EP0873039A2

EP0873039A2 - Speaker

Info

Publication number: EP0873039A2
Application number: EP98400928A
Authority: EP
Inventors: Takeshi Nakamura; Yoshiaki Heinouchi
Original assignee: Murata Manufacturing Co Ltd
Current assignee: Murata Manufacturing Co Ltd
Priority date: 1997-04-15
Filing date: 1998-04-15
Publication date: 1998-10-21
Also published as: CN1151699C; CN1214605A; EP0873039A3

Abstract

There is provided a speaker which is small and which allows the low sound range to be generated and a low sound level to be fully obtained. A speaker 10 comprises a cylindrical main body 12. Both ends of the main body 12 constitute open sections thereof and a sounding body 16 pinched by ringed first dampers 14 is secured to each of the open sections so that the inside of the main body 12 is kept air-tight. A circular first opening 18 is created at the middle part of the side of the main body 12. Bottomed cylindrical lid members 20 are secured on the outside of the two sounding bodies 16 via ringed second dampers 24 so as to keep the inside of the lid member 20 air-tight. A circular second opening 22 is created around the middle of the bottom of the lid member 20.

Description

TECHNICAL FIELD

The present invention relates to a speaker and more particularly to a speaker which is suitably used for a 3D center channel woofer which is used in some stereo playback systems.

The present invention also relates to a speaker in which a piezoelectric body made of ceramic is used and which is suitably used for a 3D center channel woofer which is one of stereo playback systems.

BACKGROUND ART

A so-called 3D system in stereo generating systems is a system for generating only the low sound range of about 100 to 150 Hz or less, whose direction is hardly perceived, by one speaker for a woofer comprising a sounding body corresponding to a left channel signal and a sounding body corresponding to a right channel signal.

FIG. 5 shows a prior art speaker for a woofer, for generating a signal in which both right and left channel signals of 100 to 150 Hz or less are combined.

The speaker 60 comprises a main body 100, a partition 200 provided within the main body 100, two sounding bodies 300 and 400 attached to the partition 200 and a duct 500 provided at the lower part of the front face of the main body 100. The sounding bodies 300 and 400 are attached so as to orient the bottom of the main body 100.

Although not shown specifically, a sound in the low sound range of the right and left channels of 100 to 150 Hz or less is output when the signal of the left channel is input to the sounding body 300 and the signal of the right channel is input to the sounding body 400 for example.

At this time, the sounds emitted directly from the sounding bodies 300 and 400 are not emitted to the outside of the main body 100. Only a resonant frequency of the duct 500 and the main body 100 is emitted to the outside of the main body 100.

However, because a sounding body, e.g. a cone plate, which allows large amplitude to be obtained has been used in the prior art speaker described above in order to obtain a large output in the low sound range, the speaker has to be large. It is conceivable to form the sounding body by a plate-like piezoelectric element to eliminate such a problem. However, when such a piezoelectric element is used as the sounding body corresponding to the prior art speaker described above, the low sound range would be insufficient in such speaker because the amplitude of displacement of the piezoelectric element is small as compared to that of the sounding body composed of the cone plate, and sufficient amplitude corresponding to the structure (size) of the prior art speaker cannot be obtained. To deal with this problem, an area of the piezoelectric element would have to be enlarged to increase the amplitude of the piezoelectric element, thus inevitably increasing the size of the speaker after all.

Further, because the low sound range depends on a resonance frequency between the duct and the main body and there has been only one resonance frequency in the prior art speaker, a sufficiently low sound level could not be obtained for the low sound range separated from the resonance frequency, and the generation level has been small in the wide sound range.

Still more, because a piezoelectric body made of ceramic is used for sounding bodies in a speaker comprising the piezoelectric body in general, the amplitude of a sound wave generated from the sounding bodies has been insufficient. Accordingly, the speaker comprising such a piezoelectric body has not been used as a speaker for a woofer for generating the low sound range since the past.

The inventor of the present invention has applied a speaker for a woofer which comprises a piezoelectric body and which can generate low frequency sounds as described in Japanese Patent Application No. 9 327091.

