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sound |
the development of the loudspeaker |
        
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 The
earliest loudspeakers were
little more than a telephone (headphone) earpiece positioned
horizontally, with a cone attached. These horns did amplify the sound in
much the same way as the horn in an acoustic gramophone amplified the
vibrations produced by the needle on a diaphram, but hi-fi it certainly
was not. The main limitation was the poor audio frequency coverage,
especially lacking in bass response. Mainly due to the necessarily
limited cone size, ‘Tinny’ was one of the words often used to
describe this sound. Cone
loudspeakers. As
radio development gathered pace, it became imperative that a better
solution was found. The first cone
type loudspeakers were of the moving iron form, where the peak of the
paper cone was attached to a strip of iron by means of an
adjustable bolt. Fixed at one end but free at the other, the iron strip
‘armature’ was placed closed to the core of a small electromagnet,
the winding of which was often fed directly from the output valve anode.
Current variations through the winding varied the magnetic pull on the
armature, so vibrating the cone to produce sound.
Invariably fitted with large cones, these loudspeakers were sensitive and suited the simple TRF designs of the late twenties/early thirties. Design weaknesses in this simple form limited the bass response due to the uneven nature of the pull on the cone – the cone rod actually moves through a slight but significant arc and excessive movement caused by low audio frequencies tended to cause the armature to contact the magnetic pole piece. Increasing the spacing between the armature and the pole piece prevented this but at the expense of sensitivity to the weaker signals. Careful manual adjustment, where provided for by a knurled knob to set for best sensitivity whilst avoiding contact, was required. Some improvement was gained with the balanced armature type (pictured right), where the armature slots between a split magnetic winding. This arrangement gave rather better control over the movement of the cone and allowed for a wider spacing of the gap between pole pieces and armature, but being less sensitive was suitable only for sets with a relatively high output power.
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The
development of the dynamic loudspeaker
produced a marked improvement over the two simpler ones. Pictured on the
right, a twin armature is fixed to a cone rod suspended by leaf springs
between the pole pieces, resulting in a linear movement of the cone with
no likelihood of contact chatter. The springs were only required to be
very light in action, resulting in a sensitive unit of quite good basic
quality. Moving
coil loudspeakers. Finally,
the moving coil loudspeaker arrived and in terms of dynamic range
immediately proved superior to the earlier efforts. The earliest units
were mains energized with a field coil creating the required magnetism.
This was probably due to the initial difficulty of making sufficiently
powerful permanent magnets. The moving coil speaker, then, both in permanent magnet (PM) and mains energized (ME) form became the preferred choice of domestic radio receiver manufacturers across the developed world, though horn loudspeakers continued to be used in cinemas where units of a sufficiently large cross-section to allow full frequency range audio reproduction could be employed. Pictured below is an early battery-energized magnet unit. This works on exactly the same principle as the mains energized type, requiring a source of direct current in order to energize the electromagnet.
One notable exception is the electrostatic loudspeaker, forms of which may occasionally be found as tweeters in quality radio sets. These utilize two plates with an air gap, rather like a simple capacitor. Variations in electric potential applied to the plates produce movement and therefore sound.
Although the electrostatic principle has been used by certain hi-fi manufacturers for full-range transducers, the moving coil loudspeaker remains the mainstay of domestic electronic equipment. All rights reserved. © VRW 2008/2010
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There
were several other attempts to overcome the dominance of the moving coil
design, but most have faded into obscurity.