Underwater Acoustic Test Research Articles

Active impedance control in a water-filled waveguide for low-frequency bioacoustic testing

A challenging problem in studying low-frequency sound reception in aquatic animals is creating a suitable underwater acoustic testing environment. For studies of sound reception in fishes, a plane traveling wave is often a desirable stimulus because the relationship between acoustic pressure and particle velocity is well known. At low frequencies, the long acoustic wavelengths in water rule out obtaining traveling waves in rectangular aquaria. In this study, a cylindrical, water-filled acoustic waveguide, with an active termination was used to generate constant frequency, plane traveling waves in the 12–400-Hz frequency range. The waveguide was constructed of acrylic tubing with an NRL J-13 transducer flanged to each end. One J-13 was the primary source and generated continuous harmonic waves. The active control system measured the transfer function between two hydrophones located inside the tube and used a pattern search algorithm to adjust the secondary source amplitude and phase in order to drive the measured transfer function to that for an unbounded wave. The active control system was able to reduce the terminating reflection coefficient to below 0.05 within the frequency range 12–400 Hz. [Work supported by ONR, Grant No. N00014-94-1-0337.]

A broad frequency anechoic lining system

Testing in indoor underwater acoustic test tanks can be limited by ambient noise and by reflections from the tank’s surfaces. The severity of reflection problems is related to a tank’s dimensions and the acoustic reflectivity of the tank’s sufaces. To reduce reflection problems, a tank may be enlarged, but in most cases, this solution is not acceptable. An alternative solution is to modify the reflectivity of the tank’s surfaces by adding an anechoic lining to the tank. A broad frequency (1 to 100 kHz) anechoic lining system was designed and developed for the 700 000 gallon Acoustic Test Facility at the Naval Undersea Warfare Center in Newport, RI. To accomplish broad frequency absorption, two parallel gradual transition absorbers are used: low-frequency, triangular wedges arranged in rows 6 in. apart and high-frequency molded pyramids spaced approximately 1 in. apart. Both the wedges and pyramids are fabricated from specially formulated, microvoided-polyurethane composites. The system is designed to provide 8 to 33 dB of echo reduction between 1 and 100 kHz. Measured prototype performance exceeded 10 dB at frequencies above 1.8 kHz. System acoustic design and materials development are discussed. The results of acoustic testing are presented and compared to design predictions.

Opportunities for industrial and academic partnering with the Naval Undersea Warfare Center Division, Newport

With the continuing defense drawdown, the Clinton administration policy of defense technology conversion, and the emphasis on development of dual use technologies, the Naval Undersea Warfare Center (NUWC) Division, Newport in Newport, RI is seeking partnerships with industry, academia, and other government activities. Division, Newport is the Navy’s principal laboratory for research, development, test and evaluation (RDT&E), engineering, and Fleet support for submarine and surface ship sonars, submarine combat and communications systems, autonomous underwater vehicles, undersea offensive and defensive weapons systems, and countermeasures associates with undersea warfare. To support this mission, the Division maintains a staff of technical experts in many areas including acoustics, signal processing, oceanography, hydrodynamics, large scale numerical analysis, structural acoustics, advanced materials, guidance and control. The Division also operates many world class acoustic and engineering facilities including acoustic test, transient flow loop, integrated transducer design, acoustic display, and propulsion test facilities. The Divison also operates several underwater acoustic measurement and test facilities such as the Atlantic Undersea Test and Evaluation Center (AUTEC). Mechanisms for industry, academia, and other Government activities to partner with the Division, make use of facilities/expert, or use Division patents are described.

Underwater acoustic test and calibration facilities in Canada

A survey of existing naval and commercial underwater acoustic test and calibration facilities in Canada is presented, with particular emphasis being placed on the calibration facilities operated by the Defence Research Establishment Atlantic. Each facility is described in terms of its geographical location and capabilities, its physical characteristics (including water depth, length, width), its weight handling capability, its data acquisition system(s), and the types of tests and calibrations conducted. Problems and deficiencies of each facility are discussed.

