Patent Publication Number: US-2022229021-A1

Title: Ultrasound probe

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of PCT International Application No. PCT/JP2019/046754 filed on Nov. 29, 2019, which is hereby expressly incorporated by reference into the present application. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an ultrasound probe. 
     BACKGROUND ART 
     As for inspection methods capable of inspecting the interior of an object nondestructively, inspection methods using an ultrasonic wave are known. For ultrasonic nondestructive inspections, an ultrasound probe using a thickness longitudinal oscillation of a piezoelectric element is typically used. 
     On the other hand, as for methods of performing generation and detection of an ultrasonic wave for a test object, such as a metal plate or tube, in a noncontact manner, methods using an electromagnetic acoustic transducer (EMAT) (referred to as EMAT hereinafter) are known (for example, refer to Patent Literature 1). An EMAT typically includes a coil through which a high frequency current flows, and a magnetic field generation portion, such as a permanent magnet, which provides a static magnetic field for a test object. The EMAT causes a Lorentz force to occur by means of the interaction between an eddy current occurring in a front surface of the test object because of the high frequency oscillation of the current flowing through the coil, and the static magnetic field caused in the test object by the magnetic field generation portion, so that the Lorentz force causes the test object to vibrate. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP 2002-277336 A 
     SUMMARY OF INVENTION 
     Technical Problem 
     For example, when performing an inspection of a concrete structure which constitutes a large-scale facility, it is desirable to use an ultrasonic wave of low frequency in consideration of a propagation loss and so on. However, there arises a problem that in order for such an ultrasound probe as above using a piezoelectric element to excite an ultrasonic wave of low frequency, it is necessary to increase the size of the ultrasound probe. 
     Further, in the case of using an EMAT, because it is necessary to cause an eddy current to occur in a front surface of a test object, as mentioned above, the test object is limited to a conductor such as a metal object. Therefore, a problem with EMATs is that it is impossible to perform an inspection on anon-conductive test object such as a concrete structure. 
     The present invention is made in order to solve the above-mentioned problems, and it is therefore an object of the present invention to provide a technique capable of achieving downsizing of an ultrasound probe and performing an ultrasonic nondestructive inspection on a non-conductive test object. 
     Solution to Problem 
     An ultrasound probe according to the present invention, includes: a metal case including a bottom wall portion having a vibration surface, and a side wall portion connected to the bottom wall portion; a coil mounted on one surface of the side wall portion; and a magnet mounted in such a way as to sandwich the coil between the magnet and the one surface of the side wall portion, in which the coil causes an eddy current to occur in the one surface of the side wall portion, and causes the bottom wall portion to vibrate, by using a Lorentz force occurring in the side wall portion because of the interaction between the eddy current and a magnetic field generated by the magnet. 
     Advantageous Effects of Invention 
     According to the present invention, downsizing of the ultrasound probe can be achieved and an ultrasonic nondestructive inspection can be performed on a non-conductive test object. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram showing the structure of an ultrasound probe according to Embodiment 1; 
         FIG. 2  is a cross-sectional view of the ultrasound probe taken along the dotted line AA′ shown in  FIG. 1 ; 
         FIG. 3  is a view showing a result of determining the shape of a metal case according to Embodiment 1 after deformation when a stress caused by a Lorentz force is applied to a side wall portion of the metal case, by means of simulation; 
         FIG. 4  is a graph showing a vibration displacement on a center line of a vibration surface of a bottom wall portion of the metal case according to Embodiment 1 and in a direction perpendicular to the vibration surface; 
         FIG. 5  is a top view and a side view showing the structure of an ultrasound probe according to a first variant of Embodiment 1; 
         FIG. 6  is a top view and a side view showing the structure of an ultrasound probe according to a second variant of Embodiment 1; 
         FIG. 7  is a cross-sectional view showing the structure of an ultrasound probe according to Embodiment 2; 
         FIG. 8  is a view showing a result of determining the shape of a metal case according to Embodiment 2 after deformation when a stress caused by a Lorentz force is applied to a side wall portion of the metal case, by means of simulation; 
         FIG. 9  is a graph showing a vibration displacement on a center line of a vibration surface of an outer bottom wall portion of the metal case according to Embodiment 2 and in a direction perpendicular to the vibration surface; 
         FIG. 10  is a top view and a side view showing the structure of an ultrasound probe according to a first variant of Embodiment 2; 
         FIG. 11  is a top view and a side view showing the structure of an ultrasound probe according to a second variant of Embodiment 2; 
         FIG. 12  is a cross-sectional view showing the structure of an ultrasound probe according to Embodiment 3; 
         FIG. 13  is a view showing a result of determining the shape of a metal case according to Embodiment 3 after deformation when a stress caused by a Lorentz force is applied to a side wall portion of the metal case, by means of simulation; 
         FIG. 14  is a graph showing a stress on a center line of a vibration surface of an outer bottom wall portion of the metal case according to Embodiment 3, the stress being on the vibration surface; 
         FIG. 15  is a top view and a side view showing the structure of an ultrasound probe according to a first variant of Embodiment 3; and 
         FIG. 16  is a top view and a side view showing the structure of an ultrasound probe according to a second variant of Embodiment 3. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, in order to explain the present invention in greater detail, embodiments of the present invention will be explained with reference to the accompanying drawings. 
     Embodiment 1 
       FIG. 1  is a schematic diagram showing the structure of an ultrasound probe  100  according to Embodiment 1.  FIG. 2  is a cross-sectional view of the ultrasound probe  100  taken along the dotted line AA′ shown in  FIG. 1 . As shown in  FIGS. 1 and 2 , the ultrasound probe  100  includes a coil  1 , magnets  2  and a metal case  3 . 
     The metal case  3  includes a bottom wall portion  3   b  having a vibration surface  3   c,  and a side wall portion  3   a  connected to the bottom wall portion  3   b.  In Embodiment 1, the side wall portion  3   a  of the metal case  3  includes a first flat-shaped side wall portion  3   d  and a second flat-shaped side wall portion  3   e  which face each other. 
     Further, in Embodiment 1, the bottom wall portion  3   b  of the metal case  3  includes a single flat-shaped bottom wall portion, and its flat surface opposite to the vibration surface  3   c  is connected to ends of the respective first and second side wall portions  3   d  and  3   e.  As a result, the metal case  3  according to Embodiment 1 has a shape of the letter U. The material of the metal case  3  is, for example, a conductive material such as aluminum. 
     The coil  1  is mounted on one surface of the side wall portion  3   a  of the metal case  3 . In further detail, in Embodiment 1, the ultrasound probe  100  includes, as the coil  1 , a first coil  1   a  mounted on a first surface of the first side wall portion  3   d,  the first surface being opposite to a second surface, of the first side wall portion  3   d,  facing the second side wall portion  3   e,  and a second coil  1   b  mounted on a first surface of the second side wall portion  3   e,  the first surface being opposite to a second surface, of the second side wall portion  3   e,  facing the first side wall portion  3   d.  Although the structure in which the ultrasound probe  100  includes the two coils: the first coil  1   a  and the second coil  1   b  is explained in Embodiment 1, the ultrasound probe  100  may include either a single coil or three or more coils. 
     In Embodiment 1, each of the first and second coils  1   a  and  1   b  is one in which a conducting wire made of metal such as copper is wound two or more times in the shape of a track. Further, each of the first and second coils  1   a  and  1   b  is connected to a not-illustrated alternating current power supply. 
     The magnet  2  is mounted in such a way as to sandwich the coil  1  between the magnet and the one surface of the side wall portion  3   a.  In Embodiment 1, the ultrasound probe  100  includes, as the magnet  2 , at least one or more magnets  2  mounted in such a way as to sandwich the first coil  1   a  between the one or more magnets and the first surface of the first side wall portion  3   d,  the first surface being opposite to the second surface, of the first side wall portion  3   d,  facing the second side wall portion  3   e,  and at least one or more magnets  2  mounted in such a way as to sandwich the second coil  1   b  between the one or more magnets and the first surface of the second side wall portion  3   e,  the first surface being opposite to the second surface, of the second side wall portion  3   e,  facing the first side wall portion  3   d.  Although four magnets  2  in total are shown in  FIG. 2 , the number of magnets  2  which the ultrasound probe  100  includes is not especially limited. 
