Tweeter Technology

Tweeter Technology

Most of the current speakers are electric speakers, and their upper frequency limit is difficult to reach 20kHz. Let's first look at the force of the electric speaker. The force of the voice coil pushing the paper plate is F, which can be decomposed into the longitudinal force Fe and the transverse force Ft. Due to the existence of Ft, the cone surface of the paper plate produces alternating bending motion, which causes the paper plate to vibrate transversely and form standing waves. The standing waves cause the frequency characteristics of the speaker to fluctuate in the high frequency band, and the characteristic curve is not uniform enough. To overcome this shortcoming, we need to try to reduce the weight of the paper plate (reduce inertia, thereby weakening transverse vibration) and increase the hardness of the paper plate (reduce the bending caused by Ft on the paper plate). For this reason, people have made many improvements to the paper plate of the high-frequency speaker, such as using metal cymbals to make paper plates, which are not only hard but also light. In addition, boron has high hardness and elasticity coefficient, making it a good material for making paper plates. First, use titanium to make a 10-20 micron thick paper plate substrate, and then place the substrate in a vacuum. At a high temperature of 2500 degrees C, bombard the boron with a strong electron beam to evaporate it and then deposit it on the surface of the titanium substrate. The upper limit frequency of the speaker made in this way can reach 36KHz.

    To further increase the upper frequency limit of the speaker, it is necessary to modify the structure. There is a speaker with an upper frequency limit of 50KHz that uses a ribbon structure, as shown in Figure 2. Six permanent magnets are divided into two groups to form a magnetic circuit system. The vibration membrane is made of a 7-8 micron thick polymer film, which is light and soft. A 10-micron thick aluminum ribbon voice coil is attached to the vibration membrane. When current flows through the voice coil, the sound membrane will generate F+ and F- forces according to the direction of the current, and there is no problem of lateral force at all. Since the diaphragm is very light, the movement of the diaphragm has almost no inertia, so the speaker frequency response is particularly good.