# A Vector Sensor-Based Acoustic Characterization System for Marine Renewable Energy
## 3 Results
### Location Estimation  
A key principle of long-distance underwater acoustic propagation is that sound travels along discrete paths between a source and a receiver. Given the uniqueness of the acoustic field between a source and receiver, a variety of techniques can be employed to determine the location of a sound source [10]. These techniques vary in hardware complexity, accuracy, and computational efficiency based on the underlying physical assumptions. The NoiseSpotter is a platform that can improve upon the method of direction of arrival estimation by using acoustic vector sensors that measure triaxial particle velocity in addition to acoustic pressure. Each vector sensor has the ability to determine the bearing of an acoustic source. With multiple time-synchronized vector sensors located on the array, the essential principle of the proposed technology is that the source can be geo-located by triangulation of particle velocity vectors measured across the array [5; 19]. The basic principle of triangulation using the NoiseSpotter's vector sensor array is illustrated in Figure 8. Here, a plane wave impinges upon a linear array of acoustic sensors. For a given angle of incidence, the signal received on each channel of the acoustic array is offset by a frequency-dependent phase relative to that received on neighboring channels. Plane-wave beamforming [20] acts by phase-shifting the signal on each channel, followed by coherently summing over the array. A larger number of array channels result in a greater array gain, as well as better suppression of incoherent random noise, while reinforcing a coherent signal. A vector sensor makes four acoustic measurements on each sensor. Therefore, a vector sensor array can exhibit four times the array gain of a traditional hydrophone array with the same number of physical sensors. Having established adequate data quality of the acoustic monitoring system, bearing estimation of the acoustic source was demonstrated. The technique for bearing estimation calculation involves segmenting time series of NoiseSpotter VSA data into discrete 1-s-long segments (20,000 points at a sampling frequency of 20 kHz). Then, 1024-point fast Fourier transforms (FFTs) are computed on each 1-s data segment, and plane-wave beamforming is applied in the frequency domain following the methods outlined by [PERSON] et al. [20]. At each frequency bin, this method yields a dominant azimuth and elevation angle for each time segment, estimates of which are then stitched together for the entire time series. Bearing estimation was conducted for multiple 6-min-long time series of controlled acoustic source transmissions that were obtained at various source receiver separations (10 m, 50 m, 100 m, 500 m, and 1 km) during in-water field testing. Figure 9 shows an example of elevation and angle estimates obtained for a 3-min-long time series, when the source was located 180 m from the receiver. Shown in the upper panel is a spectrogram of the received signal, which shows the evolution of the frequency content of received signals. Receptions of the controlled source transmissions are seen as the step-like progression in energy across frequency as an evolution of time. The frequency-domain beamforming processing is performed for the tone at 1700 Hz, selected to demonstrate that specific tones can be geolocated, while ignoring sources at other frequencies. Estimates of azimuth and elevation angle during these periods are seen to closely track the true bearing of the source. Outside of these periods where the source was transmitting, the bearing angles exhibit significantly larger deviations from the true bearing, consistent with isotropic ambient noise that has no dominant direction. Having estimated the three-dimensional bearing angle to the source, the true location of the source (Figure 10) is estimated assuming a known source depth. This assumption is valid when trying to associate  
Figure 7: Probability distribution of M2j metric, with and without a flow shield.