Abstract:
An electro-acoustic transducer comprises first and second panels ( 18 A, 18 B) each of which can be vibrated to generate sound, a frame ( 10 ) for mounting the panels, and first and second seals ( 20 A,B) arranged between the frame and the edges of the panels for holding the panels in the frame, substantially isolating the frame acoustically from the edges of the panel, and substantially sealing the frame to the edges of the panel. One or more actuators ( 22 ), such as piezoelectric elements, are provided for receiving a driving signal and vibrating in response thereto, and the actuators are mechanically and acoustically coupled to the first panel at one or more locations remote from the edges of the first panel so that the first panel vibrates in response to vibration of the actuators. The second panel is mechanically and acoustically coupled to the first panel and/or to the actuators at one or more locations remote from the edges of the second panel so that the second panel also vibrates in response to vibration of the actuator means. The acoustic properties of the panels, the seals, the actuators and the couplings can be chosen to obtain a required frequency response from the transducer. Different embodiments are described in which the panels are driven in phase, in anti-phase, and in a more complex manner.

Description:
FIELD OF THE INVENTION 
     This invention relates to electro-acoustic transducers for generating acoustic waves. An audio loudspeaker is an example of such a transducer. More particularly, the invention relates to transducers which include a panel, such as a flat panel, which can be vibrated to generate sound and one or more actuators for receiving a driving signal and vibrating in response thereto, the actuator(s) being coupled to the panel at one or more locations remote from the edges of the panel so that the panel vibrates, in response to the vibration of the actuators(s), in a multi-modal, non-pistonic, bending manner without any significant bodily translational movement of the panel. 
     DESCRIPTION OF RELATED ART 
     Such transducers are known in which the panel is made to vibrate in a multi-modal, non-pistonic manner, ie with bending vibrations in the panel rather than any significant bodily translational movement of the panel. In order to prevent any significant bodily translational movement, the panel is either rigidly clamped in a rigid frame, or is supported in a frame by a soft elastic suspension at its corners. A problem with such transducers, and with which a first aspect of the present invention is concerned, is that sound is generated from both sides of the panel and is allowed to interfere. Thus, if the panel is placed near an acoustically reflective surface, for example a wall, considerable interference can take place between the sound generated from the front of the panel and the sound generated from the back of the panel. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the present invention, a frame is provided for mounting the panel, and a seal is arranged between the frame and the edges of the panel for holding the panel in the frame, substantially isolating the frame acoustically from the edges of the panel, and substantially sealing the frame to the edges of the panel. Thus, acoustic vibrations in the air generated by the front and rear faces of the panel can be acoustically isolated and prevented from interfering, whilst acoustic reflections at the edges of the panel can be reduced. In other words, the seal can act as a barrier to acoustic vibrations passing around the panel member, and also act to damp out acoustic reflections at the interface between the panel and the frame, thus acting as a semi-anechoic termination. 
     (It should be noted that patent document JP-A-58-007999 shows a flat-plate transducer, the plate being arranged to vibrate in a single-mode, pistonic, bending manner with significant bodily translational movement of the plate, and the plate being mounted to a frame by peripheral rubber ring.) 
     Preferably, the seal comprises a strip of flexible resilient material, which may be arranged to wrap around the edges of the panel. (It should be noted that patent document JP-A-56-056095 discloses a different type of transducer, but with a vibratable plate mounted edge-to-edge inside a frame with an H-section element between the edges.) The strip may be received in a channel in the frame, and the strip may provide a channel which receives the edges of the panel. Conveniently, the strip may be formed from a length of resilient tubing, for example of silicone rubber, cut lengthwise and opened to clamp over the edges of the panel. 
     In some embodiments, the frame forms part of an enclosure disposed generally to one side of the panel. Thus, the enclosure can be arranged to absorb and damp out vibrations produced from one side of the panel, and sound can be radiated from the other side of the panel, with the seal substantially preventing sound from escaping from the enclosure to the outside. 
