HEARING AIDS 241 



anism of human speech in all the essentials of kinds of sound and of the 

 completeness of control. In the voder there is an electrical source of sound 

 corresponding to the vocal cords. These are the vowel sounds. This is 

 a relaxation oscillator which gives a saw-tooth wave of definite pitch. An- 

 other source of sound supplies the consonants. By means of keys coupled 

 to filters and attenuators the operator can simulate the sounds of speech. 

 10.5. Hearing Aids^^' 2^. — Tests made upon representative cross sections 

 of the people in this country show a very large percentage to be hard of 

 hearing. Practically all of these people may obtain satisfaction from the 

 use of a hearing aid. A hearing aid is a complete reproducing system which 

 increases the sound pressure over that normally received by the ear. 



ATTENUATOR 

 CARBON ATTENUATOR CARBON / CARBON \ 



MICROPHONE MICROPHONE/ AMPLIFIER \ MICROPHONE ATTENUATOR 



BATTERY 



A 



II L,-^,r,w,rr, TIL RECEIVER _ 



Fig. 10.11. Hearing Aids. yi. Simple hearing aid. B. Hearing aid with a carbon amplifier. 

 C. Arrangement of the components of a vacuum tube hearing aid. 



The simplest hearing aid consists of a carbon microphone, a battery, an 

 attenuator and a telephone receiver. Fig. 10. 11^^. This hearing aid will 

 give satisfactory service where the hearing loss is about 20 db. 



The hearing aid shown in Fig. 10.115 consists of a carbon microphone, 

 carbon amplifier, an attenuator, a battery and a telephone receiver. This 

 hearing aid will give satisfactory service where the hearing loss is about 

 40 db. 



The quality of the carbon type hearing aids is usually very poor, due to 

 the frequency and the amplitude distortion produced by the carbon micro- 

 phone and amplifier. 



During the past few years considerable attention has been directed to- 

 ward hearing aids employing vacuum tube amplifiers. The schematic 

 arrangement of the components of a vacuum tube hearing aid is shown in 

 Fig. 10.1 IC The quality is far superior to that of the carbon type. Fur- 

 thermore, suitable compensation circuits may be incorporated to comple- 

 ment the ear characteristics. At the present time the vacuum tube hearing 



^iTuffnell, W. L., EellLabs. Record, Vol. 18, No. 1, p. 8, 1939. 

 22Hawley, W. C, Bell Labs. Record, Vol. 18, No. 1, p. 12, 1939. 