     Next, the operation of the ultrasound probe  100  according to Embodiment 1 will be explained. The coil  1  causes an eddy current to occur in the one surface of the side wall portion  3   a,  and then causes the bottom wall portion  3   b  to vibrate, by using a Lorentz force occurring in the side wall portion  3   a  because of the interaction between the eddy current and a magnetic field generated by the magnet  2 . In other words, by using the same method as that of above-mentioned EMATs, the coil  1  causes a vibration to occur in the side wall portion  3   a  and thus causes the bottom wall portion  3   b  to vibrate. 
     In further detail, for example, when an alternating current flows through the first coil  1   a,  as shown in  FIG. 2 , an eddy current  4  of reversed polarity flowing in a direction opposite to a direction in which the alternating current flows occurs in one surface of the first side wall portion  3   d.  On the other hand, a magnet  2  shown in an upper left part of  FIG. 2  causes a static magnetic field  5  to occur in the one surface of the first side wall portion  3   d.  The interaction between the eddy current  4  and the static magnetic field  5  causes a Lorentz force  6  to occur in the one surface of the first side wall portion  3   d.    
     Here, because the alternating current flows through the first coil  1   a,  the polarity of the eddy current  4  changes in response to the polarity of the current flowing through the first coil  1   a.  Because, as a result of this, the direction of the Lorentz force  6  also changes, a vibration occurs in the first side wall portion  3   d  because of the change in the direction of the Lorentz force  6 . 
     In Embodiment 1, the coil  1  has a straight line portion  1   c  parallel to the vibration surface  3   c  of the bottom wall portion  3   b,  and the magnet  2  is mounted in such a way as to sandwich the straight line portion  1   c  of the coil  1  between the magnet and the one surface of the side wall portion  3   a.  For example, the first coil  1   a  has a straight line portion  1   c  parallel to the vibration surface  3   c  of the bottom wall portion  3   b,  and a magnet  2  is mounted in such a way as to sandwich the straight line portion  1   c  of the coil  1  between the magnet and the one surface of the first side wall portion  3   d.    
     According to the structure, the direction of the Lorentz force  6  occurring in the above-mentioned way in the first side wall portion  3   d  is perpendicular to the vibration surface  3   c  of the bottom wall portion  3   b,  and a bending vibration occurs in the bottom wall portion  3   b.  Therefore, when a not-illustrated test object is brought into contact with the vibration surface  3   c  of the bottom wall portion  3   b,  an ultrasonic wave is excited in the test object by the bending vibration. 
     Further, because the bending vibration of the bottom wall portion  3   b  responds to the high-frequency oscillation of the current flowing through the coil  1 , the frequency of the ultrasonic wave excited in the test object by the bending vibration of the bottom wall portion  3   b  is determined by the frequency of the current flowing through the coil  1 . Therefore, because the frequency of the ultrasonic wave excited in the test object does not depend on the size of the ultrasound probe  100 , it is possible to excite an ultrasonic wave of low frequency in the test object without increasing the size and the weight of the ultrasound probe  100 . 
     Next, a result of a simulation of the operation of the ultrasound probe  100  according to Embodiment 1 will be explained by reference to drawings.  FIG. 3  is a view showing a result of determining the shape of the metal case  3  after deformation when stresses caused by Lorentz forces mentioned previously are applied to the first side wall portion  3   d  and the second side wall portion  3   e  of the metal case  3 , by means of the simulation.  FIG. 4  is a graph showing a vibration displacement on a center line of the vibration surface  3   c  of the bottom wall portion  3   b  and in a direction perpendicular to the vibration surface  3   c.  The center line of the vibration surface  3   c  is a dotted line BB′ shown in  FIG. 3 . Further, the horizontal axis of the graph shown in  FIG. 4  shows positions on the center line BB′ in a case where the center of the vibration surface  3   c  is set as the point of origin, and the vertical axis of the graph shown in  FIG. 4  shows the magnitude of the vibration displacement in the direction perpendicular to the vibration surface  3   c.    
     In the vibration surface  3   c  of  FIG. 3 , a dark portion compared with the surrounding area is a bending portion. Further, it is shown in  FIG. 4  that the vibration displacement becomes larger with increasing proximity to the center of the vibration surface  3   c.  More specifically, it can be recognized that the bottom wall portion  3   b  has a bending vibration. This bending vibration makes it possible to excite an ultrasonic wave in the test object. 
     Next, a first variant of the ultrasound probe  100  will be explained by reference to a drawing.  FIG. 5  is a top view and a side view showing the structure of an ultrasound probe  101  according to the first variant. As shown in  FIG. 5 , a metal case  10  of the ultrasound probe  101  has a polygonal prism shape in which a side wall portion  10   a  is defined as a side face and a bottom wall portion  10   b  is defined as one bottom face. 
     Further, the ultrasound probe  101  includes a number of combinations each having a coil  1  and at least one or more magnets  2 , the number of combinations being equal to the number of multiple planes in an outer surface of the side wall portion  3   a.  Each of the combinations whose number is equal to the number of multiple planes is mounted on a corresponding plane out of the multiple planes in such a way that the combinations are arranged at positions which are mutually rotationally symmetric with respect to an axis passing through the center of the vibration surface  10   c  of the bottom wall portion  10   b  and being perpendicular to the vibration surface  10   c  of the bottom wall portion  10   b.    
     In further detail, in this first variant, the metal case  10  has a quadrangular prism shape in which the side wall portion  10   a  is defined as a side face and the bottom wall portion  10   b  is defined as one bottom face. The ultrasound probe  101  includes, as the coil  1 , four coils  1  each of which is mounted on a corresponding plane out of four planes in the outer surface of the side wall portion  10   a.    
     Further, the ultrasound probe  101  includes, as the magnet  2 , eight magnets  2  each of which is mounted in such a way as to sandwich a corresponding coil out of the multiple coils  1  between each of the magnets and a corresponding plane out of the four planes in the outer surface of the side wall portion  10   a.    
     In this first variant, the example in which the shape of the metal case  10  is a quadrangular prism one and the number of combinations each having a coil  1  and at least one or more magnets  2  is four is explained. However, the shape of the metal case  10  is not limited to this structure, but should just be a polygonal prism one. Further, the number of combinations each having a coil  1  and at least one or more magnets  2  which the ultrasound probe  101  includes is not especially limited. 
     According to the structure of the first variant, a previously-mentioned Lorentz force occurring in the side wall portion  10   a  can be effectively propagated to the center of the vibration surface  10   c  of the bottom wall portion  10   b.    
     Next, a second variant of the ultrasound probe  100  will be explained by reference to a drawing.  FIG. 6  is a top view and a side view showing the structure of an ultrasound probe  102  according to the second variant. As shown in  FIG. 6 , a metal case  20  has a cylindrical shape in which a side wall portion  20   a  is defined as a side face and a bottom wall portion  20   b  is defined as one bottom face. 
     Further, the ultrasound probe  102  includes multiple combinations each having a coil  1  and at least one or more magnets  2 . Each of the multiple combinations is mounted on an outer surface of the side wall portion  20   a  in such a way that the combinations are arranged at positions which are mutually rotationally symmetric with respect to an axis passing through the center of the vibration surface  20   c  of the bottom wall portion  20   b  and being perpendicular to the vibration surface  20   c  of the bottom wall portion  20   b.    
     In further detail, in this second variant, the ultrasound probe  102  includes, as the coil  1 , four coils  1  each of which is mounted on the outer surface of the side wall portion  20   a.  Further, the ultrasound probe  102  includes, as the magnet  2 , eight magnets  2  each of which is mounted in such a way as to sandwich a corresponding coil out of the four coils  1  between each of the magnets and the outer surface of the side wall portion  20   a.    
     In this second variant, the example in which the number of combinations each having a coil  1  and at least one or more magnets  2  is four is explained. However, the number of combinations each having a coil  1  and at least one or more magnets  2  which the ultrasound probe  102  includes is not especially limited. 
