Dynamic Power Measurements - Rockford Fosgate 100 Installation & Operation Manual

D P M
YNAMIC OWER EASUREMENTS
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% 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 represent the shifts in impedance
and phase that can occur in a typical loudspeaker.
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