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看AAC文档，查了下RMS Power，却发现这个东西竟是 FTC制造出来的错误名词
FTC: Federal Trade Commission
What's RMS Power or RMS Watts?
by Paul Quillen, 1993
This is an interesting subject. Both RMS Power and RMS Watts are a fiction created by the
Federal Trade Commission.
In the mid-1970's the FTC decided that they would create a standard for power ratings for power
amplifiers. Their motivation was furniture store stereo consoles rated at 800 Watts that actually
put out a few Watts…blatantly false advertising. Of course, everyone knew that the furniture
store power ratings were a joke, but the FTC saw it as an opportunity to regulate (control).
Amplifier power is typically measured across a precision resistor, typically a Dale 8 Ohm power
resistor. These are precision high-power resistors that can dissipate lots of power.
A sinusoidal waveform is applied to the amplifier input at various frequencies and the output
waveform is watched on an oscilloscope. When the output waveform starts to distort, the input
level is backed down until the measured distortion falls below a certain percentage. Then the
Voltage is measured across the 8 Ohm load resistor.
The formula for power is:
Power = V2/8
Now here's where the FTC goofed. The Voltage measured here is RMS Voltage. (RMS is the
DC heating equivalent Voltage. It's .707 X the peak voltage of a sine wave. RMS means Root
Mean Square, which means the Square Root of the Mean of the Squares of the sum of all the
voltages in the waveform.) So far so good.
The FTC then incorrectly assumed that if you were measuring RMS Voltage, the result from the
above formula would be RMS power.
The problem is that it's not. It's average power. This is clear in all electrical engineering
textbooks. There is no such thing as RMS power.
The FTC also incorrectly assumed that the measurement of the power in Watts would be RMS
Watts. It's not. It's Watts. There's no such thing as RMS Watts.
In summary, RMS Voltage is correct, but there's no such thing as RMS Power or RMS Watts.
Or stated differently, the Voltage that's measured is RMS Voltage, but the resulting power is
Average Power and it's measured in Watts.