     According to the structure of the second variant, a previously-mentioned Lorentz force occurring in the side wall portion  20   a  can be effectively propagated to the center of the vibration surface  20   c  of the bottom wall portion  20   b.    
     As mentioned above, the ultrasound probe  100  according to Embodiment 1 is provided with: the metal case  3  including the bottom wall portion  3   b  having the vibration surface  3   c,  and the side wall portion  3   a  connected to the bottom wall portion  3   b ; the coil  1  mounted on one surface of the side wall portion  3   a  ; and the magnet  2  mounted in such a way as to sandwich the coil  1  between the magnet and the one surface of the side wall portion  3   a,  and the coil  1  causes an eddy current to occur in the one surface of the side wall portion  3   a,  and then causes the bottom wall portion  3   b  to vibrate, by using a Lorentz force occurring in the side wall portion  3   a  because of the interaction between the eddy current and a magnetic field generated by the magnet  2 . 
     According to the above-mentioned structure, because an ultrasonic wave having the same frequency as the frequency of the high-frequency current flowing through the coil  1  can be caused to occur in a test object, an ultrasonic wave of low frequency can be excited without upsizing the ultrasound probe, unlike in the case of previously-mentioned ultrasound probes using a piezoelectric element. More specifically, downsizing of the ultrasound probe  100  can be achieved. Further, because downsizing of the ultrasound probe  100  can be achieved, a weight reduction of the ultrasound probe  100  can also be achieved. Further, because a test object can be vibrated by way of the vibration surface  3   c,  unlike in the case of conventional EMATs, an ultrasonic nondestructive inspection can also be performed on a non-conductive test object. 
     Further, the coil  1  in the ultrasound probe  100  according to Embodiment 1 has the straight line portion  1   c  parallel to the vibration surface  3   c  of the bottom wall portion  3   b,  and the magnet  2  is mounted in such a way as to sandwich the straight line portion  1   c  of the coil  1  between the magnet and the one surface of the side wall portion  3   a.    
     According to the above-mentioned structure, the direction of the Lorentz force  6  occurring in the side wall portion  3   a  can be caused to be perpendicular to the vibration surface  3   c  of the bottom wall portion  3   b.  As a result, a bending vibration can be caused to occur in the bottom wall portion  3   b,  and an ultrasonic wave can be excited in the test object by the bending vibration. 
     Further, the bottom wall portion  3   b  in the ultrasound probe  100  according to Embodiment 1 includes a single flat-shaped bottom wall portion. 
     According to the above-mentioned structure, the test object can be effectively vibrated by way of the vibration surface  3   c  of the flat-shaped bottom wall portion  3   b.    
     Further, the side wall portion  3   a  in the ultrasound probe  100  according to Embodiment 1 includes the first flat-shaped side wall portion  3   d  and the second flat-shaped side wall portion  3   e  which face each other, the bottom wall portion  3   b  has a flat surface connected to ends of the respective first and second side wall portions  3   d  and  3   e,  the flat surface being opposite to the vibration surface, the first coil  1   a  mounted on a first surface of the first side wall portion  3   d,  the surface being opposite to a second surface, of the first side wall portion  3   d,  facing the second side wall portion  3   e,  and the second coil  1   b  mounted on a first surface of the second side wall portion  3   e,  the first surface being opposite to a second surface, of the second side wall portion  3   e,  facing the first side wall portion  3   d,  are provided as the coil  1 , and at least one or more magnets  2  mounted in such a way as to sandwich the first coil  1   a  between the one or more magnets and the first surface of the first side wall portion  3   d,  the first surface being opposite to the second surface, of the first side wall portion  3   d,  facing the second side wall portion  3   e,  and at least one or more magnets  2  mounted in such a way as to sandwich the second coil  1   b  between the one or more magnets and the first surface of the second side wall portion  3   e,  the first surface being opposite to the second surface, of the second side wall portion  3   e,  facing the first side wall portion  3   d,  are provided as the magnet  2 . 
     According to the above-mentioned structure, a Lorentz force occurring in the first side wall portion  3   d  and a Lorentz force occurring in the second side wall portion  3   e  can be effectively propagated to the vibration surface  3   c  of the bottom wall portion  3   b.    
     Further, the metal case  10  in the ultrasound probe  101  according to Embodiment 1 has a polygonal prism shape in which the side wall portion  10   a  is defined as a side face and the bottom wall portion  10   b  is defined as one bottom face, and a number of combinations each having a coil  1  and at least one or more magnets  2 , the number of combinations being equal to the number of multiple planes in the outer surface of the side wall portion  10   a,  are provided, and each of the combinations whose number is equal to the number of multiple planes is mounted on a corresponding plane out of the multiple planes in such a way that the combinations are arranged at positions which are mutually rotationally symmetric with respect to an axis passing through the center of the vibration surface  10   c  of the bottom wall portion  10   b  and being perpendicular to the vibration surface  10   c  of the bottom wall portion  10   b.    
     According to the above-mentioned structure, a Lorentz force occurring in the side wall portion  10   a  can be effectively propagated to the center of the vibration surface  10   c  of the bottom wall portion  10   b.    
     Further, the metal case  20  in the ultrasound probe  102  according to Embodiment 1 has a cylindrical shape in which the side wall portion  20   a  is defined as a side face and the bottom wall portion  20   b  is defined as one bottom face, and multiple combinations each having a coil  1  and at least one or more magnets  2  are provided, and each of the multiple combinations is mounted on the outer surface of the side wall portion  20   a  in such a way that the combinations are arranged at positions which are mutually rotationally symmetric with respect to an axis passing through the center of the vibration surface  20   c  of the outer bottom wall portion  20   b  and being perpendicular to the vibration surface  20   c  of the outer bottom wall portion  20   b.    
     According to the above-mentioned structure, a Lorentz force occurring in the side wall portion  20   a  can be effectively propagated to the center of the vibration surface  20   c  of the bottom wall portion  20   b.    
     Embodiment 2 
     In Embodiment 1, the structure in which the bottom wall portion  3   b  of the metal case  3  includes a single flat-shaped bottom wall portion is explained. In Embodiment 2, a structure in which a bottom wall portion includes an outer bottom wall portion, an inner bottom wall portion and a connection portion will be explained. 
     Hereinafter, Embodiment 2 will be explained by reference to drawings. Components having the same functions as those of components explained in Embodiment 1 are denoted by the same reference signs, and an explanation of the components will be omitted hereinafter. 
       FIG. 7  is a cross-sectional view showing the structure of an ultrasound probe  103  according to Embodiment 2. As shown in  FIG. 7 , in comparison with the ultrasound probe  100  according to Embodiment 1, the ultrasound probe  103  includes a metal case  30  having a bottom wall portion  30   b  instead of the bottom wall portion  3   b.    
     The bottom wall portion  30   b  includes a flat-shaped outer bottom wall portion  30   d  having a vibration surface  30   c,  a flat-shaped inner bottom wall portion  30   e  whose one flat surface faces a flat surface of the outer bottom wall portion  30   d,  the flat surface of the outer bottom wall portion  30   d  being opposite to the vibration surface  30   c,  and a connection portion  30   h  that connects the outer bottom wall portion  30   d  and the inner bottom wall portion  30   e.  The inner bottom wall portion  30   e  has a first end and a second end opposite to the first end, the first end is fixed, the second end is connected to the connection portion  30   h,  and a portion between the first end and the second end is connected to an end of a side wall portion  3   a.    
     The above description that “the first end is fixed” means that the first end is fixed in such a way as not to move. A method of fixing the first end in such a way that the first end does not move is not especially limited. For example, in a case where the ultrasound probe  103  is inserted into a housing which protects the ultrasound probe  103 , the first end may be fixed by connecting the first end to the housing. 
