GB2188811A

GB2188811A - Sound output system

Info

Publication number: GB2188811A
Application number: GB08707400A
Authority: GB
Inventors: Hirokazu Negishi
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 1986-04-01
Filing date: 1987-03-27
Publication date: 1987-10-07
Anticipated expiration: 2007-03-27
Also published as: DE3710464A1; GB2188811B; JPS62232297A; JP2823157B2; GB8707400D0; DE3710464C2

Description

1 GB 2 188 811 A 1

SPECIFICATION Sound output system

Background of the Invention 5 Field of the Invention

This invention relates to an audio output system, and more particularly to a speaker system intended to reproduce a high-fidelity stereophonic sound field.

Related Background Art

An audio field has found itself in an age of CDs and DATs in which the socalled tone quality has been greatly improved. However, from the standpoint of high-fidelity stereophonic sound field 80 reproduction, a system has still been prevalent which uses as an ideal listening point only a vertex of an equilateral triangle with two hgh-fidelity tone quality audio signal output speakers, one being disposed at each end of the base of the triangle. Therefore, it does not seem that an audio system has been established which allows us to enjoy a true high-fidelity stereophonic sound in a wide field. This derives mainly from a narrow directivity of the audio output.

Figs. 9A and 913 illustrate the listening point and directivity of a conventional regular stereophonic audio output. In Figs. 9A and 9B, reference numerals 121,122,124 designate speakers and the cross- hatched portion designates a listening point.

On the other hand, it is said that a BOSE (USA) speaker system well-known as a lecture meeting hall speaker (hereinafter referred to as the PA speaker) system reproduces a balanced stereophonic sensation even at a point other than at 100 the vertex of an equilateral triangle. This is because the BOSE system reproduces a plentiful indirect sound even in a reproduction sound field to provide a wide listening point as in a regular concert hall in consideration of the importance of an indirect sound 105 (reflected sound from the wall, floor, ceiling, etc., of the field) while a regular high- fidelity stereophonic system mainly seeks the so-called direct sound (namely, sound arriving directly from speakers).

Figs. 10A, 10B illustrate the listening point and directivity of a BOSE type stereophonic audio output. Reference numerals 121', 122'and 124' designate speakers, and the cross-hatched portion 123' designates a listening point.

Problems thatthe Invention is Intended to Solve From the standpoint of high-fidelity stereophonic reproduction, BOSE's theory has a weak point. Applicant would say that they make too much account of a wide listening point, namely, of true stereophonic sound reproduction, to provide a true high-fidelity stereophonic sound. In other words, the BOSE type system would be a stereophonic system but not a high-fidelity stereophonic reproduction system. These drawbacks are mainly 125 composed of the following two.

(1) A Drawback in Phase Characteristic..

The BOSE speakers are of the type in which acoustic energy is radiated both forwardly (for direct 130 sound) and backwardly (for indirect sound). For example, BOSE 901 has small diameter speakers arranged radially on the back of the system and a single speaker on the front of the system. It is said that a multispeaker system is generally inferior in phase characteristic to a single speaker. Especially, BOSE 901 is a muffispeaker system of speakers having the same frequency band, so that it is likely to be conspicuous. In addition, the actual speaker design does not realize a sound source of the same phase in every direction although the design theory may be satisfied. Fig. 11 is a diagram showing an example of a phase distribution of an ideal BOSE type speaker. This phase characteristic is obtained by localization of a constant level of sound energy and is important together with the intensity of a sound. If we hear a sound of various mixed phases containing a large amount of reflected waves from a multispeaker system, our acoustic sense cannot accurately reproduce the sound field.

