(C) 2004 Hank Wallace
Musicians use the term ‘impedance’ in regard to their equipment and speakers. What does it mean? Let’s explore it here.
Electronic amplifiers pump electric currents into loudspeakers in a way similar to a water pump pushing water through a hose. The pump expends energy to creates pressure, and the water moves under the force of that pressure. In an amplifier, the pressure corresponds to voltage, and the water flow corresponds to current.
But you know that if you step on the hose that the water flow lessens. If you crank up the pump (or open the faucet) the water flow increases. Want even more water flow? You might need a bigger hose. There is a concept of flow capacity when we consider the hose, and that is related to impedance.
It’s easy to see that a smaller hose restricts, or impedes, the water flow. In the same way, a cable, amplifier or speaker has an impedance, though it is not quite so cleanly analogous to the diameter of the hose. Here’s where we depart into the void.
Impedance relates the ability of an electronic component or system to carry current. The lower the impedance, the less the current is impeded, and the more current can flow.
Electronics technicians and engineers learn about halfway through their first semester in college that impedance is a very important thing when one is trying to move as much power as possible from an amplifier to a speaker. This is actually true of any mechanical or electrical source and load.
It turns out that the maximum power is transferred from source to load when the two impedances are equal. For example, an amplifier with an 8 ohm output impedance will put the most power into an 8 ohm speaker.
That’s why it is important to load your amplifier with the proper impedance. Too low a speaker impedance will cause the amplifier’s output current to increase, and the voltage to drop, resulting in less power output to the speaker, and more heat dissipated in the amplifier.
Too high a speaker impedance will lower the output current, also reducing power output.
Speaker Cable Impedance
In the cables that connect your speakers to your amplifier, there is electrical resistance, another word for impedance (for our purposes here). If the cables have a high impedance relative to the amplifier output or speakers, then the cables will consume some of the power, perhaps resulting in a difference in tone.
That is why it is important to use cables that are sufficiently capable of carrying the power to your speakers. The thicker the wire, the better the cable, for a given length of and composition of wire.
For example, if your speaker is rated at 8 ohms, and there is 1 ohm of resistance in the cable, that represents about 12% of the speaker impedance. This means that a significant fraction of your sound power will be wasted in the cable as heat.
You have also seen impedance references with regard to signal cables, mixer board input and outputs, microphones, and other gear. What do these mean?
A high impedance signal connection implies that the current flowing in the wire is low (consider a constricted hose). In this case, the information in the audio signal is carried mainly by the voltage and the signal carries very little power overall. Some amplifier inputs draw practically zero current, making the whole arrangement a voltage based system.
This is good in one respect because the resistance of the wire is not too critical. Resistance is only a factor if there is a significant current flow (a simplified explanation).
However, high impedance signal connections are more susceptible to noise sources such as fluorescent lights and nearby electric motors and transformers. This is because the relationship of the impedances determines how much noise power moves between the noise source and signal connection. Take a look at the following diagram.
The noise source (in this case a fluorescent light) generates noise because of the way it works. There is an air gap of several feet between it and your high impedance signal connection, say, a guitar pickup. Even though there is no conductive path between the light and the guitar, there are phenomena called capacitance and inductance that can couple the noise from the light to the guitar.
The capacitance is very small, and the impedance of this noise source is quite large, perhaps millions of ohms. But your ears and gear are very sensitive, and if the noise is only a thousandth of your guitar signal, you can still hear it to some extent.
You can get a feel for how much noise is heard by comparing the impedance of the noise source (say, 1,000,000 ohms) with the impedance of the guitar pickups (say, 1,000 ohms). About 1/1,000th of the signal coming out of the guitar will be noise, give or take some due to the distance from the noise source and exactly what spectrum of noise it emits.
Imagine now that the guitar had an output impedance of 10 ohms, as with a shielded active pickup system. Then the ratio of noise source impedance to guitar signal impedance would be 1/100,000, not 1/1,000. This is why you end up with a lot less noise in low impedance signal circuits, and why sound engineers prefer them.
Adding shielding on a cable or noise source (wire mesh around the light fixture) increases the impedance of the noise source and has the same effect of lowering the level of noise you hear with the signal. So, to get better sound, we can either lower the signal impedance or raise the noise source impedance.
Impedance figures are good for comparing how well two devices will work together. This applies to amplifiers and speakers, and microphones and mixing board inputs. An instrument with an active (low impedance) output will be able to drive a longer cable with less loss and noise than a naked guitar pickup. Noise sources that we battle as musicians are typically very high impedance, and can be minimized by using low impedance interfaces. This also means that a high impedance microphone may not work well when plugged into a low impedance (typically 600 ohm) mixing board input.
Knowing something about impedances also allows you to detect suspect product claims. For example, a maker of shielded speaker cables might claim that the cables are lower noise than unshielded cables. That may be true, but the reduction in noise is not audible because the impedance of the speaker and amplifier are so small (8 ohms or less) compared with the impedance of typical noise sources. Sure, if you wrapped your speaker cables around your amp’s power cables, you might hear some noise with unshielded cables, but you are smart enough not to do that!
Don’t believe me? Connect 100 feet of any unshielded speaker cable to your speaker cabinet. Stretch the cable out so it can pick up whatever radiated noise happens to be in the area. Now get on your knees, real close to the cabinet, and listen to your speakers. (Your dog will suspect your sanity.) Hear anything? If not, then neither will you hear any radiated noise with your amplifier connected to the speaker cabinet!
Keep these things in mind the next time you see the word impedance!