Moving iron speaker
Encyclopedia
The earliest loudspeakers for speech and music
were moving iron speakers. These are still used today in some miniature speakers where small size and low cost count, and sound quality is unimportant. A moving iron speaker consists of a ferrous metal diaphragm or reed, and a permanent magnet. The permanent magnet has a winding of many turns of fine wire, forming a solenoid
. When an audio signal is applied to the coil, the strength of the magnetic field varies, and the diaphragm moves in response to the varying force on it. The original Bell telephone
receiver was of this form. Larger units would have a paper cone attached to a ferrous metal reed.
There are several types of moving iron speaker. Old undamped moving iron speakers have a very characteristic sound, with probably the worst technical specs of any known type of speaker usable for speech
. Damped moving iron mechanisms can provide respectable sound, and are much used in modern headphones.
Moving iron speakers were standard equipment on most pre-war radio sets
(1910s to 1930s). (The earliest morse-only radio receivers
used a sounding board & solenoid). The better moving coil speakers
were available from 1925, but like most new and better technologies they were relatively expensive.
Antique pre-war moving iron speakers also suffered the following defects:
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, as large diaphragms have too much weight for passable treble response.
This has remained a popular type of transducer design, being used in:
Poor bandwidth and modest output is a feature of most of these devices.
The downside of greater movement is greater non-linearity, thus higher distortion
.
Paper cone loaded moving iron speakers were in use pre-war. These suffered some noticeable issues:
would require 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
s, frequency ranges which they are capable of handling properly if suitably designed. However early speakers attempted to cover as much of the audio range as possible with one unit, making hf response very poor as well as lf.
.
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.
. The inductor dynamic speaker solved the worst problems of earlier moving iron types, and provided a more pleasant listening experience. The main defect of ID speakers was poor treble response, giving them a characteristic dull drone.
The rare 'inductor dynamic' moving iron speaker was the last in the moving iron line of technology. Moving coil mechanisms
provide better sound quality without the assorted downsides of moving iron, and eclipsed the inductor dynamic shortly after its introduction.
. This means the moving member resonates
freely in the audio 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 headphones
that use this technology incorporate damping
to greatly improve sound quality. Headphone
sensitivity is unimportant with modern equipment.
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 wise, 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.
Quality and output level can be significantly improved by removing most bass from the electrical input signal. This is simply achieved by using a capacitor in series with the speaker. Table-top horns can only reproduce the highest of bass frequencies, so no noticeable bass is lost by removing most of the bass input. When using a capacitor, the amplifier must be able to tolerate a capacitive load.
Nearly all modern amplifiers feed only ac to the speakers. However when using a moving iron speaker on an early radio, dc will be present, and either the polarity marked on the speaker should be observed, or means used to remove the dc component from the speaker. This may be done with a choke and capacitor.
. These use damping to achieve satisfactory sound.
, and connecting two together with no amplifier
makes a usable communications link
.
Music
Music is an art form whose medium is sound and silence. Its common elements are pitch , rhythm , dynamics, and the sonic qualities of timbre and texture...
were moving iron speakers. These are still used today in some miniature speakers where small size and low cost count, and sound quality is unimportant. A moving iron speaker consists of a ferrous metal diaphragm or reed, and a permanent magnet. The permanent magnet has a winding of many turns of fine wire, forming a solenoid
Solenoid
A solenoid is a coil wound into a tightly packed helix. In physics, the term solenoid refers to a long, thin loop of wire, often wrapped around a metallic core, which produces a magnetic field when an electric current is passed through it. Solenoids are important because they can create...
. When an audio signal is applied to the coil, the strength of the magnetic field varies, and the diaphragm moves in response to the varying force on it. The original Bell telephone
Telephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...
receiver was of this form. Larger units would have a paper cone attached to a ferrous metal reed.
There are several types of moving iron speaker. Old undamped moving iron speakers have a very characteristic sound, with probably the worst technical specs of any known type of speaker usable for speech
Speech
Speech is the human faculty of speaking.It may also refer to:* Public speaking, the process of speaking to a group of people* Manner of articulation, how the body parts involved in making speech are manipulated...
. Damped moving iron mechanisms can provide respectable sound, and are much used in modern headphones.
Moving iron speakers were standard equipment on most pre-war radio sets
Antique radio
An antique radio is a radio receiving set that is collectible because of its age and rarity. Although collectors may differ on the cutoff dates, most would use 50 years old, or the pre-World War II Era, for vacuum tube sets and the first five years of transistor sets.-Morse only sets:The first...
