RMS Power
What does RMS actually mean?
RMS stands for Root Mean Square. It is a mathematical method, not a type of power. It is normally applied to voltage or current, but for many years it has been used in the audio industry to describe amplifier and loudspeaker power.
Why? Simply because it was the best available method at the time, and no better, widely agreed standard existed.
I am often asked “What’s the RMS power?” My usual answer is that RMS is not particularly suitable for audio. If you want to understand why, read on. Otherwise, just accept that AES power is the standard you should be using for loudspeakers.
Why do we have RMS at all?
RMS comes from electrical engineering, where it works extremely well for AC power systems. It allows an AC signal to be converted into an equivalent DC value that produces the same heating effect.
In the UK, mains electricity is described as 240 V AC. That figure is already an RMS value. In reality, the waveform swings to about 339 V peak, or 679 V peak-to-peak.
The RMS figure is very useful. If an electric heater draws 10 A from a 240 V supply, we call it a 2400 W heater. The voltage and current are constant, the waveform is a steady 50 Hz sine wave, and the power delivery is continuous. This is a perfect use case for RMS.
[Image: AC sine wave showing peak, peak-to-peak, and RMS level]
Why RMS doesn’t map cleanly to audio
Audio amplifiers and loudspeakers do not operate with constant sine waves. Music is dynamic, the amplitude changes constantly, and power delivery is anything but steady.
To work around this, various test standards were created using controlled noise signals instead of tones. For loudspeakers, common examples included EIA RS-426A and IEC 268-5.
With a known test signal, it is possible to calculate an equivalent RMS value using averaging and squaring maths. This is where the idea of “RMS power” for speakers came from. However, it was never especially accurate, and often resulted in unrealistically low power ratings.
The amplifier vs speaker mismatch
Over time, it became normal to match amplifier and speaker ratings directly. For example, using a 400 W RMS amplifier with a 400 W RMS speaker.
The problem is that the two numbers were not measuring the same thing.
Amplifiers were often tested at 1 kHz using a continuous sine wave into a resistive load. This frequently overstated real-world power, because at lower frequencies (around 40–100 Hz) the power supply could not always sustain the same output. In practice, usable power at 100 Hz could be 10% lower than at 1 kHz.
Meanwhile, loudspeakers could often tolerate short-term peaks above their RMS rating. This is why users traditionally chose a slightly larger amplifier, to provide headroom.
Conclusion
RMS was not useless, but it was never a complete or accurate way to relate amplifier power to loudspeaker power. There was always an element of estimation and experience involved.
This mismatch is exactly why modern standards moved on, and why RMS power is no longer the best reference point for real-world audio systems.