This speaker will be explained with reference to FIG. 6. The speaker 1 comprises a cylindrical main body 2 having open sections at the both ends thereof. Ringed dampers 6 are provided at the open sections at the both ends of the main body 2, respectively, and sounding bodies 3 are attached to the dampers 6. The sounding bodies 3 are secured so that the inside of the main body 2 is kept air-tight. A circular first opening 2a is created at the middle part of the side of the main body 2.

Further, bottomed cylindrical lid members 4 are provided on the outside of the two sounding bodies 3 so as to cover the open sections at the both ends of the main body 2 and are secured so that the inside of the lid member 4 is kept air-tight. A circular second opening 4a is created around the middle of the bottom of the lid members 4. Sound absorbers 5 for absorbing and damping components of sound waves in the high sound range are provided within the main body 2 and the lid members 4.

Here, the sounding body 3 comprises a disc-like vibrating plate 3a made of metal and disc-like piezoelectric elements 3b are secured as a vibration source at the center part of the both main surfaces of the vibrating plate 3a on a concentric circle of the vibrating plate 3a in a bimorph structure. Although not shown specifically, the piezoelectric element 3b is constructed by forming electrode films on the both main surfaces of the disc-like piezoelectric body.

Both of the first opening 2a and the second opening 4a are formed to be smaller than an area of the vibrating plate 3a of the sounding body 3.

Then, one of the two sounding bodies 3 is connected to one input terminal (not shown) provided on the outside of the main body 2 via a lead wire (not shown) and the other one of the two sounding bodies 3 is connected to another input terminal (not shown) provided on the outside of the main body 2 via another lead wire (not shown).

When an electrical signal is input from an amplifier (not shown) to one input terminal at this time, one of the two sounding bodies 3 vibrates and when an electrical signal from the amplifier (not shown) is input to the other input terminal, the other of the two sounding bodies 3 vibrates.

Then, the sounding bodies 3 are driven so that pressure within the main body 2 is increased/decreased at the same time by the two sounding bodies 3. That is, when one channel signal, e.g. a right channel signal, of a stereo signal is input to one input terminal, the other channel signal, i.e. a left channel signal, of the stereo signal is input to the other input terminal.

By inputting signals as described above, sound waves are generated within the main body 2 by the two sounding bodies 3 and are emitted from the first opening 2a of the main body 2. Sound waves are generated also within the lid members 4 by the two sounding bodies 3 and are emitted from the second openings 4a of the lid members 4.

At this time, because the area of the first opening 2a of the main body 2 is smaller than the area of the vibrating plate 3a of the sounding body 3 and amplitude of the sound wave emitted from the first opening 2a of the main body 2 is larger than that of the sound wave generated within the main body 2 by the two sounding bodies 3, the low sound range is stressed and is emitted.

Further, because the sound waves in the middle/high sound range generated from the two sounding bodies 3 have phases which are mutually opposite and are cancelled within the main body 2, only the low sound range is stressed and is emitted from the first opening 2a.

Consequently, the speaker 1 can generate the low sound range by using the piezoelectric body and without using a large vibrating plate, a large case or a large baffle plate.

Moreover, because the sound absorbers 5 absorb and damp the high sound range component of the sound waves generated within the main body 2 and the lid member 4 by the two sounding bodies 3, the low sound range of the generated sound wave is stressed further as a result.

However, because the sounding body is composed of the plate-like piezoelectric body in the prior art speaker using the sounding body composed of the piezoelectric body, the vibration area of the sounding body is divided, in its mode, into a piston vibration area and a split vibration area depending on a frequency of signal input to the sounding body.

Then, in a speaker utilising a resonance phenomenon, although the resonance phenomenon occurs on a sound wave generated by the piston vibration, no resonance phenomenon occurs on a sound wave generated by the split vibration because it is inherently subject to high distortion.

When no resonance phenomenon occurs, the low sound range is not stressed. As a result, the middle/high sound range is stressed and the speaker cannot function as a speaker for a woofer.

In contrast to that, the prior art speaker using the sounding body composed of the piezoelectric body functions as a speaker for a woofer by utilising a resonance phenomenon defined by the volume of the main body and the shape of the first opening and a resonance phenomenon defined by the volume of the lid member and the shape of the second opening.

