Henry D. Dardy

Generalized nearfield acoustical holography for cylindrical geometry: Theory and experiment

From the measurement of the acoustic pressure on a cylindrical, two‐dimensional contour located close to the surface of an underwater, vibrating cylinder, the complete three‐dimensional sound field can be reproduced (reconstructed) with the aid of a computer. This reconstruction technique, called GENAH (generalized nearfield acoustical holography), is unlike conventional holography because it provides a super resolution image of the sound‐pressure field from the surface of the cylinder to the farfield. At the same time, GENAH reconstructs, from this two‐dimensional measurement, the vector velocity and the vector intensity fields (energy flow) in the nearfield of the source, and identifies modes of surface vibration of the cylinder. Experimental results are provided and the accuracy of GENAH is demonstrated by comparison with the two‐hydrophone technique.

Nearfield acoustical holography using an underwater, automated scanner

A computer‐controlled, three‐axis Cartesian scanning facility has been constructed in a large water tank to provide accurate preprogrammed contouring with hydrophone probes. As the scanner moves on the preprogrammed contours, usually located very close to a radiating object, the pressure field is sampled at discrete points until a two‐dimensional (2D) pressure map is obtained. This pressure map is essentially a hologram containing amplitude and phase information which can be processed with a computer using a technique called nearfield acoustical holography (NAH). This processing provides the pressure, vector velocity, and vector intensity anywhere in the space from the surface of the source to the farfield. Backward projection (reconstructing the field at the source surface) provides surface velocity and pressure (fluid loading) on the actual source and thus, through the continuity of the normal velocity, provides both the amplitude and phase of the structural vibration of the source. Through a series of ...

Low noise remote optical fiber sound detector

An optical system for frequency-modulation heterodyne detection of an acoustic pressure wave signal. An optical beam is directed into a Bragg cell outside of the fluid medium in which acoustic signals are to be detected. The Bragg cell modulates the incident beam such that two beams of different frequency exit the cell. The two beams are directed into an input optical fiber and the resultant combined beam is transmitted over a desired distance to a fiber optic transducer disposed in the fluid medium. The transducer includes two coiled optical fibers, a reference fiber and a signal fiber, each of which has a different sensitivity to incident acoustic pressure wave signals. The transmitted beam is directed from the input optical fiber through a power divider which splits the beam into two equal parts, one part passing through the reference fiber, the other part passing through the signal fiber. A filter in the signal fiber transmits only a fraction of the light at one of the two frequencies. The two parts of the split beam exiting the coiled optical fibers are coupled into another optical fiber and transmitted to a photodetector from which the output signal is processed to indicate the detection of an acoustic pressure wave signal. In a modification of the system, different polarization states are imparted with a polarizer and a half-wave retardation plate to the two beams of different frequency produced by the Bragg cell. The power divider and filter are replaced by a polarization beam splitter and another half-wave plate.

Ultrasonic and Hypersonic Studies of Vibrational Relaxation in Benzene

Ultrasonic absorption measurements have been made in benzene over the frequency range from 7 to 570 Mc/sec at 6° and 25° C. Velocity measurements at both ultrasonic and hypersonic frequencies also were made in the temperature range 6°–60°C. The observed relaxation effects may be interpreted in terms of double relaxation of the vibrational specific heat. Analysis of the absorption data indicates that the vibrational specific heat associated with all but the lowest vibrational mode relaxes in the ultrasonic range, with the measured relaxation frequency varying from 510 Mc/sec at 6°C to 560 Mc/sec at 25°C. Correlation of the ultrasonic and hypersonic data also indicates a second relaxation frequency (associated with the lowest vibrational mode) of the order of 10 Gc/sec.

Ultrahigh‐Frequency Ultrasonic Absorption Cell

An ultrasonic absorption cell has been constructed that maintains parallelism sufficient for absorption measurements well above 500 Mc/sec. Constructional details are described and results presented showing absorption in benzene, dioxane, and polysiloxane at 510 Mc/sec. The system as a whole will permit an attenuation of about 35 dB before the signal is lost in the noise. The absorption in benzene divided by the square of the frequency is substantially lower than values obtained at low frequencies, and indicates a relaxation frequency of approximately 600 Mc/sec.

Image profiles in schlieren observations of acoustic wave fronts

Image‐intensity functions have been calculated for acousto‐optic phase‐resolved schlieren in the phase grating (Raman–Nath) diffraction limit. The calculation applies to the case where a circular stop is used. Any number of diffraction orders may be stopped. The results are programmed into a laboratory GT‐44/PDP‐11 real‐time graphic system and compared to experimental observations. The latter include TV monitoring of both the diffraction orders and the image distribution and photomultiplier‐pinhole recording of the image‐intensity distribution. It appears that phase resolution will permit quantitative pressure level determinations. The results aid significantly in interpreting phase‐resolved schlieren observations from radiation and scattering mechanisms.Subject Classification: [43]35.65.

Vibrational Relaxation in Liquid Dichloromethane

Measurements of ultrasonic absorption have been conducted in liquid dichloromethane over a frequency range of 30 to 510 Mc/sec, and over a temperature range of −60° to 25°C. The relaxation is exceedingly strong. The experimental values are predictable by the assumption that the vibrational specific heat of the molecule is the chief cause of the absorption, if it is assumed that the first vibrational mode is inactive, as hypothesized by Andreae. The frequencies of relaxation, arrived at through a curve of best fit, are 171 Mc/sec at 25°C, 147 Mc/sec at 0°C, and 117 Mc/sec at −60°C. The measured velocity dispersion is in agreement with the magnitude of the relaxing specific heat of vibration.

Resonance studies in cylinders using stress induced birefringence imaging

Previous work has considered the effect of free body resonances on the scattering of an incident plane wave by a cylindrical target in water and has related the major features of the theoretical form function resonances to the experimentally obtained farfield reflection patterns. Excitation of these free body resonances is here observed for a glass cylinder in water utilizing a birefringence imaging technique. A fused quartz cylinder is insonified by a long plane propagating pulse and images of the internal stress pattern at resonance frequencies obtained under polarized light. The experimental data are in agreement with a normal mode theoretical computation of the resonance locations and predicted stress distributions for the particular modes.

Comparison of the Experimental and Theoretical Pressure Fields in the Nearfield of Ultrasonic Transducer-Lens Systems

Experimental measurements of acoustic pressure fields near the focal points of ultrasonic transducer‐lens systems are compared to calculations of a recently developed analytic technique. This method incorporates the Fresnel approximation to expand the field in terms of Gaussian–Laguerre functions to calculate the acoustic field for arbitrary placement of a lens in the nearfield of a circular transducer. The predicted field pattern symmetries near the focal region with a lens located either at one focal distance or at the transducer face are confirmed by experimental measurements made with a microsphere probe.

Nearfield of ultrasonic transducer‐lens systems: Comparison of experiment and theory

Local acoustic pressures in the nearfield of ultrasonic transducer‐lens systems were determined by measuring the scattering from a microsphere scanned through the field. The lens was placed both adjacent to and separated by a distance of one focal length from the transducer. Typical parameters were the following: frequency 5.0 MHz, transducer diameter 0.5 in., focal length 1.0 in., diameter of microsphere 0.002 in. Experimental results show remarkable comparison with the details predicted by the Gaussian‐Laguerre formulation presented elsewhere in this meeting. [This research was conducted at the Naval Research Laboratory, Physical Acoustics Branch, Washington, DC.]