The chief electrical characteristic of a dynamic loudspeaker's driver is its electrical impedance as a function of frequency. It can be visualized by plotting it as a graph, called the impedance curve.
Explanation
The most common driver type is an electro-mechanical transducer using a voice coil rigidly connected to a diaphragm (generally a cone). Other types have similar connections, though differing in detail, between their acoustical environment and their electrical properties.
The voice coil in moving coil drivers is suspended in a magnetic field provided by the loudspeaker magnet structure. As electric current flows through the voice coil (from an electronic amplifier), the magnetic field created by the coil reacts against the magnet's fixed field and moves the voice coil (and so the cone). Alternating current will move the cone back and forth.
Resonance
The moving system of the loudspeaker—consisting of the cone, cone suspension, spider, and voice coil—can be modeled as an effective mass (spring–mass system), a mass suspended by a spring. This system has a characteristic mass and stiffness, and a resonant frequency at which the system will vibrate freely.
This frequency is known as the "free-space resonance" of the loudspeaker and is designated by Fs. At this frequency, the voice coil is vibrating in the speaker's magnetic field with maximum peak-to-peak amplitude and velocity. The back EMF generated by this movement is also at its maximum. The electrical impedance of the speaker varies with the back EMF and thus with the applied frequency. The impedance is at its maximum at Fs, shown as Zmax in the graph.
For frequencies just below resonance, the impedance rises rapidly as the frequency increases towards Fs and is inductive in nature. At resonance, the impedance is purely resistive. As the frequency increases above Fs, the impedance drops—it behaves capacitively. The impedance reaches a minimum value, Zmin, at some frequency where the behaviour is fairly resistive over some range. A speaker's rated or nominal impedance (Znom) is derived from this Zmin value, explained ahead.
Beyond the Zmin point the impedance is again largely inductive and continues to rise gradually with frequency. The frequency Fs and the frequencies above and below it where the impedance is Zmax are important in determining the loudspeaker's T/S parameters. These can be used, for example, to design a suitable enclosure for the driver, especially for low frequency drivers. In fact Fs is itself one of the Thiele/Small parameters.
Load impedance and amplifiers
The variation in loudspeaker impedance is a consideration in audio amplifier design. Among other things, amplifiers designed to cope with such variations are more reliable. There are two main factors to consider when matching a speaker to an amplifier.
Minimum impedance
This is the minimum value in the impedance vs. frequency relationship, which is always higher than the DC resistance of the voice coil, i.e., as measured by an ohmmeter. Minimum impedance is significant because the lower the impedance, the higher the current must be at the same drive voltage. The output devices of an amplifier are rated for a certain maximum current level, and when this is exceeded the device(s) sometimes, more or less promptly, fail.
Nominal impedance
Due to the reactive nature of a speaker's impedance over the audio band frequencies, giving a speaker a single value for 'impedance' rating is in principle impossible, as one may surmise from the impedance vs. frequency curve above. The nominal impedance of a loudspeaker is a convenient, single number reference that loosely describes the impedance value of the loudspeaker over a majority of the audio band. A speaker's nominal impedance is defined as:
The graph shows the impedance curve of a single loudspeaker driver in free-air (unmounted in any type of enclosure). A home hi-fi loudspeaker system typically consists of two or more drivers, an electrical crossover network to divide the signal by frequency band and route them appropriately to the drivers, and an enclosure that all these components are mounted in. The impedance curve of such a system can be very complex, and the simple formula above does not as easily apply.