# A Vector Sensor-Based Acoustic Characterization System for Marine Renewable Energy
## Materials and Methods
### Hardware  
The NoiseSpotter's VSA hardware system consists of a 1.25 m \(\times\) 1.25 m fiberglass grate with three high-density polyethylene (HDPE) modular frames, each housing a vector sensor draped in custom flow noise shields (Figure 1). An optional mooring line connects the bottom platform to a surface buoy for recovery, while an optional Ethernet cable connects the data logger to the surface buoy to facilitate real-time telemetry of data digests via cellular, radio frequency, or satellite communication. A third option for the VSA hardware system is replacement of the mooring line and surface buoy with an acoustic release and pop-up buoy system for system recovery. The VSA's individual frames to house each vector sensor enable adjustment of horizontal and vertical spacing between the three vector sensors. The horizontal spacing of the system can be a maximum of 80 cm, while the vertical spacing can be a maximum of 40 cm, noting that these limitations may be exceeded with customized hardware solutions. The modularity of the system allows the system configuration to be tuned to particular frequencies of interest while minimizing spatial aliasing that can occur when the half-wavelength is smaller than the array spacing. For example, location estimates of lower-frequency sounds (<1 kHz) require at least 1-m horizontal spacing for sensors, whereas a more compact and hand-deployable arrangement of sensors can be used to geolocate higher-frequency sounds. The use of HDPE allows for improved acoustic transparency, while minimizing the potential for low-frequency acoustic scattering by other more commonly used materials such as stainless steel or aluminum that have an acoustic impedance significantly different from that of seawater. The acoustic  
Figure 1: Schematic of NoiseSpotter. impedance of seawater is nominally 1.572 g/cm\({}^{2}\cdot\)s, while the acoustic impedances for aluminum, steel, and HDPE are 17.10, 45, and 2.937 g/cm\({}^{2}\cdot\)s, respectively [13]. The flow noise shields, developed in collaboration with Noise-Control Engineering, are constructed of 1050 ballistic nylon wrapped around a baffled poly-vinyl chloride (PVC) tube inside which a vector sensor is suspended. Extensive laboratory tests of 1050 ballistic nylon revealed that it is acoustically transparent and significantly reduces flow inside the noise shield. Therefore, reduction in flow noise is expected due to reduction of flow inside the shield. Furthermore, because of its acoustic transparency, VSA-measured acoustic signals are not affected. Biofouling field tests indicated that the flow noise shields largely resisted marine growth for a period of approximately three weeks in shallow, productive marine waters. Anti-fouling paints applied to 1050 ballistic nylon mitigated the effects of biofouling for up to six weeks. The electronics system consists of three vector sensors and a data logger. The three vector sensors are from Geospectrum Technologies Ltd. (Dartmouth, Nova Scotia, Canada): two model M20-40s and one M20-100. The vector sensors are rated to a 200 m water depth. The M20-40 features a single wet-connect underwater cable that relays the four analog measurement fields (tri-axial particle velocity and omni-directional pressure) to an external amplifier circuit board. The M20-100 operates similarly to the M20-40, but is augmented with a second wet-connect underwater cable and digital amplifier circuit board to receive the digital compass output from the sensor. Power to the vector sensors is supplied via an amplifier circuit board connected to a rechargeable battery pack (32 Air; Sartek Industries Inc.), and it is relayed to the sensors via the wet-connect underwater cable. The acoustic vector sensors are sensitive to acoustic frequencies in the 50 Hz-3 kHz range, with a flat frequency response on the pressure channel, and a peak in the response at 1 kHz on the particle velocity channels.