About the Dynamic Power Measurements

DY N A M I C PO W E R ME A S U R E M E N T S

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.

Information Cubed

The data acquired in the testing procedure is then graphed in the form of a 3-dimensional cube, hence the name PowerCube.

The Phase Angle is expressed on the horizontal axis, the Output Voltage is presented on the vertical axis and the Impedance is displayed on the Z axis. Output Power, in watts, is listed on the left hand side for each impedance at each phase angle.

What is an Amplifier?

An amplifier by definition is a voltage generat- ing device, recreating the signal which is input to it identically but with increased volume. It will be connected to a reactive load (the speaker). The impedance of this load and phase of the signal passing through the load will vary, dependent upon the frequency of the input signal (music).

Audio Graph – The PowerCube™

		MODEL BEING			VOLTAGE FROM	x2 = STEREO
		TESTED			VOLTAGE FROM	x2 = STEREO
		TESTED			BATTERY	MONO = BRIDGED MONO
					BATTERY	MONO = BRIDGED MONO
	Amplifier: PUNCH 200.2 14.4V x 2					Rated Power : 100 W @ 4 Ohms
	Serial No:
	Owner	: ROCKFORD CORPORATION
	8Ω *–60°	85	W
	–30°	84	W		POWER
	0°	84	W		POWER
	0°	84	W		IN
	30°				IN
	30°	84	W	{ WATTS
E	60°	86	W
C	4Ω *–60°	162	W
N	–30°	157	W
A	–30°	157	W
A	0°	156	W
D	0°	156	W
D	30°	157	W
P E	30°	157	W
P E	60°	162	W
M	2Ω *–60°	273	W
I	–30°	258	W
	–30°	258	W
	0°	251	W					4Ω
	30°	256	W					4Ω
	60°	271	W		–60°(Cap)			2Ω
	1Ω *–60°	390	W			0°	1Ω	{
	–30°	356	W			(Ind) +60°		{
	0°	346	W		PHASE{ANGLES			Impedance
	30°	352	W
	60°	390	W

•Example of a Punch 200.2 PowerCube

50V

30V

10V

8Ω

O U T P U T V O L T A G E

Therefore, a perfect amplifier will be able to maintain the same output voltage regardless of load characteristics and will not alter the signal it is reproducing. A perfect amplifier when measured by the Audio Graph PowerCube would present data that forms a perfect cube. Unfortunately, amplifiers are not perfect. The laws of physics generally prevent it. A great amplifier is about the best one can hope to attain.

As you can see by the PowerCube and as you will experience by listening, your Punch amplifier is a GREAT AMPLIFIER!

– 46 –