Sound is a physical/mechanical phenomenon. A speaker is a device that can convert an electronic signalGenerically, a device that converts a signal from one physical form (e.g., temperature) to another (e.g., a voltage) is called a transducer. into sound. but the typical signal that comes from, say, an mp3 player, is relatively low power, too low, in fact, to "drive" speaker. To boost the signal we need to build an amplifier.

An amplifier is a feedback device.

Things to read about

Amplifier
Loudness
Operational Amplifier (op amp)
Loudspeaker