the present invention will be described in detail hereinafter with reference to embodiments illustrated in drawings . fig1 shows an electroacoustic transducer according to an embodiment of the present invention and fig2 shows a pole piece portion around which a coil is wound in fig1 . the pole piece portion 2 of the electroacoustic transducer is composed of a yoke 4 having a shape of disc and a pole 6 , and the pole 6 is attached to the central portion of the yoke 4 at a base end portion 8 thereof by way of a fixing means such as press fit etc ., the pole 6 being smaller in diameter at the base end portion 8 thereof . a coil 10 is wound around the pole 6 to be cylindrical coaxially with the pole 6 at the peripheral portion thereof . the coil 10 is made flat on the yoke 4 at the side of the base end portion 8 of the pole 6 and a side surface 14 which is different in shape from that of prior art is formed at the side of the tip end surface 12 of the pole 6 , the side surface 14 being composed of first and second side surfaces 14a and 14b which are different in shape from each other . that is , the first side surface 14a is funnel - shaped to expose a part of the tip end portion of the pole 6 , while the second side surface 14b is a flat plane conforming or adjacent to the tip end surface of the pole 6 . as a result , the coil 10 is increased in the number of effective turns ( increased portion is hatched ) by a height d ( fig2 ) in the axial direction of the pole 6 to increase a winding efficiency compared with a conventional coil 108 ( fig6 ), the upper surface of which is indicated by a broken line in fig2 . supposing that the coil 10 is the same in outer diameter and material as the conventional coil 108 , it is possible to reinforce the magnetic field by that generated by the height d . the electroacoustic transducer of this invention is the same in construction as a conventional electroacoustic transducer ( fig6 ) wherein an annular magnet 16 is provided about the pole 6 on the pole piece portion 2 and is fixed thereto . a diaphragm 18 is provided on the magnet 16 and a magnetic piece 20 is attached to the central portion of the diaphragm 18 as a means to increase the substantial vibrating mass thereof . according to this embodiment , a gap 22 is formed between the diaphragm 18 and the tip end surface 12 of the pole 6 as a means for forming a space to allow the vibration of the diaphragm 18 therein by setting the height of the magnet 16 higher than that of the pole 6 . the diaphragm 18 and the magnetic piece 20 are made of magnetic material and the magnet 16 holds the diaphragm 18 thereon by way of its magnetic function and applies a bias magnetic field to the diaphragm 18 as a means for generating a magnetic oscillation . similar to the conventional electroacoustic transducer , the pole piece portion 2 , the magnet 16 , the gap 22 , the diaphragm 18 and the magnetic piece 20 constitute a single closed magnetic path and the coil 10 , the yoke 4 and the pole 6 constitute a magnetic driving portion which converts an external electric signal into a magnetic field to be applied to the diaphragm 18 . the peripheral surface of the pole piece portion 2 and the upper surface side of the diaphragm 18 are covered by a housing 24 . the housing 24 is a molded body of non - magnetic material such as synthetic resin etc . and comprises a resonance chamber 26 formed at the upper surface side of the diaphragm 18 . a sound emitting cylinder 28 is formed in the housing 24 and a sound emitting hole 30 is formed in the sound emitting cylinder 28 for allowing the resonance chamber 26 to be open to the atmosphere and emitting a resonance sound thereto . although the sound emitting cylinder 28 and the sound emitting hole 30 are formed about the central axis of the diaphragm 18 according to this embodiment , they may be formed otherwise . when an external electric signal is applied to the terminals of the coil 10 in such an electroacoustic transducer , the coil 10 is energized in response to the level of the electric signal . as a result , the pole 6 generates an alternating magnetic field therearound which acts on the diaphragm 18 and the magnetic piece 20 . since a bias magnetic field is applied to the diaphragm 18 by the magnet 16 , the diaphragm 18 receives a vertically vibrating force in response to the frequency and level of the alternating magnetic field superposed on the bias magnetic field . as a result , the diaphragm 18 vibrates to vibrate air at the upper and lower sides of the diaphragm 18 so as to resonate the resonance chamber 26 . accordingly , the vibrating sound of the diaphragm 18 and the resonance sound of the resonance chamber 26 are emitted to the outside through the sound emitting hole 30 . since the frequency of the resonance sound is distributed in the audio range , the electroacoustic transducer is used as a sound generating means such as a buzzer , etc . in the case of the electroacoustic transducer of the present invention , the number of turns of the coil 10 is larger than the coil 108 of a conventional electroacoustic transducer by the height d and consequently generates a stronger magnetic field , which means that the magnetic force to vibrate the diaphragm 18 is stronger than that of prior art in response to the same input , so that the sound pressure of the electroacoustic transducer is reinforced . moreover , this characteristic brings on a change to the electroacoustic transducer itself or the input thereto in case of generating the same magnetic field as that of prior art . that is , electric power to be applied to the coil 10 can be reduced to generate a magnetic field equal to the conventional electroacoustic transducer ( fig6 ). furthermore , to generate a conventional magnetic field in response to a conventional input , the pole 6 can be reduced in height that much . the reduction in