In this study, the acoustic pressure in the vicinity of an operating river current turbine is measured using a freely drifting hydrophone array. Analysis of turbine sound reveals tones that vary in frequency and magnitude with turbine rotation rate, and that sockeye salmon may sense. In addition to pressure, the vertical components of particle acceleration and velocity are estimated by calculating the finite difference of the pressure signals from the hydrophone array. A method of determining source bearing using an array of hydrophones is explored. The benefits and challenges of deploying drifting hydrophone arrays for marine renewable energy converter monitoring are discussed.
This work was funded by the National Science Foundation through the Sustainability of Tidal Energy Grant and by the Department of Energy though the NNMREC Grant.
Location of Research
Kviachak River, Alaska
To quantify acoustic pressure near a river current turbine and analyze the properties of turbine-generated sound
It is not clear whether the observed tones would be detectable by fishes at the ranges that measurements were undertaken. The level of the 1/3 octave band containing the H1 tone (100 Hz) decays to near ambient conditions within 100 m downstream of the source, which provides rough boundaries to the ensonified region. Additionally, the low frequency tones seen in the pressure spectra were not clearly distinguishable in the vertical particle acceleration spectra. The vertical particle acceleration associated with the tones may be masked by the higher accelerations near the water surface. Additionally, the three-dimensional particle acceleration vector was not measured and may be greater in magnitude. Though a suitable audiogram for sockeye salmon (Oncorhynchus nerka) is unavailable, the observed tones are within the approximate hearing range of Atlantic salmon. Aside from the low frequency tones, particle acceleration associated with other sources of turbine sound is not likely to be detectable by sockeye salmon at similar ranges (e.g., the broadband “clicking” is above the upper hearing limit for these fish). Notably, the acoustic pressure levels associated with recreational fishing traffic occurs at similar frequencies to turbine operation, so a thorough accounting of contextual sources of sound during the fish migration up and down river would be required as part of a behavioral study.
Murphy, P. (2015). Estimation of Acoustic Particle Motion and Source Bearing Using a Drifting Hydrophone Array Near a River Current Turbine to Assess Disturbances to Fish. Master's Thesis, University of Washington. https://tethys.pnnl.gov/publications/estimation-acoustic-particle-motion-and-source-bearing-using-drifting-hydrophone-array