Abstract:
An inventive loudspeaker includes a diaphragm, a first excitation means for generating structure-borne sound in the diaphragm, and a second excitation means, different from the first one, for setting the diaphragm into a longitudinal vibrational motion in a direction perpendicular to the extension of the diaphragm. In accordance with the invention, the problem of insufficient bass reproduction and/or of the magnitude conflicting with invisible integration or installation is solved in that a second exciter system is introduced, which uniformly moves the diaphragm, or the plate serving as the diaphragm, forward and backward in addition to the bending waves of the structure-borne sound. The sound reproduction therefore is possible across the entire audio-frequency range without impeding the goal of invisible integration or installation.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of copending International Application No. PCT/EP03/09036, filed Aug. 14, 2003, which designated the United States and Japan, and was not published in English and is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to loudspeakers, and in particular to flat-panel loudspeakers or flat-panel sound transducers. 
     2. Description of Prior Art 
     The tendency which is evident in home entertainment products towards ever smaller and ever more compact components also applies to loudspeaker technology. The trend even goes as far as suggesting that loudspeakers should not only be small, but also “invisible” to the listener, i.e. hidden from the listener&#39;s eyes. The possibility of invisible installation is very useful particularly for multi-channel playback, such as surround, and for wave-field synthesis (WFS). The number of individual channels and thus loudspeakers required herefore rapidly amounts to more than 50 items. However, since such playback systems are also to be developed and offered for home use, and since it must be assumed that the customer, for space reasons, does not wish to fit 50 conventional loudspeakers into his/her living room for, e.g., a WFS system, alternative loudspeakers will have to be employed. 
     The aim is to design loudspeakers such that they may be integrated with other pieces of equipment or furniture, so that in this manner, they may be distributed across the rooms in an inconspicuous manner. For example, there have already been loudspeakers that act as picture frames, as monitors or even as doors of wardrobes at the same time. 
     Cone loudspeakers are not suitable for technical implementation of these “hidden” loudspeakers, since cone loudspeakers are not flat enough due to their diaphragm shape. A loudspeaker whose diaphragm is flat as a plate to start with and whose electroacoustic excitation system is as small as possible in terms of dimensions is more suitable. This principle, i.e. the use of a plate as a diaphragm in connection with the use of an excitation system, has already been employed in DE 465189, published in 1929, and its supplements DE 484409 and 484872 for acoustic shop-window advertising. Then, a window pane of a shop window served as a diaphragm which was excited by means of an attached electrodynamic excitation system so as to reproduce sound. 
     The functional mechanism underlying this principle is that an electrical signal applied to the electrodynamic excitation system is transformed to a mechanical audio-frequency vibration. At an excitation point, where the excitation system is present at or fixed to the diaphragm, this mechanical vibration is transferred to the plate serving as the diaphragm, whereby structure-borne sound is produced in the plate. It is in particular that portion of structure-borne sound which propagates in the diaphragm by means of bending waves that provides for the generation of air-borne sound. 
     With this loudspeaker principle, the generation of air-borne sound consequently is effected via the indirect way of structure-borne sound. Unlike with cone loudspeakers, the longitudinal mechanical vibrational motions of the vibrational pulses of the excitation system are not taken over by the diaphragm and immediately translated into air-borne sound, but structure-borne sound is initially created in the diaphragm, which—in particular, the ending-wave portion of same—subsequently excites the surrounding air to form longitudinal waves, or compressional waves, i.e. sound. The transformation of structure-borne sound to air-borne sound here acts like a filter in the chain of signals. As a result, only that portion of the signal to be reproduced which may propagate as structure-borne sound in the plate and may subsequently be radiated off into space is reproduced as air-borne sound. 
     Since, as has already been mentioned, that portion of structure-borne sound that propagates in the form of the bending wave makes the largest contribution to generating air-borne sound by means of a plate diaphragm, the properties of the bending wave, in particular its excitation and propagation, have a decisive impact on the design of a flat-panel loudspeaker in accordance with the bending-wave principle. If these properties are taken into consideration, this results in the fact that for broad-band sound reproduction, low-weight and large-size diaphragm plates are required. The plate size required, however, conflicts with the aim of invisible integration of the loudspeaker into the surroundings of the listener. As an example, the reproduction of the frequency range below about 200 Hz is of poor quality with relatively large plates. The reason for this is that a plate resonates only in its eigenmodes with its associated natural frequencies, and that the mode densities, i.e. the number of modes per frequency range, is decisive for sound reproduction. However, sufficient mode density has not been achieved so far below 200 Hz. 
