Impedance
These speakers present an inductive load, so speaker impedance is proportional to frequency, with deviation from this proportionality at low frequency due to winding resistance, and at high frequency due to inter-winding capacitance.
It is normal for such speakers to vary in impedance by over 100:1 across the audio spectrum.
The result of this is that even ballpark impedance matching to an amplifier is impossible. This has a major effect on frequency response, and the amplifier must be able to tolerate a very low impedance load at low frequencies.
Such devices can be used on valve (vacuum tube) amplifiers, but if used with transistors some precaution to prevent overcurrent at low frequency is often necessary, such as a series resistor or capacitor. Alternatively the amp can be chosen to drive the speaker resistance, though this will result in worse impedance mismatch and thus output power far below the amplifier design spec.
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