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E A S U R E M E N T S
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 stan-
dards. 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 manufac-
turers, 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 millisec-
onds. The signal is gradually increased in level until the amplifier's
output exceeds 1% Total Harmonic Distortion (THD). At 1% distor-
tion becomes audible, therefore, 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 (compli-
ance 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.
Appendix A
