Virtual acoustic source
As described by Huygens principle, each point of a wave front poses a starting point of an elementary wave. On the other hand, by superposition of such initial waves becomes possible completely establishing any arbitrary wave front. The resulting wave head hardly differs from the original wave front. Their virtual starting point establishes same characteristics as the regular acoustical source at this place. The localization no longer remain dependent on the listener position, like the position of a phantom acoustic source during the conventional loudspeaker reproduction. Its direction will be perceived everywhere at her virtual starting point. One simple possibility for establishing a virtual acoustic source would be assembling of a lot of single loudspeakers on a spherical surface. It's easily understandable; we perceive the origin of the radiated wave front always in the centre of the sphere.
For a limited range of l is possible establishing the wave front according to Huygens principle electronically by a loudspeaker array. The radiation of each single loudspeaker becomes a delay until the wave front of the virtual sound source would hit concerning loudspeaker:
If the loudspeakers align in a plane, the acoustic curtain becomes possible. Such solution was the dream of the scientists in the 30th of the last century already. However, the realization estimated impossible at all times due to the number channels:
The acoustic curtain idea: One Microphone provides the signal for each
assigned speaker on the opposite side of the wall. Like an acoustical
curtain, the loudspeaker wall restores the genuine wave front.
However, if we consider more exactly, we don't really need a separate channel for each speaker. The signal doesn't differ in substance, but rather in time. That's a simple task for digital signal processors at present. Besides of the formidable efficiency of such loudspeaker alignment because of the perfect air resistance adaptation of the
resulting huge membrane, a lot of new possibilities occur. The directed radiation avoid feeding the mirror sources within the playback room unwanted by energy. By that reason, the playback room acoustic is losing of influence. Moreover, becomes possible by using the reflections of the playback room reflections for simulate reflections from the recording room. That establish the impression of the concert hall size. Acoustic of the Playback room environment no longer dominate the perception. As far as the playback room becomes including into the sound field synthesis a largely complete physically restoration of the genuine sound field, comparable the Holography
approach in the optical range within reach. The principle is shown by the wave field synthesis website.