Abstract:
In order to substantially realistically pre-operatively demonstrate to patients having an impaired hearing the effect and sound impression of an least partially implantable hearing system including a first electronic audio signal processing unit, a demonstration device is provided which comprises an electromechanical transducer adapted for being non-invasively coupled from the side of the external auditory canal to at least approximately the center of the tympanic membrane and thus to the end point of the manubrium mallei for producing mechanical vibrations of the tympanic membrane, an electronic audio signal generator unit, and a second electronic audio signal processing unit connected between the audio signal generator unit and the electromechanical transducer for driving the electromechanical transducer, wherein the second audio signal processing unit corresponds to or simulates the first electronic audio signal processing unit. A further aspect of the invention is a process for preoperatively demonstrating the effect and sound impression of an at least partially implantable hearing system intended to be implanted.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention in general relates to a device and to a method for pre-operatively demonstrating at least partially implantable hearing systems for the rehabilitation of hearing disorders. More particularly, the present invention relates to a device for pre-operatively demonstrating an at least partially implantable hearing system for the rehabilitation of hearing disorders, which device includes an electromechanical transducer adapted for being non-invasively coupled from the side of the external auditory canal to at least approximately the center of the tympanic membrane and thus to the end point of the manubrium mallei for producing mechanical vibrations of the tympanic membrane, and an electronic audio signal generator unit. The present invention further is concerned with a method for pre-operatively demonstrating an at least partially implantable hearing system, which system includes an audio signal processing unit and an electromechanical transducer which is driven by the audio signal processing unit and is adapted for being coupled to a preselected coupling site, particularly to the ossicular chain, for causing mechanical vibrations of the coupling site. 
     2. Description of Related Art 
     In addition to rehabilitation of congenitally deaf persons and those who have lost their hearing using cochlear implants, for some time, there have been approaches to offer better rehabilitation than with conventional hearing aids to patients with a sensorineural hearing disorder which cannot be surgically corrected, by using partially or totally implantable hearing aids. In most embodiments the principle consists in stimulating, via a mechanical or hydromechanical stimulus, an ossicle of the middle ear or directly the inner ear, rather than via an amplified acoustic signal of a conventional hearing aid in which the amplified acoustic signal is supplied to the external auditory canal. The actuator stimulus of these electromechanical systems is accomplished by different physical transducer principles, such as, for example, by electromagnetic and piezoelectric systems. The advantage of these processes is seen mainly in the sound quality which is improved as compared to conventional hearing aids, and, in the case of totally implanted systems, in the fact that the hearing prosthesis is not visible. Such partially and fully implantable electromechanical hearing aids are described, for example, by Yanigahara et al. (Arch Otolaryngol Head Neck, Surg, Vol. 113, August 1987, pp. 869872); Hoke, M. (ed), (Advances in Audiology, Vol. 4, Karger Basel, 1988); H. P. Zenner et al. (HNO 1998, Vol. 46, pp. 844-852; H. Leysieffer et al. (“A totally implantable hearing device for the treatment of sensorineural hearing loss: TICA LZ 3001”, in HNO Vol. 46, 1998, pp. 853-863); and H. P. Zenner et al. (“Totally implantable hearing device for sensorineural hearing loss”, The Lancet, Vol. 352, November 1998, No. 9142, page 1751), as well as in numerous patent documents, among others in U.S. Pat. Nos. 5,360,388; 5,772,575; 5,814,095 and 5,984,859. 
     Recently, such partially and fully implantable electromechanical hearing aids for the rehabilitation of internal ear damages have been introduced into clinical use. In this connection it turned out to be desirable to demonstrate to the patient to be provided with the implant the improvement of hearing or the sound quality, respectively, to be expected. The known audiological methods which until now merely provide for a stimulation of the hearing by sound transmission through the air or through the human body, do not permit such a demonstration without surgical intervention. 
