Stereophonic sound output system with controlled directivity

Abstract: A sound output system has a pair of right and left speakers and a pair of audio mirrors for respectively controlling directivities of sounds which are output from the pair of speakers. The shapes or arrangement of the pair of audio mirrors are adjusted such that a difference between arrival times of the sounds which are respectively output from the pair of speakers can be compensated by a sound pressure difference due to the Haas effect in a predetermined area. Alternative means are phase difference, dipole, and asymmetrical horn loading.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a stereophonic sound output system and, more particularly, to a sound output system to reproduce a stereophonic sound field with high fidelity. In U.K. Patent Application No. 2188811A a sound output system has been proposed which controls a directional distribution of the sound from the speakers to increase the stereo listening area. The present invention is based on the realization that in the sound output system disclosed in that specification it is desirable to reduce the sound emitted in unwanted directions. The present invention relates to a stereophonic sound output system having the features of claim 1. The above and other objects and features of the present invention will become apparent from the following detailed description and the appended claims with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram showing a directivity of a conventional speaker system; Fig. 2A is a diagram showing a schematic constitution of a sound output section; Fig. 2B is a diagram showing a directivity of the sound output section of Fig. 2A; Fig. 3A is a diagram showing a constitution and a characteristic of a stereophonic sound output system, its structure being used in an embodiment of the invention; Fig. 3B is a diagram showing a constitution and a characteristic of a conventional stereophonic sound output system; Fig. 4A is a diagram showing a schematic constitution of a sound output section according to the principle of the invention; Fig. 4B is a diagram showing a directivity of the sound output section of Fig. 4A; Figures 5A and 5B are perspective and schematic plan views, respectively, of a modified speaker unit embodying the principle of the invention; Figure 6 is a side view of an acoustic mirror; and Figure 7 is a side view of a speaker unit having a pair of acoustic mirrors of the type shown in Figure 6. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Figs. 2A and 2B are diagrams showing an audio mirror speaker. Fig 3A shows the principle of a sound output system and a sound image localization capability. Figs. 2A and 2B are diagrams showing a speaker system also shown in the Patent UK-B-2188811 and its directivity in the case where the central axis of an audio mirror 21 of a conical rotary unit is made coincident with the outer periphery of a circular diaphragm 22. In the foregoing prior art patent, attention has been paid to providing a smooth directional distribution having a change of only ± 7% over a range from +30° to -30° from the front position. However, this embodiment uses a theory such that the acoustic output smoothly decreases to 70% within a range from +45° to -45° from the front position. Fig. 3A shows the state where each speaker shown in Fig. 2A is located so as to face inwards at an angle of 45°, thereby constituting a pair of speakers adapted to reproduce stereophonic sounds. The distance between the two speakers is set to 2m. Fig. 3B is a diagram showing the state where conventional speakers are arranged in a manner similar to Fig. 3A. It is now assumed that the localization capability of the acoustic system is expressed by the reproducibility of the central sound source in a manner similar to that the modulation transfer function MFT of the optical system is conveniently expressed by the resolution. Namely, the state in that the sound image can be localized at the center at a listening point is set to 1.0 and the state in that the sound image is localized on one speaker is set to 0. If the sound image is not localized as in the case of the opposite phases, it is determined that the localization is impossible. The sound image localization capability can be fundamentally considered as a mental amount. The sound image localization capability depends on the physical amount which is expressed by the sound pressures and the difference between the arrival times due to the Haas effect as the mental conversion system of both of them. The sound image localization of the stereophonic sound system is inherently regarded as an imaginary based on the illusion of the hearing sense. If the sounds of the same sound pressure simultaneously arrived from the right and left speakers, the sound image is localized at the center. Therefore, the hot spot is suitable to reproduce the stereophonic sounds. However, if the arrival times of the sounds from the right and left speakers differ, even if their sound pressures are the same, the listener strongly feels the sound which has reached first. This is called a Haas effect. Since the difference between the arrival times certainly occurs at a listening point other than the center, even if the sound pressures which are generated from the right and left speakers are the same, the sound image is shifted toward the.speaker near the listener, so that the localization capability deteriorates. However, fortunately, the Haas effect also teaches that there is the compensation effect between the time difference and the sound pressure difference and 10 msec is almost equivalent to 5 dB. For example, in