(2) A Drawback in the Control of Indirect Sounds:

In the design of a sound field, the greatest care is paid to how indirect sounds are controlled in various halls because the listening points, echoes, etc., are all influenced by indirect sounds. In this respect, the BOSE theory can be said to be correct. However, the actual state of a reproduction sound field is quite diverse, and an ideal sound field cannot possibly be reproduced by a standardized fixed type directional characteristic, i.e., BOSE type speakers. Of course, this applies to a regular high-fidelity speaker system. This regular system does not rely on the reflected sound from the back of speakers and the like, so that it is less objectionable than the BOSE system. The problem with the BOSE system is that while the BOSE system is of a speaker system which pays attention to the reflected sounds from its back, etc., the matching adjustment of the BOSE system with an important object which reflects sounds, i.e., the reproduction sound field, is only made at the setting of the system as in regular speakers.

The reason why the BOSE type speaker system is not regarded as a true high-fidelity stereophonic speaker system although it has various advantages is not due to a problem in paper, but due to the engineering defects, just described above. Eventually, now commercial available systems take much account of either high-fidelity or stereophony and there exist no true high-fidelity stereophonic systems. Namely, existing systems are either regular ones excellent in tone quality but having a problem in directivity distribution or BOSE type ones having improved directivity but a problem in tone quality (especially in phase).

Summary of the Invention

Under such background, it is therefore an object of this invention to provide a sound output system which is very excellent both in tone quality and in directivity.

Under such object, this invention employs a construction in which an acoustic reflector is provided opposed to a diaphragm of a speaker for 2 GB 2 188 811 A 2 determining the output directivity in the direction perpendicular to the diaphragm of the speaker.

Brief Description of the Drawings

Figs. 1A, 1 B illustrate the principles and directivity 70 distribution, respectively, of non-directional audio mirror speaker system as one embodiment of this invention; Figs. 2A, 213 illustrate the principles and directivity distribution, respectively, of an audio mirror speaker 75 system having a 30'single directivity as another embodiment of this invention; Figs. 3A, 313 illustrate the principles and directivity distribution, respectively, of an audio mirror speaker system having a practical 300 single directivity as a 80 further embodiment of this invention; Fig. 4 is a diagram showing the directivity distributions in 1000 Hz and 4000 Hz of a conventional regular speaker system; Fig. 5 is a diagram showing an omnidirectional 85 frequency characteristic of a conventional regular speaker system; Figs. 6A, 613 illustrate the sound source, false sound source, and radiation wave of a conventional regular speaker system, and a virtual sound source 90 of, and radiation wave form, an audio mirror speaker, respectively; Fig. 7 illustrates the principles of a hung type non directional 2-way audio mirror speaker system as an embodiment of this invention; Fig. 8 illustrates the principles of a stationary type variable-directivity 2-way audio mirror speaker system; Figs. 9A, 913 illustrate the distribution of stereophonic listening points by a conventional regular high-fidelity speaker system and the directivity distribution of the speaker, respectively; Figs. 10A, 10B illustrate the distribution of listening points of a BOSE type stereophonic system and the directivity distribution of the speaker thereof, respectively; and Fig. 11 shows an example of the distribution of phases of an ideal BOSE type speaker.

Detailed Description of Preferred Embodiments

Figs. 1A, 1 B illustrate the principles and directivity distribution, respectively, of a non-directional audio mirror speaker system as one embodiment of this invention.

As is clear in Fig. 1A, a circular diaphragm 1 on a speaker cabinet 3 is disposed opposite to a conical audio mirror 2 of the same axis of diaphragm 1 and mirror 2, and the acoustic energy radiated perpendicularly from diaphragm 1 is equally reflected radially outwardly, which is eventually equivaieritto provision of a non-directional virtual sound source. This is because the problem with tone quality, especially, with phase, is the same as with a regular high-fidelity speaker because of use of the output from a single speaker and because the perfect nondirectivity of an audio mirror can easily be realized in theory as well as in engineering.

An audio mirror high-fidelity stereophonic speaker system, as described above, is an epoch- making system which brings about the compatibility 130 of control of tone quality (especially phase) with that of directivity distribution, which cannot be solved by the conventional techniques. As in a regular highfidelity speaker system, of course, this system can be modified to form a multiway system to further improve its tone quality, provided that a (phantom) sound source is only required to be disposed on the same axis as the speakers as in the regular highfidelity speaker system.

