Frequency dependence of directional amplification at the cat's pinna.

We examined in detail the effects of changing stimulus frequency upon the inclination of the acoustical axis of the pinna and upon the solid angle (area) subtended by isoamplification contours. We measured the relative sound pressure level difference between points on a 1 m radius, coordinate sphere using the cochlear microphonic as an indicator of tympanic sound pressure. The inclination of the acoustical axis for a given frequency was found to vary with the posture of the pinna, and with the pinna in a drooped position (following midline incision) there was a frequency spreading of axial positions such that high frequency axes were inclined progressively more laterally. However, with the pinna in an upright posture the axes for all frequencies tested were relatively tightly clustered. Alternative models for sound localization can be formulated to suit either situation, but it seems likely that the cat can use the frequency spreading effect of its pinna sound transformation as a cue to location. The pinna becomes more directional at higher frequencies, and this is clearly shown when the solid angle of isoamplification contours is plotted against frequency. The inverse relationship formed was shown to be closely matched by a model based upon diffraction by the outer dimension of the pinna.