     There are approaches for testing the middle ear by direct contact with an electromechanical transducer. In conformity with Zoellner (A. Thullen, “Clinical experiences with the sound probe according to Zoellner”, Medizinal-Markt, Vol. 4, No. 12, December 1956, pages 444 and 445) a sound probe is contacted with the middle ear, particularly invasively during middle ear operations. A device for electromechanical testing of hearing (U.S. Pat. No. 5,833,626) and a device for positioning and fixing of therapeutic, surgical, or diagnostic instruments (U.S. Pat. No. 5,776,144) have been proposed for the pre-operative demonstration of implantable hearing systems and for the psychoacoustical measurement of the auditory threshold in quiet by direct mechanical stimulation of the umbo. Hofmann et al. (German Patent No. 198 21 602) propose a vibration measuring head for evaluation of the movability of the middle ear. The basic embodiment includes a transducer, particularly an electromagnetic transducer, which exclusively is operated in resonance, wherein the movability of vibratorily movable elements of the middle ear structure coupled to the actoric side of the transducer can be evaluated by means of a second measuring coil, because the dampening of the system by the middle ear structure coupled thereto is represented by a variation of the voltage generated by this coil. 
     In the meantime, the device suggested in U.S. Pat. Nos. 5,776,144 and 5,833,626 was used for clinical examination of test persons having normal hearing. The examination showed in a statistically significant manner that this method is well reproducible and valid, and can be applied without any risk for the safety of the test persons. 
     However, basically there is the problem, that when using the device and the method for patients with impaired hearing, there is an individually varying audition. The differences particularly reside in spectrally very different courses of the auditory threshold in quiet as well as possibly in a positive recruitment (increase of the steepness of the soundness perception) and a reduced frequency resolution power for above-threshold signals. The known devices and methods scarcely permit successes because an individual compensation of the respective hearing disorder, i.e. an adaptation of the electronic audio signal processing unit driving the electromechanical transducer in the sense of an adaptation of a hearing aid, can not be carried out. This necessarily results in the serious disadvantage of the proposed devices and methods that the pre-operative demonstration never provides the patient with the hearing impression he will encounter later on after implantation and individual adaptation of the implanted hearing system to his individual hearing impairment. 
     SUMMARY OF THE INVENTION 
     The primary object of the present invention is to devise a device and a method for pre-operatively demonstrating at least partially implantable hearing systems, which permit a non-invasive testing of the hearing capacity as it will be encountered after implantation and adaptation of an individual hearing system. 
     In accordance with one aspect of the invention this object is achieved by a demonstration device for pre-operatively demonstrating an at least partially implantable hearing system for the rehabilitation of hearing disorders, said hearing system including an electronic audio signal processing unit, said device comprising: 
     an electromechanical transducer adapted for being non-invasively coupled from the side of the external auditory canal to at least approximately the center of the tympanic membrane and thus to the end point of the manubrium mallei for producing mechanical vibrations of the tympanic membrane, 
     an electronic audio signal generator unit, and 
     an electronic audio signal processing unit connected between the audio signal generator unit and the electromechanical transducer for driving the electromechanical transducer, wherein the audio signal processing unit of the demonstration device corresponds to or simulates the electronic audio signal processing unit of the hearing system intended to be implanted. 
     By the demonstration device of the present invention the action and the sound impression to be expected upon implantation of the respective hearing system can be demonstrated in a very realistic manner to the patient having a hearing disorder. 
     Preferably, means are provided for adapting the audio signal processing unit of the demonstration device to the individual hearing disorder of the respective patient. 
     Furthermore, means for playing back a data carrier or a sound carrier are preferably associated to the audio signal generator unit. In this connection all types of signals may be utilized which usually are used for audiological purposes, such as pure sinusoidal sounds, narrow-band noise, wide-band noise, speech, music and so on. Also all known embodiments of data carriers and means for generating these test signals may be used, such as an analog and/or digital generation or synthesizing, an analog or digital storage in all known types of non-rewritable or rewritable analog and/or digital storage media, such as semiconductor storages, analog sound carriers (e.g. magnetic tape), audio CDs, CD-ROMs and so on. 
     In conformity with the invention means for storing a plurality of parameter sets for setting the audio signal processing unit of the demonstration device, and means for selecting and transmitting to the audio signal processing unit of the demonstration device any one of said plurality of parameter sets may be provided. In such an embodiment of the demonstration device of the invention different “standard” parameter sets for setting the audio signal processing unit of the demonstration device, in which sets the individual parameters are adapted to each other in an advantageous manner, may be determined and stored in advance. The operator of the demonstration device then can select any one or any combination of the stored parameter sets without an individual setting of individual parameters being required. Furthermore, no deepened knowledge of the effects of individual parameters or of the interaction of pluralities of parameters is necessary in order to attain more or less optimum parameter settings, so that the demonstration device then also can be properly operated by less trained personal. 