Non-directivity is not always best depending on the conditions of the reproduction sound field. Namely, to have a controlled directivity may work out right. This is because there are many conditions or limitations such as the size and shape of a reproduction sound field, the quality of the wall or the like thereof, the position where the speakers are set, the distance from the backs of the speakers, etc. In this case, the directivity distribution can be controlled as desired, by the shape of the speaker diaphragm surface, the shape of the audio mirror, and the speaker setting. Of course, the directivity distribution is related to the quantity of energy radiated to the main axis from the virtual sound source, so that selection of a wide directivity will reduce the efficiency on the main axis. Therefore, it is necessary to balance directivity with efficiency also f rom the standpoint of speaker design.

Figs. 2A, 213 illustrate a system having a secured range of 30' in directivity as another embodiment of this invention and the directivity thereof, respectively. In this case, an audio mirror 11 in which the shape of speaker diaphragm 12 is modified from a circle while keeping a symmetry of revolution assumes the shape mainly using a part of conical body of revolution.

Of course, in a practical use, it suffices forthis system to have a gentle directivity obtained by moving the main axes of the circular diaphragm and conical body of revolution of the audio mirror. The directivity is adjustable in accordance with the actual states of a listening room and by request. Figs. 3A, 313 illustrate a system of further embodiment of this invention in which the central axis of audio mirror 21 of a conical body of revolution is coincident with the outer periphery of the circular diaphragm, and the directivity thereof, respectively. As shown, although a variation of about 7% may occur in a range of up to 30', there is no practical problem at all and there is a gentle distribution of energy even outside the range of 30'.

While the features of the above audio mirror type high-fidelity stereophonic speaker system have been described mainly with reference to its equal- phase superwide angle directivity, this system has another advantage which the prior are speaker system has not reached. It is directivity which does not depend on frequency. Fig. 4 is a diagram showing directivity distributions of a conventional regular speaker in 1000 Hz and 4000 Hz. Usually, as frequency increases, directivity becomes sharper. Namely, directivity has frequency dependence. The directivity distribution of the above audio mirror type high- fidelity stereophonic speaker system is substantially constant irrespective of frequency.

3 L GB 2 188 811 A 3 This is because the directivity distribution is determined unconditionally by the relation between the shape and position of the diaphragm and audio mirror. In addition, unlike light, sound brings about a diffraction because the sound wave is long in wavelength compared to light. Generally, a long wavelength (low-pitched sound) is difficult to reflect and does not so much influence the audio mirror while medium- and high-pitched sounds can practically be handled as in the reflection of light.

This cannot be avoided as a physical phenomenon, but, fortunately, there is no practical problem, as will be described hereinafter. Fig. 5 is a diagram illustrating that reason and shows the omnidirectional frequency characteristic of a conventional regular high-fidelity speaker.

According to this characteristic, a low-pitched sound up to about 400 Hz lowers to only about 7 dB even in a direction of 90' and to only about 2-3 dB in 60'.

In other words, that low-pitched sound bends itself 85 to a considerable degree due to its diffraction without the aid of the audio mirror. Therefore, although incomplete, an audio mirror would improve the condition.

In addition, from the standpoint of a practical view 90 point, the human sensation of direction becomes duller as the sound frequency becomes lower, and the location of a sound source is very difficult especially below 200 Hz. In other words, sounds of less than 200 Hz are required only to be present, which is also the reason why the so-called 3D stereophonic system (medium- and high-pitched sounds are output f rom right and left speakers, and a low-pitched sound is output from only a single speaker in order to save the space, etc.) holds. There 100 is no problem although low-pitched sounds, especially, of less than 200 Hz may be replaced by a conventional regular wafer, using the physical phenomenon and hearing sensation, namely, the fact that the directivity of even a regular speaker is 105 widened due to low-pitched sound diffraction and that the location of the sound source becomes duller as the sound frequency becomes lower.

