200px, Moving iron speaker 200px, Moving iron sounder The moving iron speaker was the earliest type of electric

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 ...

. They are still used today in some miniature speakers where small size and low cost are more important than sound quality. A moving iron speaker consists of a

ferrous In chemistry, the adjective Ferrous indicates a compound that contains iron(II), meaning iron in its +2 oxidation state, possibly as the divalent cation Fe2+. It is opposed to " ferric" or iron(III), meaning iron in its +3 oxidation state, suc ...

-metal diaphragm or reed, a permanent

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 ...

and a coil of

insulated wire Overhead power cabling. The conductor consists of seven strands of steel (centre, high tensile strength), surrounded by four outer layers of aluminium (high conductivity). Sample diameter 40 mm A wire is a flexible strand of metal. Wire is co ...

. The coil is wound around the permanent magnet to form a

solenoid upright=1.20, An illustration of a solenoid upright=1.20, Magnetic field created by a seven-loop solenoid (cross-sectional view) described using field lines A solenoid () is a type of electromagnet formed by a helix, helical coil of wire whose ...

. When an

audio signal An audio signal is a representation of sound, typically using either a changing level of electrical voltage for analog signals, or a series of binary numbers for digital signals. Audio signals have frequencies in the audio frequency range of ro ...

is applied to the coil, the strength of the

magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...

varies, and the springy diaphragm or reed moves in response to the varying force on it. The moving iron loudspeaker

Bell A bell is a directly struck idiophone percussion instrument. Most bells have the shape of a hollow cup that when struck vibrates in a single strong strike tone, with its sides forming an efficient resonator. The strike may be made by an inte ...

telephone A telephone is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be easily heard directly. A telephone converts sound, typically and most efficiently the human voice, into e ...

receiver was of this form. Large units had a

paper Paper is a thin sheet material produced by mechanically or chemically processing cellulose fibres derived from wood, rags, grasses or other vegetable sources in water, draining the water through fine mesh leaving the fibre evenly distribu ...

cone attached to a ferrous metal reed. There are several types of moving iron speaker. Modern damped moving iron mechanisms can provide respectable sound quality, and are used in

headphones Headphones are a pair of small loudspeaker drivers worn on or around the head over a user's ears. They are electroacoustic transducers, which convert an electrical signal to a corresponding sound. Headphones let a single user listen to an a ...

. The first moving iron transducer, the telephone receiver or earphone, evolved with the first

systems in the 1870s. Moving iron horn loudspeakers developed from earphones after the first amplifying device which could drive a speaker, the

triode A triode is an electronic amplifying vacuum tube (or ''valve'' in British English) consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate (anode). Developed from Lee De Forest's ...

vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied. The type known as ...

, was perfected around 1913. They were used in

radio receiver In radio communications, a radio receiver, also known as a receiver, a wireless, or simply a radio, is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna. Th ...

s and the first

public address system A public address system (or PA system) is an electronic system comprising microphones, amplifiers, loudspeakers, and related equipment. It increases the apparent volume (loudness) of a human voice, musical instrument, or other acoustic sound sou ...

s. Moving iron cone loudspeakers appeared around 1920. Around 1930 they were replaced by the

moving coil A magnetic cartridge, more commonly called a phonograph cartridge or phono cartridge or (colloquially) a pickup, is an electromechanical transducer that is used to play records on a turntable. The cartridge contains a removable or permanentl ...

cone loudspeaker developed in 1925 by Edward Kellogg and Chester Rice. Today the moving iron driver mechanism is still used in some earphones and tiny

PC speaker A PC speaker is a loudspeaker built into some IBM PC compatible computers. The first IBM Personal Computer, model 5150, employed a standard 2.25 inch magnetic driven (dynamic) speaker. More recent computers use a tiny moving-iron or pie ...

Design

There are several variations. Each speaker has one property from each of the following groups of characteristics: * Means of restraint of the moving member: ** Diaphragm ** Springy metal reed * Acoustic loading: ** Cone ** Disc ** Horn ** Resonant chamber ** None * Drive method: ** Single ended drive ** Balanced armature ** Inductor dynamic * Damping: ** Yes ** No

Restraint

Diaphragm

Diaphragm type speakers use a thin semi-flexible

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...

disc held at its outer rim. The disc is centrally driven, bending back and forth under magnetic force. It is only practical to make small drivers with this technology, large diaphragms have too much

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 ele ...

