DYNAMIC POWER MEASUREMENTS– 20 –
Appendix A
About the Dynamic Power Measurements
The
Audio Graph PowerCube
is a test instrument used to measure the
output of an amplifier. It makes measurements in accordance with IHF-
202 standards. The IHF-202 standard is a Dynamic power measurement.
It was developed as a means of measuring power in a manner that best
represents the Real World operation of an amplifier. Many manufactur-
ers, 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 hooked up to a speaker and playing music.
Music
Music is dynamic; 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 a short
period of time and then off for a “rested” period. 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 manu-
facturers represent their amplifier’s output power in excess of 10%
distortion. They use many names for the 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) 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 to a 4 Ohm resistor is not the same as
measuring power with the amplifier hooked 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 cause phase
shifts in the signal that is passed through it. These phase shifts happen
because a speaker is an inductor (voice coil), 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.