Another feature of the audio mirror system lies in a vertical line-like distribution of virtual sound 110 sources. A regular speaker is a limited surface sound source in which sound waves from false sound sources produced from the edges of the cabinet are superimposed to confuse our acoustic sensation of locating the sound source. Especially, if 115 the distance between the centre of the sound source and the edge lines of the cabinet is substantially equal to the distance between our ears, the confusion will become greater. Fig. 6A shows this state.

In the audio mirror type, virtual sound sources are in principle distributed only in a line perpendicular to every direction, so that even a regular box type cabinet is not reflected in the audio mirror because same is conical and its axis is vertical. Figs. 6A, 6B illustrate the comparisons between virtual sound sources and between the radiated waves of a conventional regular speaker system and an audio mirror system. As will be obvious from these Figures, in the audio mirror system, there are no phantom sound sources and the sound sources are distributed vertically in a line, so that there is no expansion of radiated sound waves in the horizontal direction. Therefore, this system is optima for location of the sound sources. In addition, as described above, the fact thatthe directivity is irrespective of frequency and that a wide point phase is even results in a true high-fidelity stereophonic system which a regular system cannot reach.

The characteristics of the above audio mirror high-fidelity stereophonic speaker system are arranged to be as follows.

(1) A true high-fidelity stereophonic system from the standpoint of theory andlor engineering, namely, a speaker system of tone quality, especially, of even phases, and of controllable directivity.

(2) Substantially constant directivity distribution in the entire acoustic frequency band. (This has not been accomplished by any conventional speaker systems).

(3) Directivity distribution variable in accordance with the characteristics and conditions of the reproduction sound field.

(4) Systematizability of speakers in a multiway manner as in regular highfidelity speakers.

(5) Easy availability of perfect non-directivity.

(6) Only vertical line-like distribution of sound sources, and no false sound sources formed due to the cabinet edges.

In the above description, the spherical speaker cabinet and the planar speaker have been illustrated, but these are not essential conditions although preferable. It is natural thatthe speakers are of high-fidelity, so thatthey mustfurther be required to have characteristics required usually of high-fidelity speakers. For the material of the audio mirror, it is necessary to select the material which absorbs the least of sounds and prevents resonance within acoustic frequencies. It is also necessary that the reflective area of the audio mirror be slightly larger than the required geometric area in consideration of the diffusiveness of sound waves.

A specific example of the actual construction of the above system will now be described.

(1) A non-directivity 2-way audio mirror type highfidelity stereophonic speaker system:

Fig. 7 is a cross-sectional view of this system in which a low-mediumpitched sound planar plate speaker 33 having a diameter of 16 cm and a highpitched sound planar plate speaker 37 having a diameter of 4 cm were set in spherical speaker cabinets 32,36, respectively, so as to face downwardly. The upper portion of high-pitched sound cabinet 36 was shaped conical and placed opposite to the diaphragm of the low-mediumpitched sound speaker with the speakers 33 and 37 having the same axis. Namely, this system took the form of an integral construction 36 of the highpitched sound speaker cabinet and the lowmedium-pitched sound audio mirror. A high-pitched sound audio mirror 39 was hung from the ceiling with the axis of the conical portion of mirror 39 being coincident with that of the speaker diaphragm. This resulted in a uniform non- 4 GB 2 188 811 A 4 directivIty in the entire band of 90 Hz -20 KHz in this speaker system. It also brought about a balanced high quality stereophonic sensation over a wide listening point.

(2) A 3-way non-directivity hybrid type high fidelity stereophonic speaker system:

This was a hybrid type high-fidelity stereophonic speaker system which included a combination of a subwafer which produces sounds of 40-150 Hz and 65 the above non-directivity 2-way audio mirror type high-fidelity stereophonic speaker system. The subwafer was of the type installed on the floor and set so as to face the listening area. This resulted in a wide listening point containing a very low-pitched sound. In this case, the crossover was 12 dB/Oct.