When sound waves caused by the split vibration generated from the two sounding bodies exist within the main body of the prior art speaker, the sound waves in the middle/high sound range are cancelled within the main body and no sound wave in the middle/high sound range caused by the split vibration is emitted from the first opening because the phases of the sound waves in the middle/high sound range generated respectively from the two sounding bodies have the phases which are mutually opposite.

However, differing from the inside of the main body, only the sound wave caused by the split vibration generated from one sounding body among the two sounding bodies exists within the lid member and the sound wave in the middle/high sound range caused by the split vibration is not cancelled.

Accordingly, the sound wave in the middle/high sound range caused by the split vibration has been emitted as it is from the second opening of the lid member. This high sound range cannot be absorbed fully by the sound absorber, so that both sound waves in the high and low sound ranges have been emitted from the second opening as a result. Further, because the sound wave caused by the split vibration causes no resonance phenomenon, the sound of the speaker for the woofer is emitted with much turbidity and small low sound level.

Accordingly, it is a primary object of the present invention to eliminate the above-mentioned problems by providing a speaker which is small and which allows the low sound range to be generated and an enough low sound level to be obtained in the wide low sound range.

Further, a secondary object of the present invention is to provide a speaker which is small and which allows the low sound range to be generated and a clear low sound level to be obtained by generating only the low sound range.

DISCLOSURE OF THE INVENTION

In order to achieve the above-mentioned primary object, a speaker of the present invention comprises a cylindrical main body having open sections at the both ends thereof; two sounding bodies which are provided at open sections at the both ends of the main body and which are vibrated by electrical signals; two bottomed cylindrical lid members provided so as to cover the two sounding bodies; and a first opening provided at the side portion of the main body and a second opening provided at the bottom portion of the lid members, wherein the two sounding bodies are driven in the direction in which internal pressure of the main body and internal pressure of the lid member are increased/decreased. The speaker is characterized in that a resonance frequency within the main body is differentiated from a resonance frequency within the two lid members.

Preferably, a piezoelectric body made of ceramic is used for the sounding body.

In a preferred embodiment, one input terminal for inputting one channel signal of a stereo signal is connected to one sounding body among the two sounding bodies and another input terminal for inputting another channel signal of the stereo signal is connected to the other sounding body among the two sounding bodies.

Another object of the present invention is to provide a speaker having a cylindrical main body having open sections at the both ends thereof; two sounding bodies which are provided at open sections at both ends of the main body and which comprise a piezoelectric body which is vibrated by electrical signals; two bottomed cylindrical lid members provided so as to cover the two sounding bodies; and a first opening provided at the side portion of the main body and a second opening provided at the bottom portion of the lid members, wherein the two sounding bodies are driven in the direction in which internal pressure of the main body and internal pressure of the lid member are increased/decreased. The speaker is characterized in that a membrane is provided between the sounding body and the second opening within the lid member.

Thereby, because the plurality of resonance frequencies in the low sound range exists, the range of the low sound range is expanded and the low sound level generated by the speaker is improved in the wide low sound range.

When an electrical signal is input to the two sounding bodies so that the internal pressure of the main body is increased/decreased by the two sounding bodies, sound waves are generated within the main body from the two sounding bodies and are emitted to the outside from the first opening of the main body. At this time, because a diameter of the first opening is smaller than the sounding body, the amplitude of the sound wave emitted from the first opening becomes larger than amplitude of the sound wave generated from the sounding body.

Therefore, the speaker of the present invention allows not only the miniaturisation thereof to be achieved because it uses the plate-like piezoelectric body which is smaller and thinner than a sounding body utilising a cone plate used in the prior art speaker for a woofer, but also the speaker for a woofer which can fully generate the low sound level to be achieved.

Further, it becomes possible to obtain a speaker which allows the low sound range level fully by the simple structure and at low cost such a speaker is suitable for a 3-D center channel woofer since an acoustic low-pass filter is constructed by attaching the two sounding bodies on one main body, by taking out a sum output of the right and left channel signals and by utilising only sound pressure from the openings provided on the main body and the lid member.

