Shengke Zeng
Richard B. Beard
Biomed. Eng. and Sci. Inst., Drexel Univ., Philadelphia, PA 19104
The discovery of the quadratic property of the human auditory sensation induced by the focused amplitude-modulated (AM) ultrasound ( Zeng and Beard, ``The quadratic property of the human auditory sensation induced by focused amplitude-modulated ultrasound,'' preceding abstract, this meeting) suggests the need for preconditioning of the AM ultrasound stimuli. The preconditioning can cancel the harmonic distortion which is caused by the quadratic effect in ultrasonic hearing. This quadratic harmonic distortion slopes from about -3 dB at 250 Hz (modulation frequency) to -11 dB at 2 kHz (modulation frequency). The preconditioned AM ultrasound is in the form of [radical |f(t)[inf min]|+f(t)[radical cos (omega)t, where f(t) is the modulation signal and (omega) the angular frequency of the carrier. The non-negative of |f(t)[sub min]|+f(t) guarantees that its square root can always be obtained. The auditory perception to the preconditioned AM ultrasound is in the form of the square of the modulation signal, |f(t)[sub min]|+f(t), which eliminates the harmonic distortion. In auditory perception test, the ultrasound which is modulated by the preconditioned and unconditioned consonant--vowel--consonant (CVC) syllables is used to stimulate the human cochlea. The subject's preconditioned ultrasonic intensity thresholds for CVC syllable recognition are lower than the unconditioned ultrasonic intensity thresholds for the same CVC syllable recognition. [Work partially supported by Electro-Stim Corp.]