Dynamic Power Measurements - Rockford Fosgate Punch 40I Owner's Manual

D
YNAMIC
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 1 kHz 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
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 4Ohm speaker may experience a drop in impedance 4-6 time slower
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
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°
represent the shifts in impedance and phase that can occur in a typical
loudspeaker.
Appendix A
P
OWER
-
phase angles. These different impedances and phase angles
M
EASUREMENTS
Not Resistors
- 1 8 -
measures output