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. [1] 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). [2] (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.
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Undamped moving iron speakers suffer the following defects:
Antique pre-war moving iron speakers also suffered the following defects:
There are several variations. Each speaker has one property from each of the following groups of characteristics:
Diaphragm type speakers use a thin semi-flexible iron 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.
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.
Paper cone loaded moving iron speakers were in use pre-war. These suffered some noticeable issues:
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.
Horns were normally driven by diaphragm type drivers. The problem with horns is that reasonable bass response 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 tweeters, 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.
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.
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.
Early speakers were usually single ended drive. This simple method of operation produced copious amounts of second harmonic and intermodulation distortion.
Balanced armature moving iron speakers were developed in an attempt to reduce the high distortion 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.
These enjoyed brief success in the 1920s but were quickly eclipsed by moving coil speakers. 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.
Most moving iron speakers have no damping. 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.
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.
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 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.
Use of balanced armature mechanisms is common in modern headphones. These use damping to achieve satisfactory sound.
Moving iron diaphragm transducers are highly microphonic, and connecting two together with no amplifier makes a usable communications link.
