Page 26 - Spring 2010

Fig. 3. Measured and predicted directivity pattern for the parametric array at dif- ferent frequencies: □ –230 Hz at 200 κm; ○ – 400 Hz at 200 κm: ■ – 230 Hz at 1000 κm.
Fig. 4. Frequency dependence of parametric array gain (dB), measured at 500 km (reference level, 1 μPa-m.
axis was inclined from the horizon at the same angle, 12° allowing some energy of pump transmission to be trapped by the ocean waveguide. The horizontal directivity width for the pump transmission was approx 4° in the main lobe. The directivity pattern of parametric signal as well as propagation characteristics of sharply directed parametric radiation through synoptic ocean vortices were measured.
Harmonic sound signals of 60 sec and 180 sec duration were generated parametrically at frequencies of 230, 400 and 700 Hz. Linear frequency-modulated signals with chirp in a frequency band of 230-700 were used as well. The angular directivity pattern was measured to be almost constant in the frequency range of signals used and close to the squared pump wave angular pattern (Fig. 3). The average level of parametric signals as a function of frequency measured (Fig. 4) showed that the efficiency of parametric array increased with signal frequency. This feature of the parametric array led to an interesting result—leveling of the spectral compo- nents of the intensity with distance (Fig. 5). In this experi- ment, it is seen that the signal levels for a frequency range from 230 Hz to 600 Hz are similar at a distance of about 600
km from the source.
The ABK sonar was used to investigate synoptic eddy
structure at far distances. A region of the Kuril Strait with typical ocean eddies was chosen for this experiment. The ABK (transmitting) and the ANA (receiving) were spaced at a distance of 400 km apart in this area. The chain of ocean eddies was located in the ocean between the vessels. A para- metric signal frequency of 700 Hz (with pump frequencies of 3.6 kHz and 2.9 kHz) was transmitted from the ABK when it passed five areas corresponding to positions of the ANA indicated in Fig. 2. The map of ocean surface temperature in this region is also shown in this figure from a satellite view. The angular pattern for the parametric signal that passed through an inhomogeneous ocean has been investigated in this research (Fig. 6). The diagrams in Fig. 6 differ drastical- ly from the directivity pattern measured in a quiet part of the ocean when there were no eddies (Fig. 3). Additional lobes appeared and the angular distribution of these lobes changed with the position of the receiving vessel in the area of experiment. The directivity pattern remained sharp while the signal passed mainly in a quiet area (Fig. 6-N1) and com- pletely dispersed when it propagated through an inhomoge- neous current that was produced by eddies. It should be noted that the noise level in these experiments never exceed- ed -30 to -25 dB.
Single-mode frequency dispersion for a parametric sig- nal in a marine waveguide
In shallow water, the sound field usually consists of a series of modes exhibiting frequency dispersion of the speed of propagation of a signal. The value of the dispersion depends, among other causes, on the vertical sound speed profile. The frequency dispersion provides either a spread in time of short broadband pulses that travel long distances, or concentration of acoustic signal energy within a short time interval when the frequency modulation of the signal corre- sponds to the dispersion conditions in the medium. In the latter case, focusing of the acoustic signal or the signal com- pression in time should be considered. A parametric array
9 signal has been used to observe the compression effect. The
acoustic signal in this experiment was radiated by a narrow- beam parametric array. The sea depth at the site of the exper-
Fig. 5. Range dependence for selected frequencies of the intensity of the parametric signal:1–230Hz;2–300Hz;3–400Hz;4–500Hz;and5–600Hz.
22 Acoustics Today, April 2010