In 1954, H. Lauridsen of the Danish National Broadcasting System discovered a remarkable stereophonic effect using a single input signal. The subjective impression is that of sound coming from all directions resulting in a unique illusion and powerful sense of presence and immediacy for the listener. In order to obtain this effect the signal is applied to both ears in phase and, in addition, a delayed (50-150 ms) version of the same signal is fed to both ears in anti-phase. This experimental condition can be described in terms of two intensity responses and a differential ase response for the paths to the two ears. The intensity responses are two complementary comb-filters. The differential phase response is a meander-type function jumping discontinuously between -n/2 and +n/2 at frequencies which are integer multiples of 1/2r, where r is the delay. A number of experiments have been performed involving complementary comb-filters of different kinds and various types of differential all-pass filters in order to distinguish between the relative contributions of the intensity variations and phase variations, respectively. The result of these experiments is that -wiggly- differential phase responses alone are not sufficient to give rise to the baffling spatial illusion first reported by Lauridsen. Rather, strong spectral intensity modulations, preferably of a complementary type, are required. These findings and the fact that the original experiment can be suffessfully duplicated with delays as short as 2.5 ms suggest that the delay is not the primary agent of this effect. Instead, the simple fact that some frequencies enter the lead through one ear while others prefer the other ear is offered as an explanation for the psychoacoustic illustion of -sitting in the middle of the orchestra.-
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