How many words can you think of to describe sound? Not what a sound is, but its characteristics. Maybe loud, quiet, too loud, too quiet. Perhaps muffled, shrill. It’s easy to communicate vaguely, but it’s not easy to com-municate accurately. What does 100dB sound like? What about the pressure of 100dB or the intensity of 100dB or its reverberation time in the Albert Hall?
Our ears pick up sound pressure waves and we use a logarithmic scale to judge the difference in sound pres-sures between quiet and loud. It’s very different from describing the dimensions of a physical object, where we can measure and visualise. This difficulty in visualising and describing sound has been a challenge in acoustics, particularly where it meets and integrates with other engineering disciplines, such as in buildings.
Valuable expertise
Professor Trevor Cox, from the acoustic engineering department at the University of Salford, is a former presi-dent of the Institute of Acoustics. As well as conducting research and teaching acoustic engineering, Cox is well known for presenting radio documentaries on the topic. He says: “In architecture, it’s not common to have exper-tise about acoustics, so they need engineers to help. There is a risk that if the architect doesn’t understand acoustics sufficiently the building’s design will suffer. It can be a big problem if they don’t account for acoustics and this leads to functional failures.”
One infamous example is Bexley Business Academy in London. The £31 million college was designed by Foster and Partners. It has an open-plan atrium surrounded by classrooms partitioned by glass. However, once built and in use, pupils could not hear teachers because of the noise from the atrium and other classrooms. In 2008, an extra £800,000 was spent on modifying the building.
Typically, an acoustics engineer will spend most of their time calculating and modelling sound in a room, consult-ing with the architect and the customer to provide the best solution. They use CATT-Acoustic software and de-sign specifications to work out values such as reverberation and background noise levels.
Cox says: “You have to defend the inside space from the outside noise, including vibration isolation. Then there is sound from other places inside your building, and finally you consider the sound inside your room, modifying with absorption.
“The process and solution depend entirely on the function of the building. Buildings benefit from an acoustics engineer’s early involvement. In one case, when building a hospital, the engineer recommended changing the position of wards so they were not on the side near a busy road. Such a simple change saved substantial money on sound insulation.”
As well as materials that absorb sound, other possible modifications include baffles, rafts and arranging surfaces to reflect sound better. “Acoustics is a mature area of study,” says Cox. “A lot of the fundamentals have been known for many decades. The university has a test house for materials development and there is a lot of interest in making things such as ventilation systems quieter. There is also interest in the psychology of acoustics.”
James Beer, venue consultant at Arup, agrees that engineers have a “good handle on the physics” and that the direction of current research is into more psychological areas: “Research areas include, subjective response, how we interpret and respond to sound. Anything involving people is complex, but particularly how sound relates to what you see is interesting.”
Beer studied mechanical engineering before an enthusiasm for music and audio brought him into the field of acoustics. He is one of a team of 30 in the Arup Acoustics office in Winchester, Hampshire, which works on concert halls and theatres.
Acoustics are an integral part of this type of building project from the start. Beer says: “We were working with the architect from day one on our most recent large project, the Stormen concert hall in Bodø, Norway, bringing our acoustics perspective to the building’s design alongside the plant and equipment that will be used in it.”
The centrepiece of the Stormen building is the 944-seat main hall, the Store Sal, which can transform from a concert hall to a proscenium theatre in an hour. This is achieved using adjustable and removable 14m-high wall panels mounted onto tracks on both sides of the stage, and ceiling panels that can be rotated and lifted to pro-vide a ceiling void.
As a theatre, it has a large stage, full-height flytower and a variable-sized orchestra pit. When changed into a traditional shoebox concert hall, it can deliver symphonic performances with critically acclaimed acoustic conditions. The ability to be both a theatre and a concert hall, while retaining the best acoustics for each type, is a rare attribute. The building won an Institute of Acoustics award last year for its innovative design.
Beer says: “One of the biggest trends at the moment is for spaces that transform, as many budgets get tighter and people want to do more with their buildings.