(3) Avariable-directional 2-way audio mirror type high-fidelity stereophonic speaker system:

Fig. 8 shows the principles of this system in which a low-medium-pitched sound planar plate speaker 75 43 having a diameter of 16 cm was fixed on the upper surface of a box-like cabinet 44 and a conical audio mirror 42 was set opposite to the diaphragm of speaker 43. The audio mirror also served as a cabinet for high-pitched sound speaker 41 having a diameter of 4 cm. The entire audio mirror was movable and settable at any position between the center and end of the opposite speaker 41. High pitched sound conical audio mirror 40 was also settable at any point between the center and end of the high-pitched sound speaker.

The broken lines show the position of the audio mirror set to have non-directivity. High-pitched sound audio mirror 40 only moves horizontally above the low-medium-pitched sound audio mirror and cabinet 42 while the low-medium-pitched sound audio mirror and cabinet 42 moves inclined to the low-medium-pitched sound speaker cabinet 44. When both virtual sound sources are set so as to have the same directivity distribution, they are on the same vertical axis and become sound sources of the same phase.

Arrangement may be such that when these audio mirrors are set in the state of the same directivity distribution, an adjustment device can change the distance between the audio mirror and the speaker diaphragm so that the virtual sound sources are aligned on the same vertical axis.

(4) A 2-way audio mirror type high-fidelity stereophonic speaker system with a bass reflector: 105 In the above (3), a bass reflector duct and an aperture were provided at the front of the lowmedium sound box-like cabinet to increase a lowpitched sound band.

As described above, this invention provides a very 110 high-fidelity stereophonic acoustic reproduction output system.

Claims

1. A sound output system comprising:

a) a speaker having a diaphragm; and b) an acoustic reflector disposed opposite to said diaphragm for determining the directivity of the acoustic output from said speaker.

2. A system according to claim 1, wherein said diaphragm is of a flat plate.

3. A system according to claim 1, wherein said diaphragm is of a symmetry of revolution.

4. A system according to claim 1, 2, or 3, wherein the surface of said reflector facing said diaphragm is conical.

5. A system according to claim 1, 2,3, or 4, wherein said reflector and said diaphragm are movable relative to each other.

6. A system according to claim 1, 2,3,4, or 5, further comprising:

a low-pitched sound speaker provided separately from said speaker.

7. A system according to claim 1, 2,3,4, 5, or 6, further comprising:

a spherical cabinet in which said speaker is fixed.

8. A sound output system comprising:

a) a first and a second speaker having a first and a second diaphragm, respectively; and b) a first and a second acoustic reflector disposed opposite to said first and second diaphragms, respectively, for determining the directivity of the acoustic outputs from said first and second speakers.

9. A system according to claim 8, wherein said first and second speakers are provided for different frequency bands of acoustic output.

10. A system according to claim 8 or 9, wherein said first and second speakers and virtual sound sources of said first and second reflectors are disposed on the same axis.

11. A system according to claim 8 or 9, further comprising:

a speaker cabinet to which said first speaker is fixed and with which said second reflector is constructed integrally.

12. A system according to claim 8,9, 10 or 11, wherein said first and second speakers are movable relative to said first and second reflectors, respectively.

13. A loudspeaker unit, the unit comprising a speaker having a diaphragm and an acoustic reflectorfacing thefree surface of the diaphragm.

14. A loudspeaker unit substantially as hereinbefore described with reference to Figures 1A and 1 B; or Figures 2A and 213; or Figures 3A and 313; or Figure 7; or Figure 8.

15. A sound output system comprising an acoustic reflector provided opposite to a diaphragm of a speaker for determining the directivity of the speaker output in the direction perpendicular to the diaphragm.

Printed for Her Majesty's Stationery Office by Courier Press, Leamington Spa, 10187. Demand No. 8991685. Published by the Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.