     The audio signal processing unit of the demonstration device preferably comprises a programmable processor unit, particularly a personal computer (PC) or a digital signal processor (DSP). The presently used term “personal computer” or “PC” is to be understood as also including notebooks, laptops and the like, as well as any other “external” computers, i.e. computers which are independent of the transducer driver. 
     The programmable processor unit may be configured for carrying out the functions of audio signal generator unit as well as of the audio signal processing unit of the demonstration device. 
     In conformity with a particularly preferred embodiment of the invention the audio signal processing unit of the demonstration device comprises electronic driver means for driving the electromechanical transducer, wherein a digital-to-analog converter may be connected between the programmable processor unit and the electronic driver means. Particularly, when using as the programmable processor unit a personal computer which carries out the functions of the audio signal generator unit as well as of the audio signal processing unit of the demonstration device, the electronic driver means and the digital-to-analog converter may be integrated in a hardware interface which is connected between the personal computer and the transducer. 
     When, however, the programmable processor unit is a digital signal processor (DSP), a particularly compact demonstration device may be obtained by integrating the electronic driver means, the digital-to-analog converter and the digital signal processor in a hardware interface. In order to simplify the operation of this hardware interface, furthermore display means may be provided for displaying audio signal generation data and audio signal processing data. The display means likewise may be integrated in the hardware interface or may be connected to the latter. 
     The audio signal processing unit of the demonstration device preferably comprises electronic audio signal processing means and electronic driver means for driving the electromechanical transducer, which are at least approximately the same as electronic audio signal processing means and electronic driver means included in the hearing system intended to be implanted, and which may be integrated in an interface. 
     In order to attain an impression of the output-side deflection of the transducer which is independent from individual variations of the biological load impedance, the electromechanical transducer preferably has a mechanical source impedance which, in the entire spectral transmission range of the device, is distinctively higher than the mechanical load impedance defined by the biological system comprising tympanic membrane, ossicular chain and inner ear. 
     The examination may be carried out in a manner which is particularly comfortable to the patient, when the electromechanical transducer comprises a transducer housing which provides for an acoustical encasing that minimizes sound signals emitted by vibrating structures of the transducer to such an extent that an acoustical deafening of the contralateral, non-examined ear becomes unnecessary. 
     The electromechanical transducer may be based on the electrodynamic, electromagnetic, magnetostrictive, capacitive or piezoelectric transducer principle. Particularly preferred is a piezoelectric transducer because magnetic stray fields may be completely avoided thereby. 
     In conformity with a further embodiment of the invention, a coupling element may be provided which is adapted to be coupled to the electromechanical transducer and to be non-invasively contacted, through the external auditory canal, with at least approximately the center of the tympanic membrane and thus the end point of the manubrium mallei. Preferably, this coupling element is a rod-shaped member which is stiff in axial direction thereof and which has an actuator end remote from the transducer, which actuator end is configured for a non-traumatic mechanical contact with the center of the tympanic membrane. Advantageously, the rod-shaped coupling element is configured such that it can be easily manually flexed to adapt it to the individual geometrical configuration of the external auditory canal. 
     Preferably, the electromechanical transducer is disposed within a transducer housing configured for introduction into an inlet zone of the external auditory canal, wherein the transducer housing has geometrical dimensions which are selected such that an examining person, even when using a microscope, has an unobstructed view of the actuator end of the coupling element contacting the center of the tympanic membrane. This permits the examining person to easily introduce the device, while at the same time providing for the safety of the patient. 
     Furthermore, by connecting the coupling element to the transducer via mechanical plug-type connection means, rather than by a mechanically fixed connection, different coupling elements may be used, which elements may be easily exchanged e.g. for hygienic reasons and which may be configured as disposable articles. 
     Preferably, the electromechanical transducer, possibly in combination with the mechanical coupling element, has a first mechanical resonance frequency at the upper end of the spectral transmission range of ≧10 kHz. A broadband behavior and thus short transient times may be attained thereby. 
     In conformity with a further embodiment of the invention, positioning means are provided for positioning the electromechanical transducer with respect to the umbo. Thereby the transducer, or, when the latter is coupled to the coupling site by a mechanical coupling element, such as a coupling rod connected to the electromechanical transducer, the actor end of the coupling element may be precisely moved to the target point. 
     Fixing means are preferably provided to obtain a secure, play-free linkage of the positioning means to a human skull and thus to fix the relative spatial positions of the positioned transducer or the coupling element, respectively. 
