Acoustics

The anechoic chamber

The acoustics group operates a large anechoic chamber with a lower limiting frequency of 50 Hz and background noise levels well below the threshold of hearing.

It is a room-in-room construction: the inner room with the absorbing, 1.5 m long wedges is supported by springs and thereby isolated from the environment, providing vibration insulation for frequencies of 5 Hz and above. Equipment for multichannel playback and recording of sound is available.

The anechoic chamber is used for two different types of measurements: psychoacoustical tests and characterization of sound sources.

Psychoacoustical / listening tests

Apart from a more "technical" use of the anechoic chamber, it is suited to various listening tests for investigating human perception and rating of sound. This is not limited to plackback with one or two speakers at a fixed location, but facilities are provided for source positioning and multichannel playback.

The first picture shows a horizontal ring with up to 64 speakers allowing for simultaneous playback of sound from different positions. This rig can be used for localisation experiments in the horizontal plane.

The second picture shows the two arc positioning system, which can position a sound source at an arbitrary position on a sphere around a subject or, in this case, a dummy head.
It is used for localization experiments (the subjects indicates which position the sound seems to come from) and for measuring of head related transfer functions (HRTF).

 

 

Frequency response, directivity pattern and sound power

A "classic" use of an anechoic chamber is the characterization of sound sources and receivers, be it transducers (loudspeaker, microphones) or machinery.

Apart from the frequency response / spectrum of sources, the directivity pattern can be measured with help of a Bruel & Kjaer 9640 turntable system (max. load of 100 kg).
Source power is determined by measuring the sound pressure level or, preferably, the sound intensity on a closed surface around the equipment under test.

An example of a frequency response and directivity pattern measurement is shown below. The source was a B&K 4295 OmniSource, a (mostly) omnidirectional speaker for building acoustics purposes. The frequency response was measured on axis, the directivity pattern is for 5 kHz. The source has a limited (100 Hz - 10 kHz) frequency range and a highly irregular response: this is not a speaker for music playback. The directivity is, even at 5 kHz, mostly omnidirectional.