Acoustic Waves In Water Research Articles

Harnessing the ghost

A previous field experiment demonstrated that the interface between an air‐water mixture of low fractional air volume (say, 0.005) and air‐free water is an effective reflector of acoustic waves, suggesting that acoustic waves in water might be beamed by placing an energy source at the focus of a paraboloidal interface between an air‐water mixture and air‐free water. When this interface is concave downward, wave energy traveling upward and outward from the source at the focus is reflected downward and the surface reflection (ghost) is eliminated. Accordingly, a paraboloid reflector consisting of perforated circular air pipes was constructed and tested with a small water gun at its focus. The structure, suspended from its apex, had a height of 4 ft (1.2 m), a base diameter of 8 ft (2.4 m), and a focus 1 ft (0.3 m) below the apex. The wave field, measured with an air‐bubble stream emanating from the perforated pipes, displays a beam with a nearly constant half‐width of 22 ft (6.7 m) created by the paraboloid reflector. Signal amplitudes directly below the paraboloid on an extension of its axis are as much as 9 dB above, and off‐axis amplitudes are as much as 17 dB below, signal amplitudes for no air‐bubble stream. The source signal’s amplitude spectrum, extending to about 1.5 kHz, is skewed toward higher frequencies by the paraboloid reflector, resulting in a dominant peak frequency of about 1210 Hz. The ghost is not evident. Theoretical wave fields of beamed monofrequency source signals demonstrate that the paraboloid reflector behaves as a high‐cut filter, the band‐pass narrowing as axial distance to the paraboloid decreases and off‐axis distance increases. Axial rates of decrease of the beamed experimental signal with increasing depth, at depths of 20 ft (6.1 m) and greater below the source, compare favorably to those of beamed theoretical continuous sinusoidal signals which are nearly equal at all frequencies within the experimental signal’s frequency band. Additional research is required to determine the complete effect of the paraboloid reflector on water‐gun signal characteristics. The small prototype reflector used in this experiment is not suitable for typical seismic exploration efforts because it produces excessively high‐frequency content. A useful paraboloid reflector may have to be several times the size of this prototype.

Device for producing soundwaves in water

An apparatus for producing acoustic waves in water through sudden ejection of liquid mass from a tubular main housing. A first shuttle and second shuttle are slidably mounted inside the main housing. The first shuttle forms with the main housing a slug chamber for confining therein a liquid slug. A pneumatic source together with a pneumatically-operated valve cyclically cause the shuttles to move relative to or in locked condition with each other, thereby applying during each cycle of operation an abrupt propulsion force to the confined liquid slug which becomes expelled as a very high-velocity liquid jet through ports in the housing. A calibrated control element for providing the proper pressures to the pneumatically-operated valve ensures proper movement of the shuttles.

Whispering gallery resonances on solid elastic spheroids

Using tbe NORDA T matrix code, backscattering amplitudes versus frequency have been calculated for the axial incidence of a plane acoustic wave in water on solid tungsten carbide spheroids with a large variety of aspect ratios. Besides the usual series of broad Rayleigh wave resonances, series of narrow whispering gallery resonances are observed and investigated. The dominant mode number of each of these resonances is identified by an interpretation of the bistatic scattering pattern. For increasing aspect ratios, it is noted that an increasing number of the lowest‐order whispering gallery resonances fail to be excited. A possible explanation of this phenomenon is discussed, based on phase matching arguments for the circumferential propagation of surface waves.

Multi-Channel External Modulation Using Acousto-Optic Diffraction

Acousto-optic diffraction with multiple acoustic waves in water is examined to realize multi-channel modulation of an optical beam. Ultrasound, operating as a diffraction grating, is generated via a mode conversion from a surface acoustic wave (SAW) on the free surface of a piezoelectric plate. The operation principle and practical result are described for two independent signals in the Bragg regime. This is supported by experimental measurement.

