Dynamic Power Measurements
About the Dynamic Power Measurements
The Audio Graph PowerCube is a test instrument used to measure the out-
put 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 test-
ing, 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 mil-
liseconds) 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% 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 resis-
tor 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 sur-
round/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 represent the shifts in
impedance and phase that can occur in a typical loudspeaker.
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