Main types of speakers： There are many kinds of loudspeakers, which can be divided into electric type (i.e. moving coil type), electrostatic type (i.e. capacitive type), electromagnetic type (i.e. tongue spring type), piezoelectric type (i.e. crystal type) and other types according to the energy exchange principle. The latter two types are mostly used in rural cable broadcasting networks; According to the frequency range, it can be divided into low-frequency speakers, mid-range speakers and high-frequency speakers, which are often used as combined speakers in speakers. According to the energy exchange mechanism and structure, the electric loudspeaker has the advantages of good electroacoustic performance, firm structure and low cost, and is widely used; According to the sound radiation materials, it can be divided into paper basin type, horn type and diaphragm type; According to the shape of paper basin, it is divided into circular, oval, double paper basin and rubber folding ring; According to the working frequency, it is divided into bass, midrange and treble, and some are also divided into special recorders, special televisions, ordinary and high fidelity speakers, etc; Divide into low impedance and high impedance according to voice coil impedance; According to the effect, it is divided into direct radiation and ambient sound. Speakers are divided into built-in speakers and external speakers, and external speakers are generally referred to as speakers. Built in speaker means that the MP4 player has a built-in speaker, so that users can listen to the sound emitted by the MP4 player not only through the headphone jack but also through the built-in speaker. Mp4 player with built-in speaker can not use external speaker, but also avoid the inconvenience caused by wearing headphones for a long time.