Further, the sound wave caused by the split vibration occurring from one sounding body is emitted to the outside from the second opening provided on the lid member after being damped by the membrane, which serves as a wall within the lid member, and being transformed into a sound wave having no distortion as the displacement of the sound wave caused by the split vibration is averaged by impinging on the membrane. Accordingly, as a speaker for a woofer, the speaker of the present invention can suppress a rasping and unpleasant sound and generate a clear sound.

The specific nature of the invention, as well as other objects, uses and advantages thereof, will clearly appear from the following description and from the accompanying drawings in which like numerals refer to like parts.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view showing a structure of a speaker according to a first embodiment of the present invention;

FIG. 2 is a cross-sectional view showing the structure of the speaker according to the first embodiment of the present invention;

FIG. 3 is a perspective view showing a structure of a speaker according a second embodiment of the present invention;

FIG. 4 is a cross-sectional view showing the structure of the speaker according to the second embodiment of the present invention;

FIG. 5 is an explanatory diagram showing a structure of a prior art speaker; and

FIG. 6 is a section view showing a structure of another prior art speaker.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be explained below in detail with reference to the drawings.

FIGs. 1 and 2 show a speaker according to a first embodiment of the present invention.

A speaker 10 comprises a cylindrical main body 12. Both ends of the main body 12 constitute open sections and ringed first dampers 14 are provided at the respective open sections. A sounding body 16 is provided at the first damper 14 and another first damper 14 is provided on the outside of the sounding body 16. That is, the sounding body 16 is secured so that the inside of the main body 12 is kept air-tight while being pinched by the two first dampers 14.

Then, a circular first opening 18 is created at the middle part of the side of the main body 12. Further, ringed second dampers 24 are provided on the first dampers 14 on the outside of the two sounding bodies 16. A bottomed cylindrical lid member 20 is provided over the second damper 24. The lid-member 20 is secured so as to cover the open section of the main body 12 and to keep the inside of the lid member 20 air-tight. A circular second opening 22 is created around the middle of the bottom of the lid member 20.

Here, the main body 12 and the lid member 20 are made of a synthetic resin material such as plastic or a wood cut out into the cylindrical shape.

The sounding body 16 also comprises a disc-like vibrating plate 16a made of metal for example and disc-like piezoelectric elements 16b are secured at the middle part of both main surfaces of the vibrating plate 16a on a concentric circle of the vibrating plate 16a respectively as a vibration source in a bimorph structure. Although not shown specifically, the piezoelectric element 16b is made by forming electrode films on the both main surfaces of the disc-like piezoelectric body.

Preferably, the first damper 14 and the second damper 24 are made of a material whose mass is heavy and having a high sealability.

Further, although not shown specifically, the speaker 10 is placed on a mount (not shown) so that the first opening 18 of the main body 10 heads downward.

Then, one of the two sounding bodies 16 is connected to one input terminal (not shown) provided on the outside of the main body 12 via a lead wire (not shown) and the other of the two sounding bodies 16 is connected to another input terminal (not shown) provided on the outside of the main body 12 via another lead wire (not shown).

When an electrical signal is input from an amplifier (not shown) to one input terminal at this time, one of the sounding bodies 16 vibrates and when an electrical signal from the amplifier (not shown) is input to the other input terminal, the other of the sounding bodies 16 vibrates.

Then, the sounding bodies 16 are driven so that the internal pressure of the main body 12 is increased/decreased at the same time by the two sounding bodies 16 in the speaker 10 thus constructed. That is, when one channel signal, e.g. a right channel signal, of a stereo signal is input to one input terminal, the other channel signal, i.e. a left channel signal, of the stereo signal is input to the other input terminal.

By inputting signals as described above, sound waves are generated within the main body 12 from the two sounding bodies 16 and resonance occurs between the main body 12 and the first opening 18. Only the generated sound stressed by this resonance is emitted to the outside of the main body 12 from the first opening 18.

Here, the low sound range having no phase difference between the right channel signal and the left channel signal is emitted to the outside of the main body 12 as the generated sound of double pressure in which the right channel signal and the left channel signal are composited and sufficient sound pressure may be obtained even if the size of the main body 12 is small, so that the speaker 10 which emits fully the low sound level, even if it is small, can be obtained.

It is noted that the middle/high sound range having a phase difference between the right channel signal and the left channel signal is cancelled as the right channel signal and the left channel signal are composited, so that the low sound range of the emitted sound is stressed further.

