Abstract:
A method and apparatus for determining a position of an external transceiver ( 24 ) relative to an implanted transceiver ( 23 ) comprising means ( 30 ) for measuring the strength of a magnetic field proximal to the external transceiver ( 24 ) and means for determining a position of the external transceiver ( 24 ) relative to the implanted transceiver ( 23 ) from said measured magnetic field strength. Furthermore there is disclosed is a method and apparatus for determining a skin flap thickness of a recipient of a prosthesis including a transcutaneous link between the external transceiver ( 24 ) and the implanted transceiver ( 23 ). A skin-flap thickness meter is also provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority of and is a national stage application of PCT Application No. PCT/AU2003/001140, entitled, “Method and Apparatus for Measurement Of Transmitter/Receiver Separation,” filed on Sep. 4, 2003, which claims the priority of Australian Patent No. 2002951217, filed on Sep. 4, 2002. The entire disclosure and contents of the above applications are hereby incorporated by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for determining the position of an external transceiver relative to an implanted transceiver. The invention also relates to a method and apparatus for determining a skin flap thickness of a recipient of a prosthesis comprising a transcutaneous link provided by an external transceiver and an implanted transceiver, and to a skin-flap thickness meter. 
     BACKGROUND OF THE INVENTION 
     Hearing loss, which may be due to many different causes, is generally of two types, conductive and sensorineural. Of these types, conductive hearing loss occurs where the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded, for example, by damage to the ossicles. Conductive hearing loss may often be helped by use of conventional hearing aid systems, which comprise a microphone and an amplifier for amplifying detected sounds so that acoustic information does reach the cochlea and the hair cells. 
     In many people who are profoundly deaf, the reason for deafness is sensorineural hearing loss, which is caused by an absence of, or destruction of, the hair cells in the cochlea which transduce acoustic signals into nerve impulses. These people are thus unable to derive suitable benefit from conventional hearing aid systems, no matter how loud the acoustic stimulus is made, because there is damage to or absence of the mechanism for nerve impulses to be generated from sound in the normal manner. It is for this purpose that cochlear implant systems have been developed. Such systems bypass the hair cells in the cochlea and directly deliver electrical stimulation to the auditory nerve fibres, thereby allowing the brain to perceive a hearing sensation resembling the natural hearing sensation normally delivered to the auditory nerve. U.S. Pat. No. 4,532,930, the contents of which are incorporated herein by reference, provides a description of one type of traditional cochlear implant system. 
     Cochlear implant systems have typically consisted of two essential components, an external component commonly referred to as a processor unit and an internal implanted component commonly referred to as a stimulator/receiver unit. Traditionally, both of these components have cooperated together to provide the sound sensation to a user. 
     The external component has traditionally consisted of a microphone for detecting sounds, such as speech and environmental sounds, a speech processor that converts the detected sounds, particularly speech, into a coded signal, a power source such as a battery, and an external transmitter coil. 
     The coded signal output by the speech processor is transmitted transcutaneously to the implanted stimulator/receiver unit situated within a recess of the temporal bone of the user. This transcutaneous transmission occurs via the external transmitter coil which is positioned to communicate with an implanted receiver coil provided with the stimulator/receiver unit. This communication serves two essential purposes, firstly to transcutaneously transmit the coded sound signal and secondly to provide power to the implanted stimulator/receiver unit. Conventionally, this link has been in the form of an RF link, but other such links have been proposed and implemented with varying degrees of success. 
     The implanted stimulator/receiver unit traditionally includes a receiver coil that receives the coded signal and power from the external processor component, and a stimulator that processes the coded signal and outputs a stimulation signal to an intracochlea electrode assembly which applies the electrical stimulation directly to the auditory nerve producing a hearing sensation corresponding to the original detected sound. 
     A particular problem that the present invention seeks to address is determining a distance of separation between an external transceiver and an implanted transceiver by determining the relative position of the external transceiver to the implanted transceiver. Another problem is when an external transmitter or transceiver has been displace, for example when the external transceiver has fallen away from an optimum position upon the recipient. It is particularly relevant when the recipient, such as an infant, is unable to or unlikely to indicate such an occurrence and therefore cannot derive maximum hearing benefit from the implant system. Embodiments of the present invention may be particularly advantageous, as the distance between the transceivers impacts upon the amount of power that can be delivered to the implanted transceiver, and hence impacts upon a power source current and useful life, for instance where the power source is a battery. As such embodiments of the present invention enable a determination of the distance to be made, transmission and stimulation parameters of transmissions between the transceivers may be optimised to allow for the actual distance of separation. Optimising such parameters for the actual distance of separation leads to improved performance of the implant system, and also improves battery lifetime. 
     Embodiments of the present invention are particularly advantageous in that an actual field between the transmitter and receiver is measured. While an alternative prior art approach is to monitor a voltage standing wave ratio (VSWR) on the cable leading to the transmitter, such an approach requires an assumption that a change in the VSWR stems from an alteration in the link between the transmitter and receiver, whereas in fact such alterations in the VSWR may equally arise from a break in the cable or transmitter coil causing an open circuit or other such fault. 
     Furthermore, for the reasons given hereinbefore, the invention also seeks to provide an improved method of determining a skin-flap thickness of a recipient partly by determining the separation between the external transceiver and the implanted transceiver or stimulator/receiver. Prior art attempts at determining the separation have used battery current whereby the battery current is mapped to transceiver separation. However, measurement and monitoring of battery current may not change monotonically with varying transceiver separation, and therefore does not enable a one-to-one mapping of battery current to the separation. Conversely, measuring magnetic field strength, as with the present invention, provides a monotonic variation with transceiver separation and therefore provides a one-to-one mapping of magnetic field strength to transceiver separation. 
     Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention before the priority date of each claim of this application. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention there is provided a method of determining a position of an external transceiver relative to an implanted transceiver, the method comprising the steps of: 
     measuring the strength of a magnetic field proximal to the external transceiver; and 
     determining a position of the external transceiver relative to the implanted transceiver from said measured magnetic field strength. 
     According to a second aspect of the invention there is provided apparatus for determining a position of an external transceiver relative to an implanted transceiver, the apparatus comprising: 
     means for measuring the strength of a magnetic field proximal to the external transceiver; and 
     means for determining a position of the external transceiver relative to the implanted transceiver from said measured magnetic field strength. 
     It has been realised that the magnetic field strength between two transceivers preferably forming a transcutaneous link is a particularly useful factor in determining the relative positioning of the transceivers. In particular, the magnetic field strength changes monotonically with varying transceiver separation, thus allowing a one-to-one mapping of magnetic field strength to transceiver separation. However, other factors, such as battery current, may not change monotonically with varying transceiver separation, and thus do not enable a one-to-one mapping of battery current to transceiver separation, making it impossible to determine transceiver separation by monitoring or measuring such a factor. 