An Integro-differential Equation for Plane Waves Propagating into a Random Fluid: Asymptotic Behavior

Beran and McCoy [J. Math. Phys., 17(1976), pp. 1186–1189], [J. Acoust. Soc. Amer., 56(1974), pp. 1667–1672] have developed a mathematical model for the propagation of acoustic waves in water which incorporates the scattering effect of random microscopic variations in density (sound speed) into the classical model of geometric optics. This work characterizes the behavior of the acoustic intensity spectral density as a function of distance from the source. The problem is formulated as a Cauchy problem in $L^P $-space. It is shown that nonnegative initial profiles produce nonnegative solution profiles which conserve intensity (area under the nonnegative solution profile). The scattering effect is shown to cause dispersal (mean-square decay) and consequent loss in resolution of the wave as it moves away from the source. This loss is expected to occur more slowly than any exponential, which is the case if there were no scattering (the optical model). Some comments on the approximation of solutions are included. In particular, the last phenomenon is lost in most approximations. Finally the connection between solutions to this problem and spatially inhomogeneous Markov processes is established. Specifically, the original Cauchy problem constitutes the Kolmogorov equation associated with the process.

Frustrated-total-internal-reflection multimode fiber-optic hydrophone

A multimode fiber-optic hydrophone based upon the principle of frustrated total internal reflection has been constructed and tested. The sensitivity of the device to acoustic waves in water is in good agreement with predictions derived from its measured sensitivity to applied static displacements in air. The minimum detectable pressure for the device was 62 dB relative to 1 micro Pa at 500 Hz. Static displacements as small as 4.8 x 10(-3) A can be detected. The sensor is compatible with present multimode fiber-optic sources, detectors, and components.

Extension of synthetic aperture radar (SAR) technique to undersea applications

This paper presents results on the extension of synthetic-aperture radar (SAR) technique to underwater mapping applications. Due to the modest propagation speed of acoustic waves in water, the areal-mapping rate is found to be limited. A number of approaches are discussed in the paper which have the potential of significantly increasing the mapping rate. The map resolutions, both in azimuth and range, are derived for a synthetic array system employing multiple elements. The digital processing requirements, such as the complex multiplication rate and memory size are determined. It was found that the digital signal-processing requirements for many viable mapping systems are within the capability of existing processing hardware.

Radiation-Induced Acoustic Waves in Water

Introduction T momentum transfer caused by a pulse laser upon a surface has been studied both in vacuum and under atmospheric conditions.' These studies were concerned primarily with surfaces of solids, i.e., aluminum, tungsten, etc. When a water surface is irradiated by a powerful pulse laser, an acoustic wave has been observed to propagate into the water.' These experimental observations were made with CO2 lasers and with small spot size (diameter d<H 1 cm). Here we will consider d~~3Q cm. The phenomenology of laser induced acoustic waves in water is rather complex. For very high intensity laser beams, a plasma will be generated above the surface and will alter the characteristics of the beams. For simplicity, when the water is vaporized, the droplets in the water vapor above the surface are assumed to be of the same size as the average aerosols, i.e., 3-5 #m. From the summary of the observed breakdown data in air, the plasma formation threshold for 3-5 /mi particles is of the order of 10 W/cm. Since the formation of plasma is an inefficient mechanism in the process of radiation induced sound, we consider only laser intensities < 10 W/cm. In order to assess the acoustic signals in water, we must first establish the pressure history on the surface. Here, we will divide the acoustic signal generation into two regimes according to laser intensity. For high intensities, i.e., 10 10 W/cm, the blast wave theory will be used to give the surface pressure history. In applying this theory it is assumed that the laser energy is completely absorbed in the blast wave, and in addition, we can ignore the presence of water vapor which may be significant. For low intensities, i.e., less than 10 W/cm, a previously developed thermal conduction code will be used to predict the pressure history on the surface. Finally, knowing the surface pressure history, we can proceed to estimate the far-field signatures.

