There are a number of possibilities for rating a power source.
The most straightforward is a simple rating for continuous available power, a product of the maximum voltage (pressure) output times the maximum available current (flow) from the amplifier.
This “Volt-Ampere” rating represents the largest number that could be used to characterize the amplifier.
For the “ideal” amplifier, we could simply multiply the DC rail voltage times the maximum current available from the power supply into a purely resistive load.
Unfortunately, the only value of such a rating would be its affect on retail sales. This large number must be de-rated in light of the actual conditions under which amplifiers must operate.
So what factors serve to reduce the “ideal” output power?
First, no sound reinforcement amplifier is called upon to deliver DC voltage and current into a load. If such a signal were indeed applied to a loudspeaker, it would quickly (and silently) burn up!
Loudspeakers only vibrate with the application of alternating current, or AC. We could pulse the DC current, producing a square wave, but this still would bear little resemblance to a real-world audio waveform. Most amplifiers are rated based on their ability to pass a sinusoidal waveform.
Figure 1: A sine wave plotted as a function of time.
The sine wave, when applied to a loudspeaker, makes it move in and out like a piston. Sine waves are discrete in their spectral content, meaning that they contain only one frequency.
They are also the building blocks of more complex waveforms. Since the typical sound system must pass many thousands of frequencies, it would take a lot of sine wave testing to fully characterize an amplifier!
One possibility is to rate the amplifier using a 1 kHz sine wave, and then add an additional descriptor for how much this will vary over the bandwidth of the amplifier.
A power bandwidth of -3 dB would mean that the guaranteed power output of the amplifier is one-half of the 1 kHz rating. That seems like a big difference, but it’s not when you consider the logarithmic characteristics of human hearing. One-half power is just noticeably lower in sound level.
So let’s go with the sine wave for rating the amplifier. A 500-watt continuous amplifier will deliver 500 watts to my loudspeaker, right?
Well, only if you play sine waves through it! In reality, the power flow will be much lower. This is due to the complex nature of the audio waveforms generated by real-world program sources.
The waveforms produced by a drum kit or lead singer bear little resemblance to sine waves. They are inherently more complex. Before we take this thought further, let’s look at some characteristics that describe time-varying voltages.
