The original electrical sound-producing instrument was the Bell telephone receiver. It consists of a thin iron diaphragm fixed in front of an electromagnetic system, the edges being firmly clamped for mechanical reasons. (See TELEPHONE.) The diaphragm thus has very marked mechanical resonances; in other words it responds more to certain frequencies than to others ; and in reproducing music these frequencies are particu larly favoured, with a resulting loss of naturalness. The frequency response characteristic for an average telephone earpiece shows these resonance peaks ; which are however considerably damped down when the telephone is applied to the ear.
The first loud speaker was developed from the telephone ear piece by placing a conical horn in front of the diaphragm. The air column of the horn acted as a damping on the diaphragm, reduced the effects of the resonance peaks and concentrated the energy of sound coming from the diaphragm. The conical horn however does not deal equally with all frequencies.
The next development was that of a horn which would deal equally with all frequencies. The exponential horn, in which the area of the section perpendicular to the axis varies exponentially with the distance along the axis, is capable of doing this provided that it is sufficiently long.
An exponential horn of medium length is good for high and middle frequencies but fails in the lower frequencies. The longer the horn the lower the cut off frequency. Such a horn acts essen tially as an acoustic transformer between the confined area at the diaphragm and the open air condition at the flare of the horn. It is an ideal method of coupling a relatively small diaphragm to the open air to obtain the maximum output from the dia phragm. The subject has been dealt with fully by Hanna and Slepian.
A typical loud speaker exponential horn does not give, in actual practice, uniform characteristics when such a born is used in con junction with an electromagnetic unit of the telephone earpiece type. The resonance peaks are still present and there is a notice able lack of response in the bass frequencies. These are due to the type of unit used where diaphragm resonances are so ap parent, and also to the fact that this form of moving iron move ment is constrained and asymmetrical, and unable to have the free motion necessary to produce the lower frequencies (the lower the frequency the greater the amplitude necessary for equal pressure radiation).
This restriction does not allow the condition of linearity to be satisfied, and results in the generation of harmonics for pure low frequency input.
These difficulties are overcome by the use of an electrodynamic or moving coil type of movement in the unit. This was developed in one form by Round in 1924, and in another by Wente and Thuras in 1927. In both these cases the diaphragm is non-mag netic, comparatively rigid in itself and flexibly mounted, and driven by a coil attached to it which is suspended in a strong magnetic field.
Such a loud speaker has very much more satisfactory frequency and linearity characteristics. In the Wente type the efficiency is also greatly increased to an average value of 3o% which is a great advance beyond that for previous horn or large diaphragm types, the majority of which do not show an efficiency greater than 3%.
For the reproduction of broadcasting the horn type has been somewhat set aside and its place taken by the large diaphragm type, which, operated by some form of electrical movement, com municates the sound direct to the air. Such diaphragms are usually made in the form of paper or card cones, and in some cases these cones are designed to have particular resonances, introduced on purpose to remedy some defective part of the curve.
Where the electrical movements of these types of speakers embody moving iron armatures, lack of linearity exists just as it does for the earpiece type of movement, with consequent absence of pure low frequency reproduction. This fault has been remedied, as in the case of horn type speakers, by the electrodynamic drive. In this type a moving coil is attached to the apex of a lightly suspended paper cone diaphragm and moves in a constant strong magnetic field. The low frequency radiation is preserved by mounting the diaphragm in a baffle.
The baffle is an extension of the mounting, holding the edges of the diaphragm, which serves to lengthen the air path from one face of the diaphragm around to the other. Its function is to limit direct interaction between air pressure changes produced at the two faces of the diaphragm.
The relationship between sound pressure output and input cur rent to the coil can be made more even for the frequency range. Although the acoustic theory for the radiation due to the motion of a circular diaphragm in a baffle was worked out by Rayleigh in 1878, and the moving coil principle was demonstrated by Oliver Lodge in 1898 to vibrate a wooden board to reproduce sounds, it was not until 1925 that the now familiar moving coil type of loud speaker was developed by Rice and Kellogg.