     In conformity with a further preferred embodiment of the invention, an intermediate element is provided between the positioning means and the electromechanical transducer, wherein this intermediate element is configured and dimensioned to transmit quasi-steady-state positioning adjustments from the positioning means to the electromechanical transducer, but to sufficiently reduce the transmission of at least dynamic forces from the positioning means to the coupling element to such an extent that the risk of middle or inner ear damage is substantially reduced. 
     In the demonstration device of the invention, the transducer together with the coupling element, follows the relatively slow position changes which are called quasisteady-state here and which are caused by the actuation of the positioning means. The physician can thus guide the active end of the coupling element precisely and free of relative movements to structures in the human body, especially to the umbo, as the target point. However, in the case of an unintentional external action which generally takes place by jerks and jolts, for example by hitting the positioning means with the hand, an instrument or the like, the dynamic forces acting on the positioning means are kept away from the transducer and the coupling element at least to a substantial extent. 
     The intermediate element may be made as a spring member, which is a structurally simple approach. The spring member, the electromechanical transducer and the coupling element from a spring/mass system which preferably has a natural frequency in the range from 0.5 to 5 Hz. 
     A further aspect of the invention is a process for pre-operatively demonstrating an at least partially implantable hearing system intended to be implanted, said hearing system comprising a first audio signal processing unit having a predetermined audio signal processing behavior, and a first electromechanical transducer which is driven by said first audio signal processing unit and which is adapted for being coupled to a pre-selected coupling site for causing mechanical vibrations of the coupling site, said process comprising the steps of: 
     (a) providing a second audio signal processing unit having an audio signal processing behavior which at least approximates the audio signal processing behavior of said first audio signal processing unit, and supplying test and demonstration signals to the second audio signal processing unit to produce output signals for driving a second electromechanical transducer; 
     (b) storing the output signals produced in step (a) in a signal storage; 
     (c) repeating steps (a) and (b) with different sets of audiological adaptation parameters; 
     (d) non-invasively coupling the second electromechanical transducer from the side of the external auditory canal to at least approximately the center of the tympanic membrane and thus to the end point of the manubrium mallei; and 
     (e) applying to the second electromechanical transducer output signals stored in the signal storage for causing mechanical vibrations of the tympanic membrane. 
     Accordingly, the process of the invention is carried out in two phases. In a first phase output signals of the type produced by an audio signal processing unit of the hearing system intended to be implanted are stored in a signal storage for different sets of audiological adaptation parameters. In a second phase, the actual demonstration phase, a transducer (the second electromechanical transducer) is non-invasively coupled from the outside via the external auditory canal to at least approximately the center of the tympanic membrane of the hearing-impaired patient to whom the hearing impression of the hearing system to be implanted is to be demonstrated, whereupon output signals stored in the signal storage are applied to the second transducer to mechanically vibrate the tympanic membrane. Thereby the functions of the implant can be demonstrated to a possible implant carrier in a non-invasive but nevertheless realistic manner. 
     Different settings of the implant may be simulated and demonstrated, respectively, by applying to the second transducer output signals obtained for different sets of audiological adaptation parameters. 
     The second electromechanical transducer may be coupled to at least approximately the center of the tympanic membrane directly or via a coupling element which is introduced through the external auditory canal for contacting the tympanic membrane. 
     These and further objects, features and advantages of the present invention will become apparent from the following description when taken in connection with the accompanying drawings which, for purposes of illustration only, show several embodiments in accordance with the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a first embodiment of a pre-operative demonstration system in which the electronic audio signal processing means of the hearing device to be implanted are simulated by software. 
     FIG. 2 shows a second embodiment of a pre-operative demonstration system similar to the system of FIG.  1 . 
     FIG. 3 shows a third embodiment of a pre-operative demonstration system in which an audio signal processing unit for controlling an electromechanical transducer comprises electronic audio signal processing means as used in the hearing device to be implanted. 
     FIG. 4 shows a fourth embodiment of a pre-operative demonstration system similar to the system of FIG. 3 
     FIGS. 5 and 6 show positioning devices for positioning a coupling element of the demonstration system with respect to the umbo. 
