# Design and Implementation of a Multi-Function Hydrophone for Underwater Acoustic Application
## 1 Introduction  
The electromagnetic signal attenuates significantly in the water, hence it cannot transmit over a long range. On the contrary, the underwater acoustic signal attenuates far less than the electromagnetic signal, and as a result, underwater sounds widely exist in the ocean from the surface to the bottom. For example, the sound of raindrops and tides, the sound of marine animals, and the sound of tsunamis and under-sea earthquakes. In addition, there is plentif sound produced by human activities, for example, ship noise, signals for underwater acoustic communication, signals for underwater acoustic ranging, etc. As a result, underwater sound monitoring, storage, and processing become more and more important. More than 70% of the Earth's surface is covered by ocean and most of the ocean remains unexplored. While rapidly exploring the ocean, more and more in situ data are required to be transmitted or saved. However, devices for recording such in situ dataare not widely developed, or the function of such devices is unitary. The design and implementation of smart ocean equipment are urgent and significant issues. To date, there have been some systems for underwater acoustic applications. Passive acoustic monitoring systems are extremely valuable for long-term research in the marine environment [1]. With increasing concern about underwater acoustics, numerous studies have proposed analyzing ocean sound [2]. Some surface buoys are capable of streaming acoustic data to shore stations in real time for processing [3]. Ref. [4] described a distributed system for real-time environmental monitoring. Other methods used to measure acoustic signals are moving platforms, such as moored autonomous recorders, underwater gliders [5; 6], etc. Although the systems mentioned above easily acquire underwater acoustic signals, they have some shortcomings, for example, the size is quite big, the power consumption is high, the function is unitary, and the parameters are not configurable. Despite the passive acoustic measurements, active underwater acoustic measurement is very important for exploring the ocean. In one paper [7], a real-time underwater ranging system between commodity hydrophones was designed. To address the severe underwater multi-path, a dual-microphone optimization algorithm that identify the direct path more reliably was researched. The proposed system had median errors of 0.48-0.86 m at distances of up to 35 m.  
Underwater acoustic communication and networking have been paid more attention since more information from underwater sensors is required to be exchanged remotely. Many hydrophones have been parts of underwater acoustic MODEMs. For example, in [8], low-power and low bit error rate (BER) amplitude shift keying (ASK), acoustic MODEMs were designed by using STM32F103 (STMicroelectronics, Paris, France), the BER was about \(10^{-3}\) at the distance of 100 m, while when the distance was about 500 m, the BER was approximately \(10^{-1}\). A hydrophone was used to design on-off keying (OOK) acoustic MODEMs for video-capable with a high data rate in excess of 1 Mbps over 100 m in shallow water [9; 10]. In one paper [11], an underwater acoustic orthogonal frequency division multiplexing (OFDM) MODEM was designed, the maximum data rate could reach 5200 bps at a distance of 1000 m. Moreover, the number of null sub-carriers, pilot sub-carriers, and receivers in the MODEM in [11] were all configurable. In another paper [12], an underwater acoustic orthogonal signal division multiplexing (OSDM) MODEM was designed. The OSDM communication method is a compromise between single carriers and the OFDM, so the OSDM MODEM was state-of-the-art. Both shallow-water experiments and deep-sea experiments verified the robustness of the proposed OSDM MODEM. Currently, there is some related literature about recording underwater acoustic data streams. In one work [1], cost-efficient, innovative, and interoperable ocean passive acoustic sensor systems were developed.