Test case

PUBLISHED : Friday, 18 April, 2008, 12:00am
UPDATED : Friday, 18 April, 2008, 12:00am

Sound labs help acoustic engineers to present noise impacts and solutions to their clients

Determining the acoustic impact and requirements of a new structure is an important yet often unseen aspect of the building industry. Even the developer needs convincing that intrusive noises needs careful management. An acoustic engineer is not only required to devise a comprehensive sound solution, but also to present their idea to the client experimentally.

At a sound lab installed with testing equipment, engineers collect data and experiment with their solutions. 'The sound lab is basically the place where we do auralisation,' said William Ng Chi-wing, senior consultant at Arup. 'This means we use a special arrangement of speakers to simulate the sounds that exist in the outside world before a building is constructed. There may be a railway under a new building, for example, so we do some calculations and modelling to determine the magnitude of noise that the structure can bear. Technical data is gathered and translated into sound waves. We can then present the sounds to clients in the sound lab and they can listen and hear what the numbers actually mean.'

The sound lab is a small soundproof room with a big screen on one wall, a bank of surround-sound speakers, and a pair of computers that manipulate and then relay the data to simulate the natural sound in various scenarios. Consultants such as Dr Ng begin their research by collecting data from the site in question. If a new office complex is being built at the intersection of two busy roads, he will build up an aural picture of the location by recording the external ambient noise from various perspectives.

'We do a recording of the noise spectrum. We collect this data and use software known as Calculation of Road Traffic Noise (CRTN) to put it through different curtain walls. From the data we can work out what the noise level will be inside the building and then translate this into an audio experience.' In the case of heavy traffic noise, which is a common problem in Hong Kong, the solution often requires the use of noise-reducing glass. The more dense the glass, the less the traffic noise will affect the office workers. But using graphs and equations to convince clients to opt for high-density glass or a multiple-glass structure that costs more is not easy, so the acoustic engineers create computer mock-ups of the various solutions and demonstrate them in the sound lab. Hearing the solutions first-hand helps the client understand the impact of their decisions.

'The clients generally do not know much technical stuff, but they can hear. Acoustics are sometimes subjective and designing conservative solutions can be costly, so we demonstrate how the change of design can [limit noise] to let them feel the implication.'

At Chek Lap Kok airport, Dr Ng and his team soundproofed the 4D Extreme Screen cinema at the top of Sky Plaza in Terminal Two. He and his colleagues spent 24 hours on-site recording round-the-clock noise of aircraft taking off and landing.

'Because the airport project is already built, we can collect real data. Different aircraft and flight paths can create a different impact, so we work out which one is the noisiest for each frequency,' he said.

Using pre-existing theories on the impact of sound waves on various materials Dr Ng calculated the shape and density of the building envelope, and then used construction mock-ups to determine whether or not the noise reduction requirements could be met. 'The cinema requires a noise-free environment. So far we have not received any complaints of noise intrusion from the aircraft,' he said.