on of (d), or remote at a distance several times (d), and may simultaneously be at any azimuth angle (a) without reference to the azimuth angle subtended by the loudspeakers. In addition, the present invention is capable of image placement at any altitude angle (b). Listener distance (d) may vary from 0.5 m to 30 m or beyond, with the image apparently static in space during the variation.
Good image formation has ben achieved with loudspeaker spacings from 0.2 m to 8 m, using the same signals to drive the loudspeakers from all spacings. Azimuth angles at the loudspeakers (p) and (q) may be varied independently over a broad range with no effect on tee image.
It is characteristic of this invention that moderate changes in loudspeaker height (h) do not affect the image altitude angle (b) perceived by the listener. This is true for both positive and negative values of (h), that is to say loudspeaker placement above or below the listener's head height.
Since the image formed is extremely realistic, it is natural for the listener to turn to "look at", that is to face directly toward, the image. The image remains stable as this is done; listener azimuth angle (m) has no perceptible effect on the spatial position of the image, for at least a range of angles (m) from +120 impression of a localized sound source that listeners have no difficulty in "looking at" or pointing to th image; a group of listeners will report the same image position.
FIGS. 5a-5k shows a set of ten listening geometries in which image stability has been tested. In FIG. 5a, a plan view of a listening geometry is shown. Left and right loudspeakers 501 and 502 respectively reproduced sound for listener 503, producing a sound image 504. Sub-FIGS. 5a through 5k show variations in loudspeaker orientation, and are generally similar to sub-FIG. 5a.
All ten geometries were tested in three different listening rooms with different values of loudspeaker spacing (s) and listener distance (d), as tabulated in FIG. 5m. Room 1 was a small studio control area containing considerable amounts of equipment, room 2 as a large recording studio almost competely empty, and room 3 was a small experimental room with sound absorbing material on three walls.
For each test the listener was asked to give the perceived image position for two conditions; listener head angle (m) zero, and head turned to face the apparent image position. Each test was repeated with three different listeners. Thus, the image stability was tested in a total of 180 configurations. Each of these 180 configurations used the same input signals to the loudspeakers. In every case the image azimuth angle (a) was perceived as -60 degrees.
In FIG. 6 an image transfer experiment is shown in which a sound image 601 is formed by signals processed according to the present invention, driving loudspeakers 602 and 603 in a first room 604. A dummy head 605, such as shown for instance in German Patent 1 927 401, carries left and right microphones 606 and 607 in its model ears. Electrical signals on lines 608 and 609 from microphones 606, 607 are separately amplified by amplifiers 610 and 611, which drive left and right loudspeakers 612 and 613, respectively, in a second room 614. A listener 615 situated in this second room, which is acoustically isolated from the first room, will perceive a sharp secondary image 616 corresponding to the image 601 in the first room.
An example of the relationship of the inventive sound processor to known systems is shown in FIG. 7, in which one or more multi-track signal sources 701, which may be magnetic tape replay machines, feed a plurality of monophonic signals 702 derived from a plurality of sources to a studio mixing console 703. The console may be used to modify the signals, for instance by changing levels and balancing frequency content, in any desired ways.
A plurality of modified monophonic signals 704 produced by console 703 are connected to the inputs of an image processing system 705 according to the present invention. Within this system each input channel is assigned to an image po