Why is the power of a loudspeaker expressed in watts?

 Unlike sounds generated by human beings, and animals and materials, the sound output of a speaker is derived from an electrical signal. This electrical signal, like in other electrical appliances, represents the energy used or work done by a current flowing through it. This power, as we know,depends on the voltage, current and phase difference between the current and voltage, and is measured in watts. One watt is the energy or work done at one Joule per second. This scale makes measuriing of power output easy for commercial purposes. Courtesy : The Hindu