The use of flexible probe tubes in insertion gain measurement

Possible errors in the measurement of insertion gain due to the use of flexible probe tubes are considered and three aspects are investigated. (i) Does the probe tube create an effective vent in the earmould/ear canal system? (ii) Does compressing the probe tube with the earmould affect insertion gain measurement? (iii) Is it possible for a well-fitting earmould to completely compress the probe tube and give entirely spurious results? Comparative measurements were taken using probe tubes passed through the earmould and around the earmould. The results showed that the worst intra-subject standard deviation for the effect of repositioning the probe tube was approximately 5 dB. A statistically significant but small (1.5 dB) effect was produced by the probe tube at 500 Hz. The errors due both to repositioning the probe tube and probe tube compression are significant only at 3 kHz where measurements on one subject showed a 7 dB shift. The effects of probe tube position, probe tube compression and change in placement from the centre of the ear canal to the side produced differences which were significant at 3 and 4 kHz, but again the effect is small (3.2 dB). Complete blockage and marked compression of the probe tube can lead to large measurement errors. However, it is unlikely that an experienced operator would accept data obtained under these conditions as real. The errors involved in using a flexible probe tube placed around the earmould appear to be within limits that will not affect hearing aid prescription to any material degree.