LOUD SPEAKER, a device for converting electric energy into sound energy, the strength of sound produced being sufficient to be audible at a distance.
Loud speakers are generally used for the reproduction of speech and music in rooms, halls or the open air. Before 1918 the loud speaker consisted of a form of the ordinary electromagnetic telephone receiver, with a conical horn attached to it to concen trate the sound given out by the vibrating diaphragm.
Since that date considerable research work has been done on loud speakers, particularly in connection with broadcasting, public address work and talking pictures, and the most modern forms can reasonably be classed as musical instruments.
The results of such tests have demonstrated that in an electro acoustic system such as is used in broadcasting from the studio to the listener, the loud speaker is the least satisfactory link in the chain. But these measurements also show the way to the evolution of the perfect loud speaker. Such an instrument from the practical point of view should have the essential character istics of realism; which means that in listening to the perfect loud speaker connected to its appropriate amplifier and micro phone, and reproducing music or speech picked up by that micro phone, one would hear exactly the same as if one were placed in the position of the microphone, assuming, of course, that the microphone and amplifier are accurately converting sound pressure variations at the diaphragm of the microphone into similar voltage variations in the amplifier. Then the loud speaker would have to satisfy the following main requirements: (I) An even response for all frequencies. That is to say the sound pressure output for a given voltage input to the loud speaker or loud speaker amplifier must be the same for the whole musical range of frequencies, that is say from 25 to 12,000 cycles per second. Naturalness in reproduction, on the pressure basis, is synonymous with the treatment of all frequencies equally, which means that the frequency characteristic of the instrument is a straight line, having no resonance peaks to indicate that certain frequencies are favoured in comparison with others. (2) The relation between
input and output must be linear within the limits of use; that is to say if the electrical input is increased in any proportion, the acoustic output must also be increased in the same proportion. (3) The loud speaker must deal faithfully with transient phe nomena; in other words the response to sudden changes must be immediate. (4) The distribution of sound in various directions from the loud speaker must be equal for all frequencies. (5) The loud speaker must be reasonably efficient.
The majority of loud speakers fail to a certain extent on all these points; and a compromise is made in designing loud speakers for any particular purpose. The general uses for modern loud speakers are as follows : ) For realistic and faithful duction of speech and music in a limited space; as for instance in the case of broadcast reception in the home. Frequency char acteristic, linearity, and transient response must be good. Equal distribution and efficiency are not so important, but cost and maintenance must not be forgotten. (2) For public address work, efficiency and capability of handling great power are necessary. The remaining characteristics are not so important provided that complete intelligibility is maintained. (3) A pur pose now coming into prominence, namely in connection with talking pictures, occupies a half way position between the two given above, and requires the fulfilment of all the above-men tioned points.