The Westinghouse automated acoustic test facility

The underwater acoustic test facility at Westinghouse contains a weather-independent 50-ft-diam, 30-ft-deep redwood tank decoupled from the ground and supported on vibration isolators. Two independent test systems with separate water accesses can accommodate transducers up to 3 tons. The primary test system consists of a Digital MINC/DECLAB-23 processor interfacing to a Hewlett Packard frequency synthesizer and network analyzer. A newer test system consists of a Real Time Systems (RTS)/HP Series 9000 computer-based data acquisition system that has improved low frequency and data storage capabilities; emulating the Naval Underwater Warfare Center system in Newport, RI, but with special application software. The facility is calibrated for pulsed measurements from 200 Hz to 2 MHz. There are several power amplifiers available including an Instruments, Inc. 16.5-kVA amplifier. There are custom test fixtures for torpedo noses, imaging scanners, and other applications as needed, including an 18-in.-diam pressure vessel with an acoustically transparent dome capable of static pressures of up to 120 psi used for high power testing. Scientific Atlanta rotators with digital synchroposition indicators enable accurate plotting of narrow beam patterns and patterns with deep nulls. The work currently in progress is the addition of low-frequency acoustic intensity that will be used for radiated noise measurements.

Full-spectrum industrial-base hydroacoustic support: A survey of the NSWC Crane Division underwater test complex

The Crane Division of the Naval Surface Warfare Center operates a full-spectrum underwater acoustic test complex for engineering and production testing of air launched and hull mounted sonar transducers and related products. Primary emphasis is on accuracy and efficiency, gained predominately through extensive automation, standardization, and documentation. Facilities include various pressure/temperature-controlled tanks and lake and ocean sites. The presentation describes the attributes, limitations, data acquisition systems, and typical applications of the individual test facilities.

A precision measurement system for use in underwater acoustics measurements.

A Precision Measurement System (PMS) has been developed at NRL-USRD to serve both as a production measurement system and as a platform for ongoing research in new signal generation and measurement techniques. This system is intended as a single-platform solution to most of USRD’s underwater acoustic test needs. Calibration of transducers and measurements of acoustic coatings are included in the tests performed by the PMS. The precision measurement system offers the following features: wide range in both frequency and amplitude; built-in calibration for traceability to NIST; multiple channels for simultaneous signal measurements; a flexible combination of analog gain and filtering stages; digital technology to allow generation of complex waveforms as well as sophisticated processing of measured data; an integrated programming language interface based on VAX/VMS workstations; a variety of signal processing algorithms for wideband versus narrow band, continuous versus transient, parametric versus nonparametric signal processing techniques. Four copies of the system have been built to insure uniformity of measurements across all USRD measurement facilities. Because the system is based on VAX/VMS, a variety of software is available for plotting and reporting as well as more sophisticated offline mathematical analysis.

Development of a new underwater acoustic test and calibration center

A new underwater acoustics test and calibration center has recently been established in Cape Town, South Africa. With the 2.5 million liter main test tank measuring 20.2 × 11.2 × 11.2 m and a second tank measuring 20.7 × 3.5 × 4.2 m, the facility is amongst the largest in use today. Designed to meet the requirements of users involved in military, industrial, and research activities, the facilities incorporate some special features such as vibration isolation mounting of the tanks and the capability of supporting marine life for fisheries research. This paper presents details of this new facility and illustrates some interesting aspects of the design and construction of the tanks. The choice of instrumentation and signal processing techniques will be discussed as well as proposed methods for handling and positioning large transducers.

Development of a hydraulically powered underwater acoustic projector for use in the range of 1 to 10 Hz

Development of a hydraulically powered acoustic projector with a minimum source level of 160 dB re:1 μPa at 1 m over the 1 to 10 Hz range will be presented. This projector will be used at the Underwater Sound Reference Detachment for low‐frequency underwater acoustic testing. Design parameters and their relation to the unique capabilities of hydraulic power for this low‐frequency application will be discussed. Construction details, and acoustic as well as mechanical performance will also be presented. [Work supported by Naval Sea Systems Command, Code 63R.]