Commercial Hearing Aids Research Articles

Hearing-Aid Design and Evaluation for a Presbycusic Patient

An octogenarian male with gently sloping audiograms with average hearing threshold levels (0.5, 1.0, 2.0 kHz) of 56 dB was tested extensively with a two-channel master hearing aid. Each channel includes a headworn microphone and a button receiver in a fitted earmold. The electronics consist of laboratory-quality amplifiers and 13-octave-band equalizers. Speech-reception tests were conducted in an acoustically absorbent room with a closed set of 100 monosyllables presented live by a female talker in quiet and noise. In a single mini-experiment, three or four frequency responses were compared in two to four sessions of 1 hour with a counterbalanced and randomized design that allowed frequent rest periods. When the field-to-eardrum transfer function is matched in the hearing-aid over a bandwidth of 6.5 kHz, reception of monosyllables is superior to competing curves by 6%–15%. Competing curves have included: those of a good quality commercial hearing aid; high-frequency pre-emphasis of +8 dB/octave defined at the eardrum ; curves with less than +18 dB relative gain in the 2–4 kHz region; curves with bandwidth less than 6.5 kHz; and so on. Finally, it is suggested that the role of frequency distortion in the design of hearing aids may be understood by relating aided and unaided audibility curves for sound fields to measures of speech reception.

Comparison of Speech Perception Utilizing Monotic and Dichotic Modes of Listening

Two experiments were designed to compare speech perception, as measured by the Revised Peterson-Lehiste CNC word lists, for individuals with with bilateral approximately symmetrical sensory hearing losses, utilizing monotic and dichotic modes of listening. The monotic modes consisted of a monaural amplifier and an amplifying system of two channels to one ear (monotic V cord). Dichotic modes comprised a binaural system of amplification and a two channel amplifying system to each ear (double V cord). In Expt. 1, randomized word lists were presented through high fidelity amplifiers and earphones; in Expt. 2, commercial hearing aids were used. Statistical analysis of the data supported the following hypotheses for subjects with bilateral approximately symmetrical sensory hearing losses: (1) Dichotic modes were superior for speech perception to monotic modes of amplification; (2) The binaural mode was superior for speech perception to the two-channel modes (monotic V cord and double V cord systems) of amplification; (3) The two channel mode (monotic V cord system) was superior for speech perception to the monaural mode of amplification to the same ear.

The design of commercial hearing aids

A modern wearable hearing aid takes the form of a microphone, valve amplifier and earphone, the operating power being supplied by small batteries. Nearly all of the components are specially developed for the purpose to give the desired characteristics in the smallest possible size and this paper describes the engineering technique adopted.

Acoustic Amplification of Hearing Aids

This paper gives the results of measurements of the over-all acoustic gain shown by six commercial hearing aids. The input level was 50 db above the reference level of a General Radio sound level meter. The measurements covered the frequency range from 200 to 4096 c.p.s. A steady sound field was set up in an absorbent lined booth. A Western Electric 640A condenser transmitter was fitted into a hole at the center of a small baffle, with its diaphragm in the plane of the baffle face. The field was monitored by the sound level meter, with its microphone in a fixed position close to the edge of the baffle. The output of the condenser transmitter was fed into a measuring system consisting of an attenuator, amplifier, and indicating meter. The attenuation needed to produce a given reading on the meter was determined. The condenser transmitter was then replaced by the microphone of the hearing aid, with its diaphragm in the same position the transmitter diaphragm had occupied. The hearing aid receiver was coupled through a 2 cc closed coupler to the condenser transmitter and the attenuation required to give the same reading as before on the indicating meter was noted. The difference between the attenuations in the two cases was taken as a measure of the over-all acoustical gain afforded by the hearing aid. All of the instruments tested, except one, showed maximum amplification in the 1000 to 2000 c.p.s. range. Among the different instruments, this maximum was between 45 and 68 db. The one exception showed a maximum gain of 42 db at 700 c.p.s. At 300 c.p.s., the spread between the different instruments was from 12 to 55 db, while at 4096 c.p.s. it was from 18 to 52 db. Only one of the instruments tested showed any significant change in frequency response with change in the “tone control setting” of the instrument. In this instrument a change of 25 db in the amplification of tones below 512 c.p.s. was possible with a change of only 4 to 8 db at the higher frequencies.