Life is all about circuits. In hydraulic circuits, a pump forces a fluid to flow, such as water through pipes or soda through a straw.
In pneumatic circuits, a pump forces airflow down a path, such as the air conditioning system in your house or the air hose that you use to inflate your tires.
We all know intuitively that pumps produce a pressure and rate-of-flow of something. A path must exist for the flow to follow, whether intentional (electrical traces on a circuit card) or unintentional (a bolt of lightning through a dwelling). Where there is flow, there will also be opposition.
On earth, something always “fights back” when we try to move anything from point A to point B. This opposition to flow is called impedance. A power source (pressure and flow) and a path form a circuit. A circuit is complete when the flow ends up where it started – back at the power source.
This may be in the same physical location, such as the blood in your veins returning to the heart, or the current from a battery returning to the other terminal.
Alternately, the flow will stop when it reaches the same “potential” as the pressure reference of the source – a concept that’s a little harder to grasp but is actually just another way of saying the same thing. The circuit must be complete for flow to take place.
We can make the flow do something for us on its trip back to the source. This is the very heart of technology, harnessing the energy in the world around us to do useful things that make our lives easier.
Source Be With You
In electrical circuits, the pressure of the source (voltage) produces a rate-of-flow of electrons (current), against an opposition (impedance).
The paths are the wires and circuit board traces that we create in our electronic gadgets.
Nature is always ready to create additional paths if we provide the correct conditions. These “parasitic” paths for current flow can cause us much grief – hum and noise in the best case, loss of life in the worst.
Most audio and acoustic measurements are voltage measurements because it is the easiest parameter to measure in most circuits – just like hopping on the bathroom scales and reading the number.
Current is a bit more difficult, because the flow must pass through the meter. This is why doctors measure your blood pressure rather than the rate of blood flow, as a measurement of the latter could make you quite uncomfortable!
Once the pressure and rate of flow are known, the impedance can be calculated. Ohm’s law provides the relationship between the three parameters in electrical and acoustical circuits, and analogous laws do the same for hydraulic and pneumatic circuits.
An electrical power source is characterized by its available voltage, current and its source impedance – the “internal” opposition to current flow. The load is characterized by its impedance.
This is why the impedance measurement is so important to sound system technicians – it characterizes what the source “sees” when it peers through its terminals at the outside world.
Finally, impedance comes in two flavors – resistance and reactance. Resistance occurs in DC circuits. A DC circuit is a “one-way” street for electron flow, hence the name “direct” current.
Both resistance and reactance occur in AC (alternating current) circuits. All audio circuits are AC, while some of the support circuits along the way (i.e. wall warts, batteries, and internal power supplies) are DC circuits.
Resistance dissipates energy in the form of heat. Reactance stores energy and reflects it back to the source.
Since reactance affects circuits where the current is changing, its effects depend on how fast you want to change it. As such, reactance is “frequency dependent.” Impedance is the “catch all” term that includes the effects of resistance and reactance.
Crawl Or Climb?
The impedance meter is one of the most important tools in the tool bag. It lets us know what a power source “sees” when it is connected to a load. If the load impedance is too low for the source, then the demand for current can exceed the ability of the source to supply it, resulting in distortion or damage to the source. If the load impedance is too high, then insufficient current may flow, and the loudspeaker covering the balcony may not be loud enough.
Of course, part of the game is knowing what the impedance should be, and the other part is being able to measure it. I won’t tackle the former here, but I will provide some examples of impedance measuring tools. The resistance measurements performed by the garden variety of volt-ohmmeters should not be confused with impedance measurements.
DC resistance measurements do not include the effects of reactance so they are not suitable for some of the loads in sound systems. For loudspeakers, microphones and transformers, the reactance can be significant. The simple DC resistance measurement must be replaced by an impedance measurement.
Since impedance is frequency-dependent, the frequency-of-interest must be applied to the load and the opposition measured. The audible spectrum covers 10 octaves, so one could rightfully ask, “What frequency do I use?” The answer to this question will determine which impedance meter that you must use.
