Fluid Dynamics and Acoustics Group





The facilities used by the Virtual Acoustic Project are located at the Rayleigh Laboratories and are operated by ISVR Consultancy Services. The laboratories are well served with comprehensive control and preparation areas. A wide range of modern, highly specialised instrumentation is available.


Anechoic chamber

An anechoic chamber is a room in which the walls, floor and ceiling are lined with sound absorbing material, usually foam or glass-fibre wedges. The lining prevents the reflection of sound from the room boundaries so that 'free-field' conditions exist. The room sounds very 'dead'. Sound measurements are not influenced by the room surfaces, and an anechoic room is an ideal environment for many standard tests such as the measurement of the sound power of a machine, or measurements of microphone and loudspeaker frequency responses.

The Large Anechoic Room at ISVR is one of the largest in the UK. It was extensively refurbished during 1995/96 and the original polyurethane foam wedges were replaced with glass fibre wedges.


Built as a box within a box, it is acoustically isolated from the rest of the building and adjacent chambers by an air gap all around and is supported on vibration isolation mounts. The reinforced concrete walls are 305 mm thick.


Wall Lining

There are over 8,000 non-flammable glass-fibre cored wedges, extending 910 mm from the walls, floor and ceiling. Free-field conditions exist at frequencies above 80 Hz.


Without wedges the bare chamber is 9.15 m x 9.15 m x 7.32 m, volume 611 cubic metres. The usable space between the wedges is 7.33 m x 7.33 m x 5.50 m, giving a usable volume of 295 cubic metres


Double doors 2.0 m wide x 2.4 m high


A grid of removable floor panels can support a spread load of several tons with a minimum of interference with the anechoic nature of the chamber. An optional floor of varnished chipboard is available for measurements requiring a free field above a reflecting plane (hemi-anechoic conditions).


Listening room

This has been designed to meet the requirements of IEC 268-13 and BS 4860:Part 13 for standard listening conditions in which loudspeakers or recordings can be assessed subjectively. The acoustics are similar to a living room in a house rather than the 'deadness' of an anechoic room. Adjustable absorbent panels on the walls allow the reverberation time to be varied. An acoustically-transparent curtain near the front of the room allows 'blind' comparisons of loudspeakers.


6.52 m x 4.20 m x 2.68 m high, Volume 73 cubic metres

HRTFs measurement equipment

Measurements are carried out in the large anechoic chamber of the ISVR.

Motorised rotating arc

Two arcs were designed and built: a smaller one with a radius of 1.15 m and a larger one with a radius of 1.6 m. The resulting distance between the cone of the loudspeaker and the microphone positioned at the entrance to the blocked ear canal of the pinna is exactly 1 m. 19 loudspeakers are mounted at an equal spacing of 10º between each other. It is estimated that the positional error due to the radius of the arc and its curvature is less than 1 cm, and due to the spacing between the speakers is less than 0.5°.

The rotation of the arc is achieved by a transmission chain between the arc and a step motor. When the system was calibrated it was found that every pulse to the step motor would rotate the arc by 1/10932 of 1º. (i.e. in order to rotate the arc by 180º, 1456789 pulses are required. The rotation of the arc in this case takes only 4 minutes.

The effect of reflections due to adjacent loudspeakers mounted on the arc was investigated by covering all loudspeakers, except the one used for the measurement, with absorbent material, and also when the same loudspeaker was used for measurement of baffled pinnae in the anechoic chamber without the arc and all the loudspeakers. Differences at frequencies below 10 kHz were less than 1 dB, and variations up to 2 dB were noticed at higher frequencies.

Baffled pinna measurements

A large baffle was constructed out of thick plywood. Its large dimensions (with a maximum width of 2.3 m, and a maximum height of 2.4 m) ensured that the measurements of the acoustical response of the pinna are not distorted due to diffraction and reflection from the boundaries of the baffle. In the centre of the baffle a circular aluminium plate was positioned, able to rotate around its centre, and a rectangular opening enables the positioning of the artificial pinnae. The response of human pinnae can also be measured when subjects sit behind the baffle but these are not included in this study. An additional circular plate included an opening for the holding Electret microphone, flush mounted with the baffle plane. This was used for equalisation of the transducers for each source position in space.

MLSSA system

The Maximum Length Sequence System Analyzer (MLSSA) has been used widely in recent years in HRTF measurements. The MLS method offers a number of advantages compared to traditional frequency and time domain techniques. With the accurate positioning capability of the transducers with respect to the head/pinnae in our measurement set-up, a single channel measurement system is sufficient. Consecutive measurements with all loudspeakers and two microphones (in the case of KEMAR) were used to generate a matrix of impulse responses capturing the space with a dense resolution.

 Localisation measurement equipment

Huron 20 Digital Audio Convolution Workstation