     Thus, there is a need for a loudspeaker which is amenable, on the one hand, to invisible integration, i.e. which may be implemented to be flat and small, and which, on the other hand, enables satisfactory sound reproduction not only in the medium- and high-tone ranges, but also in the low-tone, or bass, range. 
     DE 19541197 A1 describes a cone loudspeaker having an electrodynamic vibration system, a cone-shaped diaphragm, a surround and a basket where the diaphragm is suspended above the surround. When a sound signal is applied to the vibration system, the diaphragm performs an upward movement along the center line. The diaphragm is provided with a layer of a piezoelectrical material which is also connected to the sound-signal source and experiences changes of extension in the process. Depending on whether the layer is connected to a further layer or is a bimorphous arrangement of two longitudinally and/or radially vibrating plates which are oppositely poled and glued to one another, the layer acts as a thickness vibrator or as a bending vibrator. 
     DE 19960082 A1 describes a loudspeaker having a plate diaphragm driven by a vibration drive at its back. During the vibration the plate diaphragm performs an upward movement. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to provide a loudspeaker which, at a fixed size, enables improved reproduction quality, or which enables, at a fixed reproduction quality, a more compact structure. 
     The invention provides a loudspeaker having a diaphragm; a first exciter for generating structure-borne sound in the diaphragm; and a second exciter, different from the first one, for setting the diaphragm into a longitudinal vibrational motion in a direction perpendicular to the extension of the diaphragm, the second exciter having an electrodynamic drive which comprises a first part including an oscillator coil and a second part including a magnet, one of the first and second parts being attached in a stationary manner, whereas the other part is attached to the diaphragm or contacts same. 
     An inventive loudspeaker includes a diaphragm, a first excitation means for exciting structure-borne sound in the diaphragm, and a second excitation means different from the first one for setting the diaphragm into a longitudinal vibrational motion in a direction perpendicular to the diaphragm extension. 
     In accordance with the invention, the problem that this insufficient low-tone reproduction, on the one hand, and the size which conflicts with invisible integration, or installation, on the other hand, is solved by introducing a second excitation system which uniformly moves the diaphragm, or the plate serving as the diaphragm, forwards and backwards in addition to the bending vibrations of the structure-borne sound. Thereby, sound reproduction is possible across the entire audio-frequency range without impeding the aim of invisible integration, or installation. 
     In other words, the core concept of the present invention is that broad-band reproduction may be achieved by means of a compact loudspeaker consisting of a diaphragm and an associated excitation means by using two different excitation means for exciting the diaphragm, which set the diaphragm into vibration in different manners, and are responsible for different frequency bands, or frequency ranges. One prior-art excitation means for generating structure-borne sound in the diaphragm is only responsible, according to the invention, for reproducing the high- and medium-tone range, and its task is only to excite as many bending waves in the diaphragm as possible. The low-tone range, which has been missing so far, is taken over by the excitation means added in accordance with the invention which excites the diaphragm to perform longitudinal forward and backward vibrating movements with a large stroke. In opposition to the sound generation performed by the structure-borne sound excitation means, the diaphragm is excited to perform longitudinal vibrations by the second excitation means introduced in accordance with the invention, whereby the diaphragm thus vibrates within itself in the form of bending waves and additionally moves forwards and backwards as a whole in a uniform manner. 
     The deflection of the second excitation means may be far larger than that of the bending waves of the structure-borne sound generation means. Since the diaphragm has a relatively large fictitious diaphragm surface, a large volume of air is moved by the uniform forward and backward motion of the plate. In this manner, the generation of a sufficient sound level in the bass area is clearly easier to implement than with the bending-wave principle, wherein the diaphragm deflections may also be smaller. 
     An advantage of the present invention, in turn, is that combining both excitation types, i.e. the generation of structure-borne sound and longitudinal vibrational forward and backward motion, on a diaphragm, enables a clearly better reproduction of the entire audio frequency range. 
     Since the excitation means, added in accordance with the invention, for setting the diaphragm into a vibrational forward and backward motion enables a larger diaphragm stroke in the bass range, the diaphragm surface may be reduced, while maintaining the reproduction quality. In contrast thereto, flat-panel speakers based only on production of structure-borne sound, require a very large diaphragm surface area to generate sufficient sound level in the bass area, since the small diaphragm stroke of the bending waves must be offset by as large a diaphragm surface area as possible so as to achieve the same volume displacement, which is why conventional flat-panel loudspeakers need to be relatively large. Consequently, an advantage of the present invention is also that due to its compactness, an inventive loudspeaker is more suitable for invisible integration or installation. 