Conical Lens Like Properties of Cylindrically Diverging Acoustic Waves

Geometrical-optics theory of diffraction of light by acoustic waves shows that the cylindrically diverging (converging) acoustic waves have properties like a conical lens whose axis of revolution coincides with the axis of acoustic line source. The local focal length is a function of the wave numbers of light and acoustic waves, diffraction order and the distance between the light ray and the acoustic line source. The diverging (converging) acoustic waves act like a positive (negative) conical lens for the minus order diffracted light and act like a negative (positive) conical lens for the plus order diffracted light. Some of these properties of the cylindrically diverging (converging) acoustic waves are verified by experiments using 2.4, 7.2 and 12 MHz acoustic waves in water.

Laser-induced acoustic waves in water

Acoustic waves in water may be produced by the absorption of infrared energy from a pulsed laser. The temperature distribution in water for a kilowatt pulse with a duration of the order of a millisecond can be described in terms of the beam parameters and the optical absorption coefficient of water. The expression for the temperature distribution has been solved numerically. The acoustic waves were studied by the use of hydrophones in an anechoic tank filled with tap water. Cylindrical waves with exponentially decreasing amplitudes with depth were produced by the absorption of laser radiation. The velocity of these waves was in close agreement with the velocity of acoustic waves in water.

Experiments on the Interaction of Light and Sound for the Advanced Laboratory

We describe an advanced laboratory experiment in which both Raman-Nath and Bragg diffraction of light by acoustic waves in water are observed in the sound frequency range from 5 to 45 MHz. The apparatus consists of a laser, light detector, rf power source, quartz transducer, and homemade water cell. We discuss the theory of Raman-Nath diffraction, Bragg diffraction, and the criteria for the manifestation of each. The measured Raman-Nath diffracted orders give a visual display of FM sidebands. We discuss a quantitative relationship between the incident and diffracted light and the sound for Bragg diffraction in terms of a three-wave parametric process. Stanford students in the Advanced Applied Physics Laboratory have successfully performed this experiment during the past three years.

Measured surface spectrum dependence of backscattering from rough surface

The wavelength dependence of backscatter from a smoothly undulating randomly rough surface was measured over a broad continuous range of wavelengths using acoustic waves in water. The experiment resulted in discovering a transition region in which the wavelength dependence changed abruptly from \lambda^{0} to \lambda^{+3} , apparently due to change in the surface height spectrum. The wavelength dependence corresponded to the measured surface height spectrum through the transition region in the manner predicted by the composite surface model formulated by Wright and others. A similar spectrum-dependent effect has been found in broad-spectrum electromagnetic measurements of natural surfaces.

A High-Resolution Linear Optical Scanner Using a Traveling-Wave Acoustic Lens

The focusing properties of traveling acoustic waves can be employed in line-scan systems to convert a low-resolution optical beam into a high-resolution beam, both beams scanning linearly in synchronism with the acoustic wave velocity. Used in conjunction with any low-resolution scanner, for example a simple Bragg deflection cell, the scanning lens can yield at least an order of magnitude improvement in resolution, without the need for mechanical elements such as spinning mirrors. The device is inherently achromatic, and can be used just as effectively with white light as with monochromatic laser light. Experiments with dilational acoustic waves in water, and flexural waves on thin elastic strips, have confirmed the effectiveness of this novel approach, and indicate that systems with resolution capabilities up to a few thousand spots per line can be expected. Thus, immediate applications should be found in the fields of recording and reproduction of photographic images, analog and digital data recording and display, and high-speed printing.

Motion of Bubbles in a Stationary Sound Field

Small air bubbles can be propelled through a liquid by a sound field. This paper presents observed and calculated values of the translational velocities at which bubbles smaller than resonance size are propelled through a standing acoustic wave in water and isopropyl alcohol. Bubble radii ranged from 29 to 149 μ, acoustic-pressure amplitudes ranged up to 1.1 bar, and the frequencies ranged from 23.6 to 28.3 kHz. The resulting bubble velocities, ranging to 23 cm/sec, were measured by photographing a moving bubble under stroboscopic illumination. Experiment and theory are in agreement as long as the bubble translation is rectilinear. Experimental results also indicate that the bubble translation becomes erratic when the pressure amplitude exceeds a threshold value. This threshold appears to be identical to the threshold for a similar onset of erratic dancing by bubbles that are trapped without translational motion in the sound field.

