AN INVESTIGATION INTO SOUND-ATTENUATION BY EARMOULD TUBING

A major problem in high-gain post-aural hearing aids is acoustic feedb ack. One of the sources of feedback is the plastic tubing that guides the sound from the hearing aid hook to the ear canal. The objectives o f this study were to determine the effects of increases in tubing wall thickness on the amount of acoustic feedback in the hearing aid syste m and to characterize earmould tubing. Five different samples of Tygon tubing were tested, of which four had different wall thicknesses; the study also compared Tygon tubing with medical grade PVC tubing. Measu rement of the wall thickness, taken from scanning electron micrographs , showed that the wall thickness differed from the National Associatio n of Earmould Laboratories standard by less than 6% and was well withi n the manufacturer's tolerances. Measurement of sound attenuation show ed that an increase in the tubing wall thickness of 0.330 mm (Tygon 13 Standard to Tygon 13 Double Wall) caused an increase in sound attenua tion of 2 dB, across the frequency range measured (0.5-4 kHz). It was also found that the medical grade PVC had a 5 dB greater sound attenua tion than the equivalent Tygon tubing. While these differences are qui te small, they could contribute to a reduction in acoustic feedback wh en used in conjunction with other hearing aid or earmould improvements .