     FIG. 7 shows a further embodiment of a pre-operative demonstration system in which an intermediate member is disposed between the positioning device and the transducer for attenuating the transmission of dynamic forces acting on the positioning device to the transducer. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The pre-operative demonstration system schematically shown in FIG. 1 comprises an electromechanical transducer  10  which outputs mechanical oscillations that are transmitted via a coupling element  12  to the center of the tympanic membrane (umbo)  14  by direct mechanical contact. The associated mounting means for the transducer and their interconnection are not illustrated in FIG.  1  and will be described in detail with reference to FIGS. 5 and 6. 
     The transducer  10  is controlled by electronic driver means provided in a hardware interface  18 . This interface is digitally controlled by a computer, for example personal computer (PC)  20 , via a serial interface (for example RS 232, V.24). Interface  18  includes a digital logical unit (DIG)  22  for bi-directional data communication with the personal computer, a digital-to-analog converter  24 , and a driver unit  16  which is connected to the output side of converter  24  and which is adapted to the physical principal of the electromechanical transducer  10 . In this embodiment, the audio signal processing of the implant system to be demonstrated is simulated in computer  20  purely digitally based on proper software. 
     The audiological adaptation parameters of this simulation software for adaptation to the respective individual hearing disorder of the patient can be changed via the operating unit of the computer, typically a keyboard  26 . The simulation software preferably includes a module which guides the operator, e.g. the audiologist of an examination team, in a user-friendly manner, for example in a dialogue-type process. The simulation software may operate in a true real-time mode (online) and may permit access to all possible parameters and parameter changes to be found in the respective hearing implant. 
     In conformity with a second alternative embodiment a plurality of parameter sets for different audiological adaptation profiles may be made available to the operator, and the operator selects among them the parameter set which is best suited for the respective individual hearing disorder. According to a third alternative embodiment the simulation of the audio signal processing of the respective implant system may be effected by transmitting test and demonstration signals over the real audio signal processing means of the respective implant system and by storing the resulting output signals in a signal storage unit. Preferably, the resulting output signals are digitized and stored on suitable digital data storage media. This process is repeated with different sets of audiological adaptation parameters. Then, these pre-processed audio data sets are available in the signal storage unit, for example a CD-Rom, offline, and they can be selected by the operator of the demonstration device in a user-guided manner. 
     In all the above mentioned embodiments the individual audiological adaptation to the individual hearing disorder may be carried out in communication with the patient in an interactive and iterative manner as this commonly is done in an audiological adaptation process of a conventional hearing aid. The respective patient himself also may actively engage in this adaptation process by varying parameters. The audio test signals required for the audiological adaptation are generated by the computer  20  itself, or are prepared and digitally stored in the computer, or may be transmitted to the computer from suitable data or sound carriers (for example audio-CD playback devices, magnetic tape devices and the like) via proper interfaces. 
     In the embodiment schematically illustrated in FIG. 2 no external computer, such as the PC  20  shown in FIG. 1, is used; rather the above described simulations methods as well as operation and adaptation thereof are combined in a device  30  which includes an operating unit  26  (for example a keyboard) and a hardware unit  28 . In this embodiment the hardware unit  28  comprises a digital signal processor (DSP)  32  which carries out all the above described simulation and audiological adaptation tasks. In a manner analog to the embodiment of FIG. 1, the device  30  includes a digital-to-analog converter  24  and a driver unit  16  by which the digital output signals generated by the signal processor  32  are converted into analog signals, are amplified and are applied to the electromechanical transducer  10 . 
     The device  30  further comprises a digital logical unit (DIG)  22  which represents a, preferably bi-directional, data communication interface to permit transmission of adjustment parameters and data commands as well as of externally generated audio test signals from a playback device  28  to the digital signal processor  32 , but also transmission of signals generated by the digital signal processor  32  to a display and/or recording device (not illustrated in FIG. 2) for facilitated operator guidance and for purposes of documentation. 
     Instead of simulating the operation and the signal behavior of the electronic audio signal processing means of the hearing device to be implanted, the preoperative demonstration system also may be designed such that the audio signal processing unit used to control the electromechanical transducer comprises the same audio signal processing means as provided in the hearing device to be implanted. This embodiment of the subject demonstration system is shown in FIG.  3 . 