Self-Demodulation of a Pulsed Sinusoidal Acoustic Wave in Water

Self-demodulation of a pulsed sinusoidal wave results from the nonlinear propagation of acoustic waves of large amplitude. In plane-wave theory, the time derivative of the envelope function squared increases linearily with distance until primary-wave distortion becomes important. For a collimated source, however, there are three distinct secondary-wave regions: the nearfield or “plane-wave” region; an intermediate region, the remaining collimation distance of the primary wave, where secondary-wave divergence balances its growth; the farfield, where the secondary-wave function becomes the second time derivative and both primary and secondary waves diverge. In the last region, the collimated volume tends to act as an end-fire radiator for the secondary wave.

Bistatic Measurements of Acoustic Waves Scattered from a Statistically Rough Known Surface

The complete bistatic scattering pattern of a rough surface with known statistical properties has been obtained for angles of incidence 10° apart from the vertical down to 30° from grazing using 1-MHz acoustic waves in water. Bistatic patterns corresponding to other incident angles within this range can easily be interpolated, thus giving rise to what is believed to be the first such complete set of scattering patterns. The surface was formed from a sheet of mild steel, after which the roughness was sampled along profiles and the surface statistics computed. The roughness was such that the standard deviation was approximately 12 of the incident wavelength and the correlation distance was about 18 wavelengths. The availability of these complete scattering patterns is important to the understanding of the basic scattering process. They allow, for the first time, comparison of the various scattering theories to experimental measurements for other than the plane of incidence. Also pointed out is the transition of the shape of the patterns as the incident angle is changed.

Finite Amplitude Distortion of 150-kHz Acoustic Waves in Water

Second-order acoustic theory holds that a plane sinusoidal pressure wave becomes sawtooth in shape as it propagates. Experiments showing the progressive distortion of a 150 kHz sine wave radiated from a square (40×40 cm) transducer are described. With an initial peak pressure of approximately 1 atm, the distortion becomes highly developed in a 10-m water path. However, the waveform appears to differ slightly from theory.

End-fire array of virtual acoustic sources produced by the interaction of cylindrically spreading acoustic waves

An experimental study of the interaction between cylindrically spreading acoustic waves in water is described, and experimental results are given. The experimental results are compared with theoretical work separately published by one of the authors.

The Back-scattering of Acoustic Waves in Water by an Obstacle II: Determination of the Reflectivities of Solids using Small Specimens

Two methods of measurement of the acoustic reflectivities of solids immersed in water were adapted to specimens in which either two or all three of the dimensions were small (e.g. one to ten times the incident wavelength). These methods are particularly relevant to materials which cannot be obtained in sufficient bulk or which are of a flabby nature. The latter can be largely overcome by using small samples. At a frequency of 1.48 Mc/s determinations were made of the reflectivities of wet fish bone (26%), damp fish tissue (4.6%), plasticine (44%), soft rubber (11%) and `pc' rubber (3%), and of their respective densities, whence the velocities of sound in these materials are calculated. The acoustic absorption of plasticine was also measured (19 dB cm-1). The merits of some of these materials for scale-model experiments are considered.

The Back-scattering of Acoustic Waves in Water by an Obstacle I. Design of a Scale Model and Investigation of its Validity

A scale model of a marine echo-sounder is described, in which acoustic waves are scattered by fish. The parameters are considered for a model immersed in a tank of fresh water in the laboratory. A frequency of 1.48 Mc/s was chosen. Many measurements were made on standard targets, supports for small obstacles, the sensibly plane-wave region, the variation of velocity of sound in water after refilling the tank and the signals received from a target of known acoustic back-scattering cross section at various ranges. As the experimental results show close agreement with calculations from theory, the validity of the model is established.