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Miscellaneous Receivers and Loudspeakers

In addition to the receivers and loudspeakers considered in the preceding sections, many other types have been developed.5, 32, 36

Thermal Receiver. The thermal receiver or thermophone is seldom used as a receiver, but it is employed to calibrate transmitters. A thermal receiver to be inserted into the ear for receiving telephone or radio messages was devised in 1906 by Eccles;5 it is shown in Fig 38.

Figure 36. Cross section of a typical telephone handset receiver. (Courtesy Automatic Electric Co.)

Pneumatic Loudspeakers.1 A device called a Stentorphone was invented by Gaydon.5 An air valve controlled by the electric signals to be reproduced regulated the flow of compressed air into a horn. A modern version is the Vocal-Aire loudspeaker,37 a highly successful device for producing very intense sounds. This has a good response from 250 to 5000 cycles, has an output of about 110 decibels at 30 feet. Under ideal conditions this loudspeaker has been audible at 10 miles, and it is well suited to airports, harbors, freight yards, stadiums, and similar uses.

Figure 37. Frequency response of the telephone receiver of Fig. 36. (Courtesy Automatic Electric Co.)

Frictional Loudspeakers. The Johnsen-Rahbek loudspeaker5 consisted of a small, revolving cylinder of agate attached to one side of the incoming line. The other input wire was connected to a thin metal strip bearing upon the revolving cylinder and attached to a diaphragm. The friction between the cylinder and wire is caused to vary in accordance with the impressed electrical impulses, and thus sound signals are produced. A modification of this historical device using a revolving glass disk and a cork pad called a Frenophone has been developed.38

The Talking Arc. The talking or singing arc produces sound waves when electrical impulses are properly impressed across the arc.

Receivers operating on the principle of the condenser motor element (page 111) and on the principle of the crystal motor element (page 111) have been used. Also, high-quality moving-coil receivers39 and ribbon receivers40 have been developed for special purposes.



Last Update: 2011-05-30