, and hence

inertia Inertia is the idea that an object will continue its current motion until some force causes its speed or direction to change. The term is properly understood as shorthand for "the principle of inertia" as described by Newton in his first law ...

, for passable frequency response. This has remained a popular type of transducer design, being used in: * Early (pre-war)

* Pre-war horn loaded loudspeakers * Some budget modern headphones * Most telephone earpieces until the early 1980s * Miniature bleepers Poor bandwidth and modest output are limitations of most of these devices.

Reed

Reed mechanisms use a flat strip of spring steel anchored at one end, the other end moved by the magnetic field from the voice coil. The reed moves a cone or pleated paper diaphragm.

Acoustic loading

; Cone: Cone loading is more or less always used with sprung mechanisms. The combination of these 2 permits a less rigid fixing of the driven member, permitting more movement. The cone is also better able to handle lower frequencies compared to a table-top sized horn. Consequently, these speakers have better bass response than small horn speakers.

Spring Spring(s) may refer to: Common uses * Spring (season), a season of the year * Spring (device), a mechanical device that stores energy * Spring (hydrology), a natural source of water * Spring (mathematics), a geometric surface in the shape of a h ...

force restrains the moving iron. The downside of greater movement is greater non-linearity, thus higher

distortion In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signa ...

. Paper cone loaded moving iron speakers were in use pre-war. These suffered some noticeable issues: The magnetic gap was usually manually adjustable to give good sensitivity at any given output volume. Misadjustment was thus common. The

sometimes stuck to the

if overdriven or misadjusted. The result was a loud 'whack' followed by near silence. Loud bass notes caused end-stopping. Unlike modern speakers, which are designed to produce soft end-stopping, the iron in these hitting the

pole piece A pole piece is a structure composed of material of high magnetic permeability that serves to direct the magnetic field produced by a magnet. A pole piece attaches to and in a sense extends a pole of the magnet, hence the name. Pole pieces are us ...

made a noisy resonant chattering sound. ; Disc: Less popular were paper disc loaded speakers. These required an outer frame to hold the disc. They worked similarly to cone speakers, but since the outside of the disc moved less than the centre, the disc had to be a lot larger to achieve the same volume and bass output. An example of this type is the Sterling Primax. The disc is

pleat A pleat (plait in older English) is a type of fold formed by doubling fabric back upon itself and securing it in place. It is commonly used in clothing and upholstery to gather a wide piece of fabric to a narrower circumference. Pleats are cat ...

ed for rigidity. ; Horn: Horns were normally driven by diaphragm type drivers. The problem with horns is that reasonable bass response requires impractical horn size, and the table-top sized horns popular on pre-war speakers were thus very short on bass response. In fact they were nearly devoid of it. Diaphragm driven horn speakers have been used in more modern times as midrange squawkers and

tweeter A tweeter or treble speaker is a special type of loudspeaker (usually dome, inverse dome or horn-type) that is designed to produce high audio frequencies, typically deliver high frequencies up to 100 kHz. The name is derived from the high ...

s, frequency ranges which they are capable of handling properly if suitably designed. However early speakers attempted to cover as much of the

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 ...

range as possible with one unit, making hf response very poor as well as lf. ; Chamber loading: Bleepers use a resonant chamber to increase sound output. This sacrifices bandwidth for sound pressure level (SPL). Such units are not suitable for speech. Such units are smaller and cheaper than their main competitor piezo sounders. ; No Loading: Moving iron transducers used in

normally have no attached loading (no cone, horn or disc). The volume output from these is sufficient for headphone use.

Drive method

; Single ended drive: Early speakers were usually single ended drive. This simple method of operation produced copious amounts of second harmonic and intermodulation

. ; Balanced armature: Balanced armature moving iron speakers were developed in an attempt to reduce the high

levels of single ended drive speakers. Their success was mixed, as although they did reduce the percentage of distortion, they changed that distortion from even harmonic distortion to odd, making the distortion more unpleasant in some respects. ; Inductor dynamic speaker: These enjoyed brief success in the 1920s but were quickly eclipsed by moving coil speakers. The ''inductor dynamic speaker'' solved the nonlinearity problem of earlier moving iron types, providing a more pleasant listening experience. The main defect of ID speakers was very poor treble response, giving them a characteristic dull drone. Inductor dynamic moving iron speakers are now rare. Moving coil mechanisms provide better sound quality without the assorted downsides of moving iron, and eclipsed the inductor dynamic shortly after its introduction.