     Conversely, an advantage of the present invention is that due to the combination of the two excitation means, the bass reproduction is clearly improved while the diaphragm size remains the same. The advantage of invisible integration or installation is not cancelled by this, but is supplemented by improved reproduction quality. 
     A further advantage of the present invention is that due to the fact that the longitudinal vibrational motion moves a large volume of air, the bass-reflex principle may be effectively employed, which has not led to any improvement in bass-range reproduction with previous flat-panel loudspeakers. 
     A further advantage of the present invention is that—since reproduction in the bass range is taken over by the generation of vibrational forward and backward motions of the diaphragm—the structure-borne sound generation means may also function in accordance with the piezoelectrical principle, which so far has only been possible, at the expense of bandwidth, when using only structure-borne sound generation due to the very narrow frequency range for which the piezoelectrical principle is suited. By the combination with the additional excitation system for a longitudinal vibrational motion of the diaphragm, a marked improvement in sound reproduction is achieved as a result, so that the structure-born sound generation means may function in accordance with the piezoelectrical principle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further preferred embodiments of the present invention will be explained below in more detail with reference to the accompanying figures, wherein: 
         FIG. 1   a  shows a diagrammatic partial-section side view of a flat-panel loudspeaker in accordance with an embodiment of the present invention, wherein only the plate serving as a diaphragm is shown along with the structure-borne sound generation means without the longitudinal vibration excitation means, the vibration behavior of the diaphragm, i.e. the bending waves generated by the structure-borne sound generation means, being indicated; 
         FIG. 1   b  is a diagrammatic partial-section side view of the loudspeaker of  FIG. 1   a , wherein only the plate serving as the diaphragm and the longitudinal vibration excitation means are shown rather than the structure-borne sound generation means, the vibration behavior, i.e. the forward and backward vibrational motion, of the plate due to the longitudinal vibration excitation means being indicated as well; 
         FIG. 1   c  is a diagrammatic front view of the loudspeaker of  FIGS. 1   a  and  1   b;    
         FIG. 1   d  is a diagrammatic partial-section plan view of a loudspeaker wherein the longitudinal vibration excitation means of  FIG. 1   b  and the structure-borne sound generation means of  FIG. 1   a  are combined into a loudspeaker; 
         FIGS. 2   a  and  2   b  depict diagrammatic front and partial-section plan views of a loudspeaker in accordance with a further embodiment of the present invention; 
         FIG. 3  is a diagrammatic partial-section plan view of a loudspeaker in accordance with a further embodiment of the present invention; 
         FIG. 4  is a diagrammatic partial-section plan view of a loudspeaker in accordance with a further embodiment of the present invention; 
         FIG. 5  is a diagrammatic partial-section plan view in accordance with a further embodiment of the present invention; and 
         FIG. 6  is a partial-section plan view of a loudspeaker in accordance with a further embodiment of the present invention, wherein only the structure-borne sound generation means is shown rather than the longitudinal vibration excitation means. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Before the present invention will be explained in more detail below with reference to the figures, it shall be pointed out that elements which are identical or identical in their functions are designated by the same or similar reference numerals in the drawings, and that a renewed explanation of these elements is omitted in order to avoid repetitions in the specification. 
     With regard to  FIGS. 1   a  to  1   d , the general principle of the present invention will initially be explained in more detail for a loudspeaker using an embodiment. The loudspeaker, generally indicated by  10 , essentially consists of a plate  12  serving as a diaphragm, a structure-borne sound generation means  14 , a longitudinal vibration excitation means  16 , and an excitation signal generation means  18 . 
     The structure-borne sound generation means  14  operates in accordance with the electrodynamic principle and is shown in more detail, in cross section, in  FIG. 1   a . The structure-borne sound generation means  14  includes an annular permanent magnet  20  polarized along its rotation axis, a cylindrical pole body  22  which is arranged in a centered or coaxial manner with regard the annular permanent magnet  20 , and an oscillator coil  24  extending in an annular gap of air between the pole body  22  and the permanent magnet  20 . In addition, the structure-borne sound generation means  14  which is formed as an electrodynamic drive may exhibit, for example, plate- or ring-shaped pole plates. Evidently, a different structure of the electromotive drive is also possible. That part of the structure-borne sound generation means  14  which consists of the oscillator coil  24 , on the one hand, and that part of the structure-borne sound generation means  14  which consists of the pole body  22  and the permanent magnet  20 , on the other hand, are slidable with respect to one another. The structure-borne sound generation means  14  thus formed is fixed in a centered manner at the plate  12  via the part containing the vibrating coil  22 . As will be described below, the reverse case is also feasible. Apart from that, the structure-borne sound generation means is not fixed, or is non-attached, i.e. the other part which consists of components  20  and  22  is freely moveable. 
