new home. A move for the Orchestra from Henry Wood Hall, a converted church in the city’s West End it has occupied since 1979, has been on the cards for several years, but few could have predicted the scale and intricacy of the final project. The New RSNO Centre snuggles conveniently right next to the Glasgow Royal Concert Hall, and brings new offices, an education suite, a digital centre and practice rooms right to the city centre. The project’s centrepiece, however, is the RSNO Centre’s auditorium, designed primarily as a rehearsal space for the RSNO, but also intended to function as a performance and recording venue. Engineering firm ARUP have a long track record of putting acoustic excellence at the centre of venue design, and they were intimately involved in the RSNO project. Lead acoustician Philip Wright explains the process of designing the RSNO’s acoustically efficient new auditorium, and just how he achieves the acoustic effects he desires.

What was ARUP’s involvement in the project?

At ARUP, we advise on acoustics, and also on venue design, which is equally important. The primary brief in the RSNO project was for a rehearsal space, but there were three main uses identified: rehearsal, recording and performance. We’re not talking about full orchestral performance, however, because the new auditorium is adjacent to the Glasgow Royal Concert Hall, but for smaller-scale uses – chamber orchestras, chamber music and recitals, as well as amplified events, in the Celtic Connections festival, for example.

How did you design the hall so that it fulfils its rehearsal and performance requirements?

There are essentially two modes that the hall can take. For a full symphony rehearsal, the stalls seats can be taken away and tucked under the balcony, and the two side balconies are raised up and folded away, to provide the full floor width for the Orchestra. But for performance, this isn’t a very sympathetic layout, and there’s not much audience seating capacity either, so the side panels are lowered to form side balconies, narrowing the hall at stalls level, and retractable seating rake is pulled into place. That’s also important acoustically: we simulated and listened to the effect of this in our SoundLab, which confirmed that we get stronger lateral reflections with the second, performance mode, which provide a better sound for the audience. And concert platform is also highly adjustable, taking the form of a series of motorised risers that can go from a flat floor configuration to various layouts of stepped platform.

How do you predict how the hall will work acoustically?

We have to consider how we want the hall to work in all of its configurations, and establish the basic geometry accordingly. Then there’s a whole suite of acoustic properties that acousticians use to paint a picture of how a hall is behaving: for example reverberation time, the balance of frequencies, loudness, clarity, etc. We can evaluate a lot of these properties with some high-level calculations, but to understand them more fully we use acoustic modelling, which involves building a computer model with a reasonable amount of detail using specially designed software.

Can you construct the hall from whatever material you want?

No – it’s crucial that you make the right choices in the materials you’re using. If you think of a conventional concert hall, you’d probably think of a space which includes quite a lot of timber panelling. Psychologically, it’s important to have a degree of visual warmth, and the historical associations of timber mean that audiences have certain subconscious expectations of what they will hear in that space, and that is in fact helpful. But timber panelling also has very specific and tunable physical acoustic characteristics – any panel mounted on the wall will suck a little energy out of the room, and we make use of that property to prevent the space becoming boomy. So we’d generally try to steer architects to use a certain amount of applied timber in their designs.

Is it important acoustically to have the same materials used in the same way throughout the hall?

Quite the opposite. In fact, we try to vary things between different materials and ways of using them within the hall. Any timber panel system, for example, will absorb sound more at certain frequencies than at others others. So if you had the same system throughout the hall, you’d end up with a very particular acoustic characteristic, with a dip in the response that you’d notice as a sound colouration. We need these effects to happen at a distribution of frequencies and only to a small degree, so it's important to vary system properties and materials.

What smaller details in the hall affect its acoustics?

In terms of fine tuning, there are a few features that are noteworthy. There’s an array of reflectors hanging over the performance space – they incorporate lights, but they’re primarily there for acoustic reasons. They’re adjustable to different heights, and you can also change their angle slightly, and they’re primarily there to make it easier for the musicians on stage to hear each other. There are also a series of absorbant fabric banners at the back and sides of the hall, which can be taken all the way up to the ceiling or lowered down to balcony level. They readily absorb sound, reducing the space’s reverberation time and also reducing loudness. And finally, you need to avoid lack of control in the bass, so there are some fixed low frequency absorbers hidden in the ceiling and walls.

How do those reflectors actually work?

The reflectors are designed as an array, with gaps between them, so that only a proportion of the sound energy is reflected back – we don’t want them to cut down the volume of the room. But the sound reflections they do provide come back a bit more quickly and a bit more strongly, especially over the playing area, so that the musicians can hear each other more clearly than they otherwise would. Each reflector is also curved slightly to disperse the sound reflections, so that there’s a greater evenness or overlap between the coverage of each reflector.

And what about the unevenness of the wall surfaces?

We want the sound to be evenly distributed, so it’s important that sound gets thrown about within the space. In order for that to happen, we incorporate large-scale articulation in the space – large architectural elements like balconies or columns. But equally, we want specific reflection paths to be quite strong, for example reflections from the side walls getting to the audience. If they’re too strong, however, the sound will be harsh because the two sound waves – from the musician and from the reflection – interfere with each other and cancel certain frequencies. You can temper those reflections to a certain degree, and ‘smear’ their time of arrival by articulating the wall surface, so that it’s not just one point you’re receiving the reflected sound waves from.

How is the hall protected from outside noise?

It’s not immediately obvious, but the the whole of the auditorium sits on steel springs, so that it’s fully isolated, and it's also a heavy concrete and block construction. There’s a certain amount of vibration in the ground at the site, primarily from trains, but now that the isolation is observed, the hall is immune to vibrations not only from the ground, but also from activities elsewhere in the building.

How do you test the acoustics once the modelling is completed?

Once the building is actually built, it’s a case of going in and doing live tests in the real space – but in fact, there’s not much you can do in-between designing it on paper and testing it once it’s been built. But you should be able to achieve a high level of design confidence through your modelling, calculation, and experience. We do live testing towards the end of the process, which is mostly a case of checking to see that the acoustic values are what we expect them to be. Then the musicians come in and try the space, which is a really interesting exercise – there’s always a certain amount of acclimatisation, but we’ve had very positive responses so far.

Once the hall is up and running, how can the orchestra make use of its acoustic flexibility?

We had to make the space easily changeable between rehearsal and performance mode, and it’s also important to be able to adjust the banners and reflectors very quickly, between, for example, an acoustic piece and an amplified piece. These days that’s quite straightforward with computerised presets, but you also have to establish what presets you need, so we’ve worked with the RSNO to identify their requirements there. I'm sure that over time, as the players spend more time in the space, the Orchestra will develop a comprehensive set of preferences, according to repertoire.

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