Robert Hickling

Analysis of Echoes from a Solid Elastic Sphere in Water

It is well known in sonar work that the pulse form of a direct echo from a target bears little relation to the form of the original signal. This is true even for regularly shaped bodies, such as a sphere. In this paper, the case of a homogeneous elastic sphere in water is examined theoretically and it is shown in comparison with experimental results, that the observed effects originate from vibrations induced in the sphere by the incident sound. Calculated results are presented for a variety of solid materials and it seems that echo forms could possibly provide information about the size and constitution of a sonar target.

ANALYSIS OF ECHOES FROM A HOLLOW METALLIC SPHERE IN WATER

The nature of sonar echoes from a hollow metallic sphere in water is investigated theoretically in relation to the thickness of the shell and the contents of the hollow region. Steady‐state solutions are presented for iron and aluminum spheres filled with water and for iron spheres with a vacuum inside. Calculated results for pulsed sound suggest that part of the echo originates from a type of flexural wave moving around the shell, which changes into a surface wave as the shell thickness increases. Significant differences in echo structure occur when the interior region is changed from water to a vacuum.

Effects of Thermal Conduction in Sonoluminescence

Current experimental results indicate that sonoluminescence is largely thermal in origin, with the spectra showing a direct relation between luminous intensity and the temperatures generated inside the collapsing cavitation bubbles. In the present paper, the strong dependence of the luminous intensity on the nature of the gas dissolved in the liquid is explained in terms of thermal conduction. Provided the cavitation bubbles are sufficiently small, loss of heat from the bubble into the liquid can significantly reduce the temperatures attained during collapse, so that there is a consequent reduction in the luminous intensity. This process is demonstrated analytically by means of a numerical solution of the equations of motion of a gas inside a collapsing cavitation bubble. The agreement between the theory and the observed luminous intensities for different dissolved gases is good.

Echoes from Hollow Aluminum Spheres in Water

Recorded farfield echoes from precision hollow aluminum spheres in water, with different shell thicknesses and with either water or air in the hollow interior, are compared with calculated echoes. Steady‐state echoes are investigated over a ka range of from 6 to 27, and individual pulsed echoes at ka = 20. Excellent verification is obtained for the features predicted by the theory. It is shown that the echoes are sensitive to changes in the elastic properties of the shell, particularly when the shell is thin. The experiments showed, moreover, that the echoes are dependent on the orientation of the sphere. This is due presumably to small asymmetries in the structure and properties of the shell and emphasizes again the strong interrelation between sonar echoes and the nature of the material in the target.

Scattering of Sound by a Rigid Movable Sphere

The echoes from a rigid, freely movable sphere, due to an incident train of plane waves, are computed, together with the associated rigid‐body motions of the sphere. These results are compared with those for a rigid, immovable sphere and it is shown that they differ significantly only for values of ka below about 5. The significance of the results in relation to previous work on sonar echoes from solid, elastic spheres in water is discussed. A particular case of forced motion of the rigid sphere, where the echoes diminish to zero at high frequencies, is also investigated.

Studies of sound transmission in various types of stored grain for acoustic detection of insects

Abstract In developing passive acoustic systems for detecting insect infestations in bulk-stored grain, it is advantageous to understand the transmission of sound in the grain between the insects and the sensors. In the work presented here grain is shown to be a strong acoustical absorber, the attenuation coefficient increasing roughly as the square root of the frequency. Tests with soft wheat immersed in three different gases: air, argon and carbon dioxide, support an earlier conclusion that sound is transmitted principally through the gas in the passageways between the grain kernels. The speed of sound and the attenuation coefficient were measured as a function of frequency for six different types of grain: hard and soft wheat, brown rice, soybeans, corn and sorghum. It was determined that sound is transmitted over longer distances in grains with a larger inter-kernel spacing, such as corn and soybeans. Grain depth, up to several meters, appears to have little effect on sound transmission.

Sound transmission in stored grain

Abstract The transmission of sound in stored grain was investigated to facilitate the use of sensitive microphones to detect insect infestations. Such infestations cause significant loss (about 10%, or a billion (109) dollars worth, annually in the USA), and early detection is an important factor in reducing such losses. The tests were conducted in a tank containing about 5 m3 of wheat. The phase velocity of sound and the absorption coefficient were measured as a function of frequency using a two-channel analyzer system. Data were analyzed using sound-transmission theory in porous media. It is shown that sound is transmitted principally through the air passages between the grains and that, over a distance of 1 m, grain is highly absorbing at frequencies above about 1 kH. The theory provides a measure of the average spacing between grains, which at a depth of 0.5 m was found to be about 0.6 mm.

Acoustic sensor system for insect detection

An acoustic sensor system for detection of insects in agricultural commodities. The system includes isolation structure for isolating the agricultural commodities from external noise and vibration, an improved acoustic sensor for detecting sound from within the agricultural commodities and for generating a signal in response to sound so detected, and a user recognizable output such as earphones or a light emitting diode for producing user recognizable output in response to signals generated by the acoustic sensor.

Echoes from Spherical Shells in Air

Calculated results show that the elastic reaction caused by sound waves incident on a solid spherical shell in air can have a noticeable effect on the resultant echoes. The effect appears to be strongest for thin shells made of light material. For polyethylene, for example, the effect is found to exist for all thicknesses of the shell. It is suggested that the elastic response of an acoustic target might be used by bats in echolocation.

Investigation of integration accuracy of sound-power measurement using an automated sound-intensity system

Abstract Automated sound-intensity systems have been used routinely for indoor measurement of the sound power of earthmoving equipment and automobiles. The measurement surface is a hemisphere, with sound intensity on the concrete floor assumed to be zero. The system automatically moves an array of sound-intensity probes over the hemisphere, making measurements at desired locations and computing the sound power when the measurements are completed. With an automated system, the number and distribution of measurement positions and the starting position of rotation can be changed without requiring much human labor. In the present paper, the effect on the integration accuracy of the number and distribution of measurement positions and of the starting position of rotation is investigated, using an automated system to measure the sound power of a garden tractor and a motorcycle. Based on these results, guidelines are proposed for achieving good integration accuracy with an automated system.