     In further details about the metal case  30  shown in  FIG. 7 , in Embodiment 2, the side wall portion  3   a  includes a first flat-shaped side wall portion  3   d  and a second flat-shaped side wall portion  3   e  which face each other. The inner bottom wall portion  30   e  includes a first flat-shaped inner bottom wall portion  30   f  whose one flat surface faces a flat surface of the outer bottom wall portion  30   d,  the flat surface of the outer bottom wall portion  30   d  being opposite to the vibration surface  30   c,  and a second flat-shaped inner bottom wall portion  30   g  whose one flat surface faces the flat surface of the outer bottom wall portion  30   d,  the flat surface of the outer bottom wall portion  30   d  being opposite to the vibration surface  30   c.    
     Further, in Embodiment 2, the connection portion  30   h  includes a first connection portion  30   i  that connects the first inner bottom wall portion  30   f  and the outer bottom wall portion  30   d,  and a second connection portion  30   j  that connects the second inner bottom wall portion  30   g  and the outer bottom wall portion  30   d.    
     Further, in Embodiment 2, the first inner bottom wall portion  30   f  has a first end and a second end opposite to the first end, the first end is fixed, the second end is connected to the first connection portion  30   i,  and a portion between the first end and the second end is connected to an end of the first side wall portion  3   d.  In  FIG. 7 , the first end of the first inner bottom wall portion  30   f  faces a below-mentioned first end of the second inner bottom wall portion  30   g.  The second end of the first inner bottom wall portion  30   f  faces the outside of the metal case  30 . 
     Further, in Embodiment 2, the second inner bottom wall portion  30   g  has the first end and a second end opposite to the first end, the first end is fixed, the second end is connected to the second connection portion  30   j,  and a portion between the first end and the second end is connected to an end of the second side wall portion  3   e.  In  FIG. 7 , the first end of the second inner bottom wall portion  30   g  faces the first end of the first inner bottom wall portion  30   f  The second end of the second inner bottom wall portion  30   g  faces the outside of the metal case  30 . 
     A coil  1  according to Embodiment 2 causes an eddy current to occur in one surface of the side wall portion  3   a,  and causes the outer bottom wall portion  30   d  to vibrate by way of the inner bottom wall portion  30   e  and the connection portion  30   h,  by using a Lorentz force occurring in the side wall portion  3   a  because of the interaction between the eddy current and a magnetic field generated by a magnet  2 . 
     In further detail, the ultrasound probe  103  includes, as the coil  1 , a first coil  1   a  mounted on a first surface of the first side wall portion  3   d,  the first surface being opposite to a second surface, of the first side wall portion  3   d,  facing the second side wall portion  3   e,  and a second coil  1   b  mounted on a first surface of the second side wall portion  3   e,  the first surface being opposite to a second surface, of the second side wall portion  3   e,  facing the first side wall portion  3   d.    
     Further, the ultrasound probe  103  includes, as the magnet  2 , at least one or more magnets  2  mounted in such a way as to sandwich the first coil  1   a  between the one or more magnets and the first surface of the first side wall portion  3   d,  the first surface being opposite to the second surface, of the first side wall portion  3   d,  facing the second side wall portion  3   e,  and at least one or more magnets  2  mounted in such a way as to sandwich the second coil  1   b  between the one or more magnets and the first surface of the second side wall portion  3   e,  the first surface being opposite to the second surface, of the second side wall portion  3   e,  facing the first side wall portion  3   d.    
     Next, the operation of the ultrasound probe  103  according to Embodiment 2 will be explained. First, the coil  1  causes an eddy current to occur in the one surface of the side wall portion  3   a,  and causes the bottom wall portion  30   b  to vibrate by using a Lorentz force occurring in the side wall portion  3   a  because of the interaction between the eddy current and a magnetic field generated by the magnet  2 . In further detail, for example, the first coil  1   a  causes an eddy current to occur in one surface of the first side wall portion  3   d,  and causes the first inner bottom wall portion  30   f  to vibrate by using a Lorentz force occurring in the first side wall portion  3   d  because of the interaction between the eddy current and a magnetic field generated by a magnet  2  shown in an upper left part of  FIG. 7 . 
     Because in the first inner bottom wall portion  30   f  a portion between the above-mentioned first and second ends is connected to an end of the first side wall portion  3   d,  the portion acts as the point of effort. Further, because in the first inner bottom wall portion  30   f  the first end is fixed, the first end acts as the point of fulcrum. Further, because in the first inner bottom wall portion  30   f  the second end is connected to the first connection portion  30   i,  the second end acts as the point of load. 
     More specifically, the first inner bottom wall portion  30   f  acts as a third class lever, and thus propagates a Lorentz force occurring in the first side wall portion  3   d  to the first connection portion  30   i.  As a result, in the outer bottom wall portion  30   d  connected to the first connection portion  30   i,  a vibration larger than that occurring in the bottom wall portion  3   b  according to Embodiment 1 occurs. The same goes for the second side wall portion  3   e,  the second inner bottom wall portion  30   g  and the second connection portion  30   j.  Therefore, the amplitude of an ultrasonic wave excited in a test object when the test object is brought into contact with the vibration surface  30   c  of the outer bottom wall portion  30   d  can be increased, and thus the sensitivity of an ultrasonic nondestructive inspection can be improved. 
     Next, a result of a simulation of the operation of the ultrasound probe  103  according to Embodiment 2 will be explained by reference to drawings.  FIG. 8  is a view showing a result of determining the shape of the metal case  30  after deformation when stresses caused by Lorentz forces mentioned previously are applied to the first side wall portion  3   d  and the second side wall portion  3   e  of the metal case  30 , by means of the simulation.  FIG. 9  is a graph showing a vibration displacement on a center line of the vibration surface  30   c  of the outer bottom wall portion  30   d  and in a direction perpendicular to the vibration surface  30   c.  The center line of the vibration surface  30   c  is a dotted line CC′ shown in  FIG. 8 . Further, the horizontal axis of the graph shown in  FIG. 9  shows positions on the center line CC′ in a case where the center of the vibration surface  30   c  is set as the point of origin, and the vertical axis of the graph shown in  FIG. 9  shows the magnitude of the vibration displacement in the direction perpendicular to the vibration surface  30   c.  A solid line shown in  FIG. 9  is a graph showing the vibration displacement on the center line of the vibration surface  30   c  of the outer bottom wall portion  30   d  and in the direction perpendicular to the vibration surface  30   c.  Further, in  FIG. 9 , a graph showing the vibration displacement on the center line of the vibration surface  3   c  of the bottom wall portion  3   b  according to Embodiment 1 and in the direction perpendicular to the vibration surface  3   c  is shown by a dotted line for comparison. 
     In the vibration surface  30   c  of  FIG. 8 , a dark portion compared with the surrounding area is a bending portion. Further, it is shown by the solid line of  FIG. 9  that the vibration displacement becomes larger with increasing proximity to the center of the vibration surface  30   c.  More specifically, it can be recognized that the outer bottom wall portion  30   d  has a bending vibration. Further, the solid line of  FIG. 9  shows the vibration displacement which is more than twice compared with the dotted line of  FIG. 9  showing the vibration displacement in Embodiment 1, so that the above-mentioned advantageous effects provided by the ultrasound probe  103  according to Embodiment 2 can be recognized. 
     Next, a first variant of the ultrasound probe  103  will be explained by reference to a drawing.  FIG. 10  is atop view and a side view showing the structure of an ultrasound probe  104  according to the first variant. As shown in  FIG. 10 , a metal case  40  has a polygonal prism shape in which a side wall portion  10   a  is defined as a side face. Further, a vibration surface  40   b  of an outer bottom wall portion  40   a  has a shape similar to that of a bottom face of the polygonal prism shape. 
     Although not illustrated, an inner bottom wall portion in the ultrasound probe  104  includes a single flat-shaped inner bottom wall portion having a penetrating hole in a central part thereof. In the single flat-shaped inner bottom wall portion, a previously-mentioned first end corresponds to an edge portion facing the penetrating hole, and a previously-mentioned second end corresponds to an outer edge portion located on a side opposite to the penetrating hole. 
     Further, the ultrasound probe  104  includes a number of combinations each having a coil  1  and at least one or more magnets  2 , the number of combinations being equal to the number of multiple planes in an outer surface of the side wall portion  10   a.  Each of the multiple combinations whose number is equal to the number of multiple planes is mounted on a corresponding plane out of the multiple planes in such a way that the combinations are arranged at positions which are mutually rotationally symmetric with respect to an axis passing through the center of the vibration surface  40   b  of the outer bottom wall portion  40   a  and being perpendicular to the vibration surface  40   b  of the outer bottom wall portion  40   a.    