(1910s to 1930s). (The earliest morse-only radio receivers
Continuous wave
A continuous wave or continuous waveform is an electromagnetic wave of constant amplitude and frequency; and in mathematical analysis, of infinite duration. Continuous wave is also the name given to an early method of radio transmission, in which a carrier wave is switched on and off...
used a sounding board & solenoid). The better moving coil speakers
Loudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
were available from 1925, but like most new and better technologies they were relatively expensive.
Audio
Undamped moving iron speakers suffer the following defects:- Gross non-linearityDistortionA distortion is the alteration of the original shape of an object, image, sound, waveform or other form of information or representation. Distortion is usually unwanted, and often many methods are employed to minimize it in practice...
and consequent harmonic distortionDistortionA distortion is the alteration of the original shape of an object, image, sound, waveform or other form of information or representation. Distortion is usually unwanted, and often many methods are employed to minimize it in practice... - Heavy intermodulation distortionDistortionA distortion is the alteration of the original shape of an object, image, sound, waveform or other form of information or representation. Distortion is usually unwanted, and often many methods are employed to minimize it in practice...
- Little bass response
- Poor treble response
- Strong undamped resonance at a few kHz
- Gross impedanceElectrical impedanceElectrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
mismatch - Prone to demagnetisation
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
Types of Moving Iron Speaker
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
- sprung
- Acoustic loading:
- cone
- disc
- horn
- none
- resonant chamber
- Drive method:
- single ended drive
- balanced armature
- inductor dynamic
- Damping
- none
- some
Diaphragm
Diaphragm type speakers use a thin semi-flexible ironIron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
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, as large diaphragms have too much weight for passable treble response.
This has remained a popular type of transducer design, being used in:
- early (pre-war) headphonesHeadphonesHeadphones are a pair of small loudspeakers, or less commonly a single speaker, held close to a user's ears and connected to a signal source such as an audio amplifier, radio, CD player or portable Media Player. They are also known as stereophones, headsets or, colloquially, cans. The in-ear...
- pre-war horn loaded loudspeakers
- some cost cutting modern headphonesHeadphonesHeadphones are a pair of small loudspeakers, or less commonly a single speaker, held close to a user's ears and connected to a signal source such as an audio amplifier, radio, CD player or portable Media Player. They are also known as stereophones, headsets or, colloquially, cans. The in-ear...
- most telephone earpieces until the early 1980s
- miniature bleepers
Poor bandwidth and modest output is a feature of most of these devices.
Cone & Sprung mechanism
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. Some form of spring is used to restrain the moving iron.The downside of greater movement is greater non-linearity, thus higher distortion
Distortion
A distortion is the alteration of the original shape of an object, image, sound, waveform or other form of information or representation. Distortion is usually unwanted, and often many methods are employed to minimize it in practice...
.
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 sensitivitySensitivity (electronics)The sensitivity of an electronic device, such as a communications system receiver, or detection device, such as a PIN diode, is the minimum magnitude of input signal required to produce a specified output signal having a specified signal-to-noise ratio, or other specified criteria.Sensitivity is...
at any given output volume. Misadjustment was thus common - The iron would sometimes stick to the magnetMagnetA 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, and attracts or repels other magnets.A permanent magnet is an object...
if overdriven or misadjusted. The result was a loud 'whack' followed by near silence. - loud bass notes caused end-stopping. Unlike modern speakersLoudspeakerA loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
, which are designed to produce soft end-stopping, the iron in these hitting the pole piece made a noisy resonant chattering sound.
Disc & Sprung mechanism
Less popular were 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 pleated for rigidity.Horn
Horns were normally driven by diaphragm type drivers. The problem with horns is that reasonable bass responseFrequency response
Frequency response is the quantitative measure of the output spectrum of a system or device in response to a stimulus, and is used to characterize the dynamics of the system. It is a measure of magnitude and phase of the output as a function of frequency, in comparison to the input...
would require 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
Tweeter
A tweeter is a loudspeaker designed to produce high audio frequencies, typically from around 2,000 Hz to 20,000 Hz . Some tweeters can manage response up to 65 kHz...
s, frequency ranges which they are capable of handling properly if suitably designed. However early speakers attempted to cover as much of the audio range as possible with one unit, making hf response very poor as well as lf.
No Loading
Moving iron transducers used in headphones normally have no attached loading (no cone, no horn, no disc). The volume output from these is very small, but sufficient for headphone use.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 piezo buzzers.Single ended drive
Early speakers were usually single ended drive. This simple method of operation produced copious amounts of second harmonic and intermodulation distortionDistortion
A distortion is the alteration of the original shape of an object, image, sound, waveform or other form of information or representation. Distortion is usually unwanted, and often many methods are employed to minimize it in practice...
