Speakers are divided into built-in speakers and external speakers. External speaker, which is generally referred to as the speaker, the built-in speaker refers to the MP4 player with a built-in speaker. Many speakers of the species, according to the exchange principle can be divided into electric (i.e. moving coil) and electrostatic (i.e. capacitance), electromagnetic (i.e. reed), piezoelectric (i.e. crystal type) etc..