Concert hall acoustics: Interview at Arup Acoustics' SoundLab

Arup Acoustics are one of the world's foremost acoustics practices. Just in the UK, they've been involved in the highest profile concert hall projects of recent years: The Sage, King's Place, Milton Court. I visited Arup's UKMEA leader Ian Knowles at their SoundLab facility in London, which lets the listener hear how a hall will sound from various seat positions – whether the hall exists in reality or just on a computerised drawing board. Ian and Senior Acoustic Consultant Ned Crowe talked about the huge variety in the needs of different projects and about the things that can affect sound in various types of music.

We started with a demo of SoundLab from Ned. I'm seated on a chair surrounded by a cluster of loudspeakers at different angles, staring at a large screen on which is projected video of Ned standing on the stage at one of Arup's latest ventures, the Stormen hall in Norway's Bodø. It's a hall that can be configured for any of classical music, choral, rock or pop music as well as the straight drama which was Stormen's primary purpose. I listen to the way Ned's voice changes as different lumps of hall appear and disappear: a proscenium arch, side walls, ceiling reflector arrays.  A second demo plays a Bruckner symphony, alternating between the room acoustic of the Concertgebouw and the Vienna Musikverein. A third demo starts with the direct sound of a bassoon, showing how it changes when you add in the reflections from different walls, followed by the “long tail” of reverberations – sound that has been diffused and reflected many times.

Arup: SoundLab is designed as a tool to demystify all the numbers that we model in doing acoustic design: envelopment, reverberance and early decay time, all the things that we have numbers for. Ultimately, your interest is “what does it sound like?”

The key to envelopment and clarity is how strong the reflections are, and that's a function of how far they are away from you and how diffusive the materials are.

The corner reflections are very important to get that sense of room. The bit that most people understand is the reverberation, but if you take just the direct sound and the reverberation, you could be standing in a public toilet somewhere. It doesn't give you that sense of room, it just gives you a mush.

DK: What is the language that's used between you and the client? How can a client answer the question “what do you want the room to sound like”? 

Quite often, you suggest a study tour to listen to different rooms and come to a consensus of what the client would like. From that, you distil out particular characteristics and what would suit their orchestra and the sort of music they play.

An obvious thing is clarity. So if you have an orchestra that specialises in classical music – perhaps a Sinfonietta or a 50-60 piece orchestra – clarity is much more important than in Romantic music where you need a much bigger and more reverberant sound. So you might go for a C80 [ratio of direct sound to reflections arriving later] of +2dB, whereas for a big orchestral reverberant sound, it might be -2. So we think about those sort of numbers, whereas the client says “I want a room that's going to sound like this one”.

Actually, your aural memory is terrible. What we found in SoundLab is that unless you play the music in one room after another, you've forgotten what the first room sounds like. If you actually move from room to room while the music is playing, you can suddenly hear the changes very clearly. The Musikverein is like a modern hall – fairly narrow. The Concertgebouw is much more "dreamy", much less "in your face".

DK: What happens if it’s an arts centre and there are two or three different orchestras and they play different repertoire?

A room has to be sized around an audience capacity. You’ve got to be very careful that you don’t have a big platform for a big reverberant symphonic sound in a small room, because it becomes overwhelming. You get situations where a client gets a new hall and puts on totally inappropriate things because they haven’t thought through what the hall is excellent for, what it was designed for. Part of the problem is that it can take ten years from the briefing process to completion, and quite often people change along the way. So you have to build in the flexibility; you have to almost second guess what you think the range of uses is going to be.

DK: Is there an increasing number of techniques with moving parts, like you showed in Stormen?

That’s probably the most comprehensive room we’ve worked on. It was always going to be a room to accommodate lots of different types of performances and there are two different ways that you can approach that. You can start with a dry space for drama, rock and then electronically add in reverberance with a real time system: there are about four or five of them around, very commonly used in the US, not so common over here. But when you're working for a client where symphonic repertoire is their main thing, electronic enhancement is seen as a bit of a dirty thing. In Stormen, the starting point was going to use an electronic sound enhancement system, but the client decided against this during the design process and we had to make a rapid change to the design: that's where all this moving scenery came from.

That’s quite unusual, but pretty much every large hall will have some variable element in it which might be as simple as pulling a drape out at high level or across some of the walls to reduce reverberance and raise clarity for speech and amplified rock and pop.

In chamber music rooms like the Royal Welsh College or Kings Place, it's interesting: once you have the flexibility to add a little bit of absorption here or there on a key surface, you’ll use it. So in Milton Court, for example, if we have a female soloist, we’ll drop the banners on the side walls by a metre, just to take the edge off the cue-ball reflection. When you put a string quartet in there, you take that out.

DK: What about the room being great for the orchestra to hear each other? Does this end up being a compromise with making it great for the audience?

No. We always start with the orchestra. If they can’t hear themselves and each other, they can’t play at their best, they’re going to be out of time and out of tune and then the audience isn’t getting the best music. So absolutely, we have to make it best for the audience, but the first priority is to make it best for the orchestra.

That goes for all performing arts, for drama as well, people need to be able to feel like they’re connected to the auditorium.

But it’s a careful balance. If you get all of the sound going back to the performer, you’ve got less going out to the audience. A classic case of that is the Globe Theatre. When you’re standing close to the front of the stage, you’re almost at the focal point of the room and your voice sounds very big. It feels great to be an actor there, but it isn’t so great if you’re in the audience: it can give the actor a false sense of delivery and mean they don’t project as much as they should.

For that reason, we would never design a room that was circular so you’re at the focal point, although you can do a lot with diffusing surfaces and subtle angling. We did Hall 2 at the Sage which is a beautiful room for quartets and a most unusual space – they have a quartet at the bottom and the audience sits in three balconies looking down on them; they move around during movements and you get a really different viewpoint. That’s a ten sided room: we had to deal with all the reflections to make sure you weren’t getting any nasty focuses, and it works beautifully.

DK: What other halls are favourites of yours?

Hall 1 at the Sage is a fabulous room. It's a shoebox, but it’s a very relaxed sounding room, very classical in its sound, with a string tone that's absolutely beautiful. That and Stormen are actually my favourite rooms on a symphonic scale. I think Milton Court is a cracker. It has a vitality to the room that has to be controlled – it can run away with itself. If you play the room right, I haven’t heard another hall like it: the piano sounds magnificent in that room.

DK: Finally, do you have any news on London’s putative new hall?

It's too early to say. There is a feasibility exercise still under way, it’s a job that Arup are working on. Until Brexit, we were fairly sure what was happening, and now we’re not! Hopefully, it’s part of a government strategy for growth. Wherever you do this, it brings massive economic benefits. There’s no doubt that London needs it.

If you hear a great concert in a great hall, it’s a life-changing thing. It’s not just turning up BBC Radio 3 a bit louder, it’s something quite special.