     The transducer may further include: a second panel which can be vibrated to generate sound and which is mounted to the frame; second coupling means for mechanically and acoustically coupling the second panel to the first panel and/or the actuator means at one or more locations remote from the edges of the second panel so that the second panel also vibrates, in response to vibration of the actuator means, in a multi-modal, non-pistonic, bending manner without any significant bodily translational movement of the second panel; and a second seal arranged between the frame and the edges of the second panel for holding the second panel in the frame, substantially isolating the frame acoustically from the edges of the second panel, and substantially sealing the frame to the edges of the second panel. If the first and second panels are parallel to each other, then the first and second seals can prevent sound generated in the space between the panels from escaping to interfere with sound generated from the other sides of the panels. If the first and second panels are side-by-side, then the seals have the same effect as if there were only one panel. (It should be noted that patent document WO-A-96-35313 shows a transducer having a pair of parallel panels rigidly fixed to a frame and bridged by an actuator which causes each panel to vibrate in a single mode, pistonic, bending manner with significant bodily translational movement of the panels.) 
     The first and second seals may have different acoustic isolation properties to assist in providing a flatter frequency response for the transducer as a whole, so that, for example, a trough in the frequency response of one of the panels coincides with a peak in the frequency response of the other panel. 
     A second aspect of the present invention is concerned with making other improvements to known electro-acoustic transducers of the type described in the opening paragraph. In accordance with the second aspect of the present invention, there is provided an electro-acoustic transducer, comprising: first and second panels each of which can be vibrated to generate sound; actuator means for receiving a driving signal and vibrating in response thereto; first coupling means for mechanically and acoustically coupling the first panel to the actuator means at one or more locations remote from the edges of the first panel so that the first panel vibrates, in response to vibration of the actuator means, in a multi-modal, non-pistonic, bending manner without any significant bodily translational movement of the panel; and second coupling means for mechanically and acoustically coupling the second panel to the first panel and/or the actuator means at one or more locations remote from the edges of the second panel so that the second panel also vibrates, in response to vibration of the actuator means, in a multi-modal, non-pistonic, bending manner without any significant bodily translational movement of the second panel. The first and second panels may therefore have different acoustic properties and/or the first and second coupling means may have different acoustic coupling properties to assist in providing a flatter frequency response for the transducer, again, for example, by arranging that a trough in the frequency response of one of the panels coincides with a peak in the frequency response of the other panel. 
     It should be noted that features of the first and second aspects of the invention described above may be combined in a single transducer. 
     In some embodiments of either aspect of the invention, the first and second panels are arranged face-to-face. In this case, the second coupling means may be arranged to couple the second panel mechanically and acoustically to the first panel. In one embodiment, the second panel has at least one aperture therein which receives the actuator means. Alternatively, the second coupling means may be arranged to couple the second panel mechanically and acoustically to the actuator means. In this case, the actuator means may be common to the first and second panels and may comprise at least one piezo-electric actuator bridging between the first and second coupling means. In this case, the, or at least one of the, piezo-electric actuators may comprise a stack formed of layers of piezoelectric material. 
     In other embodiments, the first and second panels are arranged side-by-side, and the second coupling means is arranged to couple the second panel mechanically and acoustically to the actuator means. (It should be noted that patent document U.S. Pat. No. 4,899,390 shows a different type of transducer, but with panels arranged side-by-side.) 
     In either case, the actuator means may comprise: a first actuator means which is coupled by the first coupling means to the first panel; and second actuator means which is coupled by the second coupling means to the second panel. In this case, the first and second actuator means may each comprise at least one piezo-electric actuator. 
     In any of the embodiments wherein a plurality of such piezo-electric actuators are provided, the, or at least two of the, piezo-electric actuators may have different electro-acoustic transducing properties, and again this may be used to achieve a flatter frequency response for the transducer as a whole. 
     Also, in any of the embodiments wherein a plurality of such piezo-electric actuators are provided, an electrical circuit may be provided for receiving an input signal and for producing therefrom at least two output signals with different amplitudes, frequency characteristics and/or phases, the output signals being supplied to different ones of the piezo-electric actuators, and again this may be used to achieve an improved frequency response for the transducer as a whole. 
     In any of the embodiments having a pair of such vibratable panels, the first and second panels may be arranged to vibrate substantially in phase with each other, and thus may act together like a single panel, but with an improved frequency response. 
     Alternatively, the first and second panels may be arranged to vibrate substantially in anti-phase with respect to each other. This may be particularly useful in the case where one of the panels faces towards an acoustically reflective surface, such as a wall, because the sound reflected from the surface can be arranged to interfere constructively with the sound radiated forwardly from the transducer. 
