The Role of the External Ear in the Hearing of the Guinea Pig

The acoustic transmission characteristics of the outer rim of the pinna (flap), the concha, and the external auditory meatus of the guinea pig were measured in the frequency range 100 Hz–15 kHz using a PDP-12 computer. The experiments were performed on anesthetized guinea pigs suspended in a sound field. The orientation of the field relative to the experimental animal was varied and the localization properties of the pinna recorded. To measure the amplitude and phase of the input and transmitted sound, two identical acoustic probes were used. One probe was placed at the entrance to the ear canal, and the other probe was surgically implanted in front of the tympanic membrane. The results of ten experiments indicate: (1) that the flap does not provide any significant amplification in the frequency range of measurement, (2) that the concha provides a significant amount of boost (10 dB) in the 5–9 kHz region, (3) that the ear canal itself can be represented by an open-ended tube of appropriate dimensions, and (4) that in the guinea pig, two pressure maxima are observable at the tympanic membrane owing to the loading effect of the middle ear. These maxima occur at approximately 2.4 and 5.0 kHz. In similar measurements on humans, no corresponding peaks have been observed. This difference can be explained by anatomical dissimilarities between the human and guinea pig middle ears.