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 at the time.
Antique pre-war moving iron speakers also suffered the following defects:
The sound of these moving iron speakers has an unmistakable character that is clearly from the early days of radio with its crude technology, but no-one I've shared this sound with has been willing to tolerate it for long. The sound of these speakers has a marked tendency to annoy.
These may sound like harsh POV words to anyone unfamiliar with early moving iron speakers, but they genuinely weren't polite to the ear.
Its not unusual for an electronics student, on hearing some of the specs of these devices, to conclude that they could not possibly have been capable of reproducing speech. Yet they do, and with a sound that can not be mistaken for anything else.
A recording of an early moving iron speaker is available here (or will be hopefully)
1: Means of restraint of the moving member:
2: Acoustic loading:
3: Drive method:
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.
Its only practical to make small drivers with this technology, as large diaphragms, while they can be made, 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.
Miniature bleepers also use a very thin diaphragm to maximise excursion and minimise power consumption.
diagram of mechanism needed here
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 charactistic 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.
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.
Modern moving iron headphones are another matter.
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. The more bass you remove, the more volume and less audible distortion you get. 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.
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.
Bleepers are resonant devices, and only produce high SPL at resonance. They are not designed to reproduce speech. Moving iron and piezo bleepers are the 2 types in most widespread use today.
These devices are also well known for producing sound on 33k/56k dial-up modems.
Miniature bleepers increase output by resonance. They also use a very thin diaphragm to maximise excursion and minimise power consumption.
The challenge of providing a calling signal on the Intercom is left to the reader for now. There are various ways to do it.