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Nov 2007

Volume 122, Issue 5, pp. 2487-EL195

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Performance prediction of passive time reversal communications

H. C. Song, W. S. Hodgkiss, and Sea-Moon Kim

J. Acoust. Soc. Am. Volume 122, Issue 5, pp. 2517-2518 (2007); (2 pages) | Cited 7 times

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A recent paper [ M. Stojanovic J. Acoust. Soc. Am. 117, 1173–1185 (2005) ] presented theoretical performance bounds of time reversal communications. In this letter, the performance of time reversal communications is evaluated using at-sea experimental data and compared to the theory for two different approaches; (1) time reversal alone and (2) time reversal combined with channel equalization. It is found that approach (1) shows a good agreement between theory and data. On the other hand, approach (2) indicates that the data is about 3–5 dB below the theory which assumes perfect knowledge of the channel, an infinite number of taps to remove the intersymbol interference, and no need for phase tracking. Taking these into consideration, the theoretical performance can provide a useful upper bound for predicting performance of time reversal communications.
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43.60.Dh Signal processing for communications: telephony and telemetry, sound pickup and reproduction, multimedia
43.60.Gk Space-time signal processing, other than matched field processing
43.60.Fg Acoustic array systems and processing, beam-forming

Flat-response sound source technique for using the two-microphone method in an impedance tube

Xiaodong Jing and Xiaofeng Sun

J. Acoust. Soc. Am. Volume 122, Issue 5, pp. 2519-2521 (2007); (3 pages)

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In this letter, to improve the acoustic impedance measurement in a tube using the two-microphone method, a sound source technique is developed that is capable of generating incident sound waves of uniform intensity over the entire frequency range of interest. The basic principle is to use a digitally synthesized compensation input signal to achieve a flat response of the sound source provided by one or more loudspeakers. The present sound source technique has been demonstrated to be very effective and easily implemented with the current impedance tube testing systems.
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43.58.Bh Acoustic impedance measurement
43.55.Ev Sound absorption properties of materials: theory and measurement of sound absorption coefficients; acoustic impedance and admittance
43.20.Mv Waveguides, wave propagation in tubes and ducts

Estimation of tissue’s elasticity with surface wave speed

Xiaoming Zhang and James F. Greenleaf

J. Acoust. Soc. Am. Volume 122, Issue 5, pp. 2522-2525 (2007); (4 pages) | Cited 5 times

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The mechanical response of tissues to external forces has gained considerable interest in medical diagnosis. One approach to imaging tissue elastic properties is to apply an external force on the surface of the body. Another approach is to generate a localized force inside the tissue with the radiation force of ultrasound. In this paper, a new method is developed to estimate tissue’s elasticity based on surface wave speed measurement. The theory of surface wave speed is developed for estimating tissue’s elasticity. Experiments are carried out on a tissue-mimicking ultrasound phantom. An amplitude modulated ultrasound signal of a few hundred hertz is used to generate a localized force in the phantom. The surface wave fields are measured with a laser vibrometer. It shows that the surface wave speed can be used for inversely estimating tissue’s elastic properties based on tissue’s surface measurement.
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43.80.Ev Acoustical measurement methods in biological systems and media
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