Could anything be more confusing than a “power rating?” The question “How many watts” can have many answers, depending on whether we are talking about amplifiers or loudspeakers.
There are also many variables regarding how power is measured and specified. The numerous variables have rendered “face value” power ratings virtually useless.
Why is “power” so hard to quantify? One reason is because it is time-dependent and constantly changing – much like the “value” of the stock market. We can look at the “highs,” “lows,” or the “average,” and all are different numbers that are useful for different things.
Another problem is that we connect power amplifiers to loudspeakers. Given the nature of what each does (and how it does it), the methods used to rate one are inappropriate for the other. A 100-watt amplifier can’t produce 100 watts of power to a loudspeaker unless it plays a very annoying test waveform. Go figure.
Even so, sound system designers must select appropriate amplifiers for the loudspeakers in their projects. The power ratings supplied by amplifier manufacturers must be interpreted and scaled to meet the requirements of the design.
Some of us are old enough to remember the “power rating wars” of the 1970s. Any rating used to sell something is suspect, especially when there are many ways to test it and many more ways to present the results.
But, amplifier power ratings don’t have to be mysterious or confusing. There are some guidelines, assumptions and rules-of-thumb that can simplify the process of amplifier selection without sacrificing accuracy.
A Reference Waveform
The determination of the power of an amplifier can be trivially simple, or very complicated. It depends on the characteristics of the audio waveform and the impedance of the load that it drives. The simplest AC waveform is a sine wave.
When applied to a loudspeaker, the sine wave makes it pump in and out like a piston – easy enough to visualize. There are several commonly used power equations. Both are pretty simple when used with sine waves driving resistive loads on a test bench. Power is typically calculated from the effective, or Root-Mean-Square (RMS) value of the audio waveform.
Don’t get nervous – this is just a way of averaging the waveform over time to come up with a value that would equal the power (or heat) generated by a constant value of direct current. As such, the RMS value of a waveform is sometimes called its “equivalent DC value” or “heating value.”
This is a very important point. Power generation and dissipation is related to heat and dependent on time. We will revisit that later.
An example will clarify this. Figure 1 shows a dummy load that can be connected to an amplifier. This allows the amplifier to produce power without making sound, which is good for the person doing the test. The loads are water heater elements submerged in oil. All of the power is converted to heat. The temperature of the load can be monitored with the built-in thermometer.
The AC waveform that has the highest heating value is the square wave, and is equivalent to DC. It shall serve as our reference. I’ll apply 10 volts RMS to the load and allow the temperature to stabilize.
Next, I’ll substitute pink noise for the square wave. The pink noise is pure chaos when compared to a sine or square wave (as are music and speech). How does one give it a value?
In order to produce the same load temperature as the square wave, I have to turn the amplifier up by 12 dB. This is called the “crest factor” of the waveform. It is subtracted from the square wave voltage level to find the “effective” or “heating value” of the pink noise. Ten volts RMS of pink noise produces the same power into the load resistor as a 10 VRMS square wave.
Figure 2 is a chart of crest factors commonly encountered in audio work.
The EIA standard (now defunct) measures the power from the amplifier using a steady 1 kHz sine wave (3 dB crest factor).
While not very realistic with regard to real-world audio applications (unless you are driving a siren), it provides a useful reference wattage that can be de-rated for complicated audio waveforms like music and speech. The EIA standard lives on as CEA-426-B R-2005.