     Further, a measurement of magnetic field strength can be performed with very little power consumption, and with very little loading effect on the transmissions between the external and implanted transceivers, thus providing the advantages of simple low current implementation. 
     The position of the external transceiver relative to the implanted transceiver may be determined simply in order to indicate whether the external transmitter has been displaced, for example where the external transceiver has fallen away from a proper position upon the recipient. Such embodiments of the present invention are particularly useful where the recipient is unlikely to indicate such an occurrence, for instance where the recipient is an infant. 
     In such embodiments of the first aspect of the present invention, the step of determining preferably further comprises a step of comparing a measured strength of magnetic field proximal to the external transceiver to a threshold value; and the method of the first aspect of the invention preferably further comprises the step of indicating that the external transceiver has been displaced when the measured strength of magnetic field proximal to the external transceiver exceeds the threshold value. The step of indicating may comprise providing an audible indication such as an alarm, a visible indication or other indication. 
     Similarly, in such embodiments of the apparatus of the second aspect of the invention, the apparatus preferably further comprises means for comparing a measured strength of magnetic field proximal to the external transceiver to a threshold value; and means for indicating that the external transceiver has been displaced when the measured strength of magnetic field proximal to the external transceiver exceeds the threshold value. The means for indicating may comprise an audible alarm, a visible indicator, or other type of indicator. 
     Alternatively, the position of the external transceiver relative to the implanted transceiver may be determined in order to estimate a distance of separation between the external transceiver and the implanted transceiver. Such embodiments of the invention may be particularly advantageous, as the distance between the transceivers impacts upon the amount of power that can be delivered to the implanted transceiver, and hence impacts upon a power source current and useful life, for instance where the power source is a battery. As such embodiments of the present invention enable a determination of the distance to be made, transmission and stimulation parameters of transmissions between the transceivers may be optimised to allow for the actual distance of separation. Optimising such parameters for the actual distance of separation leads to improved performance of the implant system, and also improves battery lifetime. 
     In such embodiments of the first aspect of the invention, the step of determining preferably further comprises mapping a measured value of magnetic field strength proximal to the external transceiver to a distance value. The step of mapping may comprise consulting a look-up table comprising a plurality of pairs of values, each pair of values mapping a particular magnetic field strength to a corresponding transceiver separation distance. 
     Alternatively the step of mapping may comprise algorithmically converting said measured value of magnetic field into a corresponding transceiver separation distance. 
     Similarly, in such embodiments of the second aspect of the invention, the apparatus preferably further comprises means for mapping a measured value of magnetic field strength proximal to the external transceiver to a distance value. The means for mapping may comprise a look-up table comprising a plurality of pairs of values of magnetic field strength to transceiver separation distance. 
     Alternatively the means for mapping may comprise means for algorithmically converting said measured value of magnetic field into a corresponding transceiver separation distance. 
     According to a third aspect of the invention there is provided a method of determining a skin flap thickness of a recipient of a prosthesis comprising a transcutaneous link provided by an external transceiver and an implanted transceiver, the method comprising the steps of: 
     measuring a strength of a magnetic field proximal to the external transceiver when the external transceiver is positioned so as to implement the transcutaneous link; and 
     determining a skin flap thickness of the recipient by determining a position of the external transceiver relative to the implanted receiver from said measured magnetic field strength. 
     According to a fourth aspect of the invention there is provided apparatus for determining a skin flap thickness of a recipient of a prosthesis comprising a transcutaneous link provided by an external transceiver and an implanted transceiver, the apparatus comprising: 
     means for measuring a strength of a magnetic field proximal to the external transceiver when the external transceiver is positioned so as to implement the transcutaneous link; and 
     means for determining a skin flap thickness of the recipient by determining a position of the external transceiver relative to the implanted receiver from said measured magnetic field strength. 
     A transcutaneous link formed by the external transceiver and the implanted transceiver may comprise an RF link. The transcutaneous link may be unidirectional, in that the external transceiver comprises a transmitter, and the implanted transceiver comprises a receiver. Alternatively, it is envisaged that the transcutaneous link may be bidirectional, in that both the external transceiver and the implanted transceiver may transmit and receive signals across the transcutaneous link. In particular, it is envisioned that the external transceiver will be operable to transmit both data and power across the transcutaneous link, and to receive data across the transcutaneous link. Similarly, it is envisioned that the implanted transceiver will be operable to receive both power and data across the transcutaneous link and to transmit data across the transcutaneous link. 
     The means for measuring the strength of the magnetic field proximal to the external transceiver may comprise a pickup coil positioned proximal to the external transceiver. Preferably, the pickup coil is positioned in a plane substantially perpendicular to a primary axis of the magnetic field produced by the transceivers. The pickup coil may comprise an open circuited single turn, positioned concentrically with turns of the external transceiver. In such embodiments, a voltage induced on the pickup coil will be indicative of a magnetic field proximal to the external transceiver, and may thus be used in determining a position of the external transceiver relative to the implanted transceiver. 
     The external transceiver will typically be capable of transmitting power and data to the implanted transceiver. However, the external transceiver is preferably capable of receiving data from the implanted transceiver. Similarly, the implanted transceiver will typically be capable of receiving power and data from the external transceiver, but is preferably also capable of transmitting data to the external transceiver. 
     It is to be appreciated that measurement of the magnetic field strength proximal to the implanted transceiver may similarly yield information regarding the position of the external transceiver relative to the implanted transceiver and is thus within the scope of the present invention. However it is unlikely that such internal field measurements will be efficient due to the limited power available to an implanted portion of a prosthesis, and the difficulty of processing and communicating such measurements from the implanted portion to the external transceiver. 
     According to a fifth aspect of the invention there is provided a skin-flap thickness meter, the meter comprising: 
     a meter transmitter coil for placement proximal to an implanted transceiver such that the meter transmitter coil and the implanted transceiver coil are separated by substantially the skin-flap thickness; 
     means for measuring a strength of a magnetic field proximal to the meter transmitter coil when the meter transmitter coil is placed proximal to the implanted transceiver; and 
     means for determining a skin flap thickness by determining a position of the meter transmitter coil relative to the implanted transceiver from said measured magnetic field strength. 
     According to a sixth aspect of the invention there is provided apparatus for determining a position of an external transceiver relative to an implanted transceiver, the apparatus comprising: 
     means for measuring the strength of a magnetic field proximal to the external transceiver; 
     means for determining a position of the external transceiver relative to the implanted transceiver from said measured magnetic field strength; 
     means for comparing a measured strength of magnetic field proximal to the external transceiver to a threshold value; 
     means for indicating that the external transceiver has been displaced when the measured strength of magnetic field proximal to the external transceiver exceeds the threshold value; and 