     In the present document, diaphragm  12  has been described, in an exemplary manner, as an upright diaphragm  12  which has a coil  24  attached to it which is immersed into an annular gap of their between a cylindrical pole body  22  and an annular permanent magnet  20 , pole body  22  and permanent magnet  20  forming a unit which is guided across oscillator coil  24  so as to be slidable, relative to same, in the direction perpendicular to the direction of extension of diaphragm  12 . The upright diaphragm is, for example, part of a wall. In this perpendicular alignment, no force which points in the direction of the normal to surface of diaphragm  12 , i.e. points in that direction in which this part may be shifted relative to the oscillator coil  24 , but only the force of gravity pointing downwards is exerted onto the non-attached parts  20 ,  22  of drive  14 . Without the excitation signal being applied, there is consequently no reason for parts  20 ,  22  to be dispensed with. In addition, this part naturally exhibits a certain amount of inertia, so that the excitation means  14 , which, as is known, is provided for generating structure-borne sound in the diaphragm  12 , i.e. mechanical waves in the grid of diaphragm  12  which propagate within same, is excited at high frequency, and so that, at a sufficient amount of inertia and/or sufficient weight of the free movable parts  20 ,  22  of the drive compared with the inertia and/or the weight of diaphragm  12 , this part will substantially not leave its position but will rather move the oscillator coil  24  forwards and backwards along with the diaphragm  12  within the gap of air, and will continue to prevent the freely movable part  20 ,  22  from being pulled down by gravity. Factors such as the elasticity of the diaphragm material play a part in how much the diaphragm  12  and, therefore, the oscillator coil  24 , is deflected, so that the oscillator coil  24  can be prevented, with appropriate care being taken, from sliding out of the gap of air of the excitation means  14 . In addition, the stroke caused by the longitudinal vibration excitation means  16  must also be taken into account to prevent the coil from being pulled out of the gap, which stops, as it were, due to the inertia of the free moveable part. This may be effected, for example, by a corresponding length of overlap of coil  24  and the air gap. In addition, an elastic connection may be provided between the two parts of drive  14  which are slidably displaceable against one another, so that the freely moving part is moved, when vibrations are present, along with the diaphragm and the part fixed to same, and additionally produces structure-borne sound in the diaphragm due to higher-frequency motions relative to the fixed part. 
     Evidently, a loudspeaker of the type shown may also be fixed in a different position, e.g. at the ceiling. In this case, however, additional provisions would have to be made for the moveable parts of drive  14  to be coupled to one another, such as via an elastic connection in addition to the mechanical air-gap oscillator-coil guide, so that the two moveable parts of drive  14  by themselves form a vibrating system, and so that the freely moveable part of drive  14  is prevented from sliding down and out of the guide by coil  24 . 
     In accordance with the electrodynamic principle, the electrodynamic drive  14  transforms an electrical excitation signal flowing through oscillator coil  24  to a mechanical relative vibrational motion between the two parts, i.e. the part fixed to plate  12  and the freely movable part. The freely moveable part advantageously exhibits sufficient inertia to effectively transmit the mechanical relative vibrational motion to plate  12 , whereby structure-borne sound and, in particular, bending waves are produced in plate  12 , as is shown in an exaggerated form in  FIG. 1   a . The oscillator coil  24  receives the excitation signal flowing through oscillator coil  24  from the excitation signal generation means  18 , which, in turn, generates same from an electrical sound signal which suitably indicates the information to be rendered. 
     The longitudinal vibration excitation means  16 , too, functions in accordance with the electrodynamic principle and is depicted in cross section in  FIG. 1   b . The longitudinal vibration excitation means  16  is arranged coaxially in relation to structure-borne sound generation means  14 . The electrodynamic drive of longitudinal vibration excitation means  16  also includes a permanent magnet  30 , a pole body  32  and an oscillator coil  34 . Oscillator coil  34  also obtains its electrical excitation signal from excitation signal generation means  18 , which generates said electrical excitation signal from the same sound signal indicating the information to be rendered. The part including the oscillator coil  34  contacts plate  12 —or is connected to it—via an adapter  36 . In other words, oscillator coil  34  is fixedly connected to adapter  36 , which extends from oscillator coil  34  in the direction of plate  12  and expands radially in the process so as to come to lie, in the idle state of loudspeaker  10 , on plate  12  along an annular excitation area of a certain diameter, or to be fixed, such as glued, to plate  12  so as to surround structure-borne sound generation means  14  together with plate  12 . In particular, adapter  36  consists of a cylinder barrel  38  of a diameter exceeding one tenth of the extension of plate  12  at the narrowest point, and of ridges  40  extending radially and connecting cylinder barrel  38  with oscillator coil  34 , such that cylinder barrel  38  is aligned coaxially to an excitation point, at which the mechanical vibration of structure-borne sound generation means  14  is exerted onto plate  12 . 