Sound waves are generated also within the lid members 20 from the two sounding bodies 16 and resonance occurs between the lid member 20 and the second opening 22. Then, only a generated sound stressed by this resonance is emitted to the outside of the lid member 20 from the second opening 22.

Here, a resonance frequency may be expressed by the following equation from the law of Helmholtz: f = c 2π πr 2 V(l+1.3r)

Where, (f) denotes a resonance frequency, (c) a speed of sound, (V) a volume of a resonator, (l) a thickness of the resonator and (r) a radius of an opening.

Then, in the speaker 10, when a resonance frequency in the main body 12 is f₁₂ and a resonance frequency in the lid member 20 is f₁₈, the range of the low sound range is widened and the low sound level generated by the speaker 10 is improved in the wide low sound range when the speaker 10 is constructed so that f₁₂ ≠ f₁₈ by increasing the volume of the main body 12 more than that of the lid member 20 for example.

Further, although not shown specifically, sound absorbers made of glass wool may be provided within the main body 12 and the lid member 20. Because they absorb and damp the middle/high sound range of the sound wave generated within the main body 12 and the lid member 20, the low sound range of the generated sound wave is stressed further.

When an electrical signal is input to the two sounding bodies 16 so that the internal pressure of the main body 12 is increased/decreased by the two sounding bodies 16 in the speaker 10 of the present invention, sound waves are generated within the main body 12 from the sounding bodies 16 and are emitted to the outside from the first opening 18 of the main body 12. At this time, because a diameter of the first opening 18 is set to be smaller than the sounding body 16, amplitude of the sound wave emitted from the first opening 18 becomes larger than that of the sound wave generated from the sounding body 16. Therefore, the present invention allows not only the miniaturisation the speaker to be achieved because it uses the plate-like piezoelectric body which is smaller and thinner than the sounding body utilising a cone plate used in the prior art speaker for a woofer, but also the speaker for a woofer which can fully generate the low sound level to be achieved by using the piezoelectric body.

Further, the speaker 10 which allows the low sound range level to be obtained fully with the simple structure and at low cost and which is suitable for a 3-D center channel woofer may be realised because an acoustic low-pass filter may be constructed by attaching the two sounding bodies 16 on one main body 12, by taking out a sum output of the right and left channel signals and by utilising only sound pressure from the openings provided on the main body 12 and the lid member 20.

It is noted that although the main body has been formed to be cylindrical in the mode for carrying out the invention, the main body may be formed into any cylindrical shape such as a square cylindrical shape other than the circular cylindrical shape. Similarly to that, the lid member may be formed into any bottomed cylindrical shape such as a bottomed square cylindrical shape, other than the bottomed circular cylindrical shape.

Further, the vibrating plate and the piezoelectric elements composing the sounding body may be formed into any shape such as that of square-plate, other than that of disc, and the vibrating plate may be formed of rubber or synthetic resin for example other than metal.

Still more, not only the sounding body using the piezoelectric elements having the bimorph structure, but also a sounding body using a piezoelectric element having a unimorph structure or a sounding body using a piezoelectric element having laminated structure made of ceramic and using three or more layers of laminated piezoelectric body layers may be used.

Next, a second embodiment of the present invention will be explained below in detail with reference to figures 3 and 4.

FIGs. 3 and 4 show a speaker according to the second embodiment of the present invention.

A speaker 10' comprises a cylindrical main body 12 having open sections at both ends thereof. Ringed first dampers 14 are provided at the open sections at the both ends of the main body 12 and sounding bodies 16 are secured so that the inside of the main body 12 is kept air-tight while being pinched by the first dampers 14. A circular first opening 18 is formed at the middle part of the side of the main body 12.

Further, a ringed second damper 24 is secured to the first damper 14 along the direction approaching to the open section of the main body 12 from the sounding body 16. A disc-like membrane 26 is secured to the second damper 24 and a bottomed cylindrical lid member 20 is provided over the membrane 26. The lid member 20 is secured so as to cover the open section of the main body 12 and to keep the inside of the lid member 20 air-tight. A circular second opening 22 is created around the middle of the bottom of each lid member 20.

