Acoustics

The anechoic chamber is probably the quietest place you’ll ever experience. It’s designed to ensure that no sound is reflected from the walls, floor or ceiling.

Constructed as a room within a room, it’s built from heavy Accrington brick and concrete with acoustic doors and seals. The chamber sits on a set of springs to reduce vibration through the foundations and every surface including the floor is covered with foam wedges to absorb sound. Users walk on a wire trampoline that is suspended above the floor and stretched between the walls with an acoustically transparent catch net below it.

This lack of acoustic interference makes the anechoic chamber ideal for a number of acoustic measurement types. For example, it can help us understand the true acoustic performance of loudspeakers or microphones. It also makes it perfect for virtual acoustics where you want to create an accurate auralisation of a concert hall or city street. Or perhaps you need to test very quiet products or simply need a space where you can see how people respond to hearing very quiet sounds.

However you choose to use it, if the environment you require for your research or product test needs to be as quiet as possible without room reflections, then the anechoic chamber is the perfect place.

Similar to the anechoic chamber, this room features sound proofing to the walls and ceiling. Unlike the anechoic chamber it has a solid floor that reflects sound. This makes it much easier to support and test heavy items such as washing machines where the measurement regulations don't require the use of the full anechoic chamber.

The room was also invaluable as part of a research project we worked on aimed at developing more accurate room acoustic prediction models. Our research led to us developing the standard method for measuring acoustic diffusion coefficients. This means it is now possible to accurately calculate how the sound in, for example, a concert hall or factory will be affected by the various surfaces or objects found in that space. As a result, architects and designers can compare the acoustic merits of different surfaces and provide accurate design specifications during the initial stages of a project.

We also have a smaller semi- anechoic room that is part of the student audio and acoustics laboratory. It is used exclusively for teaching as part of the Applications of Acoustics module and for measuring the response of loudspeakers in the Electroacoustic Transducer Design module.

Reverberation is created when sound is reflected off surfaces and objects in a room. This means sound lingers in a space even after the source has stopped making a noise. A room’s reverberation time tells us how long it takes that lingering sound to die away.

So if you’re designing a space such as a concert hall, the reverberation time needs to be the correct length to make sure that speech sounds clear and the music amazing. To ensure the correct reverberation time you need to know how the seating, carpets and other materials in the space will absorb sound. With this in mind, our reverberation room has been used to measure the absorption of materials used in buildings throughout the world. It was even used to test the BBC Studios at MediaCity.

Our researchers also used the room to develop a method for testing the sound absorption of theatre seating. This test is now utilised commercially and has been used to equip several new or refurbished theatres and concert halls such as the Royal Albert Hall and Royal Festival Hall.

Specifically designed to be very reverberant, this room features walls made of painted dense brick, a reinforced concrete sloping roof and a non-parallel pair of walls that help make the room more diffuse. The reinforced concrete floor is built on Rockwool insulation board to provide vibration isolation.

The room allows us to examine the modal behaviour of small rooms so we can learn how room geometry combines with the position of the loudspeaker and listener within it to affect the tonal balance and timbre of a sound. Within this context, we used the room to test the performance of portable vocal screens for Sound on Sound magazine.

It’s also possible to use the room to simulate different acoustic environments. For example by adding absorbers to the room we created a simulated kitchen. This was then used to record washing machine sounds as part of a government funded project exploring methods to test the sound quality of products.

Most of us have lived in a house where the walls are so ‘paper thin’ that you can hear your neighbours coming in late or the traffic starting early.

As a result companies that serve the construction sector are continuously looking for ways to reduce the amount of noise that gets into our homes, and the transmission suite is used to test how different types of wall construction can resist sound.

The transmission suite consists of source and receiving rooms with a test partition in between. The rooms are structurally isolated from each other and the surrounding building and the walls are built to resist sound better than any test partition we might want to measure.

The source room contains two dodecahedral loudspeakers that generate high-level noise. Some of this sound then passes through the test partition into the receiving room. The noise levels in the source and receiving rooms are measured and the difference in sound level between the two rooms, gives a measurement that shows how good the test partition is at resisting sound.

The transmission suite has been used to:

• Test the facade of Portcullis House, the building that provides offices for MPs next to the Houses of Parliament
• Investigate sound going around partitions or through window supports
• Measure the transmission loss and absorption of roadside barriers used on British motorways
• Develop high performance double glazing units and lightweight plasterboard partitioning
• Test acoustic doors for hotels, executive offices, studios and cinemas.

To help acoustic engineers design better sounding products, they need to know how people respond to sound. Our listening room is an acoustically neutral environment designed for perceptual testing.

Constructed as a room within a room and built from heavy Accrington brick and concrete with acoustic doors and seals, it is built on a mineral wool slab to reduce vibration through the foundations. Absorbers and diffusers are placed throughout the room allowing us to create the correct acoustic conditions for listening tests.

Capable of meeting the stringent requirements of international standards tests, the room has been used to determine, which surround sound system creates a realistic sense of being enveloped by sound, the perceptual quality of wind farm noise and the audibility of audio watermarking.

It also let us help Ofcom find out how to improve the intelligibility of television dialogue for the hearing impaired, without spoiling the enjoyment of programs for those without hearing loss. By getting young and elderly listeners to watch and score the quality and intelligibility of different TV sound tracks, we were able to create a new set of communications industry guidelines.

While it is possible to carry out similar tests in the anechoic or semi-anechoic chamber, the visual and aural environment of the listening room is more like a normal room. Since expectation and emotional state affect listener judgements, the listening room is the best place to get more natural reactions from the listener.

The audio booth features an ambisonics loudspeaker system. This allows us to test peoples’ response to surround sound in augmented and virtual reality systems.

It’s been used recently to test the intelligibility of speech in broadcast audio content for the BBC and to explore how what you see affects how good a concert hall sounds (VR). It also helped Qualcomm understand the audible quality of different Bluetooth codecs and for the BBC to see peoples’ responses to different surround sound reproduction by using brain imaging (EEG).