Click here to purchase the entire book in PDF format. Electrical Impedance Hmmmmmm... loudspeaker impedance... We're going to only look at moving coil loudspeakers. (If you want to know more about this - or anything about the other kinds of drivers, get a copy of the Borwick book I mentioned earlier.) We'll begin by looking at the impedance of a resistor: Figure 6.136: The impedance is the same at all frequencies. Next, we learned the impedance characteristic of a capacitor Figure 6.137: The higher the frequency, the lower the impedance. Finally, we learned that the impedance characteristic of an inductor (a coil of wire) is the opposite to that of a capacitor. Figure 6.138: The higher the frequency, the higher the impedance. Therefore, even before we attach a diaphragm or a magnet, a loudspeaker coil has the above impedance. The problem occurs when you add the diaphragm, magnet and enclosure which, together create a resonant frequency for the whole system. At that frequency, a little energy will create a lot of movement because the system is naturally resonating. This causes something interesting to happen: the loudspeaker starts acting more like a microphone of sorts and produces its own current in the opposite direction to that which you are sending in. This is called back EMF (Electro-Motive Force) and its end result is that the impedance of the speaker is increased at its resonant frequency. The resulting curve looks something like this (notice the ``bump'' in the impedance at the resonant frequency of the system): Figure 6.139: INSERT CAPTION Note the following things: - The curve is for a single driver in an enclosure - For a driver rated at an impedance of $8\Omega$, the curve varies from about $7\Omega$ to $80\Omega$ (at the resonance point). As you add more drivers to the system, the impedance curve gets more and more complicated. The important thing to remember is that an $8\Omega$ speaker is almost never $8\Omega$.