# A Vector Sensor-Based Acoustic Characterization System for Marine Renewable Energy
## 4 Discussion and Summary  
Ocean ambient noise levels appear to be increasing worldwide, as a result of increasing anthropogenic activities (e.g., shipping, seismic surveys, and pile-driving) [21], ocean acidification [22], and global economic growth, thereby further decreasing the ability of marine animals to communicate, find prey, and socialize [23]. In recognition of its identified importance, underwater sound is now a key ocean variable, along with the more well-known quantities such as temperature, salinity, and density [24]. Several large international efforts are now underway to identify and quantify long-term trends in underwater noise levels, such as the Oslo Paris (OSPAR) convention [25], Marine Strategy Framework Initiative, and the Joint Monitoring Program for Ambient Noise North Sea (JOMOPANS) [26]. In light of these efforts, the acoustic monitoring technology described in this paper can better help identification of noise sources through its localization capability, and it can pave the way for improved characterization of acoustic particle velocity, which is the acoustic quantity of interest to fishes and marine invertebrates [27]. Acoustic vector sensors are in use since the 1950s [28], primarily as the acoustical sensing unit on board US Navy directional sonobuoys for anti-submarine warfare. The utility of these sensors to localize acoustic sources such as baleen whales and ships was explored by a few studies [5; 10; 20; 29]. More recently, the importance of particle motion in coral reef soundscapes and its significance to fish were explored [30; 31], and these findings contributed to the increasing awareness of its biological significance. However, widespread commercial application was hampered to date by the affordability of reasonably priced sensors, lack of documentation, and challenges in suppressing motion-induced noise (flow noise) and electronic self-noise, as well as the limited application of an array of vector sensors. This paper, therefore, presents an advancement of the state of the science by developing and applying a three-dimensional array of vector sensors to monitor a variety of underwater sounds. The key scientific advancements are as follows:  
* Development of a three-dimensional vector sensor array using a modular frame whose acoustic impedance is very close to that of seawater;
* Quantitative characterization of flow noise contamination, and its suppression in an energetic channel, applied to a vector sensor;
* Application of a location estimation algorithm to an in-water acoustic source. The development of a modular hardware system based on a fiberglass bottom platform upon which an HDPE frame is mounted allows for a unit whose acoustic impedance is much closer to that of  
Figure 10: True (black) and estimated (orange) controlled source locations relative to the NoisePotter V3 (blue), which were separated from the source by 200 m. The difference between true and estimated locations is 3.6 m (2% of separation distance). seawater than other commonly used materials such as aluminum or stainless steel, thereby minimizing the possibility of acoustic scattering by the frame itself. While improper mounting of acoustic vector sensors can often result in unwanted motion of the sensor (see Reference [32] and references therein), the use of PVC pods with a robust, repeatable mounting system provides high data quality as evidenced by the measurements of nearby vessel traffic with little to no data contamination. The use of a three-dimensional array allows for inter-element separation of sensors in both the horizontal and vertical direction. This has the potential to reduce spatial aliasing through the use of multiple array spacings [10]. A horizontal array spacing of 75 cm allows for aliasing-free beamforming at frequencies of 1000 Hz or lower, while the vertical spacing of 25 cm allows for aliasing-free beamforming at frequencies of 3000 Hz or lower.