“One approach is to use electroacoustic enhancement systems – finely tuned loudspeakers that add artificial reverberation. But for many clients this proves unsatisfactory. Instead, spaces that physically transform while keeping acoustics that work are becoming popular. They are complex and challenging to design, but can now be delivered within budgets and architectural constraints.”
Arup Acoustics works closely with architects and clients. Beer says: “The idea is that you have a good balance between the architecture and the acoustics. We talk to the client about existing spaces that we like and visit with them. But acoustics is very subjective. You can’t just give numbers to a client and expect them to under-stand them like an acoustician.”
Arup has gone some way to solve this problem by developing acoustics demonstrators, Soundlabs, that are in-stalled in several of its offices. An anechoic sound recording of a source is obtained. This recording is combined mathematically with the impulse response – measurements of the reflections of sound that happen between the source and the listener in a space. This impulse response can be generated by computer predictions if it doesn’t physically exist yet. The result is played back in the Soundlab, a small room with speakers arranged at specific points on the floor and ceilings, to accurately simulate acoustic conditions. Arup calls the process an “auralisa-tion”.
Soundlabs were originally developed in the early 2000s to demonstrate performing arts spaces to clients, but the range of applications has grown, says Beer: “We use them now to demonstrate noise from infrastructure, like HS2 trains, or new airport runways.”
Soundlabs are also used as a decision-making tool within the design process, not just a marketing tool, he says. Recent projects have seen them used to demonstrate the background noise levels in a library, and the sound of rain on an exhibition hall’s plastic roof. On both occasions the auralisation helped with making a decision on the type of materials and design used. The next step for a Soundlab is to immerse people further, Beer adds, through vibration and heat.
Hearing is believing
Meanwhile, engineering consultancy Cundall has taken the idea of Soundlabs a step further by using virtual reality. The company believes its Virtual Acoustic Reality (VAR) technology will help to elevate the importance of acoustics in building projects.
Tom Nightingale, principal acoustic consultant at Cundall, says: “Acoustics is often described as a black art be-cause for every part of our discipline there are lots of different notations. Trying to talk to a non-expert can be very difficult. The main idea of our system is to help us communicate about acoustics.”
Acoustics engineers normally use spreadsheet calculations for simple rooms, determining the reverberation time by the volume of the space, the surface finish and the shape of the room. But when the calculation becomes more complex in larger spaces, the CATT-Acoustic software is used. The engineer builds the shape of the room, including material properties of surfaces, adds a sound source and receivers, and the software “ray traces” to model sound as it reflects off surfaces, including how much the materials absorb and scatter sound.
Nightingale says: “We are using the software more and more, even for buildings like schools. Ten years ago, people didn’t think much about acoustics, but studies have shown it is just as important as the other disciplines.”
Cundall’s VAR solution links CATT-Acoustic to a graphics program called Unity, which renders the room from a CAD program into virtual-reality goggles, while the sound goes into headphones. As the user moves around into different rooms in virtual reality, the sound from the source changes according to the distance and shape of the room.
“Instead of graphs and figures you can see and hear the architecture and acoustics,” says Nightingale. “We could have something being performed on the stage, and you could run around the whole of the theatre, listening in real time as the sound changes.”
Cundall says the software will help it communicate with clients and architects, justifying changes and spending on acoustic treatment or materials, not just for concert halls, but also for everyday buildings. It can be used to identify acoustic problems early on in order to save money. It will elevate the importance of acoustics within building projects. The equipment is portable and can be used by clients and other participants in a project.
Nightingale says: “Acoustics should be thought of from day one, but quite often we are brought in late. It’s often a small discipline in part of a grand project. Structures, architecture, mechanical and electrical engineering are often considered the big money spenders. Acoustics is often a finishing job. But getting us involved at the start can make life easier in the long run.”
Building acoustics is growing in importance. Advances in the ability to communicate about sound that take ad-vantage of the latest software and hardware will only help to serve that cause further.
