In brief: consider the electrical characteristics of a typical fullrange driver (speaker). Input impedance looking into the driver appears to be mostly resistive and consistent in the middle of the driver's range but becomes very high as frequency approaches the driver resonance. Looking up at the high end of the band, the driver characteristic becomes inductive as impedance rises again due to voice-coil inductance.

Consider what impedance does for power delivery if input voltage stays constant. If we apply 1 volt RMS to an 8 ohm driver we get (P = v^2/R) which is one volt squared, divided by eight ohms: 0.125W. Now let's say we're going down in frequency from the middle of the driver's range (we're assuming impedance is 8 ohms in the middle). As we approach resonance, impedance rises. Looking back at the formula for power it can be seen that as impedance rises, power delivery decreases (input voltage remaining still the same). It can also be deduced that the inductance of the voice-coil would cause a decrease in power delivery as frequency rises past the point where inductance overtakes coil resistance and impedance goes up.

What does the above mean in practical terms? It means that in many cases, the driver in question is probably being under-utilized when connected to an amplifier that maintains constant-voltage output in proportion to input voltage (very low output impedance and thus high damping factor). It is usually possible to make better use of a driver's frequency response by raising the amplifier's output impedance somewhat so that it balances the characteristic of the driver. Of course not all drivers act the same way or have the same impedance characteristic, so some way of adjusting the amplifier to suit the load is desirable.


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