Effect of canal depth on sound pressure level distribution in human bilateral ears

This study was to measure the sound pressure level distribution by ear canal resonance in the human left and right external auditory canals (EAC). The gain for different stimulus frequencies was analyzed at four different measuring depths (0.5 cm, 1.0 cm, 1.5 cm and 2.0 cm) from the entrance of the ear canal bilaterally. Comparative evaluation showed that the gain for different stimulus frequencies at a depth of 2.0 cm was consistent with the results of Dillon's study. In addition, the gain for the right EAC at 4000 Hz was larger than that of the left EAC by 1.2 dB at 0.5 cm, 1.8 dB at 1.0 cm, and 0.8 dB at 1.5 cm. This seems to suggest that gain at 4000 Hz is more affected by depth in the right EAC than in the left EAC. This study further found that the gain at the stimulus frequency of 4000 Hz was more affected by the depth than at 2000 Hz for the bilateral ear canals respectively. These gain differences between the right and left ears were statistically significant (p<0.05) at any of four measuring depths. The findings of this study may have an understanding of gain distribution to have implications for microphone placement of custom-made bilateral hearing aids (i.e. ITC or CIC) as these are placed at different depths within the ear canal.