A Mammoth Loud-speaker

Radio Record, Volume IV, Issue 34, 6 March 1931, Page 12

A Mammoth Loud--speaker

Developed in America A HUGE loudspeaker, deriving its powtr from a powerful air blast driven. by an sir-compressor, has been developed in America, forsuse in addressing outdoor gatherings. Mounted on a large truck, it brings radio programmes to a whole city. ; Sitting in a sound-proof cab. at the front of this moving sound projector, Frank Hetzel, the inventor, rode through the streets of New York City recently and spoke into a microphone. His voice was. clearly understood by people thirty-five floors above him in the surrounding tall office buildings! The device which enabled him to address the world in such stentorian tones is a powerful speech-amplifier unit, coupled to a new type of loudspeaker which imitates the action of the human throat and larynx, thus giving the same quality that enables a tenor to fill a large auditorium with a round, robust tone, yet without the least strain on the part of the singer. Designers of the exponential type of loudspeaker are familiar with the fact that the action of the loudspeaker diaphragm sets up a standing wave in the horn, and that the horn. must be quite long in order to compare favourably with the wavelength of the low bass notes. The,action of the ordinary horn may indeed be compared to that of a man whispering. The air, set in vibration by the vocal eords, transfers the sound through the throat and the mouth, and the quality of tone is controlled by the placement of the mouth and tongue. But to produce a large robust tone eapable of filling a large room, the words must be voiced-more air must be forced out by the pressure of the diaphragm, so that the air all about is get in motion. ’ This principle is the secret of this joudspeaker’s huge volume. The air, set in motion by a powerful air compressor, is controlled by a set of "vocal cords," which impress upon the moving air column the vibrations of the sound impulses originating in the speech amplifier. The sound is thus literally blown out of the loudspeaker, instead of merely setting up standing waves in the horn, and the. resulting disturbanze in the air carries over great distances. The huge instrument, built at a cost of £5000, by Hetzel Brothers; automobile brake ‘engineers, of Los Angeles, California, was intended for use in imblicising their business. However, & ‘flood of inquiries from interested business men make it seem probable that eventually the device will be commercialised.