er position 201 corresponds with position 103 and an image position 202 corresponds with image position 104 in FIG. 1. Image altitude angle (b) is measured upwardly from a horizontal line 203 through the head of listener 103 to a line 204 joining the listener's head to image position 202. It should be noted that loudspeakers 101, 102 do not necessarily lie on line 203.
Having defined th image positional parameters with respect to a reference listening configuration, we proceed to define parameters for possible variations in the listening configuration. Referring to FIG. 3, loudspeakers 301 and 302, and lines 304 and 305 correspond respectively to items 101, 102, 106, and 105 in FIG. 1. A loudspeaker spacing distance (s) is measured along line 304, and a listener distance (d) is measured along line 305. In the case that a listener is arranged parallel to line 304 along line 306 to position 307, we define a lateral displacement (e) measured along line 306. For each loudspeaker 301 and 302 we define respective azimuth angles (p) and (q) as measured counterclockwise from a line through loudspeakers 301, 302 and perpendicular to a line joining them, in a direction toward the listener. Similarly for the listener we define an azimuth angle (m) counterclockwise from line 305 in the direction the listener is facing.
In FIG. 4, a loudspeaker height (h) is measured upward from the horizontal line 401 through the head of the listener 303 to the vertical centerline of loudspeaker 302.
The parameters as defined allow more than one description of a given geometry. For example, an image position may be described as (180,0,x) or (0,180,x) with complete equivalence.
In conventional stereophonic reproduction the image is confined to lie along line 106 in FIG. 1, whereas the image produced by the present invention may be placed freely in space: azimuth angle (a) may range from 0-360 degrees, and range (r) is not restricted to distances commensurate with (s) or (d). An image may be formed very close to the listener, at a small fraction 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 contain