     At least one further panel may be provided, arranged face-to-face with respect to the first panel and/or arranged side-by-side with respect to the first panel. 
     The, or at least one of the, coupling means may be provided by bonding a respective portion of the, or the respective, actuator means to the, or the respective, panel, or may comprise a passive intermediate layer disposed between a respective portion of the, or the respective, actuator means and the, or the respective, panel. In this latter case, the, or at least one of the, intermediate layers preferably has larger lateral dimensions than the respective piezo-electric actuator and/or has a greater stiffness than the respective panel and substantially the same stiffness as the respective piezo-electric actuator. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     Specific embodiments of the present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 is a sectioned side view of a first embodiment of electro-acoustic transducer, taken along the section line  1 — 1  in FIG. 2; 
     FIG. 2 is a sectioned rear view of the electro-acoustic transducer of FIG. 1, taken along the section line  2 — 2  in FIG. 1; 
     FIG. 3 is a graph of the transfer function, as a function of frequency, of an example of the transducer of FIGS. 1 and 2; 
     FIG. 4 is a graph of the transfer function, as a function of frequency, of an example of the transducer of FIGS. 1 and 2, but modified to secure the panel rigidly to the frame; 
     FIGS. 5 to  13  are each sectioned side views of further embodiments of electro-acoustic transducers; 
     FIG. 14 is a sectioned side view of another embodiment of electro-acoustic transducer, taken along the section line  14 — 14  in FIG. 15; 
     FIG. 15 is a sectioned rear view of the electro-acoustic transducer of FIG. 14, taken along the section line  15 — 15  in FIG. 14; and 
     FIG. 16 is similar to FIG. 11, but showing the transducer adjacent a wall. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1 and 2, the first embodiment of electro-acoustic transducer in the form of a loudspeaker comprises a rectangular frame  10  which is fixed to a rectangular back panel  12  and which are designed to be hung on a wall. The front edge of the frame  10  has inwardly facing lips  14 , and battens  16  are secured around the inside of the frame  10  so that channels  17  are formed between the lips  14  and the battens  16 . The transducer also includes a rectangular vibratable panel  18 , the outside dimensions of which are slightly smaller than the inside dimensions of the rectangular frame  10 . A seal  20  is provided around the edge of the vibratable panel  18 . The seal  20  is formed from a length of silicone rubber tubing which has been cut along its length, opened out and clamped around the edges of the panel  18 . The seal  20  is engaged in the channels  17  between the lips  14  and the battens  16 , and thus the panel  18  is held in place in the frame  10 , with the seal  20  isolating the frame  10  acoustically from the edges of the panel  18  and sealing the frame  10  to the edges of panel  18 . The seal  20  permits very slight movement of the edges of the panel  18  towards and away from the back panel  12 , and also permits the edges of the panel  18  to twist slightly in the channels  17  to accommodate multi-modal bending vibration of the panel  18 . 
     An array of six piezoelectric actuators  22  are secured by adhesive  23  to the rear face of the vibratable panel  18 . The actuators  22  are connected together in parallel by wires to a source (not shown) of a high voltage audio driving signal. In response to the driving signal, the piezoelectric material of the actuators  22  bends at the frequency of the driving signal, thereby causing the panel  18  to vibrate. The panel  18  vibrates predominantly in a non-pistonic, multi-modal manner by bending, rather than by bodily translation. This bending is facilitated by the seal  20  between the frame  10  and the edges of the panel  18 . Furthermore, the seal  20  damps out acoustic reflections which may occur at the boundary between the panel  18  and the frame  10  and thus acts as a semi-anechoic termination. Such acoustic reflections would otherwise cause interference with the vibrations in the panel  18  and thus affect the performance of the transducer. 
     A cavity  24  is formed between the vibratable panel  18  and the back panel  12 , and the cavity  24  may be filled with acoustic damping material to damp out acoustic vibrations generated rearwardly from the rear face of the vibratable panel  18 . Also, the back panel  12  itself may be made from acoustic damping material. 
