In order to meet the military requirements for a headset that could be worn under the MK-1 helmet, midget hearing aid receivers were redesigned for this application. Data is presented showing characteristics of the original design adopted and of subsequent improvements resulting in improved performance. Some of the methods for securing adequate performance in quantity production and methods for 100 percent inspection of frequency characteristics are described.

The experience gained in fitting a large number of impaired persons with hearing aids is presented, showing illustrative ranges of impairment between 30 and 100 decibels fitted with three progressively-powered hearing aids. General features of such hearing aids are discussed, showing circuits and arrangements for peak clipping and automatic gain control. The microphone and air and bone receivers are described, considering features developed for providing optimum reliability in service such as moisture proofing, and design arrangements providing miniaturization concomitant with ruggedness.

An artificial mastoid for testing bone conduction receivers is described. It consists essentially of a stiff metal bar which has a fundamental resonant frequency above the measurement range, with a strain-sensitive translating element fastened to the under side and a compliant pad on top. The translating element is a slab of activated ceramic, which is essentially invariant to humidity and temperature changes. The compliant pad generally used is Koroseal No. 74, which simulates the flesh over the mastoid prominence reasonably well. Koroseal No. 15 simulates flesh better but is less rugged. Calibration methods are discussed.

The experience gained in fitting a large number of impaired persons with hearing aids is presented, showing illustrative ranges of impairment between 30 and 100 decibels fitted with three progressively powered hearing aids. General features of each hearing aid are discussed, showing the circuits and arrangements for peak clipping and automatic gain control. The microphone, air and bone receivers are described, considering features developed for providing optimum reliability in service, such as moistureproofing and design arrangements providing miniaturization concomitant with ruggedness.

The construction of an acoustically shielded bone conduction unit is described. The masses and springs have been proportioned for optimum purity of wave form at low frequency, and for extension of the high frequency range to over 12,000 cycles. The precautions taken in preparing an acoustical loudness-balance calibration are described, and the reproducibility of observations is considered. Methods of specifying the response of production units are shown.