Figs. 3A and 3B, at the listening points (E ○ and ) in the front areas which are 2m away from the right speaker, the sound from the left speaker is delayed by 2.4 msec. Assuming that the Haas effect is linear in this area, this time delay is compensated by giving the sound pressure difference of 1.2 dB, so that the sound image is returned and localized to the center. Namely, the localization capability is set to 1. It is the key point to control the sound pressure every direction, namely, to control the directivity in order to widen the hot spot to the sweet area as explained above. In the case of arranging the right and left speakers as shown in Fig. 3A, the acoustic energy which is radiated toward the listening area is the half of the whole energy and the remaining energy is useless as the direct sound. The remaining acoustic energy is reflected by the wall or the like and becomes the indirect sounds. Since these indirect sounds are very close to direct sounds in time span, they may confuse the listening sense of direction. Then there is a case that asymmetrical directivity is desirable, since it can reduce unfavourable indirect sounds. As a practical method of eliminating the indirect sounds, the audio mirror speakers each include a sound absorbing material Figs. 4A and 4B are diagrams showing the principle of the invention, following from which a sound absorbing material 23 is inserted between the speaker diaphragm and the audio mirror, thereby absorbing the acoustic energies which are generated in the unnecessary directions. Such a sound absorbing material is also used to control the directional distribution of the acoustic energy toward the listening area. However, attention needs to be paid to the point that the use of the sound absorbing material 23 results in decrease in efficiency of the sound generating power of a speaker 24 and that the sound absorbing effect changes in dependence on the kind, amount, shape, etc. of the sound absorbing material, so that this effect have frequency dependency. In the foregoing description it has been assumed that speakers having circular diaphragms are used. However other types of speaker may be used, for instance speakers could have: elliptical diaphragms, rectangular-shape diaphragms with rounded corners; square diaphragms. In addition Horn-loaded speakers could be used. There are various kinds of opening planes for Horns. Because in-phase sound produces a better sound image when reflected from the audio mirror, it is advantageous to use pistonic motion speakers. As is obvious from the above description, according to the embodiment, the compensating relation between the time difference and the sound pressure difference is derived in a wide area. Therefore, a wide sweet area is also derived at positions other than the hot spots existing on the perpendicular bisector of the line segment connecting the right and left speakers. On the other hand, in the ordinary speakers, as shown in Fig. 1, the directional distribution differs for every frequency and the peaks and dips of the sound pressure levels are large. The hot spot locates at only one point in the center. At other listening points, the sound image moves in the listening area at every frequency and cannot be localized. The characteristics of a stereo speaker system will be summarized as follows. (1) The true Hi-Fi stereophonic sounds in terms of the theory and engineering, namely, the sound quality, phase, and directional distribution are controlled. Not only the hot spot as the apex of the isosceles triangle but also the stereophonic area having a sound image localization capability of a wide range, i.e., the sweet area are obtained. (2) Since the directional distribution can be selected in accordance with the characteristic and condition of the reproduction sound field, the listener, and the like, the sweet area can be optimized. (3) The multiway network can be also realized in a manner similar to the ordinary Hi-Fi speaker. (4) The feature of the audio mirror speakers, i.e., the virtual sound sources are unconditionally determined by the shapes of the mirrors, shapes of the diaphragms, and mutual positional relations. The pseudo sound sources which are generated from the corners of the cabinets can be easily prevented. (5) The present system is also suitable for use in not only the pure audio system but also the AV or surrounding system. Practical examples in the case of actually constituting the system as mentioned above will now be described hereinbelow. (Practical Examples) (1) Sweet area stereophonic speaker system of the 10-cm full-range type: A speaker module of the 10-cm full-range type made of Jordan Watts Co., Ltd. in U.K. was attached to the closed box designated by this company and was disposed such that the speaker module is directed upward. The conical audio mirror was disposed such that its apex was positioned at the outer peripheral surface of the speaker module as shown for example in Fig. 2B. Two sets of these speakers were prepared and located in a manner such that they are directed inwardly by the angle of just 45° as shown in Fig. 3A. The distance between the right and left speakers was set to 2m. At the position away from the speakers by about 2.3m, the sound image localization capabilities were measured using the guitar solo, human voice, and saxophone solo as the sound sources. Thus, all of the three persons have confirmed that the sweet area is obviously wide as compared with the case where the ordinary speakers were used (the foregoing speakers were set by the ordinary use method). There is the interesting fact that two areas obviously exist with respect to the