     means for mapping comprises a look-up table comprising a plurality of pairs of values of magnetic field strength to transceiver separation distance. 
     According to a seventh aspect of the invention there is provided apparatus for determining a skin flap thickness of a recipient of a prosthesis comprising a transcutaneous link provided by an external transceiver and an implanted transceiver, the apparatus comprising: 
     a pick-up coil for measuring a strength of a magnetic field proximal to the external transceiver when the external transceiver is positioned so as to implement the transcutaneous link, the pickup coil being positioned in a plane substantially perpendicular to a primary axis of the magnetic field produced by the transceivers; 
     wherein a voltage induced on the pickup coil is indicative of a magnetic field proximal to the external transceiver; and 
     means for determining a skin flap thickness of the recipient by determining a position of the external transceiver relative to the implanted receiver from said measured magnetic field strength. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the invention will hereinafter be described, by way of example only, with reference to the accompanying drawings in which: 
         FIG. 1  is a pictorial representation of a cochlear implant system within which the present invention may be implemented; 
         FIG. 2  is a circuit diagram illustrating implementation of an embodiment of the present invention; 
         FIG. 3  depicts variation of magnetic field strength with transceiver separation for the embodiment of  FIG. 2 ; 
         FIG. 4  depicts variation of battery current with transceiver separation for the embodiment of  FIG. 2 ; 
         FIG. 5  is a circuit diagram illustrating a coil-off detection circuit in accordance with the present invention; 
         FIG. 6  is a circuit diagram of a circuit used for verification of coil-off detection; 
         FIG. 7  illustrates the variation of magnetic field with transceiver separation for particular values of stimulation rate and sound level for the circuit of  FIG. 6 ; 
         FIG. 8  illustrates the variation of magnetic field with transceiver separation for particular values of supply voltage and sound level for the circuit of  FIG. 6 ; 
         FIG. 9  illustrates the variation of magnetic field with transceiver separation for particular values of implanted coil tuning and sound level for the circuit of  FIG. 6 ; and 
         FIG. 10  illustrates the variation of magnetic field with transceiver separation for particular values of external coil tuning and sound level for the circuit of  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While the present invention is not directed solely to a cochlear implant, it is appropriate to briefly describe the construction of one type of known cochlear implant system with reference to  FIG. 1 . 
     Known cochlear implants typically consist of two main components, an external component including a speech processor  29 , and an internal component including an implanted receiver and stimulator unit  22 . The external component includes a microphone  27 . The speech processor  29  is, in this illustration, constructed and arranged so that it can fit behind the outer ear  11 . Alternative versions may be worn elsewhere on the recipient&#39;s body. Attached to the speech processor  29  is a transmitter coil  24  that transmits electrical signals to the implanted unit  22  via a radio frequency (RF) link. 
     The implanted component includes a receiver coil  23  for receiving power and data from the transmitter coil  24 . A cable  21  extends from the implanted receiver and stimulator unit  22  to the cochlea  12  and terminates in an electrode array  20 . The signals thus received are applied by the array  20  to the basilar membrane  8  and the nerve cells within the cochlea  12  thereby stimulating the auditory nerve  9 . The operation of such a device is described, for example, in U.S. Pat. No. 4,532,930. As depicted diagrammatically in  FIG. 1 , the cochlear implant electrode array  20  has traditionally been inserted into the initial portion of the scala tympani of the cochlea  12  up to about a full turn within the cochlea. 
     A sound processor (not shown) of the external component  29  includes an amplifier and a speech processor that uses a coding strategy to extract speech from the sounds detected by the microphone  27 . In the depicted embodiment, the speech processor of the cochlear implant can perform an audio spectral analysis of the acoustic signals and output channel amplitude levels. The sound processor can also sort the outputs in order of magnitude, or flag the spectral maxima as used in the SPEAK strategy developed by Cochlear Ltd. Other coding strategies could be employed. 
       FIG. 2  is a circuit diagram illustrating implementation of an embodiment of the present invention in a cochlear implant system of the type shown in  FIG. 1 . The speech processor of the external component  29  drives the transmitter coil  24 , which transmits power and data to receiver coil  23 , for the implanted stimulator unit  22 . In accordance with the present invention, a pickup coil  30  is provided for detecting the strength of a magnetic field proximal to the transmitter  24 . The pickup coil  30  is positioned in a plane substantially perpendicular to a primary axis of the magnetic field produced by the transmitter coil  24  and receiver coil  23 . The pickup coil comprises an open circuited single turn, positioned concentrically with turns of the transmitter coil  24 . A voltage is induced on the pickup coil which is indicative of a magnetic field strength proximal to the transmitter coil  24 . The output of the pickup coil  30  is passed through a peak detector comprising diode D and capacitor C. 
     In the present embodiment, the RF link of the implant system operates at a signal frequency of 5 MHz. The transmitter coil  24  and receiver coil  23  are stagger-tuned to achieve the bandwidth needed for a 100% amplitude modulated RF signal. The transmitter resonance circuit  24  is usually tuned below the signal frequency, while the implant receiver circuit  23  is tuned slightly above the signal frequency. As a result, the effective impedance seen by the RF drivers of the speech processor of the external component  29 , at the signal frequency, is inductive. This inductive impedance increases when the coupling between the coils  23 ,  24  is increased, by reducing the distance between the coils  23 ,  24 . As a result, the current through the transmitter coil  24 , and the magnetic field in the vicinity of the coil  24 , falls when the distance between the coils is reduced. 
     This phenomena can also be explained in terms of the interaction between the magnetic fields surrounding the transmitter coil  24  and receiver coil  23 . The magnetic field generated by the receiver coil  23  is a secondary field that opposes the primary field of the transmitter coil  24 . The interaction between the two opposite fields reduces the effective field near the transmitter coil  24 . This effect is increased as the distance between the coils  23 ,  24  is reduced. 
     The invention is based on measuring the strength of the magnetic field in the vicinity of the transmitter coil  24 . As this field increases monotonically with the distance between the transmitter coil  24  and receiver coil  23 , the measured field strength can be calibrated to estimate the distance between the coils  23 ,  24 , and also to indicate if that distance exceeds a preset value, for example if the coil has fallen off the user&#39;s head. 
     However, other factors, such as battery current, may not change monotonically with varying transceiver separation, and thus do not enable a one-to-one mapping of battery current to transceiver separation, making it impossible to determine transceiver separation by monitoring or measuring such a factor. 
     Further, a measurement of magnetic field strength can be performed with very little power consumption, and with very little loading effect on the transmissions between the external and implanted transceivers, thus providing the advantages of simple low current implementation. 
     The circuit shown in  FIG. 2  was simulated using OrCad Pspice version 9.2. The simulation model included circuit models for the CI24M implant produced by Cochlear Ltd, ESPrit 3G speech processor produced by Cochlear Ltd and a single turn pickup coil. 
     A simplified spice model was used for both the implant and the speech processor. The ESPrit 3G model included the major variable that affects and/or sets the battery current, output RF current, stimulation phase width, and intra-frame gap, as well as the RF-data mark-space ratio. The implant model, on the other hand, included all the power consuming components such as the antenna resistance, transformer losses, diode, IC consumption and stimulation current. The coupling coefficient, k, between the transmitter and receiver coils was expressed as a function of the distance d between the coils: 
               k   =     1.26     2.6   +   d         ,         
where d is in mm.
 
