Familiar audio amplifier design, and the use of Rochelle-salt-crystal mikes and ear-units, combine to produce a good sideline item for servicemen.
Radio technique at long last has come to the aid of the hard-of-hearing. The audio amplifiers that constitute such an essential element in radio reception have been refined to meet the particular needs of those whose hearing is below par. Newest developments in this direction are shown photographically in Figure A and B and by circuit in Figure 1.
Of the two types, both of which have their advantages, the battery unit meets the need for a high-fidelity hearing-aid which may be used any place without any dependence whatsoever upon a current source other than its self-contained batteries (which are said to last 1 year). This recommends the unit for use at the theatre at bridge games, etc.
An outstanding feature of the battery unit, which due to its self-contained batteries is a little larger that the A.C.-D.C unit is the exceptional fidelity it affords. First step in achieving the outstandingly excellent reproduction exhibited by the models tested in Radio-Craft laboratories is the use of a crystal microphone. Resistance-capacity coupled amplification, the second step, helps maintain this fidelity straight through to the output terminals.
The third and concluding link in this high-fidelity chain ids the use of a crystal producing unit. This may take the form of a standard earphone with which almost everyone is familiar or, it may be a crystal unit of the bone-conductor type which is designed to press against the mastoid bone in back of the ear and ordinarily is readily concealed by the hair.
No two people have identically the same response characteristics; everyone even though he may hear exactly the same sounds, mentally interprets these sounds differently. How much different then must be the sound perception characteristics of those whose hearing is impaired; some people hear sounds better in the upper frequency register, others in the middle register, still others in the lower register, and then of course persons exhibit the numerous possible combinations of these characteristics. For this reason, a “compensator” is incorporated in the amplifier circuit. Its terminals are resistor R and condenser C in Figure 1A. In a radio set this would be called the tone control. In hearing aid it becomes an individual compensator, and a convenient means of adjusting the response characteristics to suit individual ears. Potentiometer R, set by means of screwdriver from the underneath the chassis, when once adjusted to suit the owner of the hearing-aid, does not require further adjustment.
AC-DC Hearing Aid
The electric model shown schematically in Figure 1B has all the circuit elements and features mentioned above in connection with the battery model; except that load resistor is replaced by an impedance. The electric model of course may be operated on any 115V AC-DC power line but a power outlet must be conveniently available in order to operate this unit. The economy, compactness and efficiency of this unit recommended it for use by the bedridden and for use in offices. In order to reduce the weight to a minimum, filter resistors are used in place of the iron core chokes that would otherwise be needed in the power supply.
In conclusion we wish to point out that there hearing aids do not have the annoying background noise that has accompanied most preceding types of hearing aids. Furthermore both units re amazingly sensitive with sufficient output to rattle either the earphone or the bone conductor. In practice however the load upon these reproducers afforded by wearing theme prevents such rattling (which of course would only occur with the volume control near maximum). A sot rubber stud, center-perforated, improves the performance of the earphone unit.
Both units can be recommended by Servicemen as being particularly suitable, because of their exceptionally fine response characteristics, for listening to orchestral and other wide-range programs being reproduced by radio sets electric phonographs.
This article has been prepared from data supplied by courtesy of CrystalEAR, Inc.