     In this embodiment, the entire implant electronic means  34 , i.e. the audio signal processing means as well as the transducer driver means of the respective implant system (IMP), is contained, as hard- and software, in the interface  18  in the same manner as used in the respective implant system. Therefore, an online demonstration of the intended implant system with 100% identical hard- and software  34  is possible. The control of the implant hard- and software  34  and the supply of the proper audio test and demonstration signals preferably are effected via a bi-directional interface (DIG)  22  which communicates, likewise bi-directionally, with a computer  20  (for example a personal computer). The individual audiological adaptation of the system IMP to the respective hearing disorder and the generation of the audio test and demonstration signals are carried out in the same manner as described above for the embodiments of FIGS. 1 and 2. 
     The further embodiment shown in FIG. 4 is similar to the embodiment of FIG. 3, but does not use an external computer (PC). Rather, the device  30  comprises, in addition to the implant system (IMP)  34 , a microcontroller or microcomputer (μC)  36  which is controlled by an operating unit, for example a keyboard  26 . Furthermore, a display unit (not illustrated in FIG. 4) may be provided for operator guidance. The controller (μC)  36  bi-directionally controls the system IMP. The individual audiological adaptation of the system IMP to the respective hearing disorder and the generation of the audio test and demonstration signals are carried out in the same manner as described above for the embodiments of FIGS. 1 and 2. Particularly, the provision of the audio test and demonstration signals is not illustrated in FIG. 4, but may be effected in conformity with the embodiment shown in FIG.  1 . 
     The demonstration system of the present invention preferably may be used in combination with a positioning system  40  which is shown in FIG.  5  and which is of the type described in U.S. Pat. No. 5,776,144. The positioning system  40  is composed, essentially, of a linear axis mechanism  42 , a clampable ball-and-socket joint  44  and a base  46 . 
     A carriage  50  is guided, without play, in a linear guide  48  of linear axis mechanism  42 . Carriage  50  can be moved via a threaded spindle  52 . A rotary knob  54  is joined securely to threaded spindle  52 . The pitch of the threaded spindle  52  is designed to be self-locking, i.e. the pitch angle is smaller than the effective angle of friction, so that carriage  50  does not move automatically along linear guide  48  as a result of its weight. 
     The length of the path of carriage  50  moving along linear guide  48  is limited by two end stops  56 ,  58 . The upper end stop  56  is formed by a closure plate which is provided with a corresponding internally threaded hole for receiving threaded spindle  52  and which is attached to the upper end of linear guide  48 . On the one hand, the closure plate forming end stop  56  guides the threaded spindle  52  parallel to linear guide  48 , and an the other hand, this plate also prevents carriage  50  from sliding off of linear guide  48  by screwing spindle  52  out too far. Similarly, the lower end stop  58 , which is defined by a face at the lower end of rotary knob  54 , prevents threaded spindle  52  from being screwed in too far, and thus, carriage  50  from sliding out at the opposite end of linear guide  48 . 
     By turning rotary knob  54 , according to the direction of the thread and the selected pitch of threaded spindle  52 , axial displacement of the carriage  50  along guide  48  of linear axis mechanism  42  is effected. Carriage  50  can, thus, be moved continuously along the linear axis mechanism  42  between the two end stops  56  and  58 , and due to the self-locking of the threaded drive, maintains its instantaneous position. 
     Carriage  50  has a corresponding receiver  62  into which the electromechanical transducer  10  shown in FIGS. 1 to  4  can be manually inserted without play or removed therefrom. Receiver  62  for transducer  10  has an opening  64  for the coupling element  12  which is connected to transducer  10  inserted therein. The free, active end  66  of the coupling element  12  can, thus, be positioned in axial direction  68  parallel to the linear guide  48  relative to a target point  14  in and stationary with respect to body  70 , when the rotary knob  54  is turned. 
     Linear axis mechanism  42 , together with threaded spindle  52 , rotary knob  54 , carriage  50  and the transducer  10  inserted in receiver  62  and held there, is joined securely to housing  74  of the clampable ball-and-socket joint  44  using a connecting element  72 . Ball-and-socket joint  44  has a ball  76  which is securely joined via a column  80  to base  46 , and which can be clamped with reference to the housing  74  by means of a clamp screw  78 . 
     When the ball-and-socket joint  44  is unclamped, the entire linear axis mechanism  42  can be turned in all three rotary degrees of freedom  82 ,  83 ,  84  around the center of ball  76 , which is fixedly joined to the base  46 . 
     Via base  46 , positioning system  40  can be securely joined to suitable holding means. After attachment of these holding means to the body, positioning of the system attached to the holding means and subsequent clamping of clamp screw  78 , exact positioning of free, active end  66  relative to a target point  14  on the body, is thus possible without play, wherein possibly risky relative movements between the body and the free active end  66  of the coupling element are prevented. 
