External ear resonance in patients with tympanic membrane perforations.

This study investigated the effects of a tympanic membrane perforation on the external ear resonance. Measurements of external ear resonance using a probe-tube microphone system were performed in 14 patients who had medium to large unilateral tympanic membrane perforations. The contralateral normal ears of these 14 patients served as control. The results showed that there were no significant differences in the peak frequency, peak amplitude and peak sharpness between perforated and normal ears. However, intersubject variability in the resonant frequency was greater in the perforated group. In addition, the resonance curves of these two groups were substantially different. In 10 out of the 14 patients in the perforated group, the resonance curves showed 2-3 prominent peaks separated by valleys of about 10 dB reduced gain. In addition, in 11 out of 14 perforated ears, reduced responses (3.8 dB in average) occurred consistently in the lower frequency region (0.3-2 kHz). Clinically, the abnormal external ear resonance and the larger intersubject variation must be taken into consideration in fitting hearing aids for this group of patients.