Thus, the membrane 26 is positioned between the second opening 22 of the lid member 20 and the sounding body 16.

Preferably, the first damper 14 and the second damper 24 are made of a material having a large mass.

Further, the membrane 26 is preferably made of an elastic material such as a silicon rubber sheet.

Still more, although not shown specifically, the speaker 10' is placed on a mount not shown so that the first opening 18 of the main body 12 heads downward.

Then, the sounding bodies 16 are driven so that the internal pressure of the main body 12 is increased/decreased at the same time by the sounding body 16 in the speaker 10' thus constructed. That is, when one channel signal, e.g. a right channel signal, of a stereo signal is input to one input terminal, the other channel signal, i.e. a left channel signal, of the stereo signal is input to the other input terminal.

By inputting signals as described above, sound waves are generated within the main body 12 from the two sounding bodies 16 and resonance occurs between the main body 12 and the first opening 18. Only the resonant frequency generated by this resonance is emitted to the outside of the main body 12 from the first opening 18.

Here, the low sound range having no phase difference between the right channel signal and the left channel signal is emitted to the outside of the main body 12 as a double pressure resonant composited from the right and left channel signals and an enough sound pressure may be obtained even if the size of the main body 12 is small, so that the speaker 10' which emits fully the low sound level, even if it is small, can be obtained.

It is noted that the middle/high sound range having a phase difference between the right channel signal and the left channel signal is cancelled as the right channel signal and the left channel signal are composited, so that the low sound range of the sound to be emitted is stressed further.

Sound waves are generated also within the lid members 20 by the two sounding bodies 16 and resonance occurs between the lid member 20 and the second opening 22. Then, only the resonant generated by this resonance is emitted to the outside of the lid member 20 from the second opening 22.

Because the sounding body 16 is composed of the plate-like piezoelectric body in the speaker 10' of the present invention, a vibration area of the sounding body 16 is divided, in its inherent characteristics, into a piston vibration area and a split vibration area depending on a frequency of signal input to the sounding body 16.

Then, although a resonance phenomenon occurs on a sound wave generated from the piston vibration area of the sounding body 16, no resonance phenomenon occurs on a sound wave generated from the split vibration area because it has an inherently high distortion.

Here, when the sound waves caused by the split vibration generated from the two sounding bodies 16 exist within the main body 12, the sound waves in the middle/high sound range are cancelled within the main body 12 and no sound wave in the middle/high sound range caused by the split vibration is emitted from the first opening 18 because the phases of the sound waves in the middle/high sound range generated respectively from the two sounding bodies 16 are opposite from each other.

Further, differing from the inside of the main body 12, only the sound wave caused by the split vibration generated from one sounding body 16 among the two sounding bodies 16 exists within the lid member 20 and the sound wave in the middle/high sound range caused by the split vibration is not cancelled. However, the sound wave caused by the split vibration generated from one sounding body 16 within the lid member 20 is emitted to the outside from the second opening 22 of the lid member 20 after being damped by the membrane 26 provided between the lid member 20 and the second opening 22 as the membrane 26 plays a role of a wall, and being transformed into a sound wave having no distortion as its displacement caused by the split vibration is averaged by hitting to the membrane 26. In this way, the speaker 10' can suppress a rasping unpleasant sound and generate a clear sound.

Further, the membrane 26 has a function of protecting the sounding body 16 from dust and the like infiltrating through the second opening 22 as well as environmental changes around the speaker 10' such as changes in temperature.

It is noted that although not shown specifically, sound absorbers made of glass wool may be provided within the main body 12 and the lid member 20. Because they absorb and damp the middle/high sound range of the sound wave generated within the main body 12 and the lid member 20, the low sound range of the generated sound wave is stressed further.

When an electrical signal is input to the two sounding bodies 16 so that the internal pressure of the main body 12 is increased/decreased by the two sounding bodies 16, sound waves are generated within the main body 12 from the two sounding bodies 16 and are emitted to the outside from the first opening 18 of the main body 12. At this time, because a diameter of the first opening 18 is set to be smaller than the sounding body 18, amplitude of the sound wave emitted from the first opening 18 becomes larger than that of the sound wave generated from the sounding body 16. Therefore, the present invention allows not only the miniaturisation the speaker to be achieved because it uses, as the sounding body, the plate-like piezoelectric body which is smaller and thinner than the sounding body utilising a cone plate used in the prior art speaker for a woofer, but also the speaker for a woofer which can fully generate the low sound level to be achieved.

