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To begin, it's necessary to understand what sound is before one can understand ultrasound. Sound is our experience of the propagation of pressure wave through some physical elastic medium. Usually the medium is air, but a liquid works well too. A vacuum doesn't. There is no sound in space. The pressure waves are generated from some type of mechanical disturbance. Sound is a transfer of power as well. Mechanical energy is being converted to a wave form that radiates energy away from the disturbance. Mechanical vibrations become vibrating pressure waves, transferring energy to the medium and to objects that the wave contacts. Human hearing is limited. If the vibrational frequency is too fast, too high a frequency, we can't hear it. This is ultrasound, vibrations too fast for us to hear.
Human hearing can't go beyond about 18,000 vibrations per second, or 18 kHz. There are mammals that can hear well above this. Bats and whales use echolocation that can reach frequencies above 100 kHz. Higher frequencies have a shorter wavelength. A smaller size allows them to reflect from objects more readily and to give greater information about those objects. Extremely high frequencies are difficult to generate and to measure. There is an upper boundary to usable ultrasound, around 10 MHz currently. Higher frequencies are possible.
Generally, the higher frequencies are used for medical imaging, such as investigating a fetus in the mother's womb. The lower frequencies, 1 MHz or less, having longer wavelengths and greater amplitude for a given input energy, produce greater disruption of the medium. Think of fireworks, the whistlers give high pitched sharp sounds, but the boomers aren't just heard, they also move you, giving you a physical thump in the diaphragm. The greater disruption leads to increased motion and, in a liquid, the very interesting phenomenon of cavitation.
Under the right conditions, irradiation of a liquid with ultrasound leads to the formation and collapse of gas and vapor filled bubbles or cavities in the solution. The collapse of these bubbles can be violent enough to lead to interesting chemical effects.
Know of any more links about ultrasound on the web? Please share with me. Or, if you have any comments on this page. Please email. (elcooper@chem.duke.edu)
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