hearing sense. Namely, the two areas exist at the positions near the hot spot and in the outside thereof. It is now assumed that the latter area is called a Haas area. The boundary line of those two areas is clarified when the listener moves while listening to the sounds. In particular, when the listener moves from the hot spot area to the Haas area, the localization feeling momentarily disappears. However, when the listener stays here for two or three seconds, the localization feeling is recovered. It is considered that this phenomenon concerns with the pulse width or the like of the auditory nervous system. In the case of the ordinary setting of the speakers, even when the main axis of each speaker is directed inwardly by 30° and 45°, the hot spot area was narrow and no Haas area existed. (2) Use of the sound absorbing material: The arrangement described in the above item (1), was modified in that the sound absorbing material was disposed in the portion in the direction where the sound energy is not directly radiated toward the listening area: this is shown in Fig. 4A. Although the whole sound pressure level decreased by about 2.5 to 3 dB, the sounds were felt as if the sweet area was enlarged in terms of the hearing sense. In particular, it was considered that the crosstalk in the high band reduced. It has been confirmed, however, that the difference between the above items (1) and (2) excluding the sound pressure depends on the condition such as listening room or setting of the speakers. (3) Two-way type sweet area speaker system: A sub-woofer to radiate the low tones at frequencies of 150 Hz or lower was connected to the speaker module in the item (1) by use of a crossover network of 12 dB/oct. Although the sub-woofer is omnidirectional, the sweet area was almost equal to that in the item (1). The use of absorbing material has been described above with reference to Figures 4A and 4B. Referring to Figures 5A and 5B, it has been noted that unwanted dispersion/diffraction and or secondary reflection (sound waves 80) at a nearby wall 82 causes "smearing" of the sound localisation in the listening area. In order to reduce this, or prevent it happening, a mass of sound absorbing material 84 is disposed between the conical acoustic mirror 86 and the cabinet 88 for the speaker 90 in such a position as to block the sound waves which would otherwise cause such smearing. As can be seen from Figures 5A and 5B the absorbent material 84 is in the form of a sector of a circular cylinder extending through an angle of about 210 degrees and having an upper conical depression to receive the acoustic mirror 86. Referring now to the conical mirror 21 shown in Figures 2A, 5A and 5B it has been noted that there is a difference of sound localisation between a sitting position and a corresponding standing position away from the "hot spot" in the listening area. It is assumed that the reflected sound waves from the conical mirror tend to be localised in a horizontal plane. In order to provide localisation in both a sitting position and standing position, the conical mirror may be formed with a slightly curved generator such that the sound waves reflected by the mirror diverge in the vertical direction. As shown in Figure 6 the generator 92 of the mirror 86 is slightly concave, but it may, alternatively, be slightly convex. It has been found that, with such an arrangement, the sound quality is still acceptable and that localisation is less dependent on listening height. Figure 7 illustrates a multi-way speaker unit in which different acoustic mirrors 86A, 86B with slightly concave generators are disposed one above the other for reflecting the sounds from different speakers in the cabinet 88.

Claims: A stereophonic sound output system comprising a pair of left and right speakers, each speaker (SP) having a diaphragm (90) and a conical or part conical sound mirror (86) spaced from the diaphragm with an apex of the sound mirror coinciding with a rim of the diaphragm so as to define a direction in which sound is preferentially radiated towards a listening area characterised in that a sound control material (84) is inserted between the diaphragm (90) and the sound mirror in a position where sound energy is not directly radiated towards the listening area so as to control the directional distribution of the acoustic energy radiated towards the listening area by blocking the sound waves which are reflected in unnecessary directions. A system according to claim 1, wherein said pair of speakers are pistonic motion speakers. A system according to claim 1 or 2, wherein said pair of speakers are horn speakers or flat cone speakers. A system according to claim 1, 2 or 3 wherein each audio mirror (86) has an asymmetrical shape. A system according to any preceding claim, wherein the speakers (SP) are spaced apart along the base line of a triangle and the centers of the directions of the sounds which are respectively output from said pair of speakers are directed towards the apex (C) of the triangle by said audio mirrors the centers of the directions of said sounds being at 45° or less to the said base line. A system as claimed in any preceding claim, wherein the speakers are such as to output predominantly median and high tones and further comprising right and left speakers for outputting predominantly low tones. A system according to any preceding claim, wherein the audio mirror is curved (92) in an upward direction to provide divergence of the sound reflected thereby.

Inventors: NEGISHI HIROKAZU, NEGISHI, HIROKAZU
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1988
Main classification: H04R5
IPC codes: H04R5, H04S1, H04R1