A speaker or loudspeaker consists of one or more speaker drivers contained within an enclosure or covered with a grill. These drivers act as a type of transducer that converts an electrical audio signal from a computer or audio receiver into sound waves. The sound that is produced by speakers is measured in frequency and amplitude. The frequency of a speaker relates to the pitch of the sound, high or low. Some speakers have multiple cones or drivers to deal with different frequency ranges such as 'subwoofers', 'woofers' and 'tweeters' The amplitude of a speaker refers to the loudness. The loudness of a speaker is determined by the air pressure created by soundwaves when you turn up the volume. Active speakers have internal amplification and use electricity to amplify the signal. Passive speakers do not have internal amplification and require a high level of audio input which can be produced by an audio amplifier. Most speakers come in pairs with the left and right speakers using separate channels to create a natural stereo sound.