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Initial acoustic research

  Van Soest started with the simplest possible means to investigate the human capability of hearing the direction of a sound source. Remember that electronic means were either not available or primitive.

So he hang loudspeakers on the wall of an improvised sound absorbing room, tested persons and repeated the measurements in the open air. He established the great importance of the time difference between the signals received by each ear for hearing direction. The magnitude of this difference was measured with the "listening tube". This consisted of a rubber tube with wooden ear pieces at both ends. The tube was tapped with a knife like object around the centre of the tube. The blindfolded listener had to report whether he heard the sound from left, right or straight forward.
To obtain more accurate values of the more proficient listeners the centre part was replaced by an inserted and calibrated metal rod in which the speed of sound was some ten times higher, which expanded the observation scale also ten times. In this way he measured that his technical assistant perceived even a time difference of one microsecond around the centre mark on the rod.
The result was that the usually large spacing between the sound pick-up elements of a listening equipment was unnecessary for the average good listener.

A second outcome was the discovery of the detrimental effect of sound transport through metal or rubber tubes, because it caused attenuation and distortion and thus disturbed the sound image.

Van Soest combined both findings. He obtained a parabolic sound mirror with circular cross section of some 120 cm (4 ft), made of plaster covered on both sides with a sheet of paper. It was cut in two halves and each was focused directly at an ear of the listener.
Comparative tests on the Plane of Waalsdorp showed that this arrangement performed much better than the current foreign equipments.

Listening tube
The "Listening tube" used during
testing with a volunteer
(full size 89 KB)

First experiment
The first experimental listening
equipment of the Measurements
Building mounted on Goerz support
(full size 166 KB)

Comparision tests
Listening equipment's used during comparison tests
(full size 247 kb)


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