RMS

The Root Mean Square (RMS) is the mathematical method for determining the effective voltage for a continuous alternating current (AC) wave, also known as a sine wave. It is defined as the square root of the mean of the squares of the values. This is also known as the quadratic mean.

In this definition...

RMS and consumer products

RMS watts is primarily used to refer to the continuous power handling of consumer audio products, namely speakers and subwoofers. But RMS may also refer to the amount of power an amplifier can effectively output. It represents that the power a device is capable of handling over extended periods of time without compromising sound quality or causing distortion. This figure is often listed on products as “watts RMS.”

How to calculate RMS

To calculate RMS, there are three mathematical operations that must be used in tandem:

1. The first step is to determine the square of the waveform function.
2. Next, you must find the average of the resulting function from step 1 over time.
3. Finally, you will find the square root of the function you found in step 2.

RMS watts vs. peak watts

RMS watts and Peak watts both play an important role when building a sound system. These values should be used to match speakers and subwoofers with amplifiers. Products will typically be rated for both RMS and Peak watts. The difference between the two can be summed up as continuous vs. momentary.

Where RMS refers to the power a device is capable of handling over extended periods of time, Peak watts refers to the highest power level a speaker or subwoofer is capable of handling in a short burst without blowing out. Peak watts is also known as PMPO (peak momentary power). In the case of amplifiers, these terms refer to how much power an amplifier can output over extended periods of time or in a short burst before failing, respectively. Typically, Peak watts for a product will be about double the RMS.

Clearly understanding the difference between these terms is important when matching speakers and subwoofers with amplifiers. RMS ultimately means the level of power that can be sustained without causing damage to the device. Attempting to sustain peak power can easily cause damage, such as overheating wires and other components.

History of RMS in electronics

In 1974, the Federal Trade Commission (FTC) wanted to stop false-marketing from audio device manufacturers about the capabilities of their products. The resulting Amplifier Rule created a standard for prescribing continuous power measurements for sine waves in advertising.

Vangie Beal
Vangie Beal is a freelance business and technology writer covering Internet technologies and online business since the late '90s.

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