The real ear effect of adjusting the tone control and venting a hearing aid system

The effect of modifying the frequency response of a hearing aid system has previously been reported only on 2 cc couplers. Although it is recognized that coupler measurements do not accurately reflect real ear measures, they are frequently quoted when describing the effect of modifying a hearing aid system. To investigate the correlation, the real ear effect was assessed in 43 ears by measuring the effect on the insertion gain of adjusting the tone control of a hearing aid and acoustically venting the mould with a 2 mm parallel vent, singly and in combination. The results were considerably different from those measured in couplers. By comparing insertion gain with 2 cc coupler gain measurements, it was found that: (1) adjusting the tone control had a lesser mean effect than predicted; (2) venting the earmould with a 2 mm parallel vent produced a mean reduction in the frequency response at 0.75 to 1 kHz of 8 dB SPL; (3) by combining a 2 mm earmould vent and adjusting the tone control, a mean reduction in the frequency response of 10 dB SPL was produced at 0.75 to 1 kHz. In addition, the range of acoustical effects was considerable, from virtually none to greater than those predicted in coupler experiments. This was presumably due to variation in the anatomy of normal canals. It is concluded that for both research and clinical purposes, the effect of modifying the frequency response of a hearing aid system should be measured by insertion gain rather than predicted from laboratory results.