     This value of k was empirically obtained from the particular antennae used in the circuit depicted in  FIG. 2 . The peak detector decay time constant was set to 10 ms. This time constant was chosen much longer than the stimulation period of the SPEAK strategy, set to 2000 pps in the Spice model. 
     The circuit was simulated using stimulation rates from 2000 pps to 13900 pps, stimulation current ranging from 0 to 1.8 mA and link range from 1 to 20 mm. The circuit parameters shown in the following table were used to study the effect of the distance between the coils. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
               
               
                   
                 Parameter 
                 Value 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Vbatt 
                 3.0 
                 V 
               
               
                   
                 Stim rate 
                 13.9 
                 kHz 
               
               
                   
                 Phase width 
                 25 
                 us 
               
               
                   
                 Stim current 
                 1 
                 mA 
               
               
                   
                 Transmitter coil tuned freq 
                 4.8 
                 MHz 
               
               
                   
                 Receiver coil tuned freq 
                 5.25 
                 MHz 
               
               
                   
                 Pickup inductance 
                 60 
                 nH 
               
               
                   
                 Coupling coefficient of pickup coil 
                 0.7 
               
               
                   
               
             
          
         
       
     
     The simulation results are shown in  FIG. 3  and  FIG. 4 .  FIG. 3  depicts the peak detector output voltage versus link range (transmitter  24 /receiver  23  separation). This output voltage depends on the strength of the magnetic field, normal to the pickup coil  30 . In this example, the pickup coil  30  is a single track printed on a PCB upon which the transmitter coil  24  is also printed. The coupling coefficient between the transmitter  24  and pickup coil  30  is assumed to be 0.7. Higher coupling can be achieved in practice by the careful placement of the pickup coil  30  relative to the transmitter  24 . Higher output signals can also be obtained if a two-turn (or more) pickup coil is used. 
       FIG. 3  reveals that the magnetic field of the transmitter  24  increases with the distance between the transmitter coil  24  and receiver coil  23  (link range). When that distance exceeds 20 mm, the output voltage reaches about 780 mV (not shown in the figure).  FIG. 3  also reveals that the increase in magnetic field is monotonic as the link range increases from 1 mm to 10 mm. 
       FIG. 4  depicts the battery current, which reaches a peak value of 18.9 mA at 4 mm then gradually drops to 18 mA at 10 mm, and to 17 mA at 20 mm (not shown in the figure). Thus, the battery current does not vary monotonically with increasing link range between the transmitter  24  and receiver  23 . 
       FIGS. 3 and 4  clearly show that the battery current cannot be used to estimate the link range, as a given value of battery current can not be equated to a single value of transceiver separation. On the other hand, there is a one to one correlation between the output voltage of the peak detector C, D and the distance between the transmitter coil  24  and receiver coil  23 . 
     It is to be noted that the battery current is proportional to the total system power. On the other hand, the strength of the magnetic field in the vicinity of the transmitter coil  24  is proportional to the stored reactive energy. The relationship between the active and reactive energy components depends on the phase angle of the coil current relative to the driving voltage. It is this phase angle which changes with the coupling coefficient between the transmitter coil  24  and receiver coil  23 . 
     The peak detector output depends slightly on the implant power, as explained below with respect to  FIGS. 5 to 10 . The effect of the implant power on the output of the peak detector becomes negligible at maximum link range. 
     As the distance between the coils  23 ,  24  is gradually increased from minimum to maximum link range, a number of effects occur. Firstly, the power delivered to the implant  22  is reduced. Secondly, transmitter losses increase due to increased RF current. 
     These changes determine the behavior of the battery current, whereas the current through the transmitter coils  23 ,  24 , and hence the magnetic field strength, increases monotonically towards an asymptotic value. 
     The peak magnetic field, normal to the pickup coil  30 , depends on the sum of the electric fields produced by the transmitter coil  24  and receiver coil  23 . The peak magnetic field depends slightly on the stimulation parameters, namely the stimulation current and the stimulation rate. The influence of the stimulation parameters is relatively small because the stimulation power represents a small part of the total system power which includes the implant  22  and transmitter coil  24  losses, as follows:
 
Total transmitter coil power=transmitter coil losses+implant losses+stimulation power
 
     On the other hand, the ratio of the stored to dissipated energy is the effective quality factor of the loaded transmitter coil, as follows:
 
 Q =stored energy per cycle/dissipated energy per cycle=reactive power/dissipated power
 
     But Q&gt;&gt;1, therefore Reactive power&gt;&gt;dissipated power, which yields:
 
Reactive power&gt;&gt;transmitter coil losses+implant losses+stimulation power
 
     That is, Reactive power&gt;&gt;Stimulation power. 
     The magnetic field is proportional to the reactive power, which is much higher than the stimulation power. Therefore, the stimulation parameters can only have a second order effect on the peak amplitude of the magnetic field. This is in agreement with the simulation results. 
     The stimulation rate, however, has a stronger effect due to the fact that the peak detector used in  FIG. 2  is not ideal and has a finite decay time constant. 
     The significance of the above discussion is to highlight the fact that, at long link range, the peak detector output is not sensitive to the stimulation current, but is affected by the stimulation rate. This effect must be taken into account when the peak detector output is used to estimate the distance between the coils. 
     One application of the present invention is in estimating a skin flap thickness of a recipient of a cochlear implant system of the type shown in  FIG. 1 , that is, the thickness of skin between the implanted receiver coil  23  and the external transmitter coil  24 . 
     To date, estimating the skin flap thickness has been done in a clinic where the speech processor is powered from the programming system. In this case, specific stimulation parameters are used in order to achieve consistent and repeatable skin flap thickness estimates. 
     However, the circuit of  FIG. 2  can be used to estimate the skin flap thickness. A first method by which the skin flap thickness may be estimated by using the circuit of  FIG. 2 , involves using the recipient&#39;s own speech processor to create the RF magnetic field. This method requires providing a signal path from the peak detector output to the programming system. In this case, the transmitter coil is excited with maximum frame rate at a regulated supply voltage supplied by the programming system. This eliminates the dependency of the peak detector output on the stimulation rate and supply voltage. A look up table stored in the programming system can be used to map the measured voltage to skin flap thickness. 
     A second method by which the skin flap thickness may be estimated by using the circuit of  FIG. 2 , involves using a stand-alone device with built-in oscillator and voltage measurement circuit. In this second method, the stand-alone device is essentially a skin flap thickness meter. The meter contains a 5 MHz crystal oscillator with low output impedance drivers to drive a tuned transmitter coil with continuous 5 MHz square voltage. The transmitter coil contains a pickup coil and a peak detector similar to that shown in  FIG. 2 . The DC output of the peak detector is measured using a built-in analog to digital converter (ADC). The output of the ADC is converted to skin flap thickness, which is then displayed by the meter. 
     Another application of the present invention is in detecting displacement of the external transmitter  24  from the user&#39;s head, for example where the transmitter coil  24  falls off an infant&#39;s head. Such coil-off detection is based on detecting a link range greater than a set threshold value, which would typically be set to around 10-12 mm. Such a circuit solution has to be implemented on the transmitter coil and/or the speech processor. For reliable detection, the circuit has a low sensitivity to battery voltage, stimulation current, stimulation rate, ambient temperature and implant tuning. The circuit also operates without requiring precision measurement of the output voltage of the peak detector. The circuit solution is simple, uses a small number of components and has low current consumption. 
     One manner in which many or all of the above requirements may be met is by comparing the peak detector signal with another reference signal, which has all of the major characteristics of the peak detector signal except its dependency on the coil separation. The reference signal should be generated from a peak detector similar to that shown in  FIG. 2  in order to have the same decay time constant, voltage offsets and temperature characteristics as the measured signal, and should be proportional to the battery voltage to track the changes of the measured signal with the battery voltage. Further, the reference signal should vary with the stimulation rate in a manner similar to that of the measured signal, and should have low sensitivity to the implant power, especially at relatively large link ranges. 
     A simple manner in which the reference signal can be obtained comprises rectifying and peak-detecting the output of the RF drivers of the speech processor, as shown in  FIG. 5 . In  FIG. 5 , the output of the speech processor, in this instance an ESPrit 3G speech processor of the type produced by Cochlear Ltd, is full-wave rectified by D 1  and D 2 . The DC voltage across C 2  tracks the amplitude of the ESPrit 3G RF output voltage. This DC voltage can be made to vary with the stimulation rate in a manner which is similar to that of the voltage across R 1 . This is determined by the time constant:
 
τ 1   =C   2 ·( R   2   +R   3 )
 
