The chief electrical characteristic of a

dynamic loudspeaker A loudspeaker (commonly referred to as a speaker or speaker driver) is an electroacoustic transducer that converts an electrical audio signal into a corresponding sound. A ''speaker system'', also often simply referred to as a "speaker" or "l ...

's driver is its

electrical impedance In electrical engineering, impedance is the opposition to alternating current presented by the combined effect of resistance and reactance in a circuit. Quantitatively, the impedance of a two-terminal circuit element is the ratio of the com ...

as a function of

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...

. It can be visualized by plotting it as a

graph Graph may refer to: Mathematics *Graph (discrete mathematics), a structure made of vertices and edges **Graph theory, the study of such graphs and their properties *Graph (topology), a topological space resembling a graph in the sense of discre ...

, called the impedance curve.

Explanation

The most common driver type is an

electro-mechanical In engineering, electromechanics combines processes and procedures drawn from electrical engineering and mechanical engineering. Electromechanics focuses on the interaction of electrical and mechanical systems as a whole and how the two systems ...

transducer A transducer is a device that converts energy from one form to another. Usually a transducer converts a signal in one form of energy to a signal in another. Transducers are often employed at the boundaries of automation, measurement, and cont ...

using a

voice coil A voice coil (consisting of a former, collar, and winding) is the coil of wire attached to the apex of a loudspeaker cone. It provides the motive force to the cone by the reaction of a magnetic field to the current passing through it. The te ...

rigidly connected to a diaphragm (generally a

cone A cone is a three-dimensional geometric shape that tapers smoothly from a flat base (frequently, though not necessarily, circular) to a point called the apex or vertex. A cone is formed by a set of line segments, half-lines, or lines con ...

). 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 A loudspeaker (commonly referred to as a speaker or speaker driver) is an electroacoustic transducer that converts an electrical audio signal into a corresponding sound. A ''speaker system'', also often simply referred to as a "speaker" or ...

magnet A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nicke ...

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) In a real spring–mass system, the spring has a non-negligible mass m. Since not all of the spring's length moves at the same velocity v as the suspended mass M, its kinetic energy is not equal to \tfrac m v^2. As such, m cannot be simply added t ...

, a mass suspended by a spring. This system has a characteristic

mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different eleme ...

and stiffness, and a

resonant Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscilla ...

frequency at which the system will vibrate freely. This frequency is known as the "free-space resonance" of the loudspeaker and is designated by ''F''_s. At this frequency, the voice coil is vibrating in the speaker's magnetic field with maximum peak-to-peak

amplitude The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of am ...

and

velocity Velocity is the directional speed of an object in motion as an indication of its rate of change in position as observed from a particular frame of reference and as measured by a particular standard of time (e.g. northbound). Velocity i ...

. The

back EMF Counter-electromotive force (counter EMF, CEMF, back EMF),Graf, "counterelectromotive force", Dictionary of Electronics is the electromotive force (EMF) manifesting as a voltage that opposes the change in current which induced it. CEMF is the EMF c ...

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 ''F''_s, shown as ''Z''_max in the graph. For frequencies just below resonance, the impedance rises rapidly as the frequency increases towards ''F''_s and is inductive in nature. At resonance, the impedance is purely

resistive The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual parallels ...

. As the frequency increases above ''F''_s, the impedance drops—it behaves capacitively. The impedance reaches a minimum value, ''Z''_min, at some frequency where the behaviour is fairly

over some range. A speaker's rated or

nominal Nominal may refer to: Linguistics and grammar * Nominal (linguistics), one of the parts of speech * Nominal, the adjectival form of "noun", as in "nominal agreement" (= "noun agreement") * Nominal sentence, a sentence without a finite verb * Nou ...

impedance (''Z''_nom) is derived from this ''Z''_min value, explained ahead. Beyond the ''Z''_min point the impedance is again largely inductive and continues to rise gradually with frequency. The frequency ''F''_s and the frequencies above and below it where the impedance is ''Z''_max 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 ''F''_s is itself one of the Thiele/Small parameters.

Load impedance and amplifiers

The variation in loudspeaker impedance is a consideration in

audio Audio most commonly refers to sound, as it is transmitted in signal form. It may also refer to: Sound * Audio signal, an electrical representation of sound *Audio frequency, a frequency in the audio spectrum * Digital audio, representation of sou ...

