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Dynamic Power Measurements
About the Dynamic Power Measurements
The Audio Graph PowerCube is a test instrument used to measure the output of an
amplifier in accordance with IHF-202 industry standards. The IHF-202 standard is a
dynamic power measurement and was developed as a means of measuring power
in a manner that best represents the Real World operation of an amplifier. Many
manufacturers, including Rockford Fosgate, at times will measure amplifier power
into a fixed resistor (4 ohm, 2 ohm). While this method is useful in some types of
evaluation and testing, it is not representative of an amplifier that is connected to a
speaker and playing music.
Music
Music is dynamic; the sound waves are complex and constantly changing. In order
to simulate this, the IHF-202 standard calls for the input signal to the amplifier to be
a 1kHz bursted tone. This signal is input (on for 20 milliseconds) and reduced 20dB
for 480 milliseconds. The signal is gradually increased in level until the amplifier's
output exceeds 1% Total Harmonic Distortion (THD). At 1% distortion becomes
audible, therefore, any power produced above that level is considered unusable.
Many manufacturers represent their amplifiers' output power in excess of 10% dis-
tortion. They use many names for this measurement, such as Total Maximum Power
or Maximum Output Power. This is not indicative of the actual usable output power.
Listening to Loudspeakers - Not Resistors
A loudspeaker is not a resistor. A resistor's value (resistance measured in ohms) is
fixed. A loudspeaker's impedance is dynamic. It is constantly changing in value,
dependent upon the frequency of the input signal. Therefore, measuring power with
the amplifier loaded into a 4 ohm resistor is not the same as measuring power with
the amplifier connected to a 4 ohm speaker. Most people do not listen to music
through a resistor.
A 4 ohm speaker may experience a drop in impedance 4-6 times lower than its nom-
inal (printed) impedance. A speaker will also create phase shifts in the signal that is
passed through it. These phase shifts happen because a speaker is an inductor (voice
coil) and a capacitor (compliance of the surround/spider), as well as a resistor (voice
coil wire).
To simulate a speaker the Audio Graph PowerCube measures output power into 20
different loads. It tests at 8 ohms, 4 ohms, 2 ohms and 1 ohm. Each of these imped-
ances is also tested at –60°, –30°, 0°, +30° and +60° phase angles. These different
impedances and phase angles represent the shifts in impedance and phase that can
occur in a typical loudspeaker.