When you read the specifications for audio speakers, you'll likely come across the term RMS, which stands for "Root Mean Square," a method for rating a speaker's ability to handle audio signal power. Although it is technical in nature, you don't need to understand the details completely to use RMS specifications effectively to compare different audio speakers.
Average, Peak and RMS
Audio engineers measure power in terms of average, peak and RMS watts. Peak power is simply the highest reading obtained when measuring the playback of a typical song; it is the most power a speaker can handle in a loud burst of sound. By contrast, average power tells you what a speaker can manage for sustained periods. Mathematically, it is the alternating current voltage across a speaker's terminals times the current through the speaker, multiplied by the power factor, a number related to the speaker's efficiency. RMS power is the speaker current divided by 1.414, the square root of two. This gives a number somewhat smaller than the average and reflects a more accurate sustained power capability for a speaker.
Because the peak power figure is the highest of the three ratings, speaker manufacturers may use it in advertising to make a speaker "look good." Average and RMS ratings, however, are lower numbers that give you a better idea of a speaker's capabilities. Always compare the RMS rating of one model against the RMS for another; never compare RMS versus peak, as they mean different things.