     By loosening clamp screw  78  of ball-and-socket joint  44 , connecting element  72  and the linear axis mechanism  42  which is attached to it, as well as transducer  10  inserted in carriage  50 , together with coupling element  12  coupled thereto and its free, active end  66 , can be turned around the center of ball  76  of the ball-and-socket joint  44  according to all three rotary degrees of freedom  82 ,  83 ,  84 . The shown combination of clampable ball-and-socket joint  44  and linear axis mechanism  42  securely attached to it, thus enables four-axis positioning of the free, active end  66  of the selected coupling element  12  relative to any target point  14  an the body, i.e., positioning in the translatoric degree of freedom  68  and in the three rotational degrees of freedom  82 ,  83  and  84 . 
     FIG. 6 illustrates a preferred combination of the positioning system  40  of FIG. 5 and a head support  86  for positioning and fixing the transducer and the coupling element, respectively, of the presently described demonstration system. In the embodiment shown here, the base  46  of the positioning system  40  is securely joined to head support  86 . Opening width  88  of the head support  86  is, preferably, about 200 mm, and width  88  can be set, optionally and without play, via a rotary knob  90  and an interior threaded drive by moving a pair of receiving arms  92  and  94  towards (closing) or away (opening) from one another. Rotary knob  90  for adjustment of opening width  88 , in this case, can be operated either by the wearer of head support  86  himself/herself or by a qualified specialist (physician, nurse, assistant) in order to attach head support  86  to the head of the patient by clamping on both sides. Positioning system  40 , via its base  46 , is securely attached to one (arm  92 ) of the two receiving arms. This side is called the working side of the head support. A conical retaining element  96  is connected to receiving arm  92  and can be designed, for example, similar to an ear speculum. Retaining element  96  may be cardanically mounted on receiving arm  92  to allow compensation of small spatial angles. It is inserted into the external auditory canal of the wearer (patient) with visual monitoring, if necessary, with the aid of a microscope. 
     Conical retaining element  96 , moreover, has a conical inside opening  98  which provides space for the free, active end  66  of the coupling element  12  clamped in positioning system  40  and also for visual control. The positioning system  40  is mounted on the head support  86  in such a manner that the optical axis  102  of the microscope or of the unaided eye  104 , respectively, is not covered by the positioning system  40  or components thereof. 
     On the receiving arm  94  at the opposite side of head support  86 , selectively, a second conical support, similar to support  96 , or an earmuff element  100  in the form of a half shell, is attached. The second conical support or earmuff element  100  is, respectively, inserted into the auditory canal or placed over the outer part of the opposite ear. 
     When earmuff element  100  is used, as is shown in FIG. 6, some of the pre-tensioning force generated by reducing the opening width  88  is transferred over a large area to the skull bone area which surrounds the outer ear. This prevents compressive forces from being applied at points and the associated undesirable feeling of pressure associated with it, and the force applied for support is distributed over a large area of skin. 
     After inserting conical retaining element  96  into the outer auditory canal at the working side and the subsequent placement of the earmuff element  100  on the outer ear at the opposite side, by carefully reducing opening width  88  of head support  86 , the two retaining elements, i.e., retaining element  96  and earmuff element  100 , can be caused to approach one another until the entire head support  86  is clamped on the skull of the patient. By deforming earmuff element  100  and by blocking conical retaining element  96  in the outer auditory canal, a secure fitting of the entire head support  86  on the skull of the patient is ensured. After clamping head support  86  on the skull of the patient, the free active end  66  of the coupling element  12 , attached in positioning system  40 , thus can be positioned, through conical inside hole  98  in conical retaining element  96 , without play in a manner preventing relative movements between the skull and target point  14  on the skull. The set position of the positioning system can be fixed via the described clamping means of the positioning system. 
     FIG. 7 shows a further embodiment of the above described preoperative demonstration device in which the transducer  10  is connected via an intermediate element  106  to a positioning system  40 . The positioning system  40  in turn is attached to a fixing means which is only schematically shown at  108  and which makes it possible to link the positioning system  40  to the human body, especially to the human skull, securely and without play. The electromechanical transducer  10 , the output side of which is fixedly connected to a rigid coupling rod, is driven in a manner corresponding to that used in the embodiments of FIGS. 1 to  4 . 
