Use of Anechoic Baffles for Improving Range and Detection Efficiency of Acoustic Telemetry Systems

For acoustic telemetry, underwater noise can limit the detection range and efficiency of hydrophones.  For example, the high noise levels caused by flow and mechanical noise at hydroelectric facilities.  In the Federal Columbia River Power System (FCRPS), Pacific Northwest National Laboratory (PNNL) for the U.S. Army Corps of Engineers (USACE) developed the Juvenile Salmon Acoustic Telemetry System (JSATS) to track outmigrating juvenile salmon passing through hydroelectric facilities.  JSATS utilizes hydrophones, cabling, data acquisition computers, and acoustic transmitters implanted in salmon.  To improve detection of the acoustic transmitters as fish pass through hydroelectric facilities, conical baffles were installed around each hydrophone to reduce the high ambient noise levels.  These baffles, directed away from the dam, were lined with anechoic material, thus reducing dam- and flow-produced noise while increasing the signal-to-noise ratio.  To quantify the effectiveness of baffling, the detection range and efficiency (pings detected/pings transmitted) of acoustic beacons was measured at set distances from turbine intakes using both baffled and un-baffled hydrophones.  These tests were conducted at Bonneville Dam Powerhouse-1 during high-flow conditions.  In all cases, baffled hydrophones outperformed un-baffled hydrophones, greatly increasing the passing acoustic transmitter detection range and efficiency.