The modern speakers can be divided into three main categories :
2. Electrostatic, and
3. Electro Dynamic.
In crystal loudspeaker, a crystal is clamped among two electrodes across which an audio signal is applied. The crystal is also connected mechanically to the diaphragm which vibrates and thus, generates a sound output. Electrostatic loudspeaker operates on the principle that a DC voltage among two parallel metal plates causes these plates to attract or repel each other. A low frequency speaker is called a woofer and a high frequency loudspeaker is called a tweeter.