Desirable Frequency Characteristics for Hearing Aids

Abstract

The word-articulation curves of normal listeners were determined with a communication system, known as the “Master Hearing Aid,” of uniform frequency characteristic from 100 to 7000 c.p.s. and a wide range of amplification. The characteristic could be flat or else “tilted” to give a slope of either 6 db or 12 db per octave toward either the high or the low frequencies. The acoustic output was limited sharply at 112 db re 0.0002 dyne/cm2 maximum instantaneous pressure (abrupt peak clipping). Maximum word articulation scores of 95 percent or better were attained by normal listeners with all five tilts of the frequency characteristic. With the patterns that accentuated low tones, the scores fell rapidly toward zero when the input to the limiting stage was increased above the clipping threshold. Intelligibility was best maintained with 6 db per octave accentuation of high tones and next best with 12 db per octave upward tilt. With 6 db upward tilt the scores remained above 80 percent correct, even with peak clipping so severe (80 db) as to reduce the speech pattern to a series of square waves of uniform amplitude. With the same degree of distortion and a flat frequency characteristic, ordinary connected speech was still intelligible, but less so than with the rising characteristics. Twenty-five hard-of-hearing ears with various degrees and types of hearing loss were similarly tested, with a maximum instantaneous acoustic output of 124 db re 0.0002 dyne/cm2. Performance was judged on the basis of (a) maximum articulation score attained and (b) the useful operating range, i.e., the input range to the limiter over which word articulation was 50 percent or better. By each criterion the best performance is given by either the flat or the 6 db-per-octave upward tilt. The 6 db-per-octave pattern was best for the group as a whole and also for all but two of the hard-of-hearing subjects individually, regardless of the shapes of their audiograms. The addition (acoustically) of a moderate background of “static” noise lowered the articulation scores of most of the hard-of-hearing subjects more than those of normals, but the relative performance of the different frequency patterns was unchanged. A single hearing aid with a frequency characteristic adjustable between flat and the 6 db-per-octave upward tilt should provide the best compensation for all patients. If a frequency characteristic approximating tilt and range of the 6 db-per-octave upward tilt of the “Master Hearing Aid” is incorporated in a hearing aid, “fitting” would be based primarily on the maximum acoustic gain required and the maximum acoustic output tolerable to the patient, and not on the frequency characteristic of the instrument. The idea of individual selective amplification is fallacious. For our group we have been unable to devise any simple rule for “fitting” the individual audiogram that leads to any better result than the arbitrary selection of the 6 db-per-octave upward tilt for everyone. Our tentative acoustic design objectives for hearing aids, based on these experiments and the general experience of the Psycho-Acoustic and Electro-Acoustic Laboratories, are as follows: Frequency response and range: Moderate high-tone emphasis (4 to 6 db-per-octave); otherwise uniform, without marked resonant peaks or valleys, from 300 to 4000 c.p.s. Sharp cut-offs below and above this range are desirable. Tone control: Not essential. If provided it should allow selection between a flat, a rising 3 db, and a rising 6 db-per-octave frequency characteristic. Limiting of output: Preferably by compression, alternatively by simple symmetrical peak clipping. Maximum output: Semi-permanent adjustment (or separate models) at 114, 120, 126, or 132 db instantaneous re 0.0002 dyne/cm2. Maximum acoustic gain: Separate models probably desirable, lowest powered to have at least 40 db acoustic gain available, highest powered a maximum of 80 db. Gain control: Smoothly graded on approximately logarithmic scale over a 40-db range. Intrinsic noise: Must not mask speech delivered to the instrument at an input level of 30 db SPL.