     In further detail, in this first variant, the metal case  40  has a quadrangular prism shape in which the side wall portion  10   a  is defined as a side face. Further, the vibration surface  40   b  of the outer bottom wall portion  40   a  has a square shape similar to that of a bottom face of the quadrangular prism shape. 
     The ultrasound probe  104  includes, as the coil  1 , four coils  1  each mounted on a corresponding plane out of four planes in the outer surface of the side wall portion  10   a.  Further, the ultrasound probe  104  includes, as the magnet  2 , eight magnets  2  each mounted in such a way as to sandwich a corresponding coil out of the four coils  1  between each of the magnets and the corresponding plane out of the four planes in the outer surface of the side wall portion  10   a.    
     In this first variant, the example in which the shape of the metal case  40  is a quadrangular prism one and the number of combinations each having a coil  1  and at least one or more magnets  2  is four is explained. However, the shape of the metal case  40  is not limited to this structure, and should just be a polygonal prism one. Further, the number of combinations each having a coil  1  and at least one or more magnets  2 , the combinations being included in the ultrasound probe  104 , is not especially limited. 
     According to the structure of the first variant, a previously-mentioned Lorentz force occurring in the side wall portion  10   a  can be effectively propagated to the center of the vibration surface  40   b  of the outer bottom wall portion  40   a.    
     Next, a second variant of the ultrasound probe  103  will be explained by reference to a drawing.  FIG. 11  is a top view and a side view showing the structure of an ultrasound probe  105  according to the second variant. As shown in  FIG. 11 , a metal case  50  has a cylindrical shape in which a side wall portion  20   a  is defined as a side face. A vibration surface  50   b  of an outer bottom wall portion  50   a  has a shape similar to that of a bottom face of the cylindrical shape. 
     Although not illustrated, an inner bottom wall portion in the ultrasound probe  105  includes a single flat-shaped inner bottom wall portion having a penetrating hole in a central part thereof. In the single flat-shaped inner bottom wall portion, a previously-mentioned first end corresponds to an edge portion facing the penetrating hole, and a previously-mentioned second end corresponds to an outer edge portion located on a side opposite to the penetrating hole. 
     Further, the ultrasound probe  105  includes multiple combinations each having a coil  1  and at least one or more magnets  2 . Each of the multiple combinations is mounted on an outer surface of the side wall portion  20   a  in such a way that the combinations are arranged at positions which are mutually rotationally symmetric with respect to an axis passing through the center of the vibration surface  50   b  of the outer bottom wall portion  50   a  and being perpendicular to the vibration surface  50   b  of the outer bottom wall portion  50   a.    
     In further detail, in this second variant, the ultrasound probe  105  includes, as the coil  1 , four coils  1  each mounted on the outer surface of the side wall portion  20   a.  Further, the ultrasound probe  105  includes, as the magnet  2 , eight magnets  2  each mounted in such a way as to sandwich a corresponding coil out of the four coils  1  between each of the magnets and the outer surface of the side wall portion  20   a.    
     In this second variant, the example in which the number of combinations each having a coil  1  and at least one or more magnets  2  is four is explained. However, the number of combinations each having a coil  1  and at least one or more magnets  2 , the combinations being included in the ultrasound probe  105 , is not especially limited. 
     According to the structure of the second variant, a previously-mentioned Lorentz force occurring in the side wall portion  20   a  can be effectively propagated to the center of the vibration surface  50   b  of the outer bottom wall portion  50   a.    
     As mentioned above, the bottom wall portion  30   b  in the ultrasound probe  103  according to Embodiment 2 includes the flat-shaped outer bottom wall portion  30   d  having the vibration surface  30   c,  the flat-shaped inner bottom wall portion  30   e  whose one flat surface faces a flat surface of the outer bottom wall portion  30   d,  the flat surface of the outer bottom wall portion  30   d  being opposite to the vibration surface  30   c,  and the connection portion  30   h  that connects the outer bottom wall portion  30   d  and the inner bottom wall portion  30   e,  the inner bottom wall portion  30   e  has a first end and a second end opposite to the first end, the first end is fixed, the second end is connected to the connection portion  30   h,  and a portion between the first end and the second end is connected to an end of the side wall portion  3   a,  and the coil  1  causes an eddy current to occur in one surface of the side wall portion  3   a,  and causes the outer bottom wall portion  30   d  to vibrate by way of the inner bottom wall portion  30   e  and the connection portion  30   h,  by using a Lorentz force occurring in the side wall portion  3   a  because of the interaction between the eddy current and a magnetic field generated by the magnet  2 . 
     According to the above-mentioned structure, as mentioned above, the inner bottom wall portion  30   e  acts as a third class lever, and thus propagates the Lorentz force occurring in the side wall portion  3   a  to the connection portion  30   h.  As a result, in the outer bottom wall portion  30   d  connected to the first connection portion  30   h,  a vibration larger than that occurring in the bottom wall portion  3   b  according to Embodiment 1 occurs. Therefore, the amplitude of an ultrasonic wave excited in a test object when the test object is brought into contact with the vibration surface  30   c  of the outer bottom wall portion  30   d  can be increased, and thus the sensitivity of an ultrasonic nondestructive inspection can be improved. 
     Further, the side wall portion  3   a  in the ultrasound probe  103  according to Embodiment 2 includes the first flat-shaped side wall portion  3   d  and the second flat-shaped side wall portion  3   e  which face each other, the inner bottom wall portion  30   e  includes the first flat-shaped inner bottom wall portion  30   f  whose one flat surface faces a flat surface of the outer bottom wall portion  30   d,  the flat surface of the outer bottom wall portion  30   d  being opposite to the vibration surface  30   c,  and the second flat-shaped inner bottom wall portion  30   g  whose one flat surface faces the flat surface of the outer bottom wall portion  30   d , the flat surface of the outer bottom wall portion  30   d  being opposite to the vibration surface  30   c,  the connection portion  30   h  includes the first connection portion  30   i  that connects the first inner bottom wall portion  30   f  and the outer bottom wall portion  30   d,  and the second connection portion  30   j  that connects the second inner bottom wall portion  30   g  and the outer bottom wall portion  30   d,  the first inner bottom wall portion  30   f  has a first end and a second end opposite to the first end, the first end is fixed, the second end is connected to the first connection portion  30   i,  and a portion between the first end and the second end is connected to an end of the first side wall portion  3   d,  the second inner bottom wall portion  30   g  has a first end and a second end opposite to the first end, the first end is fixed, the second end is connected to the second connection portion  30   j,  and a portion between the first end and the second end is connected to an end of the second side wall portion  3   e,  the first coil  1   a  mounted on a first surface of the first side wall portion  3   d,  the first surface being opposite to a second surface, of the first side wall portion  3   d,  facing the second side wall portion  3   e,  and the second coil  1   b  mounted on a first surface of the second side wall portion  3   e,  the first surface being opposite to a second surface, of the second side wall portion  3   e,  facing the first side wall portion  3   d,  are provided as the coil  1 , and at least one or more magnets  2  mounted in such a way as to sandwich the first coil  1   a  between the one or more magnets and the first surface of the first side wall portion  3   d,  the first surface being opposite to the second surface, of the first side wall portion  3   d,  facing the second side wall portion  3   e,  and at least one or more magnets  2  mounted in such a way as to sandwich the second coil  1   b  between the one or more magnets and the first surface of the second side wall portion  3   e,  the first surface being opposite to the second surface, of the second side wall portion  3   e,  facing the first side wall portion  3   d,  are provided as the magnet  2 . 
     According to the above-mentioned structure, a Lorentz force occurring in the first side wall portion  3   d  and a Lorentz force occurring in the second side wall portion  3   e  can be effectively propagated to the vibration surface  30   c  of the outer bottom wall portion  30   d.    