     Adapter  36  does not have to exhibit, as is shown in  FIGS. 1   a  to  1   d , an annular cross section, or an circular excitation area and be formed as a ring adapter, but may also be rectangular, for example. The extension of the excitation area amounts to, e.g., between one tenth and nine tenths of the extension of plate  12  in the respective extension direction of plate  12 . Adapter  36  enables the mechanical vibration of drive  16  to lead to a longitudinal vibrational motion of plate  12  in an almost overall, i.e. translatory, manner, as will be explained below. Due to the coaxial or central symmetric structure, the influence exerted by the longitudinal vibration excitation means  16 , by means of the excitation area, or bearing surface area, on the bending waves generated by structure-borne sound generation means  14 , the bending waves propagating from the coaxial excitation point of structure-borne sound generation means  14  in a nearly isotropic manner, is reduced. 
     Supports may be arranged along the bearing surface of adapter  36  which project from adapter  36  in the direction of plate  12 , so that adapter  36  bears on plate  12 , or is attached to same, only at isolated points of support, i.e. the ends of the supports. Hereby, the influence of adapter  36  and/or of longitudinal vibration excitation means  16  on the structure-borne sound produced may be further reduced without significantly compromising the uniformity of the drive of longitudinal vibration excitation means  16 . 
     While that part of the electrodynamic drive of longitudinal vibration excitation means  16  which consists of oscillator coil  34  is connected to plate  12  via adapter  36  or is coupled to plate  12  by bearing on same, the other part consisting of magnet  30  and pole body  32  is fixed in a stationary manner, such as attached to a backpanel of the loudspeaker (not shown). In this manner, the transmission of force of the mechanical vibration produced by longitudinal vibration excitation means  16  to plate  12  is more pronounced than with structure-borne sound generation means  14 . 
     Since the structure of the loudspeaker of  FIGS. 1   a  to  1   d  has been described above, its mode of operation will be described below. In order to transform the electrical sound signal indicating the information to be rendered to air-borne sound in the form of longitudinal waves and/or compressional waves, loudspeaker  10  includes both means  14  and  16 . Both means  14  and  16  are responsible for rendering the information to be rendered for different frequency ranges, or frequency bands. Structure-borne sound generation means  14  is responsible for reproducing the high- and medium-frequency ranges, whereas longitudinal vibration excitation means  16  is responsible for the bass range. Even though it is possible to feed the electrical sound signal to the electrodynamic drives of both means  14  and  16  and thus to feed both of them with the same excitation signal, which would render means  18  superfluous, as the case may be, it is preferred that they are fed with different excitation signals deviating from one another with regard to the frequency band and being adapted in an optimum manner to the respective area of operation of means  14  and  16 , respectively. Thus/for example, means  14  obtains a higher-frequency portion of the sound signal than means  16 . The frequency range of the excitation signal for structure-borne sound generation means  14  spans, e.g., 100 Hz to 25 kHz, and preferably 150 Hz to 20 kHz, whereas the frequency range of the excitation signal for longitudinal vibration excitation means  16  spans, e.g., 10 Hz to 2 kHz and, preferably, 20 Hz to 200 Hz. For this purpose, excitation signal generation means  18  may be implemented, e.g., as a frequency-separating means. Thus, it is generally advantageous for the frequency range to include, for generating structure-borne sound, a frequency which higher than all frequencies included in the frequency range for longitudinal vibration excitation, or the frequency ranges include a first frequency at which the excitation signal for generating structure-borne sound is higher than the other excitation signal, and a second frequency, which is lower than the first frequency, at which the excitation signal for longitudinal vibration excitation is the same as the other excitation signal or is higher than same. 
     The mechanical vibrational motions produced by the excitation signal flowing through oscillator coil  24  cause structure-borne sound and, in particular, bending waves in plate  12  which are, in turn, transformed to air-borne sound at the air/plate interface. To this end, structure-borne sound generation means  14  preferably exhibits a sufficient moment of inertia. 
