A high-sensitivity piezoelectric accelerometer has been developed to monitor and characterize low-level vibration spectra in large structures. The theory of accelerometer design is traced and applied to a specific problem to develop a set of design criteria. The construction and calibration of the accelerometer is described. The accelerometer has a sensitivity of 2.82 V/g (9.0 dB re 1 V/g) at frequencies to 1 kHz. The use of the accelerometer in conjunction with a real-time spectrum analyzer to identify vibration sources and characterize their ’’signatures’’ is described.

Arlington, VA 2221712 .j 14 MONITORING AGENCY NAME & ADDRES5,~iI different from ComifolIind Oh1W,*vy, 15 SECFRIT'f CI ASS. (of this report) UNCLASSIFIED 15a DECL ASSI FICATION/ DOWNGRADING SCHEDULE Approved for public release; distribution unlimited. f, 1...~ ~ l~~. C-IS SUPPLEMENTARY NOTES 19 KEY WORDS (Continue on reverse side if necessary and Identify by block num be) Hydrophone preamplifier Electrical noise Preamplifier Self-noise 20 Ar*TRACT (Continue on reverse old* if necessary and Identify by' block number) This report presents analysis of the self-noise which determines the threshold pressure level in hydrophone designs. The noise present in the acoustic sensor, coupling network, and preamplifier input stage are represented by an equivalent circuit noise model. A model provides an effective method to predict in advance of sensor-preamplifier construction the noise levels expected for a specific d-~sign.

The direct measurement of farfield acoustic radiation from underwater transducers in a shore-based facility is often impossible because of the limited size of measurement tanks. The Trott nearfield calibration array (NFCA) concept solves this problem by allowing the farfield radiation to be determined from measurements made close to the transducer. We describe here the development of a 5- to 50-kHz synthetic cylindrical NFCA that is suitable for measurement tanks as small as about 2.5 m in each dimension and that can be operated at hydrostatic pressures up to 6.8 MPa. The NFCA is synthesized using a single line of 48 piezoelectric ceramic hydrophone elements spaced 2.54-cm apart center-to-center and amplitude shaded in a special way. The transducer to be calibrated is driven electrically and rotated near the line about an axis parallel to the line. The response of the line is measured at equally spaced angular positions, thereby creating a virtual cylindrical array surrounding the transducer. We then obtain the azimuthal farfield pattern of the transducer by computer processing the angular responses together with a set of complex shading coefficients. We evaluated the line experimentally by calibrating a large piston transducer in a cylindrical measurement tank less than 2.5 m in diameter. The resulting computed farfield patterns are in excellent agreement with corresponding patterns obtained from direct farfield measurements made in a lake.

An omnidirectional 25-kHz transducer has been designed for use in bottom-scattering measurements. The transducer can be used either as a projector or when fitted with a preamplifier, as a hydrophone. The requirements for the design are presented and solved with mathematical modeling and analysis. A comparison between theory and measured performance is given. The design of a low-noise preamplifier with integrated circuits is described. 20. DISTRIBUTION/AVAILABILITY OF ABSTRACT ra UNCLASSIFIED/UNLIMITED D SAME AS RPT. D DTIC USERS 21. ABSTRACT SECURITY CLASSIFICATION Unclassified 22a. NAME OF RESPONSIBLE INDIVIDUAL Allan C. Tims DDFORM 1473,84 MAR 83 APR edition may be used until exhausted. All other editions are obsolete. i 22b TELEPHONE (Include Area Code)

No abstract