1. Field of the Invention
The present invention relates to speakers to produce sound in a response to an audio signal, and, in particular, relates to such speakers for the production of an ambience or "surround sound" signal.
2. Background Art
Ideally, sound reproduced through a sound reproduction system, such as that of a stereo high fidelity system, a television, or the like, would sound like the original source. In part, this means that the reproduced sound should have a spatial dimension or quality in that a listener should perceive the sound as being distributed in space as it would be if listening to the original performance. Unfortunately, a problem with conventional sound reproduction systems is the tendency of the sound to be localized by the listener at the loudspeakers, or imaged at a point relative to the loudspeakers.
When a listener hears an original performance, the listener receives some acoustical signals which permit the listener to localize the source of the sound, for example, a particular singer or instrument, and other signals which provide a sense of a spatial dimension, which will be referred to from time-to-time hereinafter as "ambience". The first category of signal is comprised of those which travel along a substantially straight path from the source to the listener; the second set of signals, which are not readily localizable, are those which are reflected off of the walls, ceiling, floor, fixtures, and the like of the listening area. It is these latter singals which provide the sense of ambience or spatiality, and this quality is imparted both by their arrival at the listener from a variety of directions, and by the fact that their arrival is delayed relative to those signals which travel directly from the source to the listener; this delay is the result of the longer paths which the reflected signals must travel. The ambience signals may consequently be delayed on the order of 10 or more milliseconds as compared to the direct path signals.
A number of approaches have been proposed for reproducing the ambience signals with a sound reproduction system. Some of these have employed electronic delay circuitry to delay the signal from the left and right channels of the amplifier of a conventional stereo system, the delayed output then being supplied to dedicated right and left speakers which project the delayed signal along direct paths to the listener. While this approach has achieved some success in producing a "surround sound" effect, it possesses a number of inherent disadvantages: not only is the delay circuitry (which typically requires housing in a separate component) both relatively expensive and noisy, but the approach also ordinarily employs two dedicated "surround sound" speakers, in addition to the conventional, non-delayed speakers of the stereo system.
Another approach is disclosed in U.S Pat. No. 4,596,034, which shows a system in which each channel of a stereo system is reproduced in full by first and second transducers with the output of the first transducer being 180 degrees out of phase with repect to the output of the second transducer. The transducers are positioned such that their acoustic outputs, that is, their sound pressure lobes, are directed to either side of the listener, and a pressure minimum, or null zone, formed between the two lobes is directed towards the listener, eliminating all direct path sound so as to provide a sound field which prevents the listener from localizing the speakers. While this arrangement may help produce an enhanced sense of sound distribution and a decreased awareness that the sound is coming from speakers, this system also possesses several inherent disadvantages. The most significant of these lies in the very fact the the system is intended to prevent the listener from localizing sound for the whole of both channels of the stereo system; in other words, this arrangement renders it difficult or impossible for the listener to localize any of the acoustical signals which are reproduced by the system, regardless of whether those signals were originally recorded as direct path signals or indirect path signals. This is undesirable in that it consequently makes it impossible for the listener to localize those sources (e.g., a singer or particular instrument) where a degree of localization is desirable. The practical result is that the reproduced sound is preceived as being formless or "mushy".
Accordingly, there exists the need for a sound reproduction system which reproduces a non-localizable ambience signal, yet which also reproduces localizable direct-path acoustic signals for those sounds for which such localizability is desirable. Furthermore, there is a need for such a system which does not require noisy and expensive signal delay circuitry, and for such a system which does not require two dedicated speakers to reproduce the ambience acoustical signal.