摘要:

An electromagnetic acoustic transducer has been developed for line-focusing the shear-vertical(SV)wave in a metal. The EMAT consists of a permanent magnet to supply the bias magnetic field normal to the surface, and a meanderline coil to induce the dynamic field and eddy currents in the surface region of the sample. The meanderline spacing is continuously changed so that the generatedSVwaves from all segment sources become coherent on the focal line after traveling oblique paths. The printed circuit technique enables the fabrication of such a functionally spaced meanderline coil within 1-μm accuracy. The directivity is measured using a half-cylindrical sample of an aluminum alloy, which shows a much sharper radiation pattern of the focusing EMAT than the EMAT having a meanderline coil of constant spacings. This EMAT is then tested through detecting a shallow notch. The scattered signal from the notch is received by the same EMAT, which shows enough strength even for the notch of 0.2-mm depth.

ISSN:0001-4966    

DOI:10.1121/1.422760

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Among the various techniques available, ultrasonic Lamb waves offer a convenient method of evaluating composite materials. Since the Lamb wave velocity depends on the elastic properties of a structure, an effective tool exists to monitor damage in composites by measuring the velocity of these waves. Lamb wave measurements can propagate over long distances and are sensitive to the desired in-plane elastic properties of the material. This paper describes two studies which monitor fatigue damage and two studies which monitor thermal damage in composites using Lamb waves. In the fatigue studies, the Lamb wave velocity is compared to modulus measurements obtained using strain gage measurements in the first experiment and the velocity is monitored along with the crack density in the second. In the thermal damage studies, one examines samples which were exposed to varying temperatures for a three minute duration and the second includes rapid thermal damage in composites by intense laser beams. In all studies, the Lamb wave velocity is demonstrated to be an excellent method to monitor damage in composites.

ISSN:0001-4966    

DOI:10.1121/1.422761

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

This paper investigates the influence of structural parameters on the performance of an optical fiber wound mandrel hydrophone by using the finite element method (FEM). A hydrophone should exhibit the highest possible sensitivity, the widest possible flat frequency response, and an omni-directional sensitivity pattern within the frequency range. The parameters studied included the mandrel geometry, the thickness of the molding coated over the fiber, and the material properties of the constituent parts of the hydrophone. The analysis results showed that the pressure sensitivity of a hydrophone increases in relation to the length of the mandrel and the thickness of the molding. A higher pressure-sensitivity also requires a mandrel or molding material with a relatively low Young’s modulus and Poisson’s ratio. On the other hand, the flat frequency response of a hydrophone increases when either the mandrel length is shortened or the mandrel material is hardened. The omni-directional characteristic is also improved with a shorter mandrel. Therefore, a hydrophone with the best performance must balance the tradeoff between the geometrical and material parameters. The analysis discussion is focused on a representative specification of a frequency range of up to 5 kHz.

ISSN:0001-4966    

DOI:10.1121/1.422762

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Three-dimensional wave propagation and probe modeling in a homogeneous, linear elastic, anisotropic half-space is discussed. The probe is modeled by modifying the traction vector at the interface between an isotropic half-space and an anisotropic half-space. The traction vector is obtained by considering a plane wave in the isotropic solid incident upon the interface. The probe can be of any type, angle, frequency, and size. The probe is attached to an anisotropic half-space and the wave propagation problem is solved by means of Fourier transform techniques. Numerical examples illustrate some of the properties of the probe model and wave propagation in anisotropic solids.

ISSN:0001-4966    

DOI:10.1121/1.422763

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

A set of in-air experiments were made on two, thin, cylindrical shells to understand and control the propagation of membrane waves, which are primary determinants of target strength in the mid-frequency, high-aspect angle regimes. A circumferential source array was used to drive single modes in all wave types, compressional, shear, and flexural. Resulting waves were collected using an axial line array of laser Doppler velocimeter measurements and the wave types separated by array processing. Comparison between a clean plastic shell and a steel shell with a keel-like feature suggest on the order of 10-dB reduction of compressional wave response via coupling into higher order flexural waves by the nonaxisymmetric discontinuity. The conversion coupled with subsequent damping of the resulting flexural waves makes this a potentially useful mechanism for control of membrane waves.

