Acoustic engineers need to know how people respond to sound so they can design products to sound better. The listening room is an acoustically neutral environment for perceptual testing. In this room we might test which surround sound system creates a realistic sense of being enveloped by sound, the perceptual quality of hi-fi components or which hair dryer makes the least annoying noise.
While it is possible to carry out perceptual tests in the anechoic or semi-anechoic chamber, the visual and aural environment of the listening room is more like a normal room. Expectation and emotional state affect listener judgements, and so the listening room is often preferred to get more natural reactions. Students have also used this room, to examine the best way of designing small critical listening environments.
Ofcom, the regulator for the communications industries, wanted us to find out how to improve the intelligibility of dialogue for the hearing impaired, without overly impairing 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 come up with new guidelines to be used by the industry.
How is it made?
In this room, listeners need to compare various sounds, and the differences between the sounds can be very small. Consequently, we need a room where differences are not hidden by background noise or masked by room acoustic artefacts. For research and enterprise work, it is important that the room can meet the stringent requirements of international standards.
A ‘room within a room’ construction stops noise getting in. The room is made with heavy brick walls, reinforced concrete inner and outer roofs and double, heavy acoustic doors with rubber seals. The room is built on a mineral wool slab to reduce vibrations through the foundations. This construction doesn’t just make conditions in the listening room ideal for testing, it also prevents noise leaking out and disturbing others in the Newton Building.
At low frequency, standing wave modes in a small rooms cause problems, because the modes ring, distorting the sound. By carefully choosing room dimensions, and using resonant absorbers, the audible effects of these modes are controlled.
At mid and high frequencies, absorbers and diffusers are used to control the acoustic. Absorption and diffusion are used to treat the first order reflection paths from the walls, ceiling and floor. By delaying and attenuating the first reflections which meet the listener, the room appears bigger than it really is, removing the small ‘boxy’ sound.
The idea is not to make a completely dead room, like an anechoic chamber, because the room is meant to replicate a flawless living room and so should have a little reverberance. This is why diffusers are used alongside absorbers.
Mid-frequency reverberation time 0.27s
Room dimensions 6.6m x 5.8m x 2.8m
Background noise 5.7dBA
Tests and Standards
This room is one of few test rooms in the UK which can be used to meet the requirements of ITU-R BS 1116-1 for subjective assessments of small impairments in audio systems. This room can also be used to carry out listening tests on loudspeakers to BS 6840-13 / IEC 268-13