Impedance 101: Part Two, Pumping Up the Volume on Vectors

Impedance 101: Part Two
Pumping Up the Volume on Vectors

by Eddie Ciletti

Research for this two-part series reinforced the value of experience. My math skills might have been better in college, but impedance is one of those multidimensional “concepts” that I organically understand and better appreciate now vs. then. Using carefully chosen analogies along with three interface examples, I hope to demonstrate the common, everyday effects of impedance.

The term “interface” is equally important, because it implies the interconnection of two devices—a source and a destination—each having defined impedance. Like the time before the well-tempered clavier—when transposing a song from one key to another was not an option—there are interface combinations that beg for a “professional tuner.”

As you may recall from the last installment, I pointed out that wire is not a perfect conductor—it has resistance—and two wires translate into a complex assortment of series resistance and inductance, combined with parallel capacitance. The long and short of interfacing is simply this: Well-designed equipment can tolerate wiring variations, while other gear live and die by cable performance. Understanding what’s good, bad and potentially ugly will help to maximize performance and minimize destruction to your wallet and your sound.

EXAMPLE 1: HAUT-PARLEUR

Figure 1: Sweeping an oscillator through a two-way speaker system generates these very typical impedance and phase variations (click image).

A loudspeaker is like a drumhead, tuned real low by a soft edge-suspension material made of rubber, foam or paper. The part you can’t see is a coil of wire centered in a strongly focused magnetic field. Talk about complex impedance, here you have a mechanical resonator mounted to a resonant chamber (a cabinet) coupled with the voice coil, the inductor known as “L” in electrical circles. A loudspeaker is technically a “motor,” but it can also be used to generate electricity just as a dynamic microphone does. As an electromechanical device, it is the perfect example for making impedance tangible.

Loudspeakers come in various sizes and shapes for their respective purposes. The published AC impedance will typically be 4, 8 or 16 ohms, often referred to as “nominal,” because the magnitude changes with frequency and is therefore averaged. The DC resistance will be a different number. As you can see in Fig. 1, both impedance (the blue arrow) and phase (the red arrow) meander across the frequency spectrum for a passive, two-way monitor system. Note that the combined woofer and cabinet resonance raises the impedance to a whopping 25 ohms at about 45 Hz!

TESTS FOR RESONANCE AND DAMPING
To test for woofer resonance, simply insert a 100-ohm resistor in series—between it and the amp—and slowly sweep a sine wave oscillator from lowest frequency to the midband. You won’t need any other test equipment other than ears and eyes to find the resonant “bump.”

The next impedance demonstration also requires a speaker and an amp, but sans resistor. Assuming the power amp is connected and turned on, tap on the woofer and listen closely to the resonance. Now, disconnect one of the amp wires (or turn the amp off) while tapping and notice the difference. (Allow enough time for the amp to be fully “off.”) The transition from a tight, well-damped “tap” on paper to a less-restricted tonal “thud” should be obvious.

The woofer has a natural free-air resonance that changes once installed into a cabinet, either ported (bass reflex) or relatively airtight (air suspension). The speaker’s nominal electromechanical impedance is at least a factor of 10 higher than that of the amplifier’s source impedance. The ratio of the two is called the Damping Factor (DF), which is responsible for keeping the bump in Fig. 1 under control, unless the cable resistance becomes a contributing factor.

Note: While a power amp’s output stage is relatively simple, it can be further reduced to a single component for the purpose of defining its impedance—how the outside world sees it—the result of this reduction process, known as the Thevenin Equivalent, is typically below 1 ohm. Do not confuse this with the recommended “load” or destination impedance found on the back panel of most amplifiers.

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