     Longitudinal vibration excitation means  16  sets plate  12  into longitudinal vibrational motions  42  with a stroke which is significantly larger, e.g. more than 20 times larger can be, than the amplitude of structure-borne sound generation means  14 , such as 20 mm. This longitudinal forward and backward motion  42  performed by plate  12  immediately leads to longitudinal air-borne sound waves, or compressional waves  44 , in the bass range. So as to enable the large stroke of longitudinal vibration excitation means  16  without causing the oscillator coil  34  to no longer be able to be immersed into the field of the air gap in a perpendicular manner, and thus without causing distortions to be formed, because of the mass of the drive of longitudinal vibration excitation means  16 , longitudinal vibration excitation means  16  is fixed with that part of the drive which includes magnet  30  and pole body  32 , such as at a back-panel. Adapter  36  serves to transmit the mechanical vibrational motion of oscillator coil  34  in a manner distributed across plate  12  such that plate  12  is excited to perform essentially translatory vibrational motions in the direction perpendicular to an extension direction of plate  12 , i.e. such that the plate vibrates back and forth as a whole as much as possible. Thus, plate  12  vibrates in the form of bending waves, as is shown in  FIG. 1   a , and additionally vibrates forward and backward as a whole in a uniform manner as is shown by the double arrow  42  in  FIG. 1   b.    
     Even though it would be possible to support plate  12  only via a fixed connection via adapter  36  with that part of the drive of longitudinal vibration excitation means  16  which includes oscillator coil  34 , and to support the guide of this part in that part which includes permanent magnet  30  and pole body  32 , such as when mounting the loudspeaker at the ceiling such that it is suspended from same, it is preferred to additionally provide a bracket for plate  12 , as is the case in the following embodiments. Even though it is also possible to generate the translatory longitudinal vibrational motion  42  of plate  12  by means of the electrodynamic drive only, it is preferred for plate  12  to be suspended or journalled in an oscillatory manner such that, when plate  12  undergoes a longitudinal translation from an idle position of same in the direction perpendicular to the extension of the plate, a force caused by the suspension counteracts this translatory deflection to return the diaphragm to the idle position. In this manner, suspension and plate  12  form a vibrating system wherein plate  12  is capable of moving back and forth in a translatory manner in a direction perpendicular to the direction of extension. This vibrating system should be designed for a natural frequency near the bass range for which longitudinal vibration excitation means  16  is responsible, so as to be able to exploit the resonance step-up. 
     Several embodiments will be described below, by means of which various possibilities of suspending the plate serving as a diaphragm, of attaching the longitudinal vibration excitation means as well as of positioning same on the plate will be described. 
       FIGS. 2   a  and  2   b  show an embodiment of a loudspeaker, wherein the only differences compared with the embodiment of  FIGS. 1   a  to  1   d  are that the longitudinal vibration excitation means consists of four drives  16   a ,  16   b ,  16   c  and  16   d  which operate in an electrodynamic manner, and that plate  12  serving as the diaphragm is suspended from a frame  52  by means of a spider  50 , which frame  52 , in turn, is attached to a backpanel  54 , to which, in turn, that part of the drives  16   a - 16   d , operating in an electrodynamic manner, which includes permanent magnet  30  and core  32  is attached. 
     The spider  50  consists of elastic bands  56 , such as rubber bands, which are mounted along the circumference and which extend, in a manner in which they show the way to follow, from their mounting ends at the circumference of plate  12  in an essentially star-like manner from the center of plate  12  outwards so as to be attached at frame  52  at the other end. With regard to their attachment and spring constants, bands  56  are designed such that each part of the edge is influenced in the same manner. The fact that drives  16   a - 16   d  are attached to the backpanel, on the one hand, and that plate  12  is suspended by means of spider  50 , on the other hand, does away with the risk that due to the mass of drives  16   a - 16   d , the oscillator coils  34  of same are no longer able to be immersed perpendicularly into the field of the air gap, and that this may cause distortions. During assembly, plate  12  serving as a diaphragm, and drives  16   a  to  16   d  are preferably adjusted such that none influences the direction of motion of the other. In this manner, the mass of the diaphragm, or plate, and the mass of longitudinal vibration excitation means  16  have no influence on the direction of vibration of the excitation coils  34  of drives  16   a - 16   d . Spider  50  takes on the function of a surround which attenuates diaphragm, or plate,  12  after each deflection and takes it back to the starting position, or idle position. Backpanel  54  may serve as part of a loudspeaker housing. However, the provision of a loudspeaker housing is not necessary. Since drives  16   a - 16   d  are arranged in a centrally symmetric manner, the disturbance caused by them due to their contact, or connection, with plate  12  at the excitation points with regard to the bending waves generated by structure-borne sound generation means  14  are reduced. The excitation drives ( 16   a - 16   d ) are driven, in an in-phase manner, either by one and the same excitation signal or by such excitation signals which differ with regard to the amplitudes, so as to offset the fringe effects of diaphragm plate  12 . 
