This is from the Engineer at Texas Instruments, it also states the 1/8th rated output for 1 hour:
1.1 Power-Rating References and Basic Definitions
Application Report
SLEA047A–February 2005–Revised March 2005
Power Rating in Audio Amplifiers
Tuan Luu ............................................................................................... Digital Audio and Video Group
ABSTRACT
Average consumers often weigh heavily on cost versus power rating of audio amplifiers
as their basis for purchasing one. Depending on the marketing strategy, the power
rating methodology for audio amplifiers can vary from manufacturer to manufacturer.
The purpose of this document is to clarify three commonly used power ratings: average
power, peak power, and PMPO. The following Texas Instruments PurePath Digital™
power stages will be used for power measurement:
• TAS5142 – 4 half-bridges (2 BTL channels or 4 single-ended [SE] channels)
• TAS5152 – 4 half-bridges (2 BTL channels or 4 single-ended [SE] channels)
• TAS5186 – 6 half-bridges (6 single-ended [SE] channels)
There are a few standards that describe the power rating of an audio amplifier. The Federal Trade
Commission (FTC) establishes fair advertisement practices for home audio power ratings. This is
described in the FTC document 63FR37233, 16 CFR, Chapter 1, Part 432. Another standard is the
Electronic Industries Association (EIA) SE-101-A. For the car audio industry, some manufacturers accept
the Consumer Electronics Association CEA-2006-A standard. It defines how the amplifier should be tested
for power and signal distortion.
How can one tell a good power rating from a bad one? For a good power rating, all the specified reference
points are measurable.
Ohm’s law establishes the relationship of voltage, current and load, i.e., V = I × R, where V = voltage, I =
current in amperes and R = load resistance in ohms.
Power is energy per time and is derived as P = V × I = V2
/R = I
2
× R.
For an audio signal, the voltage is in Vrms (root-mean-square), the power is referenced to a frequency of
1,000 Hz, and the load is usually referenced to a resistive load. Thus, the power obtained is the average
power that the amplifier can sustain. The FTC requires further that the amplifier be pre-conditioned at
one-eighth of rated total power output (for a multiple-output system, all channels are on) for one hour
using a sinusoidal wave at a frequency of 1,000 Hz. The power spectrum measurement is then collected
with two channels at maximum rated power over the audio frequency range of 20 to 20,000 Hz, in ambient
still air of not less than 25°C, for the duration of not less than 5 minutes.