Further, the speaker 10' which allows the low sound range level to be obtained fully by the simple structure and at low cost and which is suitable for the 3-D center channel woofer may be realised because an acoustic low-pass filter may be constructed by attaching the two sounding bodies 16 on one main body 12, by taking out a sum output of the right and left channel signals and by utilising only sound pressure from the openings provided on the main body 12 and the lid member 20.

Although the membrane and the second damper have been separately formed in the embodiment described above, they need not be separately formed specifically and the membrane and the second damper may be moulded in a body.

As described above, according to the speaker of the present invention, there exist a plurality of resonance frequencies in the low sound range by differentiating the resonance frequency within the main body from the resonance frequencies within the two lid members, so that the range of the low sound range is widened and the low sound level generated by the speaker is improved in the wide low sound range.

Further, when an electrical signal is input to the two sounding bodies so that the internal pressure of the main body is increased/decreased by the two sounding bodies, sound waves are generated within the main body from the two sounding bodies and are emitted to the outside from the first opening of the main body. At this time, because the diameter of the first opening is smaller than the sounding body, amplitude of the sound wave emitted from the first opening becomes larger than that of the sound wave generated from the sounding body. Therefore, the present invention allows not only the miniaturisation of the speaker to be achieved because it uses the plate-like piezoelectric body which is smaller and thinner than the sounding body utilising a cone plate used in the prior art speaker for a woofer, but also the speaker for a woofer which can fully generate the low sound level to be achieved by using the piezoelectric body.

Further, the present invention provides for a speaker that allows the low sound range level to be obtained fully by the simple structure and at low cost and which is suitable for the 3-D center channel woofer, by being composed of an acoustic low-pass filter obtained by attaching the two sounding bodies on one main body, by taking out a sum output of the right and left channel signals and by utilising only sound pressure from the openings provided on the main body and the lid member.

Still more, according to the speaker of the present invention, the sound wave caused by the split vibration generated from one sounding body is emitted to the outside from the second opening provided on the lid member after being damped by the membrane which plays the role of a wall within the lid member and being transformed into a sound wave having no distortion as the displacement of the sound wave caused by the split vibration is averaged by hitting to the membrane. Accordingly, as a speaker for a woofer, the speaker of the present invention can suppress a rasping and unpleasant sound and generate a clear sound.

Claims

A speaker (10; 10'), comprising:

a cylindrical main body (12) having open sections at the both ends thereof;

two sounding bodies (16) which are provided at open sections at both ends of said main body and which are vibrated by electrical signals;

two bottomed cylindrical lid members (20) provided so as to cover said two sounding bodies; and

a first opening (18) provided at side portion of said main body and a second opening (22) provided at the bottom portion of said lid member;

said two sounding bodies being driven in the direction in which internal pressure of said main body and internal pressure of said lid member are increased/decreased;

said speaker being characterized in that a resonance frequency within said main body (12) is differentiated from a resonance frequency within said two lid members (20).
The speaker according to Claim 1, wherein a piezoelectric body made of ceramic is used for said sounding bodies (16).
The speaker according to Claim 1 or 2, wherein one input terminal for inputting one channel signal of a stereo signal is connected to one sounding body (16) among said two sounding bodies and another input terminal for inputting another channel signal of said stereo signal is connected to the other sounding body (16) among said two sounding bodies.
A speaker (10; 10), comprising:

a cylindrical main body (12) having open sections at the both ends thereof;

two sounding bodies (16) provided at open sections at both ends of said main body and which comprise a piezoelectric body which is vibrated by electrical signals;

two bottomed cylindrical lid members (20) provided so as to cover said two sounding bodies; and

a first opening (18) provided at side portion of said main body and a second opening (22) provided at the bottom portion of said lid member;

said two sounding bodies being driven in the direction in which internal pressure of said main body and internal pressure of said lid member are increased/decreased; and

said speaker being characterized in that a membrane (26) is provided between a said sounding body (16) and said second opening (22) within said lid member (20).