     When this time constant is made very small, the voltage across C 2  will strongly depend on the stimulation rate, and vice versa. 
     The voltage divider ratio R 3 /(R 2 +R 3 ) is designed such that the voltage across R 3  is substantially equal to the peak voltage across R 1  at the designated threshold for the maximum link range. The voltage across R 3  is applied to a diode-capacitor (D 4 , C 3 ) peak detector similar to that used with the pickup coil  30 . This is to match the time variation and the temperature characteristics of the measured signal and the reference signal. 
     A voltage comparator is used to compare the measured and reference signals. The output of the comparator can be used to trigger an audible alarm to alert the carers if the transmitter coil is removed. 
     The way the circuit operates is based on matching the amplitudes of the measured and reference signals at the maximum link range. Below that range, the measured signal is smaller and the output of the comparator is disasserted. However, if the separation between the transmitter coil  24  and receiver coil  23  exceeds the maximum link range, the measured signal exceeds the reference signal and triggers the comparator. 
     The recommended component values for typical circuit conditions of the ESPrit 3G are given below. 
     D 1  to D 4 : low cut-in voltage high-speed diodes 
     R 1 =R 4 =1 MΩ 
     R 2 =220 kΩ 
     R 3 ≅100 kΩ 
     C 1 =C 3 =10 nF 
     C 2 =100 pF 
     Pickup coil: printed single turn on the transmitter coil PCB. A single turn from an electrostatic shield can be used. 
     Where the transmitter coil is implemented on a printed circuit board, the circuit of  FIG. 5  can be fully integrated on the PCB of the transmitter coil. The comparator can be replaced with a low voltage-low power low speed operational amplifier. The DC power for the comparator/amplifier can be provided from the RF drivers&#39; signal using a voltage doubler circuit to provide the amplifier with positive and negative DC supply rails. A power cost will be in overloading the RF drivers with the comparator. DC power, which can be as low as 50 uA at 3V. However, this is an insignificant cost compared with the total RF power consumed by the system. The advantage of integrating the circuit on the transmitter coil is that it reduces the number of the coil cable connectors, and substantially guarantees the matching between the circuit components especially with respect to changes with temperature. 
       FIG. 6  is a circuit diagram of a circuit used for verification of coil-off detection, for use with an ESPrit 3G speech processor. The circuit of  FIG. 6  was used to investigate and verify the concept and to study the sensitivity to different circuit and stimulation parameters. The prototype was measured in a laboratory with both SPEAK and 14.2 kHz stimulation, at both quiet and loud sound environments, and at different battery voltages. 
     The circuit is designed for minimum loading on the RF drivers of the ESPrit 3G. It uses a small number of components which can be all mounted on the transmitter coil printed circuit board. The transmitter coil has 3 open tracks on each side used for electrostatic shielding. One shield track (nearly a full turn) is used as the pickup coil. 
     R 1 , R 2  and C 2  form a potential divider and a low pass filter for the RF signal on RFOUT  1 . The filter parameters are chosen such that the peak voltage across C 2  varies with the pulse width of the RF signal. This allows the output V 1  to track the RF power level at different battery voltages. In  FIG. 6  R 1  is a variable resistor to facilitate accurate adjustment for the best detection thresholds. In a non-testing circuit, it is expected that R 1  will be replaced with a fixed resistor. 
     The voltage across C 10  and the voltage across the pickup coil L 2  are peak detected using identical envelope detectors. The DC output V 1  is the coil-off detection voltage threshold. V 2  is the coil-off signal. The voltage V 2  increases as the separation between the transmitter coil and the implanted receiver coil increases. At or above the coil-off detection distance, V 2  exceeds V 1 . 
     The measurement method was as follows. The ESPrit 3G was loaded with 2 patient maps. The first was a 14.2 kpps map while the second was a SPEAK 2 kpps map. A “quiet” sound condition was simulated by removing the microphone and replacing it with a 1 kΩ resistor. A “loud” sound condition was simulated by placing a loud radio close to the microphone. The voltages V 1  and V 2  were measured under the conditions shown in table 1 below. 
     Each of the following tests was carried out at room temperature. A total of 40 tests (table 1) were carried out. During each test the distance was varied from 0 to 14 mm in 2 mm steps, after which the distance was set to more than 10 cm (simulating very large distance). These 40 tests cover the different circuit parameters, in order to demonstrate the sensitivity of the coil-detection method to these parameters. 
     At each distance, the test was repeated 4 times; at stimulation rates of 2000 pps and 14400 pps, and in both “quiet” and “loud” sound environments. The measurements were also repeated at different implant tuning frequencies of 5.1 MHz, 5.25 MHz and 5.4 MHz, and at different supply voltages of 2.7V, 3.0V and 3.3V. 
     To check the sensitivity to the transmitter coil tuning the test was repeated for implant tuning of 5.25 MHz and power supply voltage of 3V. The transmitter coil was tuned to its minimum limit and then to its maximum limit of 4.725 MHz and 4.775 MHz respectively. 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Test Conditions 
               
             
          
           
               
                   
                 Transmitter 
                 Implant Coil 
                   
                   
                   
               
               
                 Test 
                 Coil Tuning 
                 Tuning 
                   
                 Stimulation 
                 Sound 
               
               
                 Number 
                 Frequency 
                 Frequency 
                 VDD 
                 Rate 
                 level 
               
               
                   
               
             
          
           
               
                 1 
                 4.775 MHz 
                  5.1 MHz 
                 2.7 V 
                  2000 pps 
                 Quiet 
               
               
                 2 
                   
                   
                   
                   
                 Loud 
               
               
                 3 
                   
                   
                   
                 14200 pps 
                 Quiet 
               
               
                 4 
                   
                   
                   
                   
                 Loud 
               
               
                 5 
                   
                   
                 3.0 V 
                  2000 pps 
                 Quiet 
               
               
                 6 
                   
                   
                   
                   
                 Loud 
               
               
                 7 
                   
                   
                   
                 14200 pps 
                 Quiet 
               
               
                 8 
                   
                   
                   
                   
                 Loud 
               
               
                 9 
                   
                   
                 3.3 V 
                  2000 pps 
                 Quiet 
               
               
                 10 
                   
                   
                   
                   
                 Loud 
               
               
                 11 
                   
                   
                   
                 14200 pps 
                 Quiet 
               
               
                 12 
                   
                   
                   
                   
                 Loud 
               
               
                 13 
                   
                 5.25 MHz 
                 2.7 V 
                  2000 pps 
                 Quiet 
               
               
                 14 
                   
                   
                   
                   
                 Loud 
               
               
                 15 
                   
                   
                   
                 14200 pps 
                 Quiet 
               
               
                 16 
                   
                   
                   
                   
                 Loud 
               
               
                 17 
                   
                   
                 3.0 V 
                  2000 pps 
                 Quiet 
               
               
                 18 
                   
                   
                   
                   
                 Loud 
               
               
                 19 
                   
                   
                   
                 14200 pps 
                 Quiet 
               
               
                 20 
                   
                   
                   
                   
                 Loud 
               
               
                 21 
                   
                   
                 3.3 V 
                  2000 pps 
                 Quiet 
               
               
                 22 
                   
                   
                   
                   
                 Loud 
               
               
                 23 
                   
                   
                   
                 14200 pps 
                 Quiet 
               
               
                 24 
                   
                   
                   
                   
                 Loud 
               
               
                 25 
                   
                  5.4 MHz 
                 2.7 V 
                  2000 pps 
                 Quiet 
               
               
                 26 
                   
                   
                   
                   
                 Loud 
               
               
                 27 
                   
                   
                   
                 14200 pps 
                 Quiet 
               
               
                 28 
                   
                   
                   
                   
                 Loud 
               
               
                 29 
                   
                   
                 3.0 V 
                  2000 pps 
                 Quiet 
               
               
                 30 
                   
                   
                   
                   
                 Loud 
               
               
                 31 
                   
                   
                   
                 14200 pps 
                 Quiet 
               
               
                 32 
                   
                   
                   
                   
                 Loud 
               
               
                 33 
                   
                   
                 3.3 V 
                  2000 pps 
                 Quiet 
               
               
                 34 
                   
                   
                   
                   
                 Loud 
               
               
                 35 
                   
                   
                   
                 14200 pps 
                 Quiet 
               
               
                 36 
                   
                   
                   
                   
                 Loud 
               
               
                 37 
                 4.725 MHz 
                 5.25 MHz 
                 3.0 V 
                  2000 pps 
                 Quiet 
               
               
                 38 
                   
                   
                   
                   
                 Loud 
               
               
                 39 
                   
                   
                   
                 14200 pps 
                 Quiet 
               
               
                 40 
                   
                   
                   
                   
                 Loud 
               
               
                   
               
             
          
         
       
     
     The test results are set out towards the end of the present specification. The distances at which the measured signal (V 2 ) exceeds the threshold voltage (V 1 ) are highlighted in the results tables. Because the measurements were done at increments of 2 mm, the highlighted points could be equal to or exceed the correct detection point by up to 2 mm. 
       FIG. 7  illustrates the reference and measured voltages, V 1  and V 2  respectively, at 14.2 kpps and 2 kpps in quiet and loud sound environments. The battery voltage was set to 3.3V. The implanted coil was tuned to its nominal frequency of 5.25 MHz.  FIG. 7  shows that the reference voltage is automatically adjusted to a threshold distance of between 12 and 13 mm. Above this threshold, an alarm will be triggered to indicate a coil-off condition. 
       FIG. 8  depicts the reference and measured voltages, V 1  and V 2  respectively, at 14.2 kpps in quiet and loud sound environments, and at supply voltages of 3.3, 3.0 and 2.7V respectively. The implanted coil was tuned to its nominal frequency of 5.25 MHz. These results indicate the detection distance has low sensitivity to the supply voltage, as the point of intersection of the V 1  and V 2  curves varies by only small amounts. 
       FIG. 9  reveals that the coil-off detection distance is reasonably sensitive to the tuning frequency of the implanted coil. When the implant is tuned to 5.4 MHz, the detection threshold distance drops to 8.5 mm. The detection distance increases as the tuning frequency of the implanted coil is reduced to 5.1 MHz. At this frequency, the circuit will detect coil removal if the distance exceeds about 14 mm. 
     The effect of the transmitter coil tuning is shown in  FIG. 10 . The results, at 3V supply voltage and 14.2 kHz stimulation rate, indicate that varying the transmitter coil tuning from 4.725 MHz to 4.775 MHz has substantially no effect on the distance threshold. 
     SUMMARY OF RESULTS 
     High Rate Stimulation 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 Detection 
               
               
                   
                 Test # 
                 Distance 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 3 
                 10.8 
               
               
                   
                 4 
                 11.4 
               
               
                   
                 7 
                 11.8 
               
               
                   
                 8 
                 11.8 
               
               
                   
                 11 
                 14.9 
               
               
                   
                 12 
                 15 
               
               
                   
                 15 
                 9.5 
               
               
                   
                 16 
                 9.5 
               
               
                   
                 19 
                 9.6 
               
               
                   
                 20 
                 9.6 
               
               
                   
                 23 
                 11.9 
               
               
                   
                 24 
                 12.3 
               
               
                   
                 27 
                 6.3 
               
               
                   
                 28 
                 6.2 
               
               
                   
                 31 
                 6.2 
               
               
                   
                 32 
                 6.2 
               
               
                   
                 35 
                 8.5 
               
               
                   
                 36 
                 8.5 
               
               
                   
                   
               
             
          
         
       
     
     The above table shows the coil-off detection threshold distance at all combinations of supply voltage and tuning frequencies. 
     Low Rate Stimulation 
     Similar to the high rate stimulation, the lowest detection distance occurred at low battery voltage and high implant tuning frequency (tests 25, 26, 29 and 30). 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 Detection 
               
               
                   
                 Test # 
                 Distance 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 1 
                 6.5 
               
               
                   
                 2 
                 8.4 
               
               
                   
                 5 
                 7.0 
               
               
                   
                 6 
                 9.3 
               
               
                   
                 9 
                 13.7 
               
               
                   
                 10 
                 13.9 
               
               
                   
                 13 
                 6.7 
               
               
                   
                 14 
                 7.9 
               
               
                   
                 17 
                 6.9 
               
               
                   
                 18 
                 8.8 
               
               
                   
                 21 
                 12 
               
               
                   
                 22 
                 12.7 
               
               
                   
                 25 
                 5.3 
               
               
                   
                 26 
                 5.6 
               
               
                   
                 29 
                 5.5 
               
               
                   
                 30 
                 6.0 
               
               
                   
                 33 
                 9.1 
               
               
                   
                 34 
                 9.2 
               
               
                   
                   
               
             
          
         
       
     
     The measurement results discussed above show the usefulness of the coil-off detection circuit embodiment of the present invention. The method discussed has low sensitivity to most of the circuit parameters and variables, except for the implant tuning if at the upper end of the tuning range. This problem can be easily solved by adding a small DC offset to the reference voltage V 1 . By adjusting the value of that offset a detection distance in the range 8 mm to 15 mm can be achieved for all circuit conditions. 
     While an embodiment of the invention has been discussed in which a threshold detection of a coil-off condition is performed, it is to be appreciated that alternative embodiments of the present invention may be used to estimate an actual distance between implanted and external coils. For example, a look-up table may be experimentally derived from a voltage to distance calibration measurement, such as the voltage measurements revealed in  FIGS. 7 to 10 . Such a look-up table may then be used in converting measured magnetic field strengths to estimated transceiver separation values. Alternatively, a best-fit algorithm may be derived from the measured voltage/distance values, for use in converting measured magnetic field strengths to estimated transceiver separation values. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 APPENDIX 
               
               
                   
               
               
                 Test Results 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Test 1 
                 Test 2 
                 Test 3 
                 Test 4 
               
               
                   
                 2.0 kHz Quiet 
                 2.0 kHz Loud 
                 14.2 kHz Quiet 
                 14.2 kHz Loud 
               
             
          
           
               
                 Distance Mm 
                 V1 (mV) 
                 V2 (mV) 
                 V1 (mV) 
                 V2 (mV) 
                 V1 (mV) 
                 V2 (mV) 
                 V1 (mV) 
                 V2 (mV) 
               
               
                   
               
               
                 0 
                 254 
                 29 
                 306 
                 46 
                 412 
                 93 
                 463 
                 136 
               
               
                 2 
                 258 
                 42 
                 323 
                 65 
                 435 
                 108 
                 502 
                 157 
               
               
                 4 
                 248 
                 90 
                 341 
                 107 
                 453 
                 146 
                 532 
                 194 
               
               
                 6 
                 227 
                 196 
                 343 
                 177 
                 473 
                 205 
                 575 
                 267 
               
               
                 8 
                 228 
                 311 
                 283 
                 270 
                 423 
                 292 
                 519 
                 330 
               
               
                 10 
                 230 
                 387 
                 302 
                 359 
                 414 
                 388 
                 494 
                 436 
               
               
                 12 
                 227 
                 422 
                 288 
                 415 
                 425 
                 463 
                 482 
                 506 
               
               
                 14 
                 225 
                 446 
                 286 
                 462 
                 446 
                 537 
                 478 
                 566 
               
               
                 &gt;100 
                 222 
                 565 
                 270 
                 609 
                 440 
                 789 
                 457 
                 811 
               
               
                   
               
             
          
           
               
                   
                 Test 5 
                 Test 6 
                 Test 7 
                 Test 8 
               
               
                   
                 2.0 kHz Quiet 
                 2.0 kHz Loud 
                 14.2 kHz Quiet 
                 14.2 kHz Loud 
               
             
          
           
               
                 Distance Mm 
                 V1 (mV) 
                 V2 (mV) 
                 V1 (mV) 
                 V2 (mV) 
                 V1 (mV) 
                 V2 (mV) 
                 V1 (mV) 
                 V2 (mV) 
               
               
                   
               
               
                 0 
                 323 
                 56 
                 405 
                 93 
                 634 
                 262 
                 635 
                 263 
               
               
                 2 
                 329 
                 72 
                 405 
                 105 
                 664 
                 274 
                 664 
                 275 
               
               
                 4 
                 318 
                 129 
                 419 
                 155 
                 706 
                 316 
                 707 
                 318 
               
               
                 6 
                 298 
                 234 
                 415 
                 236 
                 746 
                 391 
                 746 
                 391 
               
               
                 8 
                 297 
                 367 
                 400 
                 339 
                 732 
                 477 
                 733 
                 479 
               
               
                 10 
                 300 
                 470 
                 408 
                 442 
                 673 
                 586 
                 673 
                 586 
               
               
                 12 
                 298 
                 509 
                 409 
                 512 
                 662 
                 671 
                 662 
                 671 
               
               
                 14 
                 296 
                 536 
                 412 
                 566 
                 654 
                 741 
                 654 
                 740 
               
               
                 &gt;100 
                 292 
                 654 
                 338 
                 746 
                 627 
                 1005 
                 632 
                 1010 
               
               
                   
               
             
          
           
               
                   
                 Test 9 
                 Test 10 
                 Test 11 
                 Test 12 
               
               
                   
                 2.0 kHz Quiet 
                 2.0 kHz Loud 
                 14.2 kHz Quiet 
                 14.2 kHz Loud 
               
             
          
           
               
                 Distance Mm 
                 V1 (mV) 
                 V2 (mV) 
                 V1 (mV) 
                 V2 (mV) 
                 V1 (mV) 
                 V2 (mV) 
                 V1 (mV) 
                 V2 (mV) 
               
               
                   
               
               
                 0 
                 544 
                 137 
                 633 
                 190 
                 773 
                 340 
                 773 
                 340 
               
               
                 2 
                 530 
                 131 
                 638 
                 192 
                 796 
                 335 
                 797 
                 336 
               
               
                 4 
                 553 
                 169 
                 654 
                 223 
                 835 
                 356 
                 833 
                 357 
               
               
                 6 
                 569 
                 249 
                 685 
                 294 
                 874 
                 421 
                 874 
                 422 
               
               
                 8 
                 585 
                 359 
                 645 
                 382 
                 865 
                 499 
                 865 
                 499 
               
               
                 10 
                 599 
                 473 
                 630 
                 487 
                 811 
                 607 
                 811 
                 607 
               
               
                 12 
                 605 
                 555 
                 631 
                 569 
                 802 
                 697 
                 802 
                 697 
               
               
                 14 
                 612 
                 621 
                 631 
                 635 
                 797 
                 773 
                 797 
                 772 
               
               
                 &gt;100 
                 612 
                 850 
                 623 
                 862 
                 774 
                 1034 
                 774 
                 1033 
               
               
                   
               
             
          
           
               
                   
                 Test 13 
                 Test 14 
                 Test 15 
                 Test 16 
               
               
                   
                 2.0 kHz Quiet 
                 2.0 kHz Loud 
                 14.2 kHz Quiet 
                 14.2 kHz Loud 
               
             
          
           
               
                 Distance Mm 
                 V1 (mV) 
                 V2 (mV) 
                 V1 (mV) 
                 V2 (mV) 
                 V1 (mV) 
                 V2 (mV) 
                 V1 (mV) 
                 V2 (mV) 
               
               
                   
               
               
                 0 
                 254 
                 35 
                 349 
                 74 
                 414 
                 105 
                 473 
                 152 
               
               
                 2 
                 259 
                 57 
                 290 
                 70 
                 430 
                 138 
                 485 
                 180 
               
               
                 4 
                 266 
                 112 
                 306 
                 124 
                 436 
                 194 
                 475 
                 227 
               
               
                 6 
                 224 
                 197 
                 263 
                 200 
                 424 
                 269 
                 476 
                 312 
               
               
                 8 
                 226 
                 280 
                 278 
                 283 
                 417 
                 358 
                 472 
                 409 
               
               
                 10 
                 232 
                 340 
                 272 
                 345 
                 410 
                 431 
                 464 
                 486 
               
               
                 12 
                 225 
                 338 
                 264 
                 389 
                 443 
                 524 
                 463 
                 537 
               
               
                 14 
                 223 
                 418 
                 276 
                 448 
                 439 
                 581 
                 462 
                 608 
               
               
                 &gt;100 
                 220 
                 565 
                 267 
                 612 
                 436 
                 786 
                 456 
                 813 
               
               
                   
               
             
          
           
               
                   
                 Test 17 
                 Test 18 
                 Test 19 
                 Test 20 
               
               
                   
                 2.0 kHz Quiet 
                 2.0 kHz Loud 
                 14.2 kHz Quiet 
                 14.2 kHz Loud 
               
             
          
           
               
                 Distance Mm 
                 V1 (mV) 
                 V2 (mV) 
                 V1 (mV) 
                 V2 (mV) 
                 V1 (mV) 
                 V2 (mV) 
                 V1 (mV) 
                 V2 (mV) 
               
               
                   
               
               
                 0 
                 328 
                 67 
                 415 
                 114 
                 641 
                 276 
                 637 
                 274 
               
               
                 2 
                 334 
                 100 
                 428 
                 145 
                 666 
                 326 
                 661 
                 316 
               
               
                 4 
                 334 
                 176 
                 438 
                 208 
                 679 
                 409 
                 674 
                 385 
               
               
                 6 
                 297 
                 257 
                 459 
                 310 
                 681 
                 487 
                 676 
                 478 
               
               
                 8 
                 298 
                 351 
                 490 
                 372 
                 663 
                 579 
                 660 
                 574 
               
               
                 10 
                 304 
                 422 
                 420 
                 462 
                 646 
                 664 
                 645 
                 662 
               
               
                 12 
                 299 
                 467 
                 405 
                 518 
                 640 
                 730 
                 647 
                 737 
               
               
                 14 
                 296 
                 506 
                 427 
                 592 
                 632 
                 793 
                 640 
                 801 
               
               
                 &gt;100 
                 292 
                 659 
                 432 
                 793 
                 624 
                 1009 
                 635 
                 1022 
               
               
                   
               
             
          
           
               
                   
                 Test 21 
                 Test 22 
                 Test 23 
                 Test 24 
               
               
                   
                 2.0 kHz Quiet 
                 2.0 kHz Loud 
                 14.2 kHz Quiet 
                 14.2 kHz Loud 
               
             
          
           
               
                 Distance Mm 
                 V1 (mV) 
                 V2 (mV) 
                 V1 (mV) 
                 V2 (mV) 
                 V1 (mV) 
                 V2 (mV) 
                 V1 (mV) 
                 V2 (mV) 
               
               
                   
               
               
                 0 
                 585 
                 177 
                 646 
                 216 
                 775 
                 348 
                 774 
                 345 
               
               
                 2 
                 593 
                 206 
                 660 
                 248 
                 797 
                 373 
                 797 
                 377 
               
               
                 4 
                 605 
                 264 
                 653 
                 297 
                 812 
                 437 
                 810 
                 436 
               
               
                 6 
                 601 
                 346 
                 662 
                 383 
                 815 
                 522 
                 815 
                 524 
               
               
                 8 
                 588 
                 485 
                 647 
                 462 
                 798 
                 616 
                 805 
                 610 
               
               
                 10 
                 593 
                 529 
                 642 
                 558 
                 785 
                 712 
                 790 
                 711 
               
               
                 12 
                 594 
                 593 
                 640 
                 618 
                 778 
                 780 
                 786 
                 774 
               
               
                 14 
                 594 
                 648 
                 636 
                 677 
                 773 
                 838 
                 780 
                 838 
               
               
                 &gt;100 
                 594 
                 856 
                 631 
                 874 
                 762 
                 1040 
                 771 
                 1038 
               
               
                   
               
             
          
           
               
                   
                 Test 25 
                 Test 26 
                 Test 27 
                 Test 28 
               
               
                   
                 2.0 kHz Quiet 
                 2.0 kHz Loud 
                 14.2 kHz Quiet 
                 14.2 kHz Loud 
               
             
          
           
               
                 Distance Mm 
                 V1 (mV) 
                 V2 (mV) 
                 V1 (mV) 
                 V2 (mV) 
                 V1 (mV) 
                 V2 (mV) 
                 V1 (mV) 
                 V2 (mV) 
               
               
                   
               
               
                 0 
                 256 
                 50 
                 293 
                 68 
                 417 
                 134 
                 463 
                 173 
               
               
                 2 
                 256 
                 93 
                 317 
                 129 
                 416 
                 200 
                 458 
                 242 
               
               
                 4 
                 257 
                 170 
                 285 
                 192 
                 408 
                 296 
                 456 
                 350 
               
               
                 6 
                 224 
                 259 
                 260 
                 280 
                 410 
                 396 
                 447 
                 439 
               
               
                 8 
                 229 
                 327 
                 265 
                 351 
                 412 
                 488 
                 452 
                 531 
               
               
                 10 
                 229 
                 383 
                 263 
                 406 
                 438 
                 578 
                 461 
                 610 
               
               
                 12 
                 224 
                 426 
                 263 
                 457 
                 439 
                 629 
                 452 
                 646 
               
               
                 14 
                 225 
                 466 
                 258 
                 492 
                 439 
                 671 
                 455 
                 692 
               
               
                 &gt;100 
                 222 
                 582 
                 255 
                 615 
                 440 
                 809 
                 458 
                 833 
               
               
                   
               
             
          
           
               
                   
                 Test 29 
                 Test 30 
                 Test 31 
                 Test 32 
               
               
                   
                 2.0 kHz Quiet 
                 2.0 kHz Loud 
                 14.2 kHz Quiet 
                 14.2 kHz Loud 
               
             
          
           
               
                 Distance Mm 
                 V1 (mV) 
                 V2 (mV) 
                 V1 (mV) 
                 V2 (mV) 
                 V1 (mV) 
                 V2 (mV) 
                 V1 (mV) 
                 V2 (mV) 
               
               
                   
               
               
                 0 
                 325 
                 88 
                 388 
                 124 
                 637 
                 315 
                 637 
                 316 
               
               
                 2 
                 326 
                 141 
                 388 
                 185 
                 647 
                 409 
                 648 
                 409 
               
               
                 4 
                 329 
                 226 
                 406 
                 290 
                 641 
                 517 
                 641 
                 517 
               
               
                 6 
                 293 
                 327 
                 380 
                 379 
                 636 
                 624 
                 636 
                 625 
               
               
                 8 
                 298 
                 407 
                 382 
                 470 
                 631 
                 721 
                 631 
                 721 
               
               
                 10 
                 299 
                 461 
                 395 
                 534 
                 629 
                 785 
                 629 
                 786 
               
               
                 12 
                 295 
                 509 
                 388 
                 581 
                 627 
                 842 
                 627 
                 842 
               
               
                 14 
                 294 
                 553 
                 392 
                 634 
                 628 
                 893 
                 628 
                 893 
               
               
                 &gt;100 
                 292 
                 678 
                 378 
                 758 
                 627 
                 1038 
                 627 
                 1039 
               
               
                   
               
             
          
           
               
                   
                 Test 33 
                 Test 34 
                 Test 35 
                 Test 36 
               
               
                   
                 2.0 kHz Quiet 
                 2.0 kHz Loud 
                 14.2 kHz Quiet 
                 14.2 kHz Loud 
               
             
          
           
               
                 Distance Mm 
                 V1 (mV) 
                 V2 (mV) 
                 V1 (mV) 
                 V2 (mV) 
                 V1 (mV) 
                 V2 (mV) 
                 V1 (mV) 
                 V2 (mV) 
               
               
                   
               
               
                 0 
                 578 
                 204 
                 636 
                 245 
                 777 
                 383 
                 777 
                 386 
               
               
                 2 
                 548 
                 248 
                 644 
                 326 
                 790 
                 474 
                 791 
                 474 
               
               
                 4 
                 570 
                 366 
                 620 
                 406 
                 786 
                 580 
                 786 
                 579 
               
               
                 6 
                 572 
                 462 
                 622 
                 499 
                 783 
                 675 
                 783 
                 675 
               
               
                 8 
                 594 
                 556 
                 628 
                 585 
                 781 
                 760 
                 781 
                 760 
               
               
                 10 
                 606 
                 636 
                 625 
                 654 
                 778 
                 836 
                 778 
                 836 
               
               
                 12 
                 608 
                 690 
                 624 
                 705 
                 777 
                 889 
                 777 
                 888 
               
               
                 14 
                 607 
                 734 
                 628 
                 754 
                 776 
                 933 
                 776 
                 933 
               
               
                 &gt;100 
                 610 
                 878 
                 623 
                 891 
                 774 
                 1070 
                 774 
                 1070 
               
               
                   
               
             
          
           
               
                   
                 Test 37 
                 Test 38 
                 Test 39 
                 Test 40 
               
               
                   
                 2.0 kHz Quiet 
                 2.0 kHz Loud 
                 14.2 kHz Quiet 
                 14.2 kHz Loud 
               
             
          
           
               
                 Distance Mm 
                 V1 (mV) 
                 V2 (mV) 
                 V1 (mV) 
                 V2 (mV) 
                 V1 (mV) 
                 V2 (mV) 
                 V1 (mV) 
                 V2 (mV) 
               
               
                   
               
               
                 0 
                 325 
                 71 
                 405 
                 115 
                 636 
                 286 
                 637 
                 287 
               
               
                 2 
                 330 
                 98 
                 417 
                 146 
                 659 
                 326 
                 659 
                 327 
               
               
                 4 
                 341 
                 166 
                 428 
                 208 
                 671 
                 398 
                 671 
                 398 
               
               
                 6 
                 296 
                 259 
                 423 
                 295 
                 673 
                 488 
                 673 
                 489 
               
               
                 8 
                 294 
                 342 
                 405 
                 378 
                 658 
                 578 
                 658 
                 578 
               
               
                 10 
                 301 
                 403 
                 407 
                 447 
                 639 
                 659 
                 640 
                 660 
               
               
                 12 
                 297 
                 442 
                 412 
                 500 
                 632 
                 716 
                 632 
                 716 
               
               
                 14 
                 293 
                 475 
                 408 
                 549 
                 626 
                 768 
                 627 
                 768 
               
               
                 &gt;100 
                 290 
                 600 
                 403 
                 695 
                 619 
                 943 
                 619 
                 943 
               
               
                   
               
             
          
         
       
     
     It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.