The creation of an open mastoid cavity changes the acoustic characteristics of the external ear. The aim of this study was to ascertain the acoustic change in the external auditory canal caused by an open mastoid cavity and to compare it with mastoid obliteration. The external ear resonance characteristics were measured in 40 normal adult ears, 20 ears with an open mastoid cavity and 40 ears with an obliterated mastoid. The measurement of resonance characteristics was performed using a real ear analyser. An open mastoid cavity changed the mean peak resonant frequency of the external ear from 2.1 kHz to 2.3 kHz (P < 0.02), with a mean attenuation of 8 dB SPL at 4 kHz. An obliterated mastoid produced higher resonance frequencies from 2.5 kHz to 2.8 kHz. The sound pressure gain of the external auditory canal with an open mastoid cavity was higher than with an obliterated mastoid. The author concludes that an open mastoid cavity can affect the resonance frequency, and that this effect is reduced by mastoid obliteration. Therefore, mastoid obliteration results in a more normal ear canal both anatomically and functionally.