     With reference to  FIG. 3 , a description will be given of an embodiment of a loudspeaker which differs from the loudspeaker of  FIGS. 2   a - 2   b  by a different suspension, which, however, also enables plate  12 , serving as the diaphragm, to perform a translatory longitudinal vibrational backward and forward motion in about an idle position. In this embodiment, the diaphragm  12  is spring-mounted on one axle  60 , respectively, per corner of rectangular plate  12  serving as the diaphragm. Axles  60  are firmly attached to backpanel  54 , which also has drives  16   a - 16   d  mounted to it, axles  60  protruding perpendicularly from backpanel  54  which extends parallel to plate  12 , i.e. axles  60  extending in the direction of the translatory longitudinal vibrational motion caused by drives  16   a - 16   d . Mounting plate  12  at each corner is implemented, for example, by a respective hole at each corner, through which the respective axle  60  extends. Spring-mounting plate  12  at each corner on axles  60  is achieved, for example, by coil springs  62  which surround axles  60 , are guided by them and have ends attached to the respective corner of plate  12 , and have fixed ends connected, e.g., to backpanel  54 . Evidently, any other elastic means may be employed to define a minimum of potential for the respective corner. 
     Perpendicular immersion of the spring coils of drives  16   a - 16   d  is also ensured by the suspension of  FIG. 3 . In addition, the assembly preferably is implemented, again, such that diaphragm  12  and drives  16   a - 16   d  do not mutually influence their directions of motion. As is also the case in  FIGS. 2   a  and  2   b , backpanel  54  may serve as part of a loudspeaker housing. The mass of the diaphragm and the mass of longitudinal vibration excitation means  16  exert less influence on the direction of vibration of oscillator coils  34  of drives  16   a - 16   d , i.e. they are immersed into the respective air gap just like in the non-assembled state. The coils take on the function of the surround which attenuates diaphragm  12  after each deflection and returns it to the starting position. 
     As has already been described with reference to  FIGS. 1   a - 1   d , that part of the drives of the longitudinal vibration excitation means which includes the oscillator coil may either be firmly connected to plate  12  or may only bear on same. In both cases it is preferred that during the assembly of the loudspeakers of  FIGS. 2   a ,  2   b  and  3 , the distance between diaphragm plate  12  and drives  16   a - 16   d  in the idle position of diaphragm plate  12  is set such that they just about have contact, but do not exert any forces upon one another in the idle position. In order to make it easier for the diaphragm plate to follow the motions of drives  16   a - 16   d , that part of same which includes oscillator coil  22 , or  34 , is preferably glued, for example, with plate  12 . 
       FIG. 4  shows an embodiment of a loudspeaker wherein, unlike the loudspeaker of  FIG. 3 , the drives  16   a - 16   d , which constitute the longitudinal excitation means, are not attached to the diaphragm plate  12  via the part including the oscillator coil  34 , such as via an oscillator-coil support, but via that part of the electrodynamic excitation system which includes permanent magnet  30 . Oscillator coil  34 , however, is attached to loudspeaker backpanel  54  rather than to diaphragm plate  12 . The perpendicular immersion of oscillator coil  34  into the gap of air between permanent magnet  30  and pole body  32  continuous to be provided by the suspension, i.e. axles . 60  with springs  62 , and/or spider  50 . 
       FIG. 5  shows an embodiment of a loudspeaker, wherein, like in the previous embodiments, both excitation means  14  and  16  operate in accordance with the electrodynamic principle, the electrodynamic drive of longitudinal vibration excitation means  16  using the permanent magnet of structure-borne sound generation means  14  as the magnet. With regard to suspension and structure-borne sound generation means  14 , the embodiment of  FIG. 5  corresponds to that of  FIGS. 3 and 4 . Unlike the embodiments of  FIGS. 3 and 4 , longitudinal vibration excitation means, however, only includes an oscillator coil  70  which is arranged coaxially with oscillator coil  24  of the drive of structure-borne sound generation means  14  and is attached to backpanel  54 . Both oscillator coils  24  and  70  interact with the same permanent magnet  20 . In this design, a further pole body may additionally be provided around oscillator coil  70 . Thus, oscillator coil  70  forms a circle around structure-borne sound generation means  14 . As is also the case in the embodiments of  FIGS. 2   a ,  2   b  and  3 , that part of the drive of longitudinal vibration excitation means  16  which includes oscillator coil  70  is fixed, whereas the other part is attached to diaphragm plate  12 , i.e. in the present case, the other part being permanent magnet  20  of structure-borne sound generation means  14 . By contrast, the drive of structure-borne sound generation means  14  is attached only to plate  12 , i.e. with that part which includes oscillator coil  24 . 
