The locally resonant sonic material (LRSM) is an artificial metamaterial that can block underwater sound. The low-frequency insulation performance of LRSM can be enhanced by coupling local resonance and Bragg scattering effects. However, such method is hard to be experimentally proven as the best optimizing method. Hence, this paper proposes a statistical optimization method, which first finds a group of optimal solutions of an object function by utilizing genetic algorithm multiple times, and then analyzes the distribution of the fitness and the Euclidean distance of the obtained solutions, in order to verify whether the result is the global optimum. By using this method, we obtain the global optimal solution of the low-frequency insulation of LRSM. By varying parameters of the optimum, it can be found that the optimized insulation performance of the LRSM is contributed by the coupling of local resonance with Bragg scattering effect, as well as a distinct impedance mismatch between the matrix of LRSM and the surrounding water. This indicates coupling different effects with impedance mismatches is the best method to enhance the low-frequency insulation performance of LRSM.

In this study, the effect of the emergence angle of a source array on acoustic transmission in a typical shallow sea is simulated and analyzed. The formula we derived for the received signal based on the Normal Mode indicates that the signal is determined by the beamform on the modes of all sources and the samplings of all modes at the receiving depth. Two characteristics of the optimal emergence angle (OEA) are obtained and explained utilizing the aforementioned derived formula. The observed distributions of transmission loss (TL) for different sources and receivers are consistent with the obtained characteristics. The results of this study are valuable for the development and design of active sonar detection.

The large variability of communication properties of underwater acoustic channels, and especially the strongly varying instantaneous conditions in shallow waters, is a challenge for the designers of underwater acoustic communication (UAC) systems. The use of phase modulated signals does not allow reliable data transmission through such a tough communication channel. However, orthogonal frequency-division multiplexing (OFDM), being a multi-carrier amplitude and phase modulation technique applied successfully in the latest standards of wireless communications, gives the chance of reliable communication with an acceptable error rate. This paper describes communication tests conducted with the use of a laboratory model of an OFDM data transmission system in a shallow water environment in Wdzydze Lake.

The features of respiratory noises and noises of fins for open-circuit scuba divers, indicating a multipole character of noises emission, are specified in cameral conditions. It demonstrates a possibility to detect low-frequency components of noises of fins with pressure gradient sensor in near field. A possibility of estimating the respiratory rate of an open-circuit scuba diver is demonstrated at distances up to 100 m in real sea. It gives an opportunity of estimating the bearing (time delay in a pair of hydrophones) for the open-circuit scuba diver by respiratory noises at distances up to 150 m in real sea. Thus, low-frequency underwater noises of open-circuit scuba divers may be successfully applied to monitor the safety of diving and to prevent waterside intrusion by trespassers.

In this paper, we present the methods to detect the channel delay profile and the Doppler spectrum of shallow underwater acoustic channels (SUAC). In our channel sounding methods, a short impulse in form of a sinusoid function is successively sent out from the transmitter to estimated the channel impulse response (CIR). A bandpass filter is applied to eliminate the interference from out-of-band (OOB). A threshould is utilized to obtain the maximum time delay of the CIR. Multipath components of the SUAC are specified by correlating the received signals with the transmitted sounding pulse with its shifted phases from 0 to 2π. We show the measured channel parameters, which have been carried out in some lakes in Hanoi. The measured results illustrate that the channel is frequency selective for a narrow band transmission. The Doppler spectrum can be obtained by taking the Fourier transform of the time correlation of the measured channel transfer function. We have shown that, the theoretical maximum Doppler frequency fits well to that one obtained from measurement results.

Underwater Acoustic Communications (UWAC) is an emerging technology in the field of underwater communications, and it is challenging because of the signal attenuation of the sound waves. Multiple Input and Multiple- Output (MIMO) is introduced in UWAC because of its support in enhancing the data throughput even under the conditions of interference, signal fading, and multipath. The paper presents the concept and analysis of 2× 2 MIMO UWAC systems that uses a 4- QAM spatial modulation scheme thus minimizing the decoding complexity and overcoming the Inter Channel Interference (IChI). Bit Error Rate (BER) investigation is carried out over different link distances under acoustic Line of Sight (LOS). The utilization of Zero Forcing (ZF) and Vertical-Bell Laboratories Layered Space-Time (VBLAST) equalizers, which estimates the transmitted data proves a success of removing Inter Symbol Interference (ISI). The ISI caused due to multipath effect and scattering in UWAC can be reduced by iterative process considered in VBLAST. A study is made on how the distance between the transmitter and the receiver and the Doppler Effect has its impact on the performance of the system.

The underwater acoustic communication (UAC) operating in very shallow-water should ensure reliable transmission in conditions of strong multipath propagation, significantly disturbing the received signal. One of the techniques to achieve this goal is the direct sequence spread spectrum (DSSS) technique, which consists in binary phase shift keying (BPSK) according to a pseudo-random spreading sequence.
This paper describes the DSSS data transmission tests in the simulation and experimental environment, using different types of pseudo-noise sequences: m-sequences and Kasami codes of the order 6 and 8. The transmitted signals are of different bandwidth and the detection at the receiver side was performed using two detection methods: non-differential and differential.
The performed experiments allowed to draw important conclusions for the designing of a physical layer of the shallow-water UAC system. Both, m-sequences and Kasami codes allow to achieve a similar bit error rate, which at best was less than 10 ⁻³. At the same time, the 6th order sequences are not long enough to achieve an acceptable BER under strong multipath conditions. In the case of transmission of wideband signals the differential detection algorithm allows to achieve a significantly better BER (less than 10 ⁻²) than nondifferential one (BER not less than 10 ⁻¹). In the case of narrowband signals the simulation tests have shown that the non-differential algorithm gives a better BER, but experimental tests under conditions of strong multipath propagation did not confirm it. The differential algorithm allowed to achieve a BER less than 10 ⁻² in experimental tests, while the second algorithm allowed to obtain, at best, a BER less than 10 ⁻¹. In addition, two indicators have been proposed for a rough assessment which of the detection algorithms under current propagation conditions in the channel will allow to obtain a better BER.