     In a manner similar to the embodiments of FIGS. 5 and 6, the positioning system  40  is provided with a base  110  which is coupled to the fixing means  108 . The base  110  carries a clampable ball-and-socket joint  44  which has a ball  76  and an associated ball receiver  74 . By means of a clamp screw  78 , the ball joint  44  can be locked in a position which can be set by means of a linear guide  48  which is fixedly connected to the ball  76 . A transversely extending support arm  112 , the length of which is adjustable, is attached to the linear guide  48 . The adjusted length of the support arm  112  is fixed by means of a clamping screw  114 . A linear adjustment device  116  engages the end of the support arm  112  which is remote from the linear guide. This device is connected on its end which is the bottom end in the FIG. 7 to a slide  118  to which a guide pin  120  is attached. The guide pin  120  is movably guided in a hole  122  of the support arm  112  in a direction which is essentially parallel to the longitudinal axis of the coupling rod  12 . The transducer  10  is connected to the slide  118  via the intermediate element  106 . By means of the linear adjustment device  116  the transducer  10  can be sensitively adjusted via the slide  118  and the intermediate element  106  in the longitudinal direction of the coupling rod  12 . The linear adjustment device  116  may include a hydraulic piston/cylinder arrangement which is not shown in detail and which, upon actuation on its end which is remote from the transducer  10 , allows fine adjustment of the transducer  10  together with the coupling rod  12  relative to the support arm  112  in a direction which is essentially perpendicular to the latter. 
     Furthermore, an ear speculum  96  is attached to base  110  in an easily removable manner. To secure and release the ear speculum  96  a clamp  124  which interacts with the base  110  and the ear speculum  96  is used. The ear speculum  96  accommodates the part of the coupling rod  12  remote from the transducer  10 , wherein the longitudinal axis of the coupling rod  12  can be aligned with the longitudinal axis of the ear speculum. Optionally, the ear speculum  96  can be cardanically supported on the base  110  to compensate for small spatial angles. 
     When the ball joint  44  is unclamped, the linear adjustment device  116  can be turned around the center of the ball  76  in all three rotational spatial degrees of freedom. The mutual distance of the longitudinal axes of the linear guide  48  and the coupling rod  12  can be adjusted when the clamping screw  114  is loosened. By attaching the fixing means  108  to the body of the test person, positioning of the system attached to the fixing means, subsequent clamping of the clamping screws  78  and  114  and corresponding adjustment of the linear adjustment device  116  is possible. Thus, exact, play-free positioning of the free actuator end  66  of the coupling rod  12  relative to the umbo as the target point on the body is possible, wherein the free actuator end  66  preferably is spherical. The position of the free actuator end  66  can be checked, for example, by a microscope. The mutual offset of the coupling rod  12  and the positioning means  40  ensures that the optical axis  102  of the microscope or the naked eye of the physician is not covered by the positioning system itself or by its components. 
     In the illustrated embodiment, the intermediate element  106  consists of two simple flexional springs arranged in parallel, of which in the figure only one can be seen, while the other extends offset normal to the plane of the figure and behind the spring to be seen. The intermediate element  106 , the electromechanical transducer  10 , and the coupling rod  12  form a spring/mass system which is preferably designed such that it has a natural or resonant frequency (or, in the case of several natural frequencies, a lowest first natural frequency) in the range from 0.5 to 5 Hz. In this way, dynamic forces having a frequency higher than this natural frequency (such forces can occur, for example, by accidental impacts against the positioning means  40 ), are transmitted, if at all, only in a substantially attenuated manner from the positioning means  40  to the coupling rod  12 . The coupling rod  12 , however, normally follows the quasi-steady-state positioning adjustments of the positioning means  40 . If, however, the transducer  10 , during positioning, inadvertently comes too close to the target point, the flexional springs which form the intermediate element  106  can deflect and in this way, also counteract damage to the middle and/or inner ear. 
     The intermediate element  106  may basically also be constructed in a different manner. For example, the intermediate element  106  may comprise a force limiter, for example in the form of a friction or induction coupling, which allows transmission of forces only up to a predetermined upper limit. 
     While several embodiments in accordance with the present invention have been shown and described, it is understood that the invention is not limited thereto, and is susceptible to numerous changes and modifications as known to those skilled in the art. Therefore, this invention is not limited to the details shown and described herein, and includes all such changes and modifications as encompassed by the scope of the appended claims.