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.
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.