101: Part Two
by Eddie Ciletti
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 devicesa source and a destinationeach having defined impedance. Like the time before the well-tempered clavierwhen transposing a song from one key to another was not an optionthere 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 conductorit has resistanceand 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 whats good, bad and potentially ugly will help to maximize performance and minimize destruction to your wallet and your sound.
EXAMPLE 1: HAUT-PARLEUR
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 seriesbetween it and the ampand slowly sweep a sine wave oscillator from lowest frequency to the midband. You wont 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 speakers nominal electromechanical impedance is at least a factor of 10 higher than that of the amplifiers 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 amps output stage is relatively simple, it can be further reduced to a single component for the purpose of defining its impedancehow the outside world sees itthe 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.
Reprinted with permission from Eddie Ciletti, Tangible Technology, June, 2001
© 2001, All Rights Reserved