Damping

A lot of moving iron speakers have no

damping 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 i ...

. This means the moving member resonates freely in the

band. This is bad news for sound quality, but introducing damping heavily reduces sensitivity. This was impractical in pre-war times when amplification was very expensive, so moving iron has a history of being used with no damping. Modern

that use this technology incorporate damping to greatly improve sound quality. The reduced sensitivity isn't a problem with modern equipment.

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 Capacitance is the capability of a material object or device to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized are ...

. 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 In electronics, impedance matching is the practice of designing or adjusting the input impedance or output impedance of an electrical device for a desired value. Often, the desired value is selected to maximize power transfer or minimize si ...

to an

amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost t ...

is impossible. This has a major effect on

frequency response In signal processing and electronics, the frequency response of a system is the quantitative measure of the magnitude and phase of the output as a function of input frequency. The frequency response is widely used in the design and analysis of s ...

, and the amplifier must be able to tolerate a very low impedance load at low frequencies. Such devices can be used on

valve A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically fitting ...

(vacuum tube) amplifiers, but if used with

transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch ...

s some precaution to prevent overcurrent at low frequency is often necessary, such as a series

resistor A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active e ...

capacitor A capacitor is a device that stores electrical energy in an electric field by virtue of accumulating electric charges on two close surfaces insulated from each other. It is a passive electronic component with two terminals. The effect of ...

. 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.

Defects

Undamped moving iron speakers suffer the following defects: * Gross non-linearity and consequent harmonic distortion * Heavy

intermodulation distortion Intermodulation (IM) or intermodulation distortion (IMD) is the amplitude modulation of Signal (electrical engineering), signals containing two or more different frequencies, caused by non-linear, nonlinearities or time variance in a system. ...

* Little

bass Bass or Basses may refer to: Fish * Bass (fish), various saltwater and freshwater species Music * Bass (sound), describing low-frequency sound or one of several instruments in the bass range: ** Bass (instrument), including: ** Acoustic bass gui ...

response * Poor treble response * Strong undamped

resonance 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 oscil ...

at a few kHz * Gross impedance mismatch * Prone to de magnetisation Antique pre-war moving iron speakers also suffered the following defects: * With cone speakers, tendency of the moving iron to stick to the pole piece, resulting in a 'whack' sound followed by very little sound output. * Noisy chattering when presented with a loud bass note * Need for adjustment * Horn loudspeakers (popular at the time) were directional

Uses

; Impedance: Early moving iron speakers were normally high impedance, being designed to be connected directly to the output

with no

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' ...

or dc blocking. Quality and output level can be significantly improved by removing most bass from the electrical input signal. This is simply achieved by using a

in series with the

speaker Speaker may refer to: Society and politics * Speaker (politics), the presiding officer in a legislative assembly * Public speaker, one who gives a speech or lecture * A person producing speech: the producer of a given utterance, especially: ** In ...

. Table-top horns can only reproduce the highest of bass frequencies, so no noticeable bass is lost by removing most of the bass input. This reduces diaphragm or reed excursion, reducing harmonic and intermodulation

. ; Polarity: Polarity matters when dc is present in the speaker. Wrong polarity weakens the magnetic field, and can sometimes demagnetise the permanent

, leaving the speaker non-functional. When powered with ac only, polarity is a non-issue. Nearly all modern amplifiers feed only ac to the speakers. However, when using a moving iron speaker on an early radio, dc is usually present, and either the polarity marked on the speaker should be observed, or means used to remove the dc component from the speaker. This was often done with a choke and

. ; Headphones: Use of balanced armature mechanisms is common in modern

. These use damping to achieve satisfactory sound. ; Simple intercoms: Moving iron diaphragm transducers are highly microphonic, and connecting two together with no

makes a usable

intercom An intercom, also called an intercommunication device, intercommunicator, or interphone, is a stand-alone voice communications system for use within a building or small collection of buildings which functions independently of the public telephon ...

References

External links

Picture of a cone speaker
{Dead link, date=April 2020 , bot=InternetArchiveBot , fix-attempted=yes
Picture of a Sterling Primax disc speaker

Loudspeaker technology