SOUND PRESSURE TRANSFORMATION AT THE PINNA OF MUS-DOMESTICUS

Sound pressure transformation properties at the pinna of laboratory mi ce Mus domesticus were studied by measuring the sound pressure level o f a continuous tone at a series of frequencies at the tympanic membran e as a function of the position of a sound source under free-field sti mulation conditions, The spectral transformation, the interaural spect ral difference, the isopressure contours and the interaural pressure d ifference contours were plotted, Sound pressure transformation functio ns showed some prominent spectral notches throughout the frequency ran ge tested (10-80kHz). However, the notch frequency did not appear to b e systematically related to sound direction, The study of interaural p ressure difference demonstrated that, when delivered from some within the ipsilateral frontal hemisphere. the pressure at the tympanic membr ane of certain frequencies may be lower than that determined at the co rresponding contralateral angles, For each sound frequency tested, the re was an angle (the acoustic axis) within the ipsilateral frontal hem isphere from which the delivered sound reached a maximal pressure leve l at the tympanic membrane, However, the acoustic axis often changed t o a new angle after removal of the ipsilateral pinna, In addition, sou nd delivered from the acoustic axis did not always generate a maximal pressure transformation, The isopressure contours determined within 2- 5dB of the maximal pressure were circumscribed, and their contained an gular areas were found to decrease with increasing sound frequency, Th e 2dB maximal pressure area may appear at more than one angular Remova l of the frequencies, modification of pinna posture contours irregular ly and split the 2dB maximal pressure area into several parts, The sou nd pressure difference determined between the angles of maximal and mi nimal sound pressure (the maximal directionality) increased with sound frequency regardless of pinna posture, Acoustic gain of the pinna at the acoustic axis reached 6-12dB, depending upon sound frequency, Howe ver, the pinna gain was not always maximal at the acoustic axis for a given frequency.