Directional hearing in the grass frog (Rana temporaria L.): I. Mechanical vibrations of tympanic membrane

The vibration characteristics (amplitude and phase as a function of frequency) of the tympanic membrane in the grass frog were measured using a laser-doppler velocity meter. It was tested to what extent the frog's acoustic system behaves as a pressure gradient receiver. This might clarify how the frog localizes sound. Using a closed sound system the membrane was stimulated at three different entrances: in front of the membrane, at the contralateral ear and from inside the mouth. A combination of these can describe the motion of the membrane under free field conditions. It is found that the sound entrance from inside the mouth will give almost identical vibration characteristics as stimulation in front of the membrane. This can yield a perfect gradient receiver mechanism, when the frog opens its mouth. It is doubted however whether the frog in nature needs to open its mouth for localization of sound. With mouth closed the effectiveness of the gradient receiver will be determined by the transmission characteristics of sound across the tissues of the mouth. The entrance of sound via the contralateral ear is only effective at frequencies between 800 and 1600 Hz. At those frequencies crosstalk between the membranes is however not more than -4 to -8 dB. This is subject to changes in the acoustic properties of the mouth cavity and can possibly be altered by the frog in free nature.