Page 25 - Spring 2010

to the dissipative term is the Reynolds number, Re. Thus: ,
where u0 is the sound wave amplitude, and ω is the circular frequency of the wave. If Re> 1 then the nonlinear effects in a sound wave dominate the dissipation, and nonlinear effects are essential in this case, while Re< 1 means that nonlinear effects are weak and the wave decays before nonlinear distor- tion develops. One can see that the Reynolds number Re increases with sound amplitude and decreases with wave fre- quency.
Historically nonlinear acoustics experiments were con- ducted in ultrasonic frequency ranges with large wave num- bers and therefore it was necessary to apply very high sound
3
intensities to observe nonlinear effects. Thus, nonlinear
acoustics became a synonym of high intensity acoustics, with
many practical applications that can be found in science such
4
as medical ultrasound. But one could increase the Reynolds
number without the sound intensity increasing by decreasing the frequency of the sound. Therefore, nonlinear effects could be realized for low frequency acoustics in the atmos- phere or the ocean.
Because long-range ocean investigations require low fre- quency sound signals that can propagate long distances with- out severe attenuation,5 the usefulness of the parametric array for this purpose bears discussion. The main question that arises when comparing specific features of parametric and conventional acoustic arrays is: could the low-energy effi- ciency of parametric arrays be compensated for by its sharp directivity pattern and wide frequency band?
Long range ocean sounding by a parametric array
Parametric arrays are widely used in marine research for examining the layers under the bottom surface however, the PA’s acoustic characteristics may also make it “a perfect tool for ocean acoustics.” 5 Long range acoustic propagation in the ocean is characterized by strong mode coupling .6 Resolving the travel time variability in several tenths of milliseconds for multi-paths in the ocean waveguide usually requires sophis- ticated signal processing techniques or single mode excita- tion. Parametric array experiments have shown that its direc- tivity pattern can be very sharp (1°-3° in angular resolution) and almost independent of the wave frequency. It is possible therefore, that parametric arrays may provide the broad fre- quency band, single mode acoustic source needed for propa- gation in shallow water waveguides.
To our knowledge, there was only one actual long range ocean experiment using a parametric array for up to 1000 km range signal propagation.7,8 This experiment was performed in the early 1990s during the cruise of the Russian research ships R/V Academician B. Konstantinov (ABK) and R/V Academician N. Andreyev (ANA) in the region of Kamchatka and Kuril in the Pacific (Fig. 2). The 6 m long and 2 m high side-looking array in the bow of the ABK was used as a para- metric array with pump wave power of 20 kW at a mean fre- quency of 3 kHz. This array transmitted parametric signals in a frequency range of 230-700 Hz. The vertical pump wave directivity pattern had a width of 12° in the main lobe and its
TUNE INTO ZERO’s
SOUND SOLUTIONS
ZERO is a world-wide leader in high-performance acoustical control for doors, windows and walls. Nobody does sound control better — we use advanced technology and testing to master the challenges of creating an effective barrier and preventing gaps in that barrier for the life of the assembly. Our systems are rated for use in sound studios and recording facilities, music halls, etc — up to 55 STC. Let us help you close the door on noise — contact us for a copy of our 20 page Sound Control brochure, and our 72 page Product Catalog, or visit our website.
1-800-635-5335 / 718-585-3230 FAX 718-292-2243 zero@zerointernational.com www.zerointernational.com
Fig. 2. Satellite view of Kamchatka and Kuril islands, where a parametric array has been used in an experiment for long distance ocean sounding. Temperature inho- mogeneities (variation in the blue false colors) can be seen in the ocean stream next to Kuril islands (brown color).
The parametric array and long-range ocean research 21