Werner G. Neubauer

Ultrasonic reflection of a bounded beam at Rayleigh and critical angles for a plane liquid‐solid interface

Limited beams of low MHz ultrasound are directed at plane interfaces between water and different solids. Reradiated energy is observed by means of schlieren visualization outside the region predicted by Schoch for incidence at the Rayleigh angle. Some of the solids have properties that would cause prediction of a beam displacement many times that for aluminum. Previous thoery would predict the reflected beam to emerge totally separated from the incident beam for several of the materials used, but no such separation is observed. It is concluded that the reradiated field is composed of specular reflection and Rayleigh‐wave radiation at and near the Rayleigh angle. These two radiations are out of phase at low MHz frequencies, and where they coexist and are of equal amplitude, which occurs within the specular region, a null strip occurs. This strip is sharply defined at exactly the Rayleigh angle. Surface waves (sometimes called pseudosurface waves) are also generated at the longitudinal and shear critical an...

Acoustic reflection from elastic spheres. I. Steady‐state signals

Measurements are made of the acoustic reflected pressure from aluminum and tungsten‐carbide spheres for ka values between 5 and 20. Only the farfield monostatic case is considered. Experimental values are compared to a harmonic‐wave solution. Excellent agreement is observed when the wave speeds in the sphere material are adjusted by an amount smaller than the known uncertainty in the wave‐speed values in the solids. The position of rapid changes in the reflection solution with ka is found to be very sensitive to shear‐wave speed and insensitive to longitudinal‐wave speed. The sensitivity to ambient temperature of the position of a selected minimum in the reflection solution is examined computationally for an aluminum sphere. A shift in the value of ka at which a minimum is calculated is also observed experimentally. Long pulses are used to approximate steady‐state conditions and agreement with the steady‐state theory shows that this approximation is adequate.

Observation of Waves Radiated from Circular Cylinders Caused by an Incident Pulse

Schlieren visualization and hydrophone measurements are used to observe the radiated wavefronts which result when an acoustic pulse is incident on a metal cylinder in water. The range of size parameter ka from 138 to 1419 is considered. The wavefront positions are traced by the refraction, internal reflection, and radiation of shear and compressional waves. In the case of solid cylinders, many wavefronts display an apparent circumferential property derived from the incidence of energy from the normal to the appropriate critical angle. Identification of one of these wavefronts as resulting from previously identified “Rayleigh‐type” wave propagation and a single incident angle is denied, although the circumferential property is verified. A previously identified faster circumferential wave is attributed to a composite wavefront resulting from direct compressional transmission and an increasing number of its internal reflections. Other wavefronts depending on mode conversions are also identified. A mechanism ...

Acoustic reflection from layered elastic absorptive cylinders

The reflection of plane waves by a layered cylindrical shell in water is investigated theoretically. Either or both of the two layers of elastic material may be absorptive. The solution as a function of frequency is examined for several combinations of inner and outer layers.

Acoustic reflection from elastic spheres and rigid spheres and spheroids. II. Transient analysis

Curves relating the reflected acoustic pressures to frequency for a rigid sphere and spheroid and for elastic spheres of aluminum, brass, and tungsten carbide in water are obtained. Experimental measurements using single short acoustic pulseforms are compared with theory. Excellent agreement is obtained for the limited ranges of ka over which the experiments were done. Only the case of monostatic reflection is considered.

Pulsed Circumferential Waves on Aluminum Cylinders in Water

Three types of waves that contribute to the total acoustic diffracted field of an aluminum cylinder in water are isolated by generating each wave to the exclusion of the other two. The “Franz‐type” or “creeping” wave is generated separately on the outside of aluminum cylinders with ka values ranging from 54 to 1008. The circumferential‐wave speed is found to be 1% less than that of the free waterborne wave, in agreement with the theory for a rigid cylinder. The attenuation of these waves on aluminum cylinders is significantly less than that predicted by the theory for the rigid cylinder. Another circumferential wave, with approximately a 30° incidence and emergence angle, is found to propagate on the inside of the curved boundary. This wave has an attenuation ranging between 0.10 and 0.18 Np/rad and has a speed of 2.5 times that of the free waterborne wave. This is classified as a “Rayleigh type” wave. A third wave is observed that is similar to the wave generated at 30° incidence but has a 15° incidence and emergence angle. The attenuation is between 0.08 and 0.14 Np/rad and has a speed of 6.5 times that of the free waterborne wave. Experimental measurements of differential‐scattering cross section are compared with those calculated by means of the creeping‐wave formulation.

Acoustic reflection from cylinders—nonabsorbing and absorbing

The reflection of an acoustic plane wave by an elastic cylinder of infinite length, which may absorb energy, is calculated and measured. Computations of reflection by an infinitely long elastic cylinder as a function of frequency and angle, are compared with experimental data taken using finite cylinders immersed in water. Good agreement is obtained.

Relationship between acoustic reflection and vibrational modes of elastic spheres

The behavior of the solution to acoustic backscattering by elastic spheres in water expressed as a form function vs ka, relative to the normal modes of vibration of solid homogeneous elastic spheres is examined. Specific free modes of vibration are identified with significant changes in form function. The solution for spheres of tungsten carbide, aluminum, and brass are considered for values of ka as high as 28. Minima in the form function usually occur at or near frequencies at which sphere resonances occur. Form functions are examined which are computed from mixing rigid and elastic terms in the waveharmonic series. In this way it is possible to see the effect of an isolated mode of vibration on the form function that describes the backscattering. Calculated resonance widths as well as observed pulse decay times allow determination of analytical and experimental Q’s of isolated resonances.Subject Classification: [43]20.30, [43]20.60, [43]20.50; [43]40.20.

Measurement of Rayleigh phase velocity and estimates of shear speed by schlieren visualization

A schlieren technique is used to give a direct accurate measurement of Rayleigh phase velocity on various materials. Plane solid interfaces underwater are insonified by finite acoustic beams. Sound incident at the Rayleigh angle produces a null strip in the radiated field which uniquely identifies the Rayleigh angle. This null strip is caused by the mutual cancellation of equal amplitude specular and Rayleigh radiations which are 180° out of phase. Rayleigh phase velocity is then calculated from the measured Rayleigh angle by a simple equation. The measured Rayleigh velocities are used to give estimates of shear velocity, and these estimates are compared with direct shear velocity measurements. The attenuation of the Rayleigh wave in the low‐MHz region is due to radiation into the water and is found to be directly proportional to frequency and inversely proportional to material density as predicted by theory.

Monostatic reflection of a plane wave from an absorbing sphere

The monostatic reflection from a lucite sphere in water is measured and compared with the exact classical scattering theory. Experimental results do not agree with theory which neglects absorption, in direct contrast to the excellent agreement found when metal spheres are used as targets. The theory is modified to include the effects of absorption of shear and compressional waves in lucite, and agreement between experiment and the modified theory is demonstrated.Subject Classification: 20.30, 20.15.