Physical, psychoacoustic and musical factors of the amplifier power required to produce a particular "volume level" with your music system. In objective terms, the intensity of a sound is measure as a sound-pressure level (SPL) which can be compared to other sound levels on a logarithmic scale of decibels (dB). As shown in the accompanying illustration, the range of pressures - from the threshold of hearing to the point where sound becomes painful - covers a span of approximately 130 dB, representing a power ratio of ten trillion to one. A 1-dB change in loudness is about the smallest that can easily be perceived by the unaided ear.
Because the ear follows a generally logarithmic response rather than a linear one in translating SPL changes into changes in perceived loudness, doubling the power of the sound (a change of 3 dB) does not produce the sensation of twice the loudness. The increase is certainly audible, but it seems rather slight. In order for a sound to appear subjectively twicce as loud as another, the power must be increased by a factor of ten (10 dB). Thus, while 2 watts (or less, as we'll see below) will create a rather high sound level on most component systems, doubling the power to 4 watts would make only a small loudness increase, and doubling the appearent loudness would requite a power increase to 20 watts.
The music itself imposes still further demands on amplifier power because of its high peak-to-average levels. The initial few milliseconds of a musical note can easily demand ten times or more as a whole. Thus if the average power required for very loud music reproduction was 20 watts, the actuall momentary peak levels would require 200 watts. Because the peaks are so fleetingly brief in time, the ear does not respond to them fully, but the amplifier and loudspeakers do - or try to! If the amplifier cannot deliver the needed power - or if the speaker cannot handle it - the peaks are clipped, and the result is distortion - which depending upon a complex of factors, may or may not be audible.