       FIG. 6  shows an embodiment of a specific form of attachment of structure-borne sound generation means  14  to plate  12  serving as the diaphragm. Instead of attaching the oscillator coil to diaphragm plate  12  via an annular oscillator-coil support in an excitation region, as has been done in the previous examples, the embodiment of  FIG. 6  provides an oscillator-coil support  80  which supports oscillator coil  24  and exhibits, on that side facing diaphragm plate  12 , a cone-shaped part, the peak of the cone being connected to diaphragm  12 . Thereby, an optimum dot excitation of plate  12 , serving as the diaphragm, to form bending waves, and a higher top cut-off frequency of the structure-borne sound generation means are achieved. 
     Finally it shall be pointed out that it is possible to produce an inventive loudspeaker with a housing, wherein the plate serving as the diaphragm is suspended at the housing by means of air-tight suspension so as to seal the housing in an air-tight manner. To enable this, a special surround may be used, such as a continuous elastic band stretching from the circumference of plate  12  to the circumference of a respective recess of the loudspeaker box. For very heavy diaphragm plates, or combinations of diaphragm plate and glued-on excitation systems, the surround may also be supported, in addition, by the spring-axle suspension of  FIG. 3  or by the spider suspension of  FIGS. 2   a  and  2   b . Since sufficient air volume is moved by the longitudinal translatory motion of the entire diaphragm, the bass reflex principle may additionally be used here. For this purpose, a hole for the reflection channel is integrated into the housing, for example on the side. 
     Even though only one structure-borne sound generation means was provided in each of the above embodiments, it shall be pointed out that in addition, several such means may be employed. Here, distribution around the center of the diaphragm plate is preferred. However, both in the case of having only one structure-borne sound generation means as well as in the case of having several structure-borne sound generation means, a decentralized arrangement at a distance from the center is also possible. The arrangement should be selected such that the bending waves are excited in an optimum manner. 
     In addition, for setting the diaphragm plate into longitudinal backward and forward vibrational motions, provision may be made not only of one or four drives, but of any number desired. When using several such longitudinal oscillatory drives, they are advantageously arranged such that the diaphragm plate is driven in a manner which is uniform across the entire surface. With several drives, the adapter may be dispensed with, such as is also the case with the examples of  FIGS. 2-4 . If several such longitudinal oscillatory drives are to be arranged, they are preferably always arranged in a central symmetric manner relative to the diaphragm plate. The use of several longitudinal vibrational drives increases the potential sound level of the loudspeaker. 
     In addition, it shall be pointed out that the above variations of the embodiments of  FIGS. 1   a  to  6  may be combined with one another in any manner desired, both with regard to suspension, positions of the drives as well as mounting the parts of the drive which are movable relative to one another. 
     With regard to the above description of  FIGS. 2   a  to  5  it shall also be pointed out that instead of the elastic, or oscillatory, suspension of the diaphragm plate by means of the elastic means described above, i.e. elastic bands  56  and springs  62 , provision may also be made for elastic suspension or attachment of the drives of the longitudinal vibration excitation means, whereas the diaphragm plate is only guided by axles  60  or is free. 
     In addition, provision may also be made for other drives than those described above, drives which are based on a transducer principle different from the electrodynamic principle. In particular, the drive used for the generation of structure-borne sound could also be implemented as operating in accordance with the piezoelectrical principle, such as a piezocrystal which is connected to the diaphragm on the one side and to a weight on the other side, and which is freely movable apart from that. 
     Finally it shall also be pointed out that it is also possible for the structure-borne sound generation means to not be firmly connected to the diaphragm, but to be held such that it is suspended from above at a specific height by a suitable device, but otherwise to be held in a freely moveable manner in the longitudinal direction of vibration of the vertically aligned diaphragm so as to bear upon the diaphragm in the idle position. 
     While this invention has been described in terms of several preferred embodiments, there are alterations, permutations, and equivalents which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and compositions of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the present invention.