"Time-averaged holography" and "holographic interferometry" enable recording of the complete vibration pattern of a surface within several seconds. The results appear in the form of fringes. Vibration amplitudes smaller than 100 nm are not readily measurable by these techniques, because such small amplitudes produce variations in gray level, but not fringes. In practice, to obtain clear fringes in these measurements, stimulus sound pressures higher than 100 dB SPL must be used. The phase of motion is also not obtainable from such fringe techniques. In this study, a sinusoidal phase modulation technique is described, which allows detection of both small amplitudes of motion and their phase from time-averaged speckle pattern interferometry. In this technique, the laser injection current is modulated and digital image processing is used to analyze the measured patterns. When the sound-pressure level of stimuli is between 70 and 85 dB SPL, this system is applied to measure the vibratory response of the tympanic membrane (TM) of guinea pig temporal bones at frequencies up to 4 kHz where complicated vibration modes are observed. The effect of the bulla on TM displacements is also quantified. Results indicate that this system is capable of measuring the nanometer displacements of the TM, produced by stimuli of 70 dB SPL.

Although some techniques for the measurement of tympanic membrane (TW vibration already exist, their sensitivity is poor, and they need high SPL for measurement. In this study, using the time-averaged electric speckle pattern interferometry, the sinusoidal phase modulation technique by a direct modulation of laser injection current and digital image processing technique, an attempt is made to develop a system for measuring small vibration in nanometer order. Then, by this system, the vibratory responses of a guinea pig's TM are measured.