     The frame  10  should be as rigid as possible, and may be made of, for example, wood, metal or plastics material. As mentioned above, the panel  18  must be able to vibrate, and it may be made of any suitable rigid, but resilient, material, such as plastics, wood, card, cardboard, or a composite material consisting of two lightweight skins of high stiffness (Young&#39;s modulus) separated and connected by a lightweight core of either an open or closed cell. The panel  18  and frame  10  may be painted, can have a picture applied thereto, or can be suitably decorated in some other manner in order to provide an unobtrusive, aesthetically pleasing and decorative panel. 
     The frequency response of the transducer is dependent, amongst other things, upon the size, shape, density and stiffness of the vibratable panel  18 , the sizes, shapes, positions and number of the piezoelectric actuators  22 , the bonding of each of the piezoelectric actuators  22  to the panel  18  by the adhesive  23 , the compliance of the seal  20 , and the damping provided by the cavity  24  and back panel  12 . Accordingly, the frequency response of the transducer can be adjusted by changing these parameters. 
     FIG. 3 illustrates the transfer function of an example of the transducer of FIGS. 1 and 2. As can be seen, the frequency response of the transducer is reasonably flat, which is a desirable feature for loudspeaker applications for the transducer. By comparison, FIG. 4 illustrates the transfer function for an example of the transducer which was similarly constructed, except that the panel  18  was secured to the frame  10  without the use of a seal  20 . As can be seen, the frequency response of this latter transducer is not so flat and has a poorer performance at low frequencies, particularly below 1 kHz. 
     It has been found that an additional unexpected advantage of the strip of flexible resilient material is that unwanted sibilance is removed and the quality of sound radiated from the transducer is improved. 
     FIG. 5 shows another embodiment which is similar to the embodiment of FIGS. 1 to  2 , except that first and second vibratable panels  18 A,B are provided, and the fixed back panel  12  is omitted. It should be understood that such a fixed back panel  12  may be added to damp out acoustic vibrations generated rearwardly from the combination of the vibratable panels  18 A,B, as described above with reference to FIGS. 1 and 2. The inner face of the first vibratable panel  18 A is secured by adhesive  23  to a first face of each of the piezoelectric actuators  22 , and the inner face of the second vibratable panel  18 B is secured by adhesive  23  to the other face of each of the piezoelectric actuators  22 . Accordingly, the piezoelectric actuators  22  directly drive the two vibratable panels  18 A,B in phase. Each of the vibratable panels  18 A,B has a respective seal  20 A,B provided around its edges, and both of the seals  20 A,B are engaged in a common channel  17  provided in the frame  10 . 
     The frequency responses of the two panels  18 A,B may be made to differ so as to achieve a flatter frequency response for the transducer as a whole. For example, peaks in the frequency response of one of the panels  18 A,B can be arranged to coincide with troughs in the frequency response on the other panel, thereby providing a flatter frequency response for the transducer as a whole. This may be done by constructing the two panels  18 A,B from different materials having different stiffnesses and/or densities, by using panels  18 A,B having different thicknesses and/or face areas, by using seals  20 A,B having different stiffnesses and/or by using different adhesives to bond the piezoelectric actuators  22  to the two panels  18 A,B. 
     FIG. 6 shows a further embodiment which is similar to the embodiment of FIG. 5, except that five of the vibratable panels  18 A-E are provided parallel to each other. Each panel  18 A-E has a respective seal  20 A-E, and the seals  20 A-E are engaged in a common channel  17  in the frame  10 . Adjacent pairs of the panels  18 A-E are secured by adhesive  23  to the opposite faces of each of six piezoelectric actuators  22  therebetween. 
     In addition to the steps described above for affecting the overall frequency response of the transducer, with the embodiment of FIG. 6, the overall frequency response may also be affected by using piezoelectric actuators  22  between some of the adjacent pairs of vibratable panels  18 A-E which have a different thickness to that of the piezoelectric actuators  22  between others of the adjacent pairs of vibratable panels  18 A-E, and/or by employing different numbers of the piezoelectric actuators  22  between different adjacent pairs of the panels  18 A-E. 
     FIG. 7 shows another embodiment which is similar to the embodiment of FIG. 5, except that only the first vibratable panel  18 A is directly driven by the piezoelectric actuators  22 . The second vibratable panel  18 B is acoustically coupled to the first vibratable panel  18 A by one or more acoustic links  26  and is held spaced apart from the piezoelectric actuators  22 . Accordingly, the second vibratable panel  18 B is indirectly driven by the piezoelectric actuators  22  via the first vibratable panel  18 A and the acoustic link(s)  26 . 
