Sound Field Visualization

methods for producing a visible pattern of a sound field. It is used in studying the distribution of the quantities that characterize sound fields of complex shape, to visualize ultrasonic images produced by means of ultrasonic focusing systems, and for ultrasonic flaw detection and medical diagnosis.

So-called sonorous figures (or Chladni figures) are the simplest example of sound field visualization. A distribution pattern of the sound pressure may be produced, for example, with a small sound receiver by using it to scan the field being studied; a point source of light, whose brightness is modulated by the voltage at the output of the sound pickup and whose movement is synchronized with the sound-pressure pickup, is used for visualization. Electroacoustic image converters are a much improved version of such a sound field visualization method: the sound-pressure distribution is converted by a piezoelectric plate into the corresponding distribution of the electric potential on its surface, which is read by an electron beam, and a visual image of the sound field is produced on the screen of a cathode-ray tube by ordinary television techniques (as is done in sonoscopes). A change in the medium’s density in the sound field causes a change in the index of refraction for light rays, which can be detected by purely optical means, such as the shadow and phase-contrast methods, or by the diffraction of light by ultrasound; all of these methods are widely used to study ultrasonic fields of complex shape.

The surface relief method and the Rayleigh disk are used in ultrasonic flaw detection. The surface relief method is based on the property of a free fluid surface of distending slightly under the action of sound rays incident upon it from within. The relief thus produced is readily visible under oblique illumination. The Rayleigh disk method is based on the property of small plates freely suspended in a sound field of rotating so that they are parallel to the sound wave front; it can be produced in a suspension of small aluminum plates in a mixture of water and xylene. In the absence of sound, the plates are oriented randomly, creating a dull gray surface when illuminated; upon the action of a sound wave, some of them assume a definite orientation, so that their reflected light causes a visible image of the sound field to appear against the gray background.

Sound field visualization methods also exist that are based on the secondary effects produced by the propagation of in-tense sound waves in fluids: the thermal effect, the degassing of fluids, the acceleration of diffusion processes, acoustic cavitation, and the effect on a photosensitive layer. For example, to implement the thermal method a thin screen of highly absorbent sound material is placed in the field being studied. The nonuniform heating of the screen by the absorbed ultrasonic beams can be visualized by using thermally sensitive paint or image converters that are sensitive to infrared rays or by excitation or extinguishing of a luminescent screen. The acceleration of photographic development is the basis of the photographic diffusion method of sound field visualization, in which ordinary exposed photographic paper is immersed in a weak developing solution; in the places of action of ultrasound, the diffusion of the developer into the emulsion is greatly accelerated and the paper blackens rapidly.