In the last several weeks I have been asked numerous questions about a subject that we have a particular interest in at our A2000 Test Lab. This subject relates to factors that influence the ultimate quality of our systems and, in particular, the workings of speakers and amplifiers. Yet with all its importance, I have found through questioning installers and competitors on the Web that this subject is little understood. So I figure it is a good time for a combination of a physics lesson with a little electrical theory thrown in for good measure.

And The Subject Is???
Now for starters, I am not going to mention the subject. I am going to explain it in a very basic form and work up to the more complex details. When we are done, it will be obvious why this subject is so important. So stick with me and pay attention to the keywords that are in bold italics, and everything should be clear when we are through.

Imagine that we have a large heavy object that we need to move. It is sitting on the floor and is too heavy to lift. The only way we can move it is by exerting a force against it. Let’s say that to move the object we have to exert a force of 100 pounds. As we push against this object it resists our efforts to move due to the friction of sliding across the floor. But, as long as we push hard enough, we can overcome the friction of sliding and the object will move.

Now, if we want to figure how much work we have done, we can calculate how much weight we have moved over a given distance in a certain amount of time, and that will tell us how much work we have done. As long as we continue to push the object and it resists our pushing, we are doing work. As soon as we stop exerting force on the object it will sit at rest wherever we leave it. The reason we have to do work is because the object is sliding on the floor and the friction caused by the sliding creates unwanted heat.

Suppose we create another situation where we also have to exert a similar force against another object. Let’s say that this object is a large spring that requires the same 100 pounds of force to compress. From the standpoint of the person doing the pushing, as long as we are pushing against the spring, we probably feel as though we are doing the same amount of work as moving the previous object. The only difference is that now the same force is exerted, but there is no heat generated by friction of an object sliding on the floor. In the case of the spring, the force we apply is stored in the spring, and as soon as we stop pushing on the spring, we become aware of the fact that the spring is now reacting to our pushing by pushing back against us. Unlike the previous object, if we cease to exert force on the spring, it will return to its original position and we have accomplished nothing. Because the stored energy in the spring is returned to its original state, technically speaking, we have done no work.

Electrical Analogies
We have covered the most basic mechanical aspects of work, so let’s look at the all-important electrical analogies. Imagine we have a source of AC or DC electrical power and we hook it to a resistive load. Depending on the voltage potential, current will attempt to flow throughout that circuit and only the resistance of the load limits the amount of current. The resulting flow of power will be turned into heat in that load. Imagine the load is a headlight bulb in our car. The resistance of the filament causes it to heat and glow. When the element heats to a certain temperature it gives off light. The work done in the resistance of the bulb’s filament is measured in watts.

In electrical terms we measure work in watts, and to find the amount of work done we simply multiply the voltage by the current and we obtain the amount of wattage. The wattage multiplied by time gives us the total energy. In engineering terms, this is known as real power.

As long as we have a resistive load we can multiply volts times amps to get wattage. But just like in the mechanical example where we have resistive and reactive elements (the object on the floor and the spring), we also have resistive and reactive loads in the electrical realm. Reactive loads consist of circuits that contain inductive and capacitive elements. Accurately calculating wattage in such loads is not so easy if AC is flowing. Just like the mechanical spring, if we have a purely reactive load, the energy that is input to the load is returned to the source and no wattage is ever dissipated in the load. We can measure the voltage and current, but we cannot multiply them to get wattage. We now have to apply a term known as Volt-Amperes. Believe it or not, engineers label this measurement imaginary power!

Imaginary Power and Real Life
Why is imaginary power important, and what does it have to do with car sound? Simply put, it is common industry practice to measure amplifier power in watts. Amplifiers are tested into resistive loads with a value equal to their intended speaker loads, and the wattage is extrapolated from the current and voltage applied to those resistive loads. But a speaker is anything but a resistor. In fact, a typical car audio woofer with a large voice coil and powerful magnet is a highly reactive device. The mass of the cone and the spring-like action of the suspension and air of the box cause the speaker to move whenever power is applied — after all, we do want speakers that move, don’t we? So when these speakers start moving, they store energy, and when they change direction, that energy gets fed back into the amplifier in a form known as back EMF (electromotive force).

The reason this is so prevalent in speakers is because they actually resemble little power generators. The voice coil of the speaker is suspended in a strong magnetic field — that is a definition of an electric generator. To see an example of this, take two speakers of similar size and wire them together. Move the cone of one of them back and forth with your hand and watch the other cone move. The movement of the second speaker is a result of the power generated by the first speaker’s coil! So when an amplifier is driving a reactive speaker as opposed to a resistor, things can be more difficult. The resistor just sits there and turns the electrical power into heat. The speaker, on the other hand, reacts by trying to push current into and pull current out of the output stage of the amp. Since the current is not dissipated as excess heat in the speaker, it sometimes causes additional heating when it is directed back into the amplifier.

Resistive and Reactive Ratings
When we publish amp tests in CAR SOUND & Performance and AUTOMEDIA, we always list resistive ratings as well as reactive ratings. The purpose of listing the resistive ratings is so you can compare the figures to the manufacturer’s ratings and to those of other magazines. But we feel that the really important numbers are the reactive power ratings. As far as we are concerned, testing an amplifier into a resistive load is the equivalent of taking a submarine out to sea and not submerging it under water. Our lab is the only domestic test facility that performs reactive testing, but we feel it is the only true way to see how an amp will perform when driving today’s massive woofers with giant coils and magnets.

We have seen many amps that do a splendid job when driving resistors. But we have seen many of these same amps perform miserably when asked to drive reactive loads. And occasionally there is the super amp that can actually do better when driving a reactive load — but these amps amount to less than 1 in 50.

So what’s our word of advice? If you really want to know how an amp can really drive speakers at high power, read the reports carefully and always check the reactive test, as it’s the only one that really counts. And if you haven’t seen the data published here, there might be a good reason to be wary of an amp that looks great driving only light bulbs!