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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 chang-
ing. 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, there-
fore, any power produced above that level is considered not usable . Many
manufacturers represent their amplifiers' output power in excess of 10%
distortion. 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 nominal (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 impedances is also tested at –60 , –30 , 0 , +30 and
+60 phase angles. These different impedances and phase angles repre-
sent the shifts in impedance and phase that can occur in a typical
loudspeaker.
– 33 –
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