Page 28 - Spring 2010

ity of signal propagation increases with frequency.
Experiments have shown that acoustic pulses received at dif-
ferent distances varied in the shape of the pulse as it propa-
gated in the waveguide. Pulse duration was reduced by a half
or greater when the signal propagated in the waveguide over
a distance of at least 3 km. To achieve complete synchronici-
ty of arrival times of all the frequency components of the sig-
nal, a special type of frequency modulation would be neces-
sary corresponding to the characteristic features of disper-
sion in the waveguide. Since the dispersion of the signal
propagation speed depends nonlinearly on frequency, the
frequency modulation should also be nonlinear to obtain the
maximum compression of the signal. The limiting duration
almost without side lobes). Our experiments demonstrated
that a parametric array may provide the capability to investi-
gate the crosswise currents in an ocean flow that is very tur-
bulent. Parametric arrays may be considered to be a promis-
ing tool for multi-frequency acoustical tomography tech-
niques, especially for monitoring the range-dependent tem-
perature and current distribution in a complex ocean envi-
ronment using only one acoustical path and scintillation
11
supports the development of a full scale experimental model of a high-power parametric array for long distance ocean monitoring; specifically for the Fram Strait environment con- ditions. This project is ongoing in the N. Andreyev Acoustics Institute (Moscow, Russia) and in the Taganrog Technology Institution (Taganrog, Russia). The parametric array, devel- oped using the principles of nonlinear acoustics, promises better experimental techniques for long-range multi-fre- quency acoustics experiments in complex ocean environ- ments, when single mode transmission coupled with an ocean waveguide is needed.AT
References
1 P. J. Westervelt, “Parametric acoustic array,” J. Acoust. Soc. Am. 15(1) 35−40 (1969).
2 V. A. Zverev, “How the idea of parametric acoustics array appear,” Acoust. Phys. 45(5), 684−692 (1999).
3 A. A. Atchley, “Not your ordinary sound experience: A nonlin- ear acoustics primer,” Acoust. Today 1(1), 19−24 (2005).
4 M. R. Bailey, V. A. Khokhlova, O. A. Sapozhnikov, S. G. Kargl, and L. A. Crum, “Physical mechanism of the therapeutic effect of ultrasound (A Review),” Acoust. Phys. 49(4), 369–388 (2003).
5 L. M. Brekhovskikh. Ocean and Human Being Present and Future (Nauka, Moscow, 1987).
6 W. Munk, P. Worcester, and C. Wunch, Ocean Acoustics Tomography (Cambridge University Press, Cambridge, MA, 1995).
7 I. B. Esipov, S. V. Zimenkov, A. I. Kalachev, and V. E. Nazarov, “Sensing of an ocean eddy by directional parametric radiation,” Acoust. Phys. 39(1), 89−90 (1993).
8 I. B. Esipov, A. I. Kalachev, A. D. Sokolov, A. M. Sutin, and G. A. Sharonov, “Long range propagation experiments with a power- ful parametric source,” Acoust. Phys. 40(1), 61−64 (1994).
9 I. B. Esipov, O. E. Popov, V. A. Voronin, and S. P. Tarasov, “Dispersion of the signal of a parametric array in shallow water,” Acoust. Phys. 55(1), 76–80 (2009).
10 B. K. Novikov, O. V. Rudenko, V. I. Timoshenko, Nonlinear Underwater Acoustics, (AIR-Press, New York, 1987).
11 I. Fuks, M. Charnotskii, and K. Naugolnykh, “A multi-frequency scintillation method for ocean flow measurement,” J. Acoust. Soc. Am. 109(6), 2730–2738 (2001).
of the signal τ is in inverse relation to the effective frequency -1
band Δf of its spectrum τ~(Δf ) . On the other hand, the sig- nal duration T of the pulse under the condition of its com- plete compression at a distance L is determined by the fre- quency dispersion ∂c/∂f of the propagation speed c and signal bandwidth Δf
techniques.
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Thus, in the case of the signal compression due to waveguide dispersion, the signal intensity may increase by factor of
Hence, the effect of an increase in intensity is proportional to the distance traveled by the signal, the value of the waveguide dispersion, and the square of the signal frequency band. At the same time, an increase occurs in the signal-to-noise ratio by the recording equipment during the signal reception time. The compression of the signal while it propagates in a ocean waveguide leads to relative signal intensity gain T / τ that increases with signal propagation distance L (Eq. 4). This can be most pronounced in the case of long-range propagation of a single mode, wide frequency band signal in an ocean wave- guide.
Conclusion
Long-range ocean sounding may be based on the use of low frequency sound. Such waves may provide a favorable condition for development of nonlinear effects and the appli- cation of parametric arrays. These arrays differ from conven- tional arrays by their small size (transmitting aperture dimensions 1-3 m), a broad frequency band of the signal transmitted (50 Hz -1000 Hz), and a very sharp directivity pattern in the whole frequency range (2°- 3° in the main lobe,
24 Acoustics Today, April 2010