Now speakers are mostly electricity-powered loudspeaker, the upper limit frequency is difficult to reach 20 KHZ, let's take a look at the force of the dynamic speaker, the voice coil paper plates push force F, F Ft Fe can be decomposed into vertical force and horizontal force, due to the existence of the Ft paper cone produces alternating bending movement, so that the paper plates to produce transverse vibration and form a standing wave, The standing wave makes the frequency characteristic of the loudspeaker fluctuate in the high frequency band and the characteristic curve is not uniform enough. To overcome this shortcoming, try to reduce the weight of the paper plate (reduce inertia, thus reducing lateral vibration) and increase the hardness of the paper plate (reduce the bending of the paper plate caused by FT).
For this reason, people have made a lot of improvements to the paper plate of high frequency loudspeaker, such as using metal cymbals to make the paper plate, which is not only hard but also light. In addition of boron is very high hardness and elasticity coefficient, is a good material to make paper plates, first made of titanium into 10-20 microns thick plate substrate, then put the substrate in a vacuum, under the high temperature of 2500 degrees C, with a strong electron beam bombardment boron, make its deposition in the titanium substrate surface after evaporation, so make a speaker is 36 KHZ upper limit of the frequency.
If you want to further improve the upper limit frequency of the loudspeaker, it is necessary to transform the structure. One loudspeaker with an upper frequency of 50 kHz uses a ribbon structure, as shown in Figure 2. Six permanent magnets are divided into two groups to form the magnetic circuit system. The vibration film is made of 7-8 micron thick polymer film, which is light and soft. The vibration film is attached with an aluminum ribbon voice coil 10 micron thick. When the current flows through the voice coil, the film will generate F+ and F- forces depending on the direction of the current. There is no transverse force problem at all. Because of the diaphragm's very light mass, the diaphragm moves with little inertia, so the speaker frequency response is particularly good.