Koss 76 Speaker Impedance, by Paul Bergman, The ideal, and the practical, What is impedance?

SPEAKER IMPEDANCE

determining the impedance reflected back to the amplifier. For example, as a woofer cone moves back and forth, acting as a linear motor, it also acts as a generator, actually generating a voltage that is opposite to that coming from the amplifier. That this complicates things is an understatement.

It must also be evident that, in a speaker that combines resistance, induc- tance and resistance, the total impedance cannot be a single number, since it will inevitably vary with frequency. This is not typically taken into consideration by designers of amplifiers, who test their designs by loading them with an 8 ohm resistor, possessing neither capacitance nor inductance, and having a constant impedance at all frequencies.

The ideal, and the practical

The closer a speaker is to a pure

NUTS&BOLTSFEEDBACK

What is the impedance of your loudspeakers? Is it 8 ohms? Or 4 ohms? Perhaps you know that

it is not just a single number, but what difference does it make anyway?

At the very least you are no doubt aware that a speaker with a very low impedance can present a problem for an amplifier, and potentially can damage it. Think about the fact that short-circuit- ing an amplifier output can either break it, or blow a fuse, or trigger a protection circuit. The lower the impedance of a loudspeaker, the closer it comes to being a short circuit. Some amplifiers can drive a load of 2 Ω or even 1 Ω, but most will not. (The Greek letter Omega is of course the symbol for resistance). In any case, low impedance may not be your only worry.

With this issue, UHF intends to begin publishing impedance curves for loud- speakers reviewed, and for that reason I have been asked to explain speaker impedance, and also to suggest a simple manner of measuring a speaker's imped- ance. “Simple” in this case means using a minimum of specially-purchased equip- ment, though in day-to-day operation it is less simple than using a purpose-built instrument that can spit out a complete impedance graph in a few seconds. Yes, impedance measurements result in a

18 ULTRA HIGH FIDELITY Magazine

graph, not just the single figure usually found in loudspeaker literature, but let me begin with some basic concepts.

What is impedance?

If a loudspeaker were to be driven by DC (direct current) we could speak simply of its resistance. The speaker’s internal wiring has a certain (low) resis- tance, as does the fine wire that makes up each driver’s voice coil. However loud- speakers are intended to be driven by AC (alternating current), whose frequency of alternation is that of the sound we are attempting to reproduce. Thus we need to take into account the speaker’s induc- tance and capacitance. The voice coil is an inductor, and the internal wiring may be as well. Inductance can be thought of as

aresistance that is frequency-dependent, with its ohm value rising as frequency drops. Most crossover networks include capacitors, which introduce capacitance. A capacitor can also be thought of as a frequency-dependent resistor, whose ohm value rises with frequency. Since a capacitor’s impedance characteristic is exactly opposite to that of an inductor, it is easy to see how capacitors and induc- tors can be combined to make filters.

I shall add, without great elabora- tion, that these are not the only factors

by Paul Bergman

resistance, the more confidence an amplifier designer can have that his product will behave in the customer’s home exactly as it did on the test bench. That said, few loudspeakers are very much like resistors at all, and so in fact amplifiers must be designed to operate with impedances that are vastly different from that ideal resistor. What is more, the designer cannot know in advance the characteristics of the speakers that will be used with his product.

To see what he (and we) are up against, let us look at the impedance curve of a small two-way speaker, which has a famous name I do not propose to reveal. It is shown on the next page.

The curve has been drawn by a technique I shall describe presently (see Measuring Impedance on page 20). Most speakers, I might add by way of explanation, have a considerable peak in impedance at the point of resonance of the woofer and cabinet. The one I have arbitrarily selected has only a small rise, centred around 100 Hz, which would be the practical lower limit of its bass response.

The manufacturer’s nominal imped- ance rating is 4 Ω, but you need only glance at the curve to see that it deviates from that rating quite considerably. It dips to about 3 Ω at 16 Hz, which should present little problem for an amplifier designed the least bit competently.