Fundamentals of hearing-aid design

A HEARING AID is a miniature sound-amplifying system which is worn on the person in order to help overcome the handicap of impaired hearing. To be effective a hearing aid must be capable of amplifying those frequencies necessary for intelligibility of speech and reproducing them at a high-intensity level so as to make normal conversation audible to the person with impaired hearing. Modern hearing-aid amplifiers are of the vacuum-tube type, using small tubes designed specifically for them. A complete hearing aid consists of a microphone, a vacuum-tube amplifier, a receiver, and the necessary battery supply and connecting cords. Usually the microphone and amplifier unit are housed in a single case and the receiver and battery supply are connected to the amplifier by means of flexible cords. In order to determine the amount of amplification necessary, the type of frequency response needed, and the sound-pressure output required, it will be necessary to consider in some detail the different types of partial deafness and the characteristics of speech and hearing.