Generally speaking, the term ultrasound is applied to all the sound waves with the frequency above the frequency detectable by the human ear (above 20 kHz). In medicine, the ultrasound employed is anywhere in the range between 1 MHz and 20 MHz.
The high-frequency sound waves are emitted and their echoes received by means of a transducer probe - a probe generally consisting of a piezoelectric crystal that can convert electrical energy into mechanical energy (sound waves) and emit it. It is used in, but not limited to, medical diagnostic ultrasonography, ultrasonic cleaning, disintegration, healing by means of therapeutic ultrasound, in welding and as a means of communication.
The medical ultrasonography is generally used to observe the internal organs: the heart, the liver and the abdominal organs and any changes that may occur in these organs. It is also widely employed in imaging the fetus during the pregnancy where the development of the fetus is tracked and it can also help determine the sex of the baby.
In ultrasound cleaning, frequencies between 20kHz and 40 kHz are generally employed. Here, the ultrasound is used in cleaning of jewelry, lenses and other optical devices, medical instruments and even to remove the mineral deposits from the teeth (use in dentistry).
The use in ultrasound disintegration is reflected in the probe producing series of alternating high and low-pressure sound waves. During the low-pressure stage, small vacuum bubbles are created in the specimen and these burst during the high-pressure stage. Common application of the technique is in breaking down of hard or biological materials, such as cellular structures, when there is a need for isolation and analysis of intracellular material.
In ultrasound welding, the high-frequency sounds are applied to make a permanent, solid-state weld between two pieces of materials. In ultrasound welding, the materials can be dissimilar and no other means of joining them is necessary (glues, bolts, etc.). The ultrasound is applied only after the pieces of material have been joined by means of contact and pressure, to assure even contact before the welding stage begins.
Ultrasounds also have possible use in communications. Namely, a graphene probe would be employed to produce or detect the ultrasound waves. This technology has been proposed in underwater communication, since radio-waves generally do not travel well through liquids.
Although they have a wide array of possible uses, the ultrasounds are not harmless though. Occupational exposure to long term ultrasound operating at energy levels higher than 120 dB may lead to hearing loss. Nevertheless, the use in medicine, industry, communications and warfare makes the ultrasound one of the best techniques available today.
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