242 



MISCELLANEOUS TRANSDUCERS 



aids are considerably larger in size and weight and are more costly to oper- 

 ate than the carbon type. 



The air conduction receiver is shown in Fig. \0A2A. A molded plug fits 

 the ear cavity and holds the receiver in place. Due to the small size and 

 method of mounting, the receiver is quite inconspicuous. 



MOLDED EAR PIECE 



MAGNETS' 

 COILS 





ASSEMBLY 



TOP VIEW COVER REMOVED 



T.3-^ 

 ^ 



■DIAPHRAGM 

 — COIL 



MAGNET 



ACTUATOR 



CROSS-SECTIONAL VIEW 



A 



CROSS -SECTIONAL VIEW 



B 



Fig. 10.12. Hearing aid telephone receivers. A. Air conduction receiver. B. Bone conduction 



receiver. 



In certain types of deafness, the middle ear, which consists of a series of 

 bones that conduct the sound to the inner ear, is damaged while the inner 

 ear, consisting of the nerves, is normal. Under these conditions sound 

 vibrations may be transmitted through the bones of the head to the inner 

 ear by means of a bone conduction receiver. Fig. 10.125. Usually the face 

 of the receiver is placed against the mastoid bone behind the ear. 



10.6. Sirens^^ (Compressed Air Loud Speakers). — The simplest siren 

 consists of a revolving disk perforated with a ring of equally spaced holes 

 which interrupt a jet of air from a tube placed close to one side of the disk. 

 The fundamental frequency of the successive puffs of air issuing through 

 the holes is equal to the product of the number of holes and the revolutions 

 per second of the disk. The wave form, of course, depends upon the shape 

 of the holes in the disk and the shape of the projection of the air tube upon 

 the disk. The pressure of the air supply in large sirens is usually very high, 

 of the order of 100 pounds per square inch. In the smaller sirens the air 

 pressure is supplied by a single stage centrifugal pump and the supply 

 pressure is of the order of a pound per square inch. Small sirens are used 

 by police cars, ambulances and fire engines for signalling the approach of 

 these vehicles. Large power sirens are used on firehouses, lighthouses and 

 lightships. 



''■'^ Wood, " A Textbook of Sound,' Macmillan Co., New^ York. 



