The first step to an understanding of ultrasonics is to understand the physics of sound. Sound is vibration conducted from a vibrating or pulsing source. Sound is transmitted by all matter with variations in efficiency and speed depending on the nature of the matter and its environment. In general, solids are the best sound conductors followed by liquids and, finally, gasses. Sound does not exist in a vacuum.
Vibrating objects are the most common sources of sound. Consider a tuning fork, a bell, and a radio speaker - all vibrate to produce sound and are sound transmitters. The vibrating object must be in contact with a material that conducts vibration for sound to exist. In the accompanying illustration, the sound transmitter is a vibrating diaphragm. The small circles represent atoms or molecules of a material that is a sound conductor. Notice that the atoms or molecules are each connected to the neighboring atoms or molecules by a spring which represents the way the atoms or molecules react with each other due to inter-molecular and atomic forces. As one atom or molecule is displaced or pushed by the force of the diaphragm as it moves from left to right, its neighboring atom or molecule is given a nudge but does not react instantly - the force must be transmitted by the interaction of forces between the two which is represented by the spring. The spring is initially compressed as the first particle is pushed toward the second and is resisted by the inertia of the second. The energy stored in the spring, in turn, finally pushes the next particle along releasing the compression of the spring to return the system to equilibrium. The now displaced second particle reacts with its neighbor, a third particle, and the process continues as a wave (yes, similar to the "wave" in a football stadium) until displacement initiated by the movement of the diaphragm either reaches the end of the conducting media or is dampened and absorbed by the internal friction of the conducting media. When the vibrating diaphragm reverses direction and moves from right to left taking the adjacent particle with it, the reverse of the above action takes place. The small spring representing the forces between the particles is pulled in tension and, in turn, pulls the adjacent particle toward it to again equalize the system. This motion, too, moves from particle to particle through the media. Soon, however, the vibrating diaphragm again reverses its direction and displaces the particles to the right compressing the springs to nudge the motion through the conducting media. The traveling zones of pressure are known as "compressions" and the areas of tension are known as "rarefactions" in the world of sound. Individual atoms or molecules are only displaced far enough to transmit the "wave" of compressions and rarefactions which travels through the material. Although vibrating objects are by far the most common sources of sound, areas of compression and rarefaction can be created by a source that creates a single or multiple pulses of pressure. The sound we know as thunder results from a single compression created as air is heated and expands in the area of the electrical discharge which we know as lightning. The instantaneous expansion of air creates a pulse of pressure. Another such single pulse source is the sound heard as a board falls flat against a cement floor. A compression is formed as the air between the board and the floor is squeezed out from between them as the board falls. The lingering sounds (the "rumble" of thunder for example) are echos as the single compression bounces around and reflects from the surrounding landscape and other objects. Something to think about - Can you think of other common sources of single compression sound? A siren produces multiple compressions to create sound. In its simplest form, a siren consists of a source of compressed air (sort of a "canned" compression) which is released in bursts as a disc with perforations rotates in front of a nozzle. In this case, of course, the "rarefactions" are areas of atmospheric pressure between the compressions produced by the bursts of compressed air. The effect is the same, waves of compression alternating with waves of lower pressure (in this case atmospheric) to create sound.
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