Acoustic behavior of the outer ear of the guinea pig and the influence of the middle ear

The sound-pressure transformation of the pinna and the ear canal of the guinea pig has been measured in the frequency range 1–15 kHz. Below 1 kHz, the outer ear does not cause any significant modification in sound pressure. Between 3 and 10 kHz, the pinna provides a boost in pressure which can specifically be attributed to resonance in the concha. This boost, however, is dependent on the location of the sound source with respect to the pinna. The largest amplification is obtained when the speaker is pointed directly into the entrance of the ear canal. The acoustic behavior of the ear canal of the guinea pig is similiar to that of a rigid circular cylinder, open at both ends, about 9 mm long and 2.5 mm in diameter. The pressure transformation between the eardrum and the entrance to the ear canal is strongly dependent on the loading effect of the middle ear. Thus, experimentally induced changes in the impedance of the middle ear, and natural changes due to disease alter the sound transmission across the eardrum of this animal. In normal animals, the eardrum pressure is roughly 5 dB higher than the free-field pressure in the frequency range 1–10 kHz. This boost is attributed partly to the effects of the body and partly to the sound-pressure transformation by the outer ear.