Electrical characteristics of dynamic loudspeakers

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. 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. 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.