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 An analog ohmmeter An ohmmeter is an electrical instrument that measures electrical resistance (the opposition offered by a circuit or component to the flow of electric current). Multimeters also function as ohmmeters when in resistance-measu ...

. 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 Reactive may refer to: *Generally, capable of having a reaction (disambiguation) *An adjective abbreviation denoting a bowling ball coverstock made of reactive resin *Reactivity (chemistry) *Reactive mind *Reactive programming See also *Reactanc ...

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: :

Z_\mathrm = 1.15 \cdot Z_\mathrm

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. The nominal impedance rating of consumer loudspeakers systems can aid in choosing the correct loudspeaker for a given amplifier (or vice versa). If a home hi-fi amplifier specifies 8 ohm or greater loads, care should be taken that loudspeakers with a ''lower'' impedance are not used, lest the amplifier be required to produce more current than it was designed to handle. Using a 4 ohm loudspeaker system on an amplifier specifying 8 ohms or greater could lead to amplifier failure.

Impedance phase angle

Impedance variations of the load with frequency translate into variation in the

phase Phase or phases may refer to: Science *State of matter, or phase, one of the distinct forms in which matter can exist *Phase (matter), a region of space throughout which all physical properties are essentially uniform * Phase space, a mathematic ...

relationship between the amplifier's

volt The volt (symbol: V) is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745–1827). Defin ...

age and

current Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stre ...

outputs. For a resistive load, usually (but not always) the voltage across the amplifier's output devices is maximum when the load current is minimum (and the voltage is minimum across the load) and vice versa, and as a result the power dissipation in those devices is least. But due to the complex and variable nature of the driver/crossover load and its effect on the phase relationship between the voltage and current, the current will not necessarily be at its minimum when the voltage across the output devices is maximum - this results in increased power dissipation in the amplifier output stage which manifests as heating in the output devices. The phase angle varies most near resonance in moving coil loudspeakers. If this point is not taken into consideration during the amplifier design, the amplifier may overheat causing it to shut down, or cause failure of the output devices. See Power factor for more detail.

Damping issues

A loudspeaker acts as a generator when a coil is moving in a magnetic field. When the loudspeaker coil moves in response to a signal from the amplifier, the coil generates a back EMF that resists the amplifier signal and acts as a "brake" to stop the coil movement. The braking effect is critical to speaker design, in that designers leverage it to ensure the speaker stops making sound quickly and that the coil is in position to reproduce the next sound. The electrical signal generated by the coil travels back along the speaker cable to the amplifier. Well-designed amplifiers have low output impedance so that this generated signal has little effect on the amplifier. Characteristically, solid state amplifiers have had much lower output impedances than

tube Tube or tubes may refer to: * ''Tube'' (2003 film), a 2003 Korean film * ''The Tube'' (TV series), a music related TV series by Channel 4 in the United Kingdom * "Tubes" (Peter Dale), performer on the Soccer AM television show * Tube (band), a ...

amplifiers. So much so, that differences in practice between a 16-ohm nominal impedance driver and a 4-ohm nominal impedance driver have not been important enough to adjust for. The amplifier

damping factor Damping is an influence within or upon an oscillatory system that has the effect of reducing or preventing its oscillation. In physical systems, damping is produced by processes that dissipate the energy stored in the oscillation. Examples inc ...

, which is the ratio of the nominal load impedance (driver voice coil) to amplifier output impedance, is adequate in either case for well-designed solid state amplifiers. Tube amplifiers have sufficiently higher output impedances that they normally included multi-tap output

transformer A transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple circuits. A varying current in any coil of the transformer produces a varying magnetic flux in the transformer' ...

s to better match to the driver impedance. Sixteen ohm drivers (or loudspeakers systems) would be connected to the 16-ohm tap, 8 ohm to the 8 ohm tap, etc. This is significant since the ratio between the loudspeaker impedance and the amplifier's impedance at a particular frequency provides damping (i.e., energy absorption) for the back EMF generated by a driver. In practice, this is important to prevent ''ringing'' or ''overhang'' which is, essentially, a free vibration of the moving structures in a driver when it is excited (i.e., driven with a signal) at that frequency. This can be clearly seen in ''waterfall'' measurement plots. A properly adjusted damping factor can control this free vibration of the moving structures and improve the sound of the driver.

References

External links

Article about effect of speaker impedance on amplifiers

{{DEFAULTSORT:Electrical Characteristics Of Dynamic Loudspeakers Loudspeaker technology