ISSN:0001-4966    

DOI:10.1121/1.422764

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Sound radiation by a simply supported unbaffled plate is presented using a double layer integral representation of the acoustic pressure. Both the pressure jump and the plate displacement are developed in series of the simply supported plate modes. This leads to modal coupling coefficients, which are numerically calculated for this specific unbaffled problem. The unbaffled problem is then transformed in the presentation of the baffled one which allows us to define original modal radiation impedance coefficients to the unbaffled plate. Baffled and unbaffled radiation impedances are then compared versus nondimensional frequency, in real and imaginary parts for a set of representative modes. An approximate expression for the direct unbaffled radiation impedance is presented, which is useful for light fluid loading. Finally, the influence of the baffle on sound radiation both in light and in heavy fluid, on the acoustic radiated power, the plate velocity, and the radiation factor is investigated.

ISSN:0001-4966    

DOI:10.1121/1.422765

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

An original technique of measuring reflection (or transmission) matrices of duct discontinuities is described. The technique is based on measuring the acoustic pressure by microphone pairs in two closed cross sections of a constant area circular duct, which separates incident and reflected modes after a modal decomposition by a Fourier–Lommel transform. Theory, experimental setup, and data treatment are presented. Then experimental reflection and conversion coefficients at an open duct end for plane wave and higher order modes are presented as an application and compared with analytical results.

ISSN:0001-4966    

DOI:10.1121/1.422766

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

The problem addressed in this paper is whether higher order correlation detectors can perform better in white noise than the cross correlation detector for the detection of a known transient source signal, if additional receiver information is included in the higher order correlations. While the cross correlation is the optimal linear detector for white noise, additional receiver information in the higher order correlations makes them nonlinear. In this paper, formulas that predict the performance of higher order correlation detectors of energy signals are derived for a known source signal. Given the first through fourth order signal moments and the noise variance, the formulas predict the SNR for which the detectors achieve a probability of detection of 0.5 for any level of false alarm, when noise at each receiver is independent and identically distributed. Results show that the performance of the cross correlation, bicorrelation, and tricorrelation detectors are proportional to the second, fourth, and sixth roots of the sampling interval, respectively, but do not depend on the observation time. Also, the SNR gains of the higher order correlation detectors relative to the cross correlation detector improve with decreasing probability of false alarm. The source signal may be repeated in higher order correlations, and gain formulas are derived for these cases as well. Computer simulations with several test signals are compared to the performance predictions of the formulas. The breakdown of the assumptions for signals with too few sample points is discussed, as are limitations on the design of signals for improved higher order gain. Results indicate that in white noise it is difficult for the higher order correlation detectors in a straightforward application to achieve better performance than the cross correlation.

ISSN:0001-4966    

DOI:10.1121/1.422767

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Reference-beam detection is inherently superior to the knife-edge detection currently in use in scanning laser acoustic microscopy. This new detector makes use of a reference beam, retarded 90 degrees, which is mixed coherently in a photodiode with the acoustically obtained image-modulated beam. The new detector has an isotropic transfer function which is circularly symmetrical around its highest value, namely the zero-frequency point in the spatial spectrum. This property makes it possible to detect spatial frequencies in all directions simultaneously and with equal sensitivity and simplifies the associated electronics. It also makes possible the employment of acoustic evanescent-wave detection so that ultrasound of low temporal frequency can be used and at the same time high spatial frequencies can be detected for obtaining high resolution. Oblique insonification, required for best operation in the knife-edge detector, is thus not preferred in the reference-beam detector and the resultant Doppler shift in the detected frequency of the transmitted zero-order acoustic waves is avoided.

ISSN:0001-4966    

DOI:10.1121/1.422768

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Auscultation of lung sounds in patient transport vehicles such as an ambulance or aircraft is unachievable because of high ambient noise levels. Aircraft noise levels of 90–100 dB SPL are common, while lung sounds have been measured in the 22–30 dB SPL range in free space and 65–70 dB SPL within a stethoscope coupler. Also, the bandwidth of lung sounds and vehicle noise typically has significant overlap, limiting the utility of traditional band-pass filtering. In this study, a passively shielded stethoscope coupler that contains one microphone to measure the (noise-corrupted) lung sounds and another to measure the ambient noise was constructed. Lung sound measurements were made on a healthy subject in a simulated USAF C-130 aircraft environment within an acoustic chamber at noise levels ranging from 80 to 100 dB SPL. Adaptive filtering schemes using a least-mean-squares (LMS) and a normalized least-mean-squares (NLMS) approach were employed to extract the lung sounds from the noise-corrupted signal. Approximately 15 dB of noise reduction over the 100–600 Hz frequency range was achieved with the LMS algorithm, with the more complex NLMS algorithm providing faster convergence and up to 5 dB of additional noise reduction. These findings indicate that a combination of active and passive noise reduction can be used to measure lung sounds in high noise environments.

ISSN:0001-4966    

DOI:10.1121/1.422769

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP