Patent Publication Number: US-9408003-B2

Title: Hearing aid with an antenna

Description:
RELATED APPLICATION DATA 
     This application claims priority to and the benefit of Danish Patent Application No. PA 2013 70664 filed on Nov. 11, 2013, pending, and European Patent Application No. 13192316.1 filed on Nov. 11, 2013, pending. The entire disclosures of both of the above applications are expressly incorporated by reference herein. 
     TECHNICAL FIELD 
     The present disclosure relates to the field of hearing aids having antennas, especially adapted for wireless communication, such as for wireless communication with accessory and/or other hearing aids. 
     BACKGROUND 
     Hearing aids are very small and delicate devices and comprise many electronic and metallic components contained in a housing small enough to fit in the ear canal of a human or behind the outer ear. The many electronic and metallic components in combination with the small size of the hearing aid housing impose high design constraints on radio frequency antennas to be used in hearing aids with wireless communication capabilities. 
     Moreover, the antenna in the hearing aid has to be designed to achieve a satisfactory ear-to-ear performance despite the limitation and other high design constraints imposed by the size of the hearing aid. 
     SUMMARY 
     It is an object to overcome at least some of the disadvantages as mentioned above, and it is a further object to provide a hearing aid. The hearing aid comprises a hearing aid assembly having a first side and a second side, a signal processor, and a wireless communications unit. The wireless communications unit is connected to the signal processor. The hearing aid comprises an antenna for emission and reception of an electromagnetic field. The antenna is connected to the wireless communications unit and the antenna has an excitation point. A first branch of the antenna extends from the excitation point and a second branch of the antenna extends from the excitation point. At least a part of the second branch extends from the first side to the second side. The second branch has at least one ground connection. 
     Typically, the antenna is configured so that current flowing in the antenna forms standing waves along the length of the antenna. The length of an antenna may for example be tailored so that the length of the antenna equals a quarter wavelength of the desired electromagnetic field, or any multiple, or any odd multiple, thereof. In one or more embodiments, an absolute relative difference between the total length of the antenna and the wavelength may be less than a threshold, such as less than 10%, 25%, etc. In some embodiments a total length of the antenna is between three quarters of a wavelength and five quarters of a wavelength. 
     In some embodiments, a current in the antenna may have a maximum in the second branch, such as for example in the part of the second branch which extends from the first side to the second side. 
     The first end may be free, so that the first end may be a free end or an open end. If the first end is free, the current at the end of the first branch may be near zero. Alternatively, the first end may be interconnected with the excitation point via a third branch. The third branch may be different from the first branch. The current in the third branch may have a local maximum near the excitation point, such as a further local maximum. In some embodiments, the third branch extends along the first side of the hearing aid assembly. 
     Likewise, the second end may be free, so that the second end may be a free end or an open end. If the second end is free, the current at the end of the second branch may be near zero. Alternatively, the second end may be interconnected with the excitation point via a fourth branch. The fourth branch may be different from the second branch. In some embodiments, the fourth branch extends along the second side of the hearing aid assembly. 
     In one or more embodiments, the first and/or second branch may form a loop. The loop formed by the first and/or the second branch may return to the excitation point. An advantage of a loop formed by the first and/or the second branch is that it may provide a relatively long total length of the antenna and therefore may improve the ear-to-ear performance of the hearing aid. In some embodiments, the first and/or second branch may be a plate or a dish of conductive material. 
     In some embodiments, the first antenna branch may form a loop along the first side and/or the second antenna branch may form a loop along the second side. 
     At least a part of the second branch extends from the first side to the second side. The part of the second antenna branch may thus extend from proximate the first side of the hearing aid assembly to proximate the second side of the hearing aid assembly, such as from adjacent the first side to adjacent the second side, or the at least part of the second branch may extend from a point or position at or along the first side to a point or position at or along the second side. 
     In some embodiments at least another part of the second branch extends on the second side. 
     At least a part of the first branch may extend along the first side, and/or at least a part of the second branch may extend along the second side. The first side may be a longitudinal side of the hearing aid assembly and the second side may be another longitudinal side of the hearing aid assembly. The first side may be opposite the second side. The second branch may be partly parallel to the first branch. In some embodiments, the part of the first branch extending along the first side of the hearing aid, and the part, i.e. the other part, of the second branch extending along the second side of the hearing aid may be symmetric parts, i.e. so that the said parts form symmetric antenna structures about a plane through the antenna, and/or so that the said parts may have an, at least substantially, same shape. 
     In general, various branches of the antenna may be formed having different geometries, the branches may be wires or patches, bend or straight, long or short as long as they obey the above relative configuration with respect to each other. In some embodiments, a total length of the antenna is between three quarters of a wavelength and five quarters of a wavelength. 
     The hearing aid may be a behind-the-ear hearing aid configured to be positioned behind the ear of the user during use, and the first side may be a first longitudinal side of the hearing aid and the second side may be a second longitudinal side of the hearing aid. The antenna may be accommodated in the housing with its longitudinal direction along the length of the housing. Preferably, the antenna is accommodated within the hearing aid housing, preferably so that the antenna is positioned inside the hearing aid housing without protruding out of the housing. 
     Typically, an excitation point is electrically connected to a source, such as the wireless communication unit, such as a radio chip, such as a transceiver, a receiver, a transmitter, etc. The antenna may be excited using any conventional means, using a direct or an indirect or coupled feed, and for example be fed using a feed line, such as a transmission line. The current induced in the antenna may have a first local maximum at a proximate excitation point of the antenna. 
     The first branch of the antenna may extend from the excitation point to a first end of the antenna, and the second branch of the antenna may extend from the excitation point to a second end of the antenna. The antenna may be structured with two branches extending from the same excitation point. 
     A first distance from the excitation point to the first end may be smaller than a second distance from the excitation point to the second end. In some embodiments, the relative difference between the first distance and the second distance may be less than 25%, such as less than 10%. The distance may be measured along the first branch and along the second branch, respectively. 
     In some embodiments, the excitation point may be provided at an edge part of the hearing aid assembly. The excitation point may be interconnected with the wireless communications unit for example via transmission lines. 
     The antenna may be configured with a length and a structure so that a current in the antenna may have a magnitude of zero at a point on the first branch and/or at a point on the second branch. 
     The hearing aid with the antenna may be configured so that the second branch of the antenna has a ground connection. By providing a ground connection at the second branch, the antenna may have better tuning properties, and may be less dependent on the ground potential of the printed circuit board. 
     The antenna may be interconnected with the ground plane at some point along the second branch, for example by providing a transmission line from the antenna to a ground plane of the hearing aid, such as for example to a printed circuit board of the hearing aid. 
     The ground connection may be provided along the second side of the hearing aid, such as at any point of the antenna extending along the second side of the hearing aid. In some embodiments, the ground connection is provided at at least a distance of ⅛ of a wavelength from the second end. 
     In one or more embodiments, the hearing aid with the antenna may be structured so that the antenna excitation point is positioned at the first side of the hearing aid and the antenna ground connection is positioned at the second side of the hearing aid. 
     The part of the second branch extending from the first side to the second side, may extend from the first side to the second side via a midpoint of the hearing aid. The midpoint may be an absolute geometric midpoint, or the midpoint may be an approximate midpoint provided within an interval, such as within an interval of +/−5%, +/−10%, +/−15%, etc. of the geometric midpoint. Typically, the midpoint will be a midpoint at an outer side of the hearing aid assembly for the antenna to extend from the first side to the second side via the midpoint. 
     The midpoint of the hearing aid may be positioned at a partition plane for the hearing aid, so that the partition plane defines a partitioning of the hearing aid in a first part and a second part. The partition plane may partition the hearing aid in two equal parts, and may e.g. define a middle of the hearing aid. 
     A distance from the midpoint to the excitation point and a distance from the midpoint to the ground connection may be of equal size. The relative difference between the distance from the midpoint to the excitation point and the distance from the midpoint to the ground connection may be less than a threshold T2. The threshold T2 may be e.g. 25%, or 10%. 
     In some embodiments, the first antenna branch has a first length and the second antenna branch has a second length, and wherein the sum of the first length and the second length may correspond to at least 90% of a total length of the antenna. 
     The length of the first branch and/or the length of the second branch may be at least λ/4, such as substantially λ/4, such as at least λ/4+/−10%. 
     The first length may correspond to the second length, so that the first and second branches have a same length, or the first length of the first branch may be different from the length of the second branch. 
     The first branch may have a first length and the second branch may have a second length. The first length may be different from the second length, and in one or more embodiments, the second length may be longer than the first length. The length of the first or the second branch may be equal to, such as substantially equal to λ/4, where λ corresponds to the frequency of the wireless communications unit. The first length and/or the second length may be at least λ/4. 
     The antenna may be a monopole antenna. 
     The hearing aid disclosed herein may be configured for operation in ISM frequency band. Preferably, the antennas are configured for operation at a frequency of at least 1 GHz, such as at a frequency between 1.5 GHz and 3 GHz such as at a frequency of 2.4 GHz. 
     A hearing aid with an assembly, the assembly includes: a first side; a second side; a signal processor; a wireless communications unit, the wireless communications unit being connected to the signal processor; and an antenna for electromagnetic field emission and electromagnetic field reception, the antenna being connected to the wireless communications unit, the antenna having an excitation point; wherein a first branch of the antenna extends from the excitation point and a second branch of the antenna extends from the excitation point, at least a part of the second branch extending from the first side to the second side, and wherein the second branch has at least one ground connection. 
     Optionally, at least a part of the first branch extends along the first side, and/or wherein at least a part of the second branch extends along the second side. 
     Optionally, the ground connection is at the second side. 
     Optionally, the first branch of the antenna extends from the excitation point to a first end, and wherein the second branch of the antenna extends from the excitation point to a second end. 
     Optionally, the first end and/or the second end is free, or wherein the first end and/or the second end is interconnected with the excitation point via a third and/or forth branch. 
     Optionally, an interconnection with a ground plane is at at least a distance of ⅛ of a wavelength of an electromagnetic field emitted by an antenna from the second end. 
     Optionally, the antenna is a monopole antenna. 
     Optionally, the excitation point is at the first side of the assembly. 
     Optionally, the third branch is different from the first branch, and/or wherein the forth branch is different from the second branch. 
     Optionally, the first branch forms a loop and/or the second branch forms a loop. 
     Optionally, the first side is opposite the second side, and wherein the first side is a first longitudinal side of the assembly and the second side is a second longitudinal side of the assembly. 
     Optionally, a part of the first branch extends along the first side, a part of the second branch extends along the second side, and the part of the first branch and the part of the second branch are symmetric. 
     Optionally, the hearing aid is a behind-the-ear hearing aid configured to be positioned behind an ear of a user during use, and wherein the first side is a first longitudinal side of the hearing aid and the second side is a second longitudinal side of the hearing aid. 
     Optionally, the at least a part of the second branch extending from the first side to the second side, extends from the first side to the second side via a midpoint of the hearing aid. 
     Optionally, a relative difference between (1) a distance from the midpoint to the excitation point and (2) a distance from the midpoint to the at least one ground connection is less than a threshold. 
     Other aspects and features will be evident from reading the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a block-diagram of a hearing aid, 
         FIGS. 2 a - b    show schematically an exemplary implementation of a hearing aid comprising an antenna according to an embodiment of the present disclosure, 
         FIG. 3  shows schematically an exemplary implementation of a hearing aid comprising an antenna according to an embodiment of the present disclosure, 
         FIG. 4  shows schematically an exemplary implementation of a hearing aid comprising an antenna according to an embodiment of the present disclosure, 
         FIGS. 5 a  and 5 b    show schematically an exemplary implementation of an antenna for a hearing aid according to an embodiment of the present disclosure, 
         FIG. 6  shows schematically an exemplary implementation of an antenna according to an embodiment of the present disclosure, 
         FIG. 7  shows schematically an exemplary implementation of an antenna according to an embodiment of the present disclosure, 
         FIG. 8  is a 3D illustration of a behind-the-ear hearing aid having an exemplary antenna, 
         FIGS. 9 a - b    show a hearing aid positioned on the right and left ear of a user&#39;s head with the hearing aid comprising an antenna according to an embodiment of this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments are described hereinafter with reference to the figures, in which exemplary embodiments are shown. The claimed invention may, however, be embodied in different forms and should not be construed as being limited to the embodiments set forth herein. Like reference numerals refer to like elements throughout. Like elements will, thus, not be described in detail with respect to the description of each figure. It should also be noted that the figures are only intended to facilitate the description of the embodiments. They are not intended as an exhaustive description of the claimed invention or as a limitation on the scope of the claimed invention. In addition, an illustrated embodiment needs not have all the aspects or advantages shown. An aspect or an advantage described in conjunction with a particular embodiment is not necessarily limited to that embodiment and can be practiced in any other embodiments even if not so illustrated, or if not so explicitly described. 
     In the following the embodiments are described primarily with reference to a hearing aid, such as a binaural hearing aid. It is however envisaged that the disclosed features and embodiments may be used in combination with any aspect described herein. 
     As used herein, the term “antenna” refers to an electrical device which converts electric power into radio waves. An antenna, such as an electric antenna, may comprise an electrically conductive material connected to e.g. a wireless communications unit, such as a radio chip, a receiver or a transmitter. 
       FIG. 1  shows a block-diagram of a hearing aid. In  FIG. 1 , the hearing aid  10  comprises a microphone  11  for receiving incoming sound and converting it into an audio signal, i.e. a first audio signal. The first audio signal is provided to a signal processor  12  for processing the first audio signal into a second audio signal compensating a hearing loss of a user of the hearing aid. A receiver is connected to an output of the signal processor  12  for converting the second audio signal into an output sound signal, e.g. a signal modified to compensate for a users hearing impairment, and provides the output sound to a speaker  13 . Thus, the hearing instrument signal processor  12  may comprise elements such as amplifiers, compressors and noise reduction systems etc. The hearing aid may further have a feedback loop for optimizing the output signal. The hearing aid has a wireless communication unit  14  (e.g. a transceiver) for wireless communication interconnected with an antenna  15  for emission and reception of an electromagnetic field. The wireless communication unit  14  may connect to the hearing aid signal processor  12  and an antenna  15 , for communicating with external devices, or with another hearing aid, located at another ear, in a binaural hearing aid system. 
     The specific wavelength, and thus the frequency of the emitted electromagnetic field, is of importance when considering communication involving an obstacle. In one or more embodiments, the obstacle is a head with a hearing aid comprising an antenna located closed to the surface of the head. If the wavelength is too long such as a frequency of 1 GHz and down to lower frequencies greater parts of the head will be located in the near field region. This results in a different diffraction making it more difficult for the electromagnetic field to travel around the head. If on the other hand the wavelength is too short, the head will appear as being too large an obstacle which also makes it difficult for electromagnetic waves to travel around the head. An optimum between long and short wavelengths is therefore preferred. In general the ear to ear communication is to be done in the band for industry, science and medical with a desired frequency centred around 2.4 GHz. 
       FIG. 2 a    shows schematically an embodiment of a hearing aid  20  comprising an antenna  25 , a wireless communications unit  24  and a ground plane  26 . Antenna  25  comprises an excitation point  23 , a first branch  21 , and a second branch  22 . The first branch  21  extends from the excitation point  23 . The second branch  22  extends from the excitation point  23 . The first branch  21  and the second branch  22  may extend from the excitation point  23  in different directions. The excitation point  23  is connected to the wireless communications unit  24  via a transmission line  27 . A part  221  of the second branch  22  extends from a first side of the hearing aid  20  to a second side of the hearing aid  20 . 
       FIG. 2 b    shows schematically another embodiment of a hearing aid  20 . The hearing aid  20  corresponds to the hearing aid in  FIG. 2 a   . Additionally, the hearing aid as shown in  FIG. 2 b    has a ground connection  223 , connecting the second branch  22  with the ground plane  26  which may be a printed circuit board. The ground connection  223  is positioned across from the excitation point for the antenna, and the distance from the midpoint  222  to the excitation point  23  and the distance from the midpoint  222  to the ground connection  223  may be substantially equal. 
     A distance from the midpoint  222  to the excitation point  23  and a distance from the midpoint  222  to the ground connection  223  may be of equal size. The relative difference between the distance from the midpoint  222  to the excitation point  23  and the distance from the midpoint  222  to the ground connection  223  may be less than a threshold T2. The threshold T2 may be e.g. 25%, or 10%. 
     The partition plane  224  may be a symmetry plane  224  for the antenna  25  so that the shape of the first branch  21  of the antenna is symmetric with the shape of the second branch  22  of the antenna with respect to the symmetry plane  224 , irrespective of the ground connection  223 . The partition plane  224  may extend exactly mid through the hearing aid, or the partition plane may extend anywhere between a first side of the hearing aid and a second side of the hearing aid. 
     In general, various branches of the antenna may be formed with different geometries, they may be wires or patches, bend or straight, long or short as long as they obey the above relative configuration with respect to each other such that the antenna comprises an excitation point, a first branch of the antenna extending from the excitation point and a second branch of the antenna extending from the excitation point and such that the first branch has a first end, the first end being free or being interconnected with the excitation point via a third branch and such that at least a part of the second branch extends from the first side to the second side. 
       FIG. 3  shows schematically an embodiment of a hearing aid  30  according to the present disclosure. The hearing aid  30  comprises an antenna  35 . The antenna  35  comprises an excitation point  33 , a first branch  31 , and a second branch  32 . The first branch  31  extends from the excitation point  33 . The second branch  32  extends from the excitation point  33 . The second branch  32  comprises a part  321  that extends from the first side to the second side, wherein the part  321  extends from the excitation point  33  to the second side in a curve. The first branch  31  and/or the second branch  32  may have any width and/or any shape configured according to hearing aid restrictions and/or antenna optimization. 
       FIG. 4  shows schematically an embodiment of a hearing aid  40  according to the present disclosure. The hearing aid  40  comprises an antenna  45 . The antenna  45  comprises an excitation point  43 , a first branch  41 , and a second branch  42 . The first branch  41  extends from the excitation point  43  to a first end  412 . The second branch  42  extends from the excitation point  43  to a second end  422 . In  FIG. 4 , the second branch  42  comprises a part  421  that extends from a first side of the hearing aid  40  to a second of the hearing aid  40 . The part  421  extends from the excitation point  43  positioned at an intersection of the first branch  41  with the second branch  42 , wherein the part  421  extends from a first side to a second side directly from the excitation point to thereby obtain a high current at the bridge. The first end  412  and/or the second end  422  may be a free end. The current is seen to be zero at the free ends  412 ,  422  of the antenna  45 . The ends  412 ,  422  may also be open or have an infinite impedance. Alternatively, the first end  412  and/or the second end  422  may be interconnected with the excitation point  43  via a third and/or forth branch. The third branch may be different from the first branch, and/or the forth branch may be different from the second branch. 
       FIG. 5 a    shows schematically an embodiment of a hearing aid having an antenna according to the present disclosure. The antenna  55  comprises an excitation point  53 , a first branch  51 , and a second branch  52 . The first branch  51  has a first length and the second branch  52  has a second length. The first length and the second length are seen to be different. The second length is longer than the first length. In  FIG. 5 a   , a first distance d1 from the excitation point to the first end is smaller than a second distance d2 from the excitation point to the second end. The first or second length may be equal to the first distance d1 or the second distance d2 respectively. The distance is typically measured along the first branch  51  and the second branch  52 , respectively. 
     The relative difference between the first distance d1 and the second distance d2 may be less than a threshold T1. The threshold T1 may be e.g. 25%, or 10%. The antenna  55  may be formed so that the distances d1 and d2 fulfil the following: 
                         d   2     &gt;     d   1       ,       d   1     ≈       1   4     ⁢   λ         ⁢     
     ⁢       0   &lt;              d   1     -     d   2         d   2            &lt;     T   1       ,       T   1     =     25   ⁢   %       ,     10   ⁢   %               (   1   )               
wherein λ is the wavelength. In one or more embodiments, the first length and/or the second length is at least λ/4.
 
       FIG. 5 b    shows schematically another embodiment of a hearing aid having an antenna according to the present disclosure. The antenna  55  comprises an excitation point  53 , a first branch  51 , and a second branch  52 . The first branch  51  has a first length and the second branch  52  has a second length. The first length and the second length are seen to be similar or identical. The second length is the same length as the first length. In  FIG. 5 b   , a first distance d1 from the excitation point to the first end is the same as a second distance d2 from the excitation point to the second end. The first or second length may be equal to the first distance d1 or the second distance d2 respectively. The distance is typically measured along the first branch  51  and the second branch  52 , respectively. 
     The length of the first and/or second branches  51 ,  52  is at least λ/4 (where λ is the resonance wavelength for the wireless communications unit). 
       FIG. 6  shows schematically an embodiment of a hearing aid having an antenna according to the present disclosure. The antenna  65  comprises an excitation point  63 , a first branch  61 , and a second branch  62 . The first branch  61  is a plate. The second branch  62  comprises a plate and a bridge  621 . The bridge  621  is a conducting element connecting the two plates, i.e. the first branch  61  and the second branch  62 . In one or more embodiments, the length of the antenna branch may be measured along a top part of a plate forming the first and/or second branch  61 ,  62  is at least λ/8 and the length along a side part of a plate forming the first and/or second branch  61 ,  62  is at least λ/8, thus having a total first and/or second length along the current path of at least λ/4. 
       FIG. 7  shows schematically an embodiment of a hearing aid having an antenna according to the present disclosure. The antenna  75  comprises an excitation point  73 , a first branch  71 , and a second branch  72 . The first branch  71  forms a loop. The second branch  72  forms a loop and further comprises a bridge  721 . The length d3 of the loop forming part of the second branch  72  may be small or it may be greater than λ/4. If the length d3 is greater than λ/4, the current has a zero at a point on the loop. The exact location of the zero depends on the magnitude of the current at the start of the loop (where the loop of the second branch  72  connects with the bridge  721 ) and the length d3 of the loop. 
       FIG. 8  is a 3D illustration of an exemplary behind-the-ear hearing aid having an antenna. 
       FIG. 8  shows a behind-the-ear hearing aid  110  configured to be positioned behind the ear of the user during use. The behind-the-ear hearing aid  110  comprises an antenna  115 , a wireless communication unit  119  (e.g. a radio chip) with a transmission line  119   a  to an antenna  115 , a battery  116 , a signal processor  117  and a sound tube  118  leading to the entrance of the ear canal. The antenna  115  comprises an excitation point  113 , a first branch  111 , and a second branch  120 . The second branch  120  comprises a part  121  extending from a first side  130  of the hearing aid assembly to a second side  140  of the hearing aid assembly. The first side  130  of the hearing aid assembly is opposite the second side  140  of the hearing aid assembly  110 . The excitation point  113  is at the first side  130  of the hearing aid assembly. The first branch  111  may in one or more embodiments be a first structure, such as a first resonant structure, provided proximate the first side  130  of the hearing aid, and the second part  120  of the antenna  115  may in one or more embodiments a second structure, such as a second resonant structure, provided proximate a second side  140  of the hearing aid. At least a part of the first branch  111  extends on the first side  130 . At least a part of the second branch  120  extends on the second side  140 . The first side  130  or the second side  140  is positioned parallel with the surface of the head of the user when the hearing aid is worn in its operational position by the user. The first side  130  is a first longitudinal side of the hearing aid  110 . The second side  140  is a second longitudinal side of the hearing aid  110 . 
       FIGS. 9 a - b    show an exemplary behind-the-ear hearing aid worn in its operational position by a user.  FIG. 9 a    shows the behind-the-ear hearing aid  150  placed on the right ear of the user. The behind-the-ear hearing aid  150  comprises an antenna  155 . 
     The antenna  155  comprises a first branch  151  and a second branch  152 . The first branch  151  of the antenna is on the side of the hearing aid  150  facing away from the head of the user. 
       FIG. 9 b    shows the behind-the-ear hearing aid  150  placed on the left ear of the user. 
     In  FIG. 9 b   , the second branch  152  (i.e. the other branch than the one shown in  FIG. 9 a   ) is on the side of the hearing aid  150  facing away from the head of the user. 
       FIGS. 9 a - b    illustrates the symmetry of the antenna implemented in a hearing aid according to this disclosure. The hearing aid disclosed herein is configured to be operational whether it is placed on the right ear or on the left ear. 
     The antenna  155  emits an electromagnetic field that propagates in a direction parallel to the surface of the head of the user when the hearing aid housing is positioned in its operational position during use, whereby the electric field of the emitted electromagnetic field has a direction that is orthogonal to, or substantially orthogonal to, the surface of the head during operation. In this way, propagation loss in the tissue of the head is reduced as compared to propagation loss of an electromagnetic field with an electric field component that is parallel to the surface of the head. Diffraction around the head makes the electromagnetic field emitted by the antenna propagate from one ear and around the head to the opposite ear. 
     Although particular embodiments have been shown and described, it will be understood that it is not intended to limit the claimed inventions to the preferred embodiments, and it will be obvious to those skilled in the art that various changes and modifications may be made without department from the spirit and scope of the claimed inventions. The specification and drawings are, accordingly, to be regarded in an illustrative rather than restrictive sense. The claimed inventions are intended to cover alternatives, modifications, and equivalents.