Biophysics of the ensiferan ear - I. Tympanal vibrations in bushcrickets (Tettigoniidae) studied with laser vibrometry

1. Laser vibrometry is used for observing vibrations of tympanal membranes. With this technique, the velocity and phase lag of vibrations can be measured from areas of 5-100 μm in diameter. The resolution of the apparatus is 10 Ångströms (vibration amplitude) in 'real time' broad-band operation (up to 100 kHz), but with averaging or selective filtering it is possible to perform more sensitive measurements. 2. At 'low' frequencies (1-3 kHz) the sound acting upon the back of the bushcricket tympanum has almost the same numerical pressure as that acting on the front. The ear, therefore, is an ideal pressure gradient receiver. 3. At 'high' frequencies (above 10 kHz) the sound pressure acting on the back of the tympanum is somewhat larger than that acting on the front. The membrane vibrates in its basic mode (like a piston) in the entire frequency range investigated (1-40 kHz). 4. These properties are for ears with open 'hearing trumpets' (i.e., the horn-shaped trachea conducting sound to the back of the tympanal membranes). When the opening of the hearing trumpet is closed by blocking it with wax, the ear becomes a pressure receiver (at least at 'low' frequencies). It is now more sensitive to 'low' frequencies and less sensitive to 'high' frequencies. 5. The vibrations of a membrane backed by a horn-shaped structure are considered theoretically. The expected variation in amplitude and phase angle of the driving force is calculated for systems in which sound is conducted through the horn-shaped structure with different amounts of gain.