height corresponds to reduction in the number of turns of the coil 10 by the height d , so that it is possible to reduce the electroacoustic transducer in height and dimensions . still furthermore , the coil 10 in the height d effectively makes use of a space at the rear side of the diaphragm 18 and does not prevent the vibration of the diaphragm 18 at all . it is because the vibration of the diaphragm 18 is maximum at the central portion thereof and is reduced toward the peripheral portion thereof . consequently , the increase of the coil 10 by the height d increases the driving force thereof while generating the vibration of the diaphragm 18 similar to that of the conventional one . although in the electroacoustic transducer illustrated in fig1 and 2 , the second side surface 14b of the coil 10 conforms to the tip end surface 12 of the pole 6 , it may project from the tip end surface 12 of the pole 6 toward the diaphragm 18 or retract therefrom as far as it is adjacent to the tip end surface 12 . still furthermore , although the terminals are not shown in this embodiment , they may be formed of the ends of the coil 10 or may be formed as lead terminals at the rear side of the yoke 4 with an intervening insulator provided . fig3 a and 3b show a method of winding a coil in the electroacoustic transducer according to an embodiment of the present invention . it employs a holding member 31 for holding the pole piece portion 2 . the holding member 31 is a chuck for holding the pole piece portion 2 around which the coil 10 is to be wound . the holding member 31 comprises a recess 34 for holding the yoke 4 of the pole piece portion 2 at the front side thereof and an axial portion 36 at the rear side thereof . the axial portion 36 is connected to a rotating means such as a motor etc ., not shown , thereby to be rotated as indicated by an arrow n in accordance with the number of turns of the coil 10 . a shaping member 32 for shaping the side surface 14 is set on the tip end surface side of the pole 6 of the pole piece portion 2 to be confronted with the holding member 31 . the shaping member 32 comprises a recess 38 formed at a position corresponding to the tip end surface 12 of the pole 6 and a shaping surface 40 around the recess 38 as illustrated in fig3 b . in the case of this embodiment , the shaping surface 40 is composed of a first shaping surface 40a and a second shaping surface 40b . that is , the internal surface of the yoke 4 of the pole piece portion 2 which is held by the holding member 31 and the first and second shaping surfaces 40a and 40b define a space 43 in which the coil 10 is to be wound . the first shaping surface 40a is conical to form the first side surface 14a of the side surface 14 . the height of the first shaping surface 40a equals to the height d of the first side surface 14a . the second shaping surface 40b is formed flat to correspond to the second side surface 14b so as to form a surface perpendicular to the central axis of the coil 10 . in this embodiment , the second shaping surface 40b forms the side surface which is on a plane conforming or adjacent to the end surface of the pole 6 . an axial portion 42 is provided at the rear portion of the shaping member 32 . the axial portion 42 is supported to be rotatable by the rotation of the pole piece portion 2 . before the coil 10 is wound around the pole 6 , the pole 6 is attached to the yoke 4 to integrally form the pole piece portion 2 . the yoke 4 of the pole piece portion 2 is embedded in the recess 34 of the holding member 31 to be held thereby while the recess 38 of the shaping member 32 is fitted onto the tip end surface 12 side of the pole 6 . thereafter a wire 44 to form the coil 10 is introduced from a bobbin 46 to the pole 6 side and the holding member 31 is rotated by way of the axial portion 36 . as a result , the wire 44 is wound around the pole 6 to gradually form the coil 10 as the holding member 31 is rotated and the first and second side surfaces 14a and 14b are formed on the shaping surface 40 of the shaping member 32 at the tip end surface 12 side of the pole 6 . in this case , a shape fixing agent may be dropped to the coil 10 to fix the same in shape . in case the wire 44 is beforehand coated with the shape fixing agent to form the coil 10 having a stable shape , dropping such a shape fixing agent is not necessary . winding the coil 10 using such a shaping member 32 can increase the number of turns of the coil 10 without wasting a winding space for holding the pole 6 by the shaping member 32 and thereby increases the winding efficiency of the coil 10 on the pole 6 . moreover , assembling the pole piece portion 2 and winding the coil 10 may be continuously performed for automation . although the second shaping surface 40b of the shaping member 32 is on the same plane as the bottom surface of the recess 38 according to this embodiment , it is not a necessary condition , and in case the second side surface 14b of the coil 10 is not on the same plane as the tip end surface 12 of the pole 6 , they are arranged properly relative to each other as occasion demands . although this embodiment exemplifies a case wherein the ends of the coil 10 are used as terminals , notches or through holes may be formed in the holding member 31 for passing lead terminals in case the same are provided on the rear side of the yoke 4 and the existence of the lead terminals formed on the yoke 4 does not matter at all in holding the yoke 4 by the holding member 31 . the surfaces of the first and second shaping surfaces 40a and 40b may be subjected to teflon coating or mirror finish to be easily separated from the first and second side surfaces 14a and 14b of the coil 10 after the same has been wounded . fig4 a and 5b show the method of winding a coil in the electroacoustic transducer according to another embodiment of the present invention . as illustrated in fig4 a base plate 48 made of insulating material is provided at the rear side of the yoke 4 of the pole piece portion 2 . terminals 50 and 52 to be connected to the end portions of the coil 10 are provided upright at the rear side thereof . the pole 6 is provided upright at the upper surface side of the yoke 4 by piercing the central portions of the yoke 4 and the base plate 48 at the base end portion 8 thereof having a columnar shape . u - shaped notches 54 and 56 are formed in the yoke 4 of the pole piece portion 2 and the base plate 48 at the respective sides thereof between the terminals 50 and 52 . these notches 54 and 56 constitute a means for passing the wire 44 between the pole 6 and the terminals 50 and 52 . a chuck 320 is provided as the shaping member 32 which is the shaping means of the coil 10 as well as the holding means of the tip end portion of the pole 6 of the pole piece portion 2 . the tip end portion of the pole 6 of the pole piece portion 2 is held by a pawl portion 322 which is closably divided into multiple pieces , e . g ., three pieces . the pawl portion 322 of the chuck 320 comprises a holding projection 41 at the side of the end surface thereof , the end surface of the holding projection 41 being composed of a first shaping surface 40a forming a conical surface arranged at the central portion thereof and a second shaping surface 40b forming a flat surface arranged around the first shaping surface 40a . that is , the first and second shaping surfaces 40a and 40b and the internal surface of the yoke 4 define the space 43 in which the coil 10 is wound . moreover in this embodiment , the chuck 320 is provided with a pin 324 which is surrounded by the multiply divided pawl portion 322 . the pin 324 is freely slidable to determine the holding length of the pole 6 by the position of the tip end thereof . projecting the pin 324 facilitates the separation of the pole piece portion 2 from the chuck 320 after the coil 10 has been wound . when the coil 10 is wound around the pole 6 , as illustrated in fig5 a , the starting portion 44e of the wire 44 is retained by a retaining member 60 , then the tip of the wire 44 is wound around the terminal 52 and is introduced to the pole 6 side by way of the terminal 50 through the u - shaped notch 54 , thereafter the chuck 320 is rotated in the direction indicated by the arrow n to wind the wire 44 around the pole 6 in the space 43 to form the coil 10 in a predetermined shape . then the tip of the wire 44 is introduced to the terminal 50 side by way of the u - shaped notch 54 to be wound therearound so as to complete the winding process . in this case , fig5 b illustrates the introduction and drawing out of the wire 44 between the pole 6 side and the terminal 50 and 52 side by way of the u - shaped notch 54 of the pole piece portion 2 and the arrow indicates the direction thereof . the wire 44 can be wound around the pole 6 starting on the surface thereof along the first and second shaping surfaces 40a and 40b by rotating the chuck 320 in the direction of the arrow n so that the coil 10 as high as the pole 6 is formed with the peripheral surface of the tip end surface 12 side of the pole 6 exposed as illustrated in fig2 . as described above , employing the chuck 320 to hold the pole 6 of the pole piece portion 2 for winding the coil 10 therearound as illustrated in fig2 obviates the holding member 31 . it simplifies not only the whole device but also the winding process since the process for the holding member 31 is eliminated . moreover , in case the pole 6 of the pole piece portion 2 is held by the chuck 320 as in this embodiment , the yoke 4 side of the pole piece portion 2 can be a free end , which has an advantage that even if bar terminals are provided thereon , there is no need to pass the same through the holding member 31 side of the pole piece portion 2 . also in this method , however , the coil 10 may be wound around the pole piece portion 2 while the yoke 4 side thereof is held by the holding member 31 . holding the pole 6 of the pole piece portion 2 at both ends thereof in this way will be able to restrain vibration due to the rotation thereof and further increase the winding accuracy . moreover , also in this embodiment , the first and second shaping surfaces 40a and 40b of the pawl portion 322 may be subjected to teflon coating or mirror finish to facilitate the separation thereof from the first and second side surfaces 14a and 14b of the coil 10 after the same has been wound . since the pawl portion 322 of the chuck 320 is divided into multiple portions such as three , the tip end portion of the pole 6 of the pole piece portion 2 can be held or released by closing or opening the divided portions . upon completion of winding the coil 10 , the pawl portion 322 may be opened to let the pole piece portion 2 drop by gravity , but in case it won &# 39 ; t drop , a means such as air blast may be used to help it to drop . as described above , the electroacoustic transducer according to the present invention can increase the winding efficiency of a coil on the pole of the pole piece portion by effectively making use of a given limited space without securing a particular space for winding the coil therein , so that it is possible to obtain a high sound pressure , miniaturize and flatten the electroacoustic transducer and automate the winding process of the coil . moreover , the method of winding a coil in the electroacoustic transducer according to the present invention can increase the winding efficiency of the coil and speed up the winding process so as to increase mass productivity . although the features of the present invention have been described with reference to preferred embodiments , it is to be understood that many variations and changes are possible in the invention without departing from the scope thereof .