Search

The sound velocity in ice is of great significance to the acoustic propagation characteristics of ocean channels under ice cover and the acoustic characteristics of underwater targets. Freshwater ice has a uniform crystal structure under normal conditions. The freezing process of sea ice is more complicated compared to fresh water ice. The sea ice contains a lot of bubbles, brine, solid particles and other impurities. Due to the large difference in the structure and composition of ice in different states, it is very different from the common sound transmission media in the past. Therefore, in order to accurately describe the acoustic propagation characteristics of the ocean channel under the ice cover, it is necessary to measure the longitudinal sound velocity in the ice accurately. A direct method for measuring the speed of longitudinal waves in ice is studied in this paper. The propagation time of high-frequency pulses in the ice is used in this research to determine the speed of longitudinal waves. For the same piece of ice, the method of averaging multiple measurements was used to reduce the measurement error. Six tests were performed on typical ice with different structural characteristics. The six tests were the fresh water in Songhua Lake area of Jilin Province in 2019, the high mud content sea ice in the coastal area of Bohai Bay in 2020, the low mud content sea ice in Liaodong Bay area in 2021, the artificial quick-frozen sea ice and fresh water ice at -20°C, and the summer Arctic sea ice obtained in the eleventh Arctic scientific survey in 2020. The sound velocity of longitudinal waves in ice was measured at 12kHz, 25kHz, 50kHz, 75kHz, and 100kHz. The acoustic properties of ice are determined by its own state. The measurement results have certain fluctuations, because the test results were closely related to the sample test environment and test process. The measurement results of longitudinal wave sound velocity were described by the measurement mean value and standard deviation in order to scientifically express the longitudinal wave sound velocity in ice. The test results showed that the longitudinal sound velocity in high mud content sea ice was about 3360±162m/s, 3270±78m/s in low mud content sea ice, 3769±54m/s in artificial sea ice, 3772±57m/s in lake ice, 3729±23m/s in artificial fresh water ice, and 3893±86m/s in Arctic ice. Under the same measured temperature, the compression wave sound velocities in most ice media are stable at 3750 m/s. The research methods and results in this paper provide references for related research on the acoustic properties of ice media, the acoustic propagation characteristics under ice cover and target acoustic characteristics in ice area.

The complex underwater noise field is formed in the process of the ship's constant volume sound source sailing. Due to the large number of machinery and equipment on the ship, and the navigation state of the ship on the sea is easy to change, these factors make it difficult to obtain the low-frequency radiated noise characteristics of the ship in a stable state. Therefore, it is an effective way to understand and master the characteristics of low-frequency underwater radiated noise field by using directional sound source in controllable state. Dipole transducer is a typical directional sound source with horizontal or vertical octagonal directivity, which is widely used in seismic and logging research. However, few dipole transducers are used in underwater acoustic research. The reason is: the directional emission of underwater acoustic low-frequency transducer is a big problem in the current transducer design. The directional emission technology of low-frequency dipole transducer is related to the distance / wavelength of the radiation surface. Because the position relationship of the radiation surface is fixed, it can only form the ideal directivity at a specific frequency. This brings great inconvenience to the application of underwater acoustic low- frequency dipole transducer. Firstly, the mathematical model of sound field radiated by acoustic dipole is established; then, the common non-directional low-frequency sound source is used to construct an acoustic dipole source. By adjusting the distance between two low-frequency sound sources, low-frequency dipole signals with different frequencies can be emitted. The lake test results show that the maximum difference of horizontal directivity of dipole source is less than 0.5dB and the depth of concave point is more than 21dB; finally, the test scheme in shallow water is designed, and the low-frequency narrow-band signals of 63Hz and 80Hz are emitted by the acoustic dipole source in uniform towing state, and the received signals of the bottom mounted hydrophone are analyzed and processed. The results of marine test show that the horizontal directivity of the dipole sound source constructed by two low-frequency sound sources can meet the requirements of acoustic test at multiple frequencies. The dipole transducer constructed in this paper has the advantages of simple structure, convenient equipment connection and wide signal frequency band, which can be applied in the study of underwater acoustic field characteristics of ships. Using the dipole sound source constructed in this paper, the sound propagation characteristics of underwater directional sound sources such as ships in the far field can be studied.

As a kind of commonly background signal, Ocean ambient noise is interference in the underwater acoustic channel. Although there are a lot of marine noise sources, the parameter influencing ocean ambient noise field are closely related to the state of the marine environment. From the perspective of theory, the change law of the ocean ambient noise at a fixed-point was difficult to describe. And from the perspective of experiment, some characteristic of the accurate ocean ambient noise cannot be measured at all. And even if the ocean ambient noise have been obtained, the data was still useless. Therefore, this paper would make an intensive study on the random characteristic of ocean ambient noise, and discuss the statistical analysis method, which including the mathematical expectation, the variance, the power spectral density and so on. In this paper, the bottom-placed hydrophones were employed to measure the marine environmental noise for 12 months in a shallow sea continuously. Then the test data was correspondingly processed, and the spectral characteristic of the ocean ambient noise in high frequency 10∼35 kHz was accordingly obtained. The results of this study would be of great significance for the offshore platform in shallow sea.

[Objectives] Measurement of acoustic characteristics of underwater targets is usually affected by the sea surface, seabed and external noise sources. It is difficult to obtain the free-field acoustic radiation characteristics of the targets. In order to solve the problem in acquiring the free-field acoustic radiation characteristics of the underwater targets in a half space whose boundary is the sea surface or the seabed where there is noise source,[Methods] the sound field recovery method based on the boundary element method is utilized, taking the typical underwater cylindrical shell as example, the free-field acoustic radiation characteristics of the cylindrical shell affected by the sea-surface/seabed or affected by both the sea-surface/seabed and external noise sources are separately recovered.[Results] The numerical simulation result shows that sea surface/seabed and external noise sources could greatly affect the measurement of the radiation sound field of an underwater target. But the sound power of recovery sound field separated by sound field recovery method based on the boundary element method is consistent with the sound power of free field. At the same time, the sound pressure, the directivity and other characteristics of field point are basically the same.[Conclusions] Therefore, the free-field acoustic radiation characteristics of the underwater targets could be effectively obtained by sound field recovery method based on boundary element method.