located at the connection place the assembly structure 31 includes an inner ring 311 and an outer ring 312 and bottoms of the inner ring 311 and the outer ring 312 are formed with an assembly slot 313 the cross section of assembly structure 31 can be in a shape of reverse v or reverse u and the assembly slot 313 is used to emplace and assemble with a top part of the voice coil 4 as the piston style movement of sound membrane 2 is completely dependent on a push of the voice coil 4 the ideal assembly structure of voice coil 4 and sound membrane 2 is very important in the present invention the connection place of the voice coil 4 is located in the assembly slot 313 at the inner bottom of the sound membrane 2 which can be provided with a good pushing and connection effect compared to the conventional structure that the voice coil is connected at a side of the sound membrane the present invention is provided with a better assembly effect the aforementioned structure can be also implemented at an embodiment as shown in fig8 and fig8 a wherein an assembly structure 41 is located at a top part of the voice coil 4 the cross section of an assembly slot 413 of that assembly structure 41 can be in a shape of v or.