Shengke Zeng Richard B. Beard
Biomed. Eng. and Sci. Inst., Drexel Univ., Philadelphia, PA 19104
The interference demodulation by the nonlinearity of hydrophones cannot be ignored in parametric demodulation of amplitude-modulated (AM) ultrasound in the near-field media, where the intensity of the primary incident waves on hydrophones is very high. The interference demodulation, which is caused by the second-order sensitivity of a hydrophone, N(f), is proportional to the square of the incident pressure p[sup 2] [Moffett and Blue, Naval Underwater System Center, Tech. Memo. 801150 (1980); Humphrey and Hsu, Proceeding of Specialists Conference on Underwater Acoustic Applications (1980)]. It is found in the measurement of parametric demodulation of AM ultrasound by a Bruel & Kj(ae ligature)r 8103 hydrophone that the N(f) is not only the function of the carrier frequency f, but also the function of the modulation frequency F. At a fixed carrier frequency f[sub 0]=2.3 MHz, the interference demodulation can be observed at the modulation frequencies below 6 kHz, and becomes stronger toward low modulation frequencies at a distance of 150 mm between the transducer and the hydrophone. The interference demodulation by the nonlinearity of the hydrophone can be distinguished from parametric demodulation by observing the directivity pattern difference between them. N(f, F) is observed in the above parametric demodulation to be proportional to F[sup 1.32] [S. Zheng, Ph.D. dissertation, Drexel University (1993)]. [Work partially supported by Electro-Stim Corp.]