DYNAMIC POWER MEASUREMENTS
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.
