The basis of the design is that the mass of the movement is the controlling factor rather than the elasticity and the damping. On this basis moving coil loud speakers have, comparatively, ex tremely good frequency characteristics. There is no doubt that the good reproduction which can be obtained has been responsible for a considerable increase of interest by the musical public in broadcast transmission and reception.
The departure from the straight line in the frequency character istic is due to two factors. A "breaking up" of the cone when vibrating at high frequency is responsible for one main and several subsidiary peaks in the range of frequencies above i,000 cycles; and mechanical resonance in the suspension of the cone at very low frequency can result in a certain "boomiiiess" in the reproduction. Careful design can however reduce very consider ably these two defects.
Another form of electrodynamic large diaphragm type has been developed by Siemens and Halske. The electrical circuit consists of a zig zag metal strip mounted by its edge on a rectangular diaphragm. This is suspended over an electromagnet with parallel gaps, and poles alternately magnetised north and south; each portion of the strip fits into its own magnetic gap with the result that when current passes through the strip the diaphragm moves as a whole and does not tend to break up at certain frequencies. The reproduction is good, and a considerable amount of poWer can be radiated.
Another type of loud speaker which has not yet attained great success is made in the form of a condenser. One metallic plate is fixed, and the other is made of foil and held close to the fixed one, but insulated from it. The point of interest is that the electrical force is thus applied over the whole of the diaphragm, but the efficiency is not very great and the condenser loud speaker is comparatively insensitive.
Measurement of the Characteristics of Loud Speakers.— It is possible to obtain the frequency characteristic of loud speak ers, and this forms a useful basis of comparison. (See MICRO PHONE for methods used.) The same methods give the contour surfaces of distribution for various frequencies. It is also possible to measure on an oscillograph the degree of departure from linearity. These measurements have already received considerable attention from loud speaker designers.
The importance of response to transient phenomena however has not been fully realised and very little work on the subject has been carried out. There is no doubt however that this re quirement is just as necessary for realistic musical reproduction as a good frequency characteristic. There is thus considerable room for improvement in loud speakers, particularly in regard to frequency characteristics, transient response and efficiency.