.
Balanced armature
Balanced armature moving iron speakers were developed in an attempt to reduce the high distortionDistortion
A distortion is the alteration of the original shape of an object, image, sound, waveform or other form of information or representation. Distortion is usually unwanted, and often many methods are employed to minimize it in practice...
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 speakersLoudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
. The inductor dynamic speaker solved the worst problems of earlier moving iron types, and provided a more pleasant listening experience. The main defect of ID speakers was poor treble response, giving them a characteristic dull drone.
The rare 'inductor dynamic' moving iron speaker was the last in the moving iron line of technology. Moving coil mechanisms
Loudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
provide better sound quality without the assorted downsides of moving iron, and eclipsed the inductor dynamic shortly after its introduction.
Damping
Most moving iron speakers have no dampingDamping
In physics, damping is any effect that tends to reduce the amplitude of oscillations in an oscillatory system, particularly the harmonic oscillator.In mechanics, friction is one such damping effect...
. This means the moving member resonates
Damping
In physics, damping is any effect that tends to reduce the amplitude of oscillations in an oscillatory system, particularly the harmonic oscillator.In mechanics, friction is one such damping effect...
freely in the audio band. This is bad news for sound quality, but introducing damping
Damping
In physics, damping is any effect that tends to reduce the amplitude of oscillations in an oscillatory system, particularly the harmonic oscillator.In mechanics, friction is one such damping effect...
heavily reduces sensitivity
Sensitivity (electronics)
The sensitivity of an electronic device, such as a communications system receiver, or detection device, such as a PIN diode, is the minimum magnitude of input signal required to produce a specified output signal having a specified signal-to-noise ratio, or other specified criteria.Sensitivity is...
. This was impractical in pre-war times when amplification was very expensive, so moving iron has a history of being used with no damping
Damping
In physics, damping is any effect that tends to reduce the amplitude of oscillations in an oscillatory system, particularly the harmonic oscillator.In mechanics, friction is one such damping effect...
.
Modern headphones
Headphones
Headphones are a pair of small loudspeakers, or less commonly a single speaker, held close to a user's ears and connected to a signal source such as an audio amplifier, radio, CD player or portable Media Player. They are also known as stereophones, headsets or, colloquially, cans. The in-ear...
that use this technology incorporate damping
Damping
In physics, damping is any effect that tends to reduce the amplitude of oscillations in an oscillatory system, particularly the harmonic oscillator.In mechanics, friction is one such damping effect...
to greatly improve sound quality. Headphone
Headphones
Headphones are a pair of small loudspeakers, or less commonly a single speaker, held close to a user's ears and connected to a signal source such as an audio amplifier, radio, CD player or portable Media Player. They are also known as stereophones, headsets or, colloquially, cans. The in-ear...
sensitivity is unimportant 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.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 wise, 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.
Impedance
Early moving iron speakers were normally high impedance, being designed to be connected directly to the output triode with no 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 capacitor in series with the speaker. Table-top horns can only reproduce the highest of bass frequencies, so no noticeable bass is lost by removing most of the bass input. When using a capacitor, the amplifier must be able to tolerate a capacitive load.
Polarity
Polarity matters when dc is present in the speaker. Wrong polarity will cause weakened magnetic field and poorer performance, and can demagnetise the permanent magnet, 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 will be present, and either the polarity marked on the speaker should be observed, or means used to remove the dc component from the speaker. This may be done with a choke and capacitor.
Headphones
Use of balanced armature mechanisms is common in modern headphonesHeadphones
Headphones are a pair of small loudspeakers, or less commonly a single speaker, held close to a user's ears and connected to a signal source such as an audio amplifier, radio, CD player or portable Media Player. They are also known as stereophones, headsets or, colloquially, cans. The in-ear...
. These use damping to achieve satisfactory sound.
Simple Intercoms
Moving iron diaphragm transducers are highly microphonicMicrophonics
Microphonics describes the phenomenon where certain components in electronic devices transform mechanical vibrations into an undesired electrical signal...
, and connecting two together with no amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
makes a usable communications link
Intercom
An intercom , talkback or doorphone is a stand-alone voice communications system for use within a building or small collection of buildings, functioning independently of the public telephone network. Intercoms are generally mounted permanently in buildings and vehicles...
.
External links
- Picture of Cone speaker: link
- Picture of Sterling Primax disc speaker
- Diagram of mechanism
- 2"-3" diaphragm transducer + horn, a very popular form of pre-war loudspeaker.]
