Speakers:

The modern speakers can be divided into three main categories :

1.      Crystal,

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.