moving coil

Moving iron speaker

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.

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.

Audio

Undamped moving iron speakers suffer the following defects:

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 speakers (popular at the time) were directional

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)

Types of Moving Iron Speaker

There are several variations. Each speaker has one property from each of the following groups of characteristics:

1: Means of restraint of the moving member:

  • diaphragm
  • sprung

2: Acoustic loading:

  • cone
  • disc
  • horn
  • none
  • resonant chamber

3: Drive method:

  • single ended drive
  • balanced armature
  • inductor dynamic

4: Damping

  • none
  • some

Diaphragm

  • Picture of 1.5" - 2" diaphragm transducer with no loading, as used in most telephones until the early 1980s

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:

  • early (pre-war) headphones
  • pre-war horn loaded loudspeakers
  • some cost cutting modern headphones
  • 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

  • Picture of Cone speaker: link for now

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:

  • the magnetic gap was usually manually adjustable to give good sensitivity at any given output volume. Misadjustment was thus common
  • The iron would sometimes stick to the magnet 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 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

2"-3" diaphragm transducer + horn, a very popular form of pre-war loudspeaker.

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.

No Loading

Picture of pre-war headphones (which were called phones or telephones at the time, just to confuse things)

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

Miniature 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 bleepers, hence their widespread use.

Miniature bleepers also use a very thin diaphragm to maximise excursion and minimise power consumption.

Single ended drive

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

diagram of mechanism needed here

Balanced armature

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.

Diagram of mechanism

Inductor Dynamic Speaker

  • Picture 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.

  • Diagram of ID mechanism here

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.

Damping

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.

Others

Some other types of 1920s speakers

Technical Specs

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.

Frequency Response

Frequency response is pointless to quantify for old moving iron devices, as the frequency response was inevitably terrible.

Modern moving iron headphones are another matter.

Use of Moving Iron speakers

Historic

Impedance and polarity should be considered, and perhaps a little mental preparation for the grim sound that awaits.

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. Either of the following enables their use on a modern transistorised amplifier:

  • use a small transformer to give an approximate match of speaker dc resistance to amplifier output impedance.
  • Connect the speaker direct and turn the volume up high. This will often give sufficient output for use, if not full volume.

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.

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.

Bleepers

  • Pic of miniature diaphragm type transducer, typically 10-12mm diameter with resonant chamber loading.

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.

Circuit design

Moving iron bleepers are microphonic, thus resonance can be improved by using some of this microphonic signal for feedback. This can be achieved by driving the transducer via a resistor, and either:

  • using an opamp to compare the signal on each side of that resistor. The signal on the bleeper side will contain more microphonic signal than on the amplifier side.
  • More simply, using the composite signal on the bleeper side of the resistor for feedback.

Square wave drive gives more rms output power than sine drive, is acceptable for bleepers, and semi-squared drive simplifies amplifier design.

Headphones

Use of balanced armature mechanisms is common in modern headphones. These use damping to achieve satisfactory sound.

Simple Intercoms

Moving iron diaphragm transducers are highly microphonic, and connecting 2 together with no amplifier makes a usable communications link.

The challenge of providing a calling signal on the Intercom is left to the reader for now. There are various ways to do it.

Be aware that if left connected, the pair of earpieces are effectively a listening device, and the other party can hear you at any time.

See also

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