     In addition to the features described above which affect the overall frequency response of the transducer, with the embodiment of FIG. 7, the overall frequency response can also be affected by the degree of acoustic coupling provided by the or each acoustic link  26 , and the number and positions of the acoustic links  26 . 
     FIG. 8 shows a yet further embodiment which is similar to the embodiment of FIG. 7 except that the second vibratable panel  18 B is formed with holes  28  in which the piezoelectric actuators  22  are received without contact. The acoustic link(s)  26  can therefore be made thinner than in the embodiment of FIG. 7, and may be provided by blobs of adhesive. Accordingly, the embodiment of FIG. 8 can be manufactured with a slimmer profile than the embodiment of FIG.  7 . It will also be noted that in the embodiment of FIG. 8, a single seal  20  may be employed which embraces both vibratable panels  18 A,B. 
     FIG. 9 shows yet another embodiment which is similar to the embodiment of FIG. 8, except that the second vibratable panel  18 B is provided by a thinner membrane which conforms to the rear surface of the first panel  18 A and the piezoelectric actuators  22  which are fixed thereto. The membrane  18 B is bonded to the first panel  18 A at regions  30  intermediate the piezoelectric actuators  22 . Alternatively or additionally, bonded regions  30  may be provided on the piezoelectric actuators  22 . The bonded regions  30  may be at odd spots over the panel structure. The membrane  18 B may be of an accoustically lossy material, such as felt. A single seal  20  is shown in FIG. 9, which embraces the edges of the first panel  18 A and the membrane  18 B. Alternatively, the edge of the membrane  18 B may be arranged to stop short of the edge of the first panel  18 A, with the seal then embracing only the edge of the first panel  18 A. 
     FIG. 10 shows another embodiment which is similar to the embodiment of FIG. 5, except that the first and second panels  18 A,B are spaced wider apart, and each of the piezoelectric actuators  22  of FIG. 5 is replaced by a piezoelectric stack  32 . Each of the stacks  32  has a plurality of parallel layers  34  of piezoelectric material which are bonded together so as to bend in response to an applied electrical signal, and this can provide an enhanced driving force to the vibratable panels  18 A,B. 
     FIG. 11 shows a further embodiment which is similar to the embodiment of FIG. 10, except that the seals  20 A,B of the vibratable panels  18 A,B are engaged in respective channels  17 A,B in the frame  10 , and each of the vibratable panels  18 A,B is provided with its own piezoelectric actuators  22 A,B, rather than sharing the piezoelectric stacks  32  of FIG. 10 with each other. 
     In addition to the steps described above which affect the overall frequency response of the transducer, with the embodiment of FIG. 11, the overall frequency response can also be affected by using piezoelectric actuators  22 A, 22 B for the two vibratable panels  18 A,B which differ, for example with regard to number, shape, size and position. 
     FIG. 12 shows another embodiment which is similar to the embodiment of FIG. 5, except that (a) a frame  10  and seals  20 A,B are not provided, and instead the vibratable panels  18 A,B are joined at their edges by a peripheral sealing member  36  between the vibratable panels  18 A,B, and (b) the piezoelectric actuators  22  are not bonded by adhesive  23  directly to the vibratable panel  18 A, but instead are acoustically coupled to the vibratable panel  18 A by respective intermediate layers  38 . The intermediate layers  38  have larger lateral dimensions than their respective piezoelectric actuators  22  and are of a material which has substantially the same stiffness as the piezoelectric actuators  22  and a greater stiffness than the panel  18 A. It has been found that these intermediate layers  38  can provide more effective acoustic coupling between the piezoelectric actuators  22  and the panel  18 A. 
     It will be appreciated that, in addition to the steps described above which can be taken to affect the overall frequency response of the transducer, with the embodiment of FIG. 12, the overall frequency response can also be affected by the choice of the size, thickness and stiffness of the intermediate layers  38 . 
     FIG. 13 shows yet another embodiment which is similar to the embodiment of FIG. 12, except that such intermediate layers  38 A,B are provided between the piezoelectric actuators  22  and both of the vibratable panels  18 A,B. With this embodiment, the transfer function of the transducer can be improved, and yet the overall frequency response can be flattened by employing a variety of intermediate layers  38  having different characteristics. 