     Further, the metal case  40  in the ultrasound probe  104  according to Embodiment 2 has a polygonal prism shape in which the side wall portion  10   a  is defined as a side face, the vibration surface  40   b  of the outer bottom wall portion  40   a  has a shape similar to that of a bottom face of the polygonal prism shape, and a number of combinations each having a coil  1  and at least one or more magnets  2 , the number of combinations being equal to the number of multiple planes in the outer surface of the side wall portion  10   a,  are provided, and each of the combinations whose number is equal to the number of multiple planes is mounted on a corresponding plane out of the multiple planes in such a way that the combinations are arranged at positions which are mutually rotationally symmetric with respect to an axis passing through the center of the vibration surface  40   b  of the outer bottom wall portion  40   a  and being perpendicular to the vibration surface  40   b  of the outer bottom wall portion  40   a.    
     According to the above-mentioned structure, a Lorentz force occurring in the side wall portion  10   a  can be effectively propagated to the center of the vibration surface  40   b  of the outer bottom wall portion  40   a.    
     Further, the metal case  50  in the ultrasound probe  105  according to Embodiment 2 has a cylindrical shape in which the side wall portion  20   a  is defined as a side face, the vibration surface  50   b  of the outer bottom wall portion  50   a  has a shape similar to that of a bottom face of the cylindrical shape, and multiple combinations each having a coil  1  and at least one or more magnets  2  are provided, and each of the multiple combinations is mounted on the outer surface of the side wall portion  20   a  in such a way that the combinations are arranged at positions which are mutually rotationally symmetric with respect to an axis passing through the center of the vibration surface  50   b  of the outer bottom wall portion  50   a  and being perpendicular to the vibration surface  50   b  of the outer bottom wall portion  50   a.    
     According to the above-mentioned structure, a Lorentz force occurring in the side wall portion  20   a  can be effectively propagated to the center of the vibration surface  50   b  of the outer bottom wall portion  50   a.    
     Embodiment 3 
     In Embodiment 2, the structure in which in the inner bottom wall portion  30   e,  the first end is fixed, the second end is connected to the connection portion  30   h,  and a portion between the first end and the second end is connected to an end of the side wall portion  3   a  is explained. In Embodiment 3, a structure in which in an inner bottom wall portion, a first end is fixed, a second end is connected to an end of a side wall portion, and a portion between the first end and the second end is connected to a connection portion will be explained. 
     Hereinafter, Embodiment 3 will be explained by reference to drawings. Components having the same functions as those of components explained in Embodiment 1 or 2 are denoted by the same reference signs, and an explanation of the components will be omitted hereinafter. 
       FIG. 12  is a cross-sectional view showing the structure of an ultrasound probe  106  according to Embodiment 3. As shown in  FIG. 12 , in comparison with the ultrasound probe  103  according to Embodiment 2, a metal case  60  in the ultrasound probe  106  has a bottom wall portion  60   b  instead of the bottom wall portion  30   b.    
     The bottom wall portion  60   b  includes a flat-shaped outer bottom wall portion  60   d  having a vibration surface  60   c,  a flat-shaped inner bottom wall portion  60   e  whose one flat surface faces a flat surface of the outer bottom wall portion  60   d,  the flat surface of the outer bottom wall portion  60   d  being opposite to the vibration surface  60   c,  and a connection portion  60   h  that connects the outer bottom wall portion  60   d  and the inner bottom wall portion  60   e.  The inner bottom wall portion  60   e  has a first end and a second end opposite to the first end, the first end is fixed, the second end is connected to an end of a side wall portion  3   a,  and a portion between the first end and the second end is connected to the connection portion  60   h.    
     The above description that “the first end is fixed” means that the first end is fixed in such a way as not to move. A method of fixing the first end in such a way that the first end does not move is not especially limited. For example, in a case where the ultrasound probe  106  is inserted into a housing which protects the ultrasound probe  106 , the first end may be fixed by connecting the first end to the housing. 
     In further details about the metal case  60  shown in  FIG. 12 , in Embodiment 3, the side wall portion  3   a  includes a first flat-shaped side wall portion  3   d  and a second flat-shaped side wall portion  3   e  which face each other, and the inner bottom wall portion  60   e  includes a first flat-shaped inner bottom wall portion  60   f  whose one flat surface faces a flat surface of the outer bottom wall portion  60   d,  the flat surface of the outer bottom wall portion  60   d  being opposite to the vibration surface  60   c,  and a second flat-shaped inner bottom wall portion  60   g  whose one flat surface faces the flat surface of the outer bottom wall portion  60   d,  the flat surface of the outer bottom wall portion  60   d  being opposite to the vibration surface  60   c.    
     Further, in Embodiment 3, the connection portion  60   h  includes a first connection portion  60   i  that connects the first inner bottom wall portion  60   f  and the outer bottom wall portion  60   d,  and a second connection portion  60   j  that connects the second inner bottom wall portion  60   g  and the outer bottom wall portion  60   d.    
     Further, in Embodiment 3, the first inner bottom wall portion  60   f  has a first end and a second end opposite to the first end, the first end is fixed, the second end is connected to an end of the first side wall portion  3   d,  and a portion between the first end and the second end is connected to the first connection portion  60   i.  In  FIG. 12 , the first end of the first inner bottom wall portion  60   f  faces a below-mentioned first end of the second inner bottom wall portion  60   g.  The second end of the first inner bottom wall portion  60   f  faces the outside of the metal case  60 . 
     Further, in Embodiment 3, the second inner bottom wall portion  60   g  has the first end and a second end opposite to the first end, the first end is fixed, the second end is connected to an end of the second side wall portion  3   e,  and a portion between the first end and the second end is connected to the second connection portion  60   j.  In  FIG. 12 , the first end of the second inner bottom wall portion  60   g  faces the first end of the first inner bottom wall portion  60   f  The second end of the second inner bottom wall portion  60   g  faces the outside of the metal case  60 . 
     A coil  1  according to Embodiment  3  causes an eddy current to occur in one surface of the side wall portion  3   a,  and causes the outer bottom wall portion  60   d  to vibrate by way of the inner bottom wall portion  60   e  and the connection portion  60   h,  by using a Lorentz force occurring in the side wall portion  3   a  because of the interaction between the eddy current and a magnetic field generated by a magnet  2 . 
     In further detail, the ultrasound probe  106  includes, as the coil  1 , a first coil  1   a  mounted on a first surface of the first side wall portion  3   d,  the first surface being opposite to a second surface, of the first side wall portion  3   d,  facing the second side wall portion  3   e,  and a second coil  1   b  mounted on a first surface of the second side wall portion  3   e,  the first surface being opposite to a second surface, of the second side wall portion  3   e,  facing the first side wall portion  3   d.    
     Further, the ultrasound probe  106  includes, as the magnet  2 , at least one or more magnets  2  mounted in such a way as to sandwich the first coil  1   a  between the one or more magnets and the first surface of the first side wall portion  3   d,  the first surface being opposite to the second surface, of the first side wall portion  3   d,  facing the second side wall portion  3   e,  and at least one or more magnets  2  mounted in such a way as to sandwich the second coil  1   b  between the one or more magnets and the first surface of the second side wall portion  3   e,  the first surface being opposite to the second surface, of the second side wall portion  3   e,  facing the first side wall portion  3   d.    
     Next, the operation of the ultrasound probe  106  according to Embodiment 3 will be explained. First, the coil  1  causes an eddy current to occur in the one surface of the side wall portion  3   a,  and causes the bottom wall portion  60   b  to vibrate by using a Lorentz force occurring in the side wall portion  3   a  because of the interaction between the eddy current and the magnetic field generated by the magnet  2 . In further detail, for example, the first coil  1   a  causes an eddy current to occur in one surface of the first side wall portion  3   d,  and causes the first inner bottom wall portion  60   f  to vibrate by using a Lorentz force occurring in the first side wall portion  3   d  because of the interaction between the eddy current and a magnetic field generated by a magnet  2  shown in an upper left part of  FIG. 12 . 
     Because in the first inner bottom wall portion  60   f  the previously-mentioned second end is connected to an end of the first side wall portion  3   d,  the second end acts as the point of effort. Further, because in the first inner bottom wall portion  60   f  the previously-mentioned first end is fixed, the first end acts as the point of fulcrum. Further, because in the first inner bottom wall portion  60   f  a portion between the first and second ends is connected to the first connection portion  60   i,  the portion acts as the point of load. 
     More specifically, the first inner bottom wall portion  60   f  acts as a second class lever, and thus propagates a Lorentz force occurring in the first side wall portion  3   d  to the first connection portion  60   i.  As a result, in the outer bottom wall portion  60   d  connected to the first connection portion  60   i,  a vibration larger than that occurring in the bottom wall portion  3   b  according to Embodiment  1  occurs. The same goes for the second side wall portion  3   e,  the second inner bottom wall portion  60   g  and the second connection portion  60   j.  Therefore, the amplitude of an ultrasonic wave excited in a test object when the test object is brought into contact with the vibration surface  60   c  of the outer bottom wall portion  60   d  can be increased, and thus the sensitivity of an ultrasonic nondestructive inspection can be improved. 
     Next, a result of a simulation of the operation of the ultrasound probe  106  according to Embodiment 3 will be explained by reference to drawings.  FIG. 13  is a view showing a result of determining the shape of the metal case  60  after deformation when stresses caused by Lorentz forces mentioned previously are applied to the first side wall portion  3   d  and the second side wall portion  3   e  of the metal case  60 , by means of the simulation.  FIG. 14  is a graph showing a stress on the outer bottom wall portion  60   d,  the stress being on a center line of the vibration surface  60   c  of the outer bottom wall portion  60   d.  The center line of the vibration surface  60   c  is a dotted line DD′ shown in  FIG. 13 . Further, the horizontal axis of the graph shown in  FIG. 14  shows positions on the center line DD′ in a case where the center of the vibration surface  60   c  is set as the point of origin, and the vertical axis of the graph shown in  FIG. 14  shows the magnitude of a stress on the outer bottom wall portion  60   d.  A solid line shown in  FIG. 14  is a graph showing the stress on the outer bottom wall portion  60   d,  the stress being on the center line of the vibration surface  60   c  of the outer bottom wall portion  60   d.  Further, in  FIG. 14 , a graph showing the stress on the bottom wall portion  3   b,  the stress being on the center line of the vibration surface  3   c  of the bottom wall portion  3   b  according to Embodiment 1, is shown by a dotted line for comparison. 
     In the vibration surface  60   c  of  FIG. 13 , a dark portion compared with the surrounding area is a bending portion. Further, it can be recognized from the comparison between the solid and dotted lines in  FIG. 14  that the stress on the outer bottom wall portion  60   d  in the center and its vicinity (±1.5 mm) of the vibration surface  60   c  is larger than that on the bottom wall portion  3   b  according to Embodiment 1, and thus the vibration is centralized to the center and its vicinity of the vibration surface  60   c.  More specifically, the previously-mentioned advantageous effects provided by the ultrasound probe  106  according to Embodiment 3 can be recognized. 
     Next, a first variant of the ultrasound probe  106  will be explained by reference to a drawing.  FIG. 15  is atop view and a side view showing the structure of an ultrasound probe  107  according to the first variant. As shown in  FIG. 15 , a metal case  70  has a polygonal prism shape in which a side wall portion  10   a  is defined as a side face. Further, a vibration surface  70   b  of an outer bottom wall portion  70   a  has a shape similar to that of a bottom face of the polygonal prism shape. 
     Although not illustrated, an inner bottom wall portion in the ultrasound probe  107  includes a single flat-shaped inner bottom wall portion having a penetrating hole in a central part thereof. In the single flat-shaped inner bottom wall portion, a previously-mentioned first end corresponds to an edge portion facing the penetrating hole, and a previously-mentioned second end corresponds to an outer edge portion located on a side opposite to the penetrating hole. 
     Further, the ultrasound probe  107  includes a number of combinations each having a coil  1  and at least one or more magnets  2 , the number of combinations being equal to the number of multiple planes in an outer surface of the side wall portion  10   a.  Each of the multiple combinations whose number is equal to the number of multiple planes is mounted on a corresponding plane out of the multiple planes in such a way that the combinations are arranged at positions which are mutually rotationally symmetric with respect to an axis passing through the center of the vibration surface  70   b  of the outer bottom wall portion  70   a  and being perpendicular to the vibration surface  70   b  of the outer bottom wall portion  70   a.    
     In further detail, in this first variant, the metal case  70  has a quadrangular prism shape in which the side wall portion  10   a  is defined as a side face. Further, the vibration surface  70   b  of the outer bottom wall portion  70   a  has a square shape similar to that of a bottom face of the quadrangular prism shape. 
     The ultrasound probe  107  includes, as the coil  1 , four coils  1  each mounted on a corresponding plane out of four planes in the outer surface of the side wall portion  10   a.  Further, the ultrasound probe  107  includes, as the magnet  2 , eight magnets  2  each mounted in such a way as to sandwich a corresponding coil out of the four coils  1  between each of the magnets and the corresponding plane out of the four planes in the outer surface of the side wall portion  10   a.    
     In this first variant, the example in which the shape of the metal case  70  is a quadrangular prism one and the number of combinations each having a coil  1  and at least one or more magnets  2  is four is explained. However, the shape of the metal case  70  is not limited to this structure, and should just be a polygonal prism one. Further, the number of combinations each having a coil  1  and at least one or more magnets  2 , the combinations being included in the ultrasound probe  107 , is not especially limited. 
     According to the structure of the first variant, a previously-mentioned Lorentz force occurring in the side wall portion  10   a  can be effectively propagated to the center of the vibration surface  70   b  of the outer bottom wall portion  70   a.    
     Next, a second variant of the ultrasound probe  106  will be explained by reference to a drawing.  FIG. 16  is atop view and a side view showing the structure of an ultrasound probe  108  according to the second variant. As shown in  FIG. 16 , a metal case  80  has a cylindrical shape in which a side wall portion  20   a  is defined as a side face. A vibration surface  80   b  of an outer bottom wall portion  80   a  has a shape similar to that of a bottom face of the cylindrical shape. 
     Although not illustrated, an inner bottom wall portion in the ultrasound probe  108  includes a single flat-shaped inner bottom wall portion having a penetrating hole in a central part thereof. In the single flat-shaped inner bottom wall portion, a previously-mentioned first end corresponds to an edge portion facing the penetrating hole, and a previously-mentioned second end corresponds to an outer edge portion located on a side opposite to the penetrating hole. 
     Further, the ultrasound probe  108  includes multiple combinations each having a coil  1  and at least one or more magnets  2 . Each of the multiple combinations is mounted on an outer surface of the side wall portion  20   a  in such a way that the combinations are arranged at positions which are mutually rotationally symmetric with respect to an axis passing through the center of the vibration surface  80   b  of the outer bottom wall portion  80   a  and being perpendicular to the vibration surface  80   b  of the outer bottom wall portion  80   a.    
     In further detail, in this second variant, the ultrasound probe  108  includes, as the coil  1 , four coils  1  each mounted on an outer surface of the side wall portion  20   a.  Further, the ultrasound probe  108  includes, as the magnet  2 , eight magnets  2  each mounted in such a way as to sandwich a corresponding coil out of the four coils  1  between each of the magnets and the outer surface of the side wall portion  20   a.    
     In this second variant, the example in which the number of combinations each having a coil  1  and at least one or more magnets  2  is four is explained. However, the number of combinations each having a coil  1  and at least one or more magnets  2 , the combinations being included in the ultrasound probe  108 , is not especially limited. 
     According to the structure of the second variant, a previously-mentioned Lorentz force occurring in the side wall portion  20   a  can be effectively propagated to the center of the vibration surface  80   b  of the outer bottom wall portion  80   a.    
     As mentioned above, the bottom wall portion  60   b  in the ultrasound probe  106  according to Embodiment  3  includes the flat-shaped outer bottom wall portion  60   d  having the vibration surface  60   c,  the flat-shaped inner bottom wall portion  60   e  whose one flat surface faces a flat surface of the outer bottom wall portion  60   d,  the flat surface of the outer bottom wall portion  60   d  being opposite to the vibration surface  60   c,  and the connection portion  60   h  that connects the outer bottom wall portion  60   d  and the inner bottom wall portion  60   e,  the inner bottom wall portion  60   e  has a first end and a second end opposite to the first end, the first end is fixed, the second end is connected to an end of the side wall portion  3   a,  and a portion between the first end and the second end is connected to the connection portion  60   h,  and the coil  1  causes an eddy current to occur in the one surface of the side wall portion  3   a,  and causes the outer bottom wall portion  60   d  to vibrate by way of the inner bottom wall portion  60   e  and the connection portion  60   h,  by using a Lorentz force occurring in the side wall portion  3   a  because of the interaction between the eddy current and a magnetic field generated by the magnet  2 . 
     According to the above-mentioned structure, the inner bottom wall portion  60   e  acts as a second class lever, and thus propagates the Lorentz force occurring in the side wall portion  3   a  to the connection portion  60   h,  as mentioned above. As a result, in the outer bottom wall portion  60   d  connected to the connection portion  60   h,  a vibration larger than that occurring in the bottom wall portion  3   b  according to Embodiment 1 occurs. Therefore, the amplitude of an ultrasonic wave excited in a test object when the test object is brought into contact with the vibration surface  60   c  of the outer bottom wall portion  60   d  can be increased, and thus the sensitivity of an ultrasonic nondestructive inspection can be improved. 
     Further, the side wall portion  3   a  in the ultrasound probe  106  according to Embodiment 3 includes the first flat-shaped side wall portion  3   d  and the second flat-shaped side wall portion  3   e  which face each other, the inner bottom wall portion  60   e  includes the first flat-shaped inner bottom wall portion  60   f  whose one flat surface faces a flat surface of the outer bottom wall portion  60   d,  the flat surface of the outer bottom wall portion  60   d  being opposite to the vibration surface  60   c,  and the second flat-shaped inner bottom wall portion  60   g  whose one flat surface faces the flat surface of the outer bottom wall portion  60   d,  the flat surface of the outer bottom wall portion  60   d  being opposite to the vibration surface  60   c,  the connection portion  60   h  includes the first connection portion  60   i  that connects the first inner bottom wall portion  60   f  and the outer bottom wall portion  60   d  and the second connection portion  60   j  that connects the second inner bottom wall portion  60   g  and the outer bottom wall portion  60   d,  the first inner bottom wall portion  60   f  has a first end and a second end opposite to the first end, the first end is fixed, the second end is connected to an end of the first side wall portion  3   d,  and a portion between the first end and the second end is connected to the first connection portion  60   i,  the second inner bottom wall portion  60   g  has a first end and a second end opposite to the first end, the first end is fixed, the second end is connected to an end of the second side wall portion  3   e,  and a portion between the first end and the second end is connected to the second connection portion  60   j,  the first coil  1   a  mounted on a first surface of the first side wall portion  3   d,  the first surface being opposite to a second surface, of the first side wall portion  3   d,  facing the second side wall portion  3   e,  and the second coil  1   b  mounted on a first surface of the second side wall portion  3   e,  the first surface being opposite to a second surface, of the second side wall portion  3   e,  facing the first side wall portion  3   d,  are provided as the coil  1 , and at least one or more magnets  2  mounted in such a way as to sandwich the first coil  1   a  between the one or more magnets and the first surface of the first side wall portion  3   d,  the first surface being opposite to the second surface, of the first side wall portion  3   d,  facing the second side wall portion  3   e,  and at least one or more magnets  2  mounted in such a way as to sandwich the second coil  1   b  between the one or more magnets and the first surface of the second side wall portion  3   e,  the first surface being opposite to the second surface, of the side wall portion  3   e,  facing the first side wall portion  3   d,  are provided as the magnet  2 . 
     According to the above-mentioned structure, a Lorentz force occurring in the first side wall portion  3   d  and a Lorentz force occurring in the second side wall portion  3   e  can be effectively propagated to the vibration surface  60   c  of the outer bottom wall portion  60   d.    
     Further, the metal case  70  in the ultrasound probe  107  according to Embodiment  3  has a polygonal prism shape in which the side wall portion  10   a  is defined as a side face, the vibration surface  70   b  of the outer bottom wall portion  70   a  has a shape similar to that of a bottom face of the polygonal prism shape, and a number of combinations each having a coil  1  and at least one or more magnets  2 , the number of combinations being equal to the number of multiple planes in the outer surface of the side wall portion  10   a,  are provided, and each of the combinations whose number is equal to the number of multiple planes is mounted on a corresponding plane out of the multiple planes in such a way that the combinations are arranged at positions which are mutually rotationally symmetric with respect to an axis passing through the center of the vibration surface  70   b  of the outer bottom wall portion  70   a  and being perpendicular to the vibration surface  70   b  of the outer bottom wall portion  70   a.    
     According to the above-mentioned structure, a Lorentz force occurring in the side wall portion  10   a  can be effectively propagated to the center of the vibration surface  70   b  of the outer bottom wall portion  70   a.    
     Further, the metal case  80  in the ultrasound probe  108  according to Embodiment  3  has a cylindrical shape in which the side wall portion  20   a  is defined as a side face, the vibration surface  80   b  of the outer bottom wall portion  80   a  has a shape similar to that of a bottom face of the cylindrical shape, and multiple combinations each having a coil  1  and at least one or more magnets  2  are provided, and each of the multiple combinations is mounted on the outer surface of the side wall portion  20   a  in such a way that the combinations are arranged at positions which are mutually rotationally symmetric with respect to an axis passing through the center of the vibration surface  80   b  of the outer bottom wall portion  80   a  and being perpendicular to the vibration surface  80   b  of the outer bottom wall portion  80   a.    
     According to the above-mentioned structure, a Lorentz force occurring in the side wall portion  20   a  can be effectively propagated to the center of the vibration surface  80   b  of the outer bottom wall portion  80   a.    
     It is to be understood that any combination of two or more of the above-mentioned embodiments can be made, various changes can be made in any component according to any one of the above-mentioned embodiments, or any component according to any one of the above-mentioned embodiments can be omitted within the scope of the present invention. 
     INDUSTRIAL APPLICABILITY 
     Because the ultrasound probe according to the present invention can achieve downsizing thereof and can perform an ultrasonic nondestructive inspection on a non-conductive test object, the ultrasound probe according to the present invention can be used for ultrasonic nondestructive inspections. 
     REFERENCE SIGNS LIST 
       1  coil,  1   a  first coil,  1   b  second coil,  1   c  straight line portion,  2  magnet,  3  metal case,  3   a  side wall portion,  3   b  bottom wall portion,  3   c  vibration surface,  3   d  first side wall portion,  3   e  second side wall portion,  4  eddy current,  5  static magnetic field,  6  Lorentz force,  10  metal case,  10   a  side wall portion,  10   b  bottom wall portion,  10   c  vibration surface,  20  metal case,  20   a  side wall portion,  20   b  bottom wall portion,  20   c  vibration surface,  30  metal case,  30   b  bottom wall portion,  30   c  vibration surface,  30   d  outer bottom wall portion,  30   e  inner bottom wall portion,  30   f  first inner bottom wall portion,  30   g  second inner bottom wall portion,  30   h  connection portion,  30   i  first connection portion,  30   j  second connection portion,  40  metal case,  40   a  outer bottom wall portion,  40   b  vibration surface,  50  metal case,  50   a  outer bottom wall portion,  50   b  vibration surface,  60  metal case,  60   b  bottom wall portion,  60   c  vibration surface,  60   d  outer bottom wall portion,  60   e  inner bottom wall portion,  60   f  first inner bottom wall portion,  60   g  second inner bottom wall portion,  60   h  connection portion,  60   i  first connection portion,  60   j  second connection portion,  70  metal case,  70   a  outer bottom wall portion,  70   b  vibration surface,  80  metal case,  80   a  outer bottom wall portion,  80   b  vibration surface, and  100 ,  101 ,  102 ,  103 ,  104 ,  105 ,  106 ,  107 ,  108  ultrasound probe.