Design and Application of Autonomous Underwater Acoustic Recorder

The goal of this work is to design and fabricate a autonomous acoustic recording system which is low cost, reconfigurable and portable for acoustic research. In the commercial market, some off-the-shelf audio systems are taken as a core unit and modified into underwater acoustic logger. Generally, these consuming products have some built-in filters and compression algorithms such that the recorded signals have some unknown distortion. It is not recoverable either. Our system consists of four components, i.e., a PC-104 single-board computer (Celeron 1G), a 12-bit A/D converter (PCM-3718HO), an 20/40 dB amplifier (provided by APL, University of Washington) and power management circuit board. Currently, the throughput of data stream attains to 69 kHz. Six scalable A/D channels are available to share the total bandwidth. A C program with multiple threads is developed to control the I/O's, digitize the underwater acoustic signals and stream data to the hard disk continuously. At present, two ITC-6050C hydrophones are connected to the system. One hydrophone is sampled with zero gain,the other is sampled with zero and 20 dB gain. Running on 35.2 AH, 16 V lithium cells,the system can operate about 4.5 hours before the depletion of power. The system was tested with compressed air nozzle jetting noise (broad-band white noise) to calibrate the hydrophones in the experimental sink. In order to verify its performance, this system works with an autonomously recording unit Bioprobe simultaneously for comparison. We can verify whether the results are coincided with each other. Further, A field test was conducted in shallow bay area. A fishing boat (37 tons, 6-cylinder diesel engine) traveling at constant speed was used as the sound source, and ran both parallel and perpendicular to shore. Bubble noise generated by the ship propeller propagated in the very shallow water waveguide (5 to 13 meters).Thus, these cases show results for both range-independent and range-dependent scenarios. The source track were recorded by using a GPS recorder on the boat, data processing resulted in range-frequency plot shows that the interference pattern reported in the literature was captured. Aided by numerical simulations, it is able to investigate the phenomenon of propagation of sound for wedge with lossy bottom. This system, connected with multiple hydrophones, will be applied in the research for target detection, environmental measurement and monitor, the harbor protection, and so on.