Rockford Fosgate 250.2 manual About the Dynamic Power Measurements, Music, Information Cubed

DYNAMIC POWER MEASUREMENTS

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 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
						Audio Graph – The PowerCube™
the name PowerCube.
				MODEL BEING			VOLTAGE FROM		x2 = STEREO
The Phase Angle is expressed on the horizontal axis,				TESTED
							BATTERY	MONO = BRIDGED MONO
the Output Voltage is presented on the vertical axis			Amplifier: PUNCH 200.2 14.4V x 2						Rated Power : 100 W @ 4 Ohms
and the Impedance is displayed on the Z axis.			Serial No:
			Owner	: ROCKFORD CORPORATION
Output Power, in watts, is listed on the left hand side			8Ω*–60°	85	W
for each impedance at each phase angle.			–30°	84	W		POWER			50V
			0°	84	W						G E
							IN
			30°
				84	W	{WATTS
What is an Amplifier?	C E		60°	86	W						A
			4Ω*–60°	162	W					30V	L T
An amplifier by definition is a voltage generating	N		–30°	157	W						O
	A										V
			0°	156	W
	P E D										P U T
device, recreating the signal which is input to it			30°	157	W
			60°	162	W					10V
identically but with increased volume. It will be	M		2Ω*–60°	273	W						T
											O U
	I		–30°	258	W					8Ω
connected to a reactive load (the speaker). The			0°	251	W
										4Ω
			30°	256	W
impedance of this load and phase of the signal			60°	271	W		–60°(Cap)	0°		2Ω
			1Ω*–60°	390	W				1Ω	{
passing through the load will vary, dependent upon			–30°	356	W				(Ind) +60°
the frequency of the input signal (music).			0°	346	W		PHASE{ANGLES			Impedance
			30°	352	W
			60°	390	W
Therefore, a perfect amplifier will be able to main-		• Example of a Punch 200.2 PowerCube
tain 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!

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