     FIGS. 14 and 15 show a further embodiment which is similar to the embodiment of FIGS. 1 and 2, except that three such vibratable panels  18 A,B,F of decreasing size are provided, arranged side by side. The frame  10  has a first horizontal dividing member  10 A below which the larger vibratable panel  18 A is located and above which the medium-sized and smaller panels  18 B,F are located. The frame  10  also has a second vertical dividing member  10 B between the medium-sized panel  18 B and the smaller panel  18 F. The frame  10 , 10 A,B provides channels  17 A,B,F which receive seals  20 A,B,F around the edges of the three panels  18 A,B,F. Various shapes and sizes of piezoelectric actuators  22 A,B,F are bonded by adhesive  23  to the three vibratable panels  18 A,B,F, and the piezoelectric actuators  22 A,B,F are connected together by wires  24  in parallel so that the three panels  18 A,B,F vibrate in-phase. As may be appreciated, the three panels  18 A,B,F will provide their highest responses in the lower, mid and upper portions, respectively, of the audio spectrum. 
     In the embodiments described above with respect to FIGS. 5 to  15 , identical in-phase signals are applied to the piezoelectric actuators, and the vibratable panels  18  are arranged to vibrate in phase with each other. Other arrangements may be employed. For example, the embodiment of FIGS. 14 and 15 may be modified to include a conventional passive 3-way crossover circuit having a common input and a low-range output connected to the piezoelectric actuators  22 A of the larger vibratable panel  18 A, a mid-range output connected to the piezoelectric actuators  22 B of the medium-sized panel  18 B and a high-range output connected to the piezoelectric actuators  22 F of the smaller vibratable panel  18 F. Other, more elaborate, circuits may also be used to alter the phases, amplitude and/or frequencies of the signals applied to the actuators on different vibratable panels, and indeed on the same vibratable panel, so as to achieve a desired frequency response for the transducer as a whole in the listening space in which it is situated. 
     Also, some of the embodiments described above may be modified so that pairs of the vibratable panels vibrate in anti-phase with respect to each other. This may be desirable when the electroacoustic transducer is situated near to an acoustically reflective surface such as a wall. Sound generated by the vibratable panel which is facing towards the wall will be reflected off the wall and will interfere, constructively and/or destructively, with the sound generated by the vibratable panel which is facing away from the wall. In some cases, a simple anti-phase relationship between the vibrations of the forwardly and rearwardly facing vibratable panels will produce good results. In other cases, the phase-frequency relationship between the vibrations of the forwardly and rearwardly facing panels may be tailored by more complex circuitry in order to achieve better results. 
     In one embodiment which achieves a simple anti-phase relationship between the vibrations of the forwardly and rearwardly facing panels, the embodiment described above with reference to FIG. 10 is modified so that the stacks  32  of layers  34  of piezoelectric material expand and contract in the direction between the vibratable panels  18 A,B in response to the applied electrical signal, rather than bending. Accordingly, the panels  18 A,B will vibrate in anti-phase. 
     In another embodiment which achieves the simple anti-phase relationship, the embodiment of FIG. 11 is modified by reversing the electrical connections to each of the piezoelectric actuators  22 B attached to the rearwardly facing vibratable panel  18 B. Accordingly, referring to FIG. 16, when the forwardly facing vibratable panel  18 A responds to a fundamental signal to bend to the left, as shown by the arrows  40 , the rearwardly facing vibratable panel  18 B will respond to the same signal by bending to the right, as shown by the arrows  42 . The rearwardly directed sound will be reflected by the acoustically reflective wall  44  to produce sound as indicated by the arrows  46  which will, when the transducer is situated close to the wall  44 , constructively reinforce the sound generated by the forwardly directed vibratable panel  18 A over most of the audio spectrum. 
     It should be noted that the embodiments of the invention have been described above purely by way of example and that many modifications and developments may be made to them. 
     For example, the intermediate layers  38  described with reference to FIGS. 12 and 13 may be used with any of the other embodiments of the invention. Also, the seals  20 , 20 A,B described above with reference to FIGS. 1 to  11  and  14  to  16  may be employed in the embodiments of FIGS. 12 and 13, and the sealing members  36  described above with reference to FIGS. 12 and 13 may be used with the other embodiments.