Ultrasound refers to a sound wave with a frequency greater than the upper limit of human hearing, which is generally over 20 kHz.
In general, humans can hear sounds with a frequency between 20 Hz and 20 kHz.
Any sound with a frequency higher than 20,000 hertz (20 kHz = 20,000 cycles of oscillation per second) cannot be sensed by a human's ear. Among audible sounds not higher than 20 kHz, those not intended to be heard by humans have also come to be called ultrasound.
Some animals including bats and dolphins rely on ultrasound to live.
Bats emit ultrasound from their mouths and receive its echo. From the time interval between sending the sound and receiving the echo as well as the angle of the echo received, they can determine the distance to targets and their location. Bats can thus sense the terrain and detect their prey.
Similarly dolphins use ultrasound to capture the surrounding conditions and communicate with their peers.
Ultrasound travels through various media including gases, liquids and solids, but cannot travel through a vacuum. The speed of sound varies by the medium it travels through. Sound is likely to travel faster through solids, followed by liquids and gases.
For example, the speed of sound in the air is about 340 meters per second (m/s). That in water is about 1530 m/s and that in iron as high as about 5,850 m/s.
Another typical property of sound is that its energy is more likely to be lost in gases while it travels through liquids or solids more efficiently.
The type of sound waves also depends on the medium. Sound travels through the air and liquids as longitudinal waves (i.e., waves vibrating in the same direction as that of propagation). Through solids, however, it can be transmitted as both longitudinal waves and transverse waves (i.e., waves vibrating at the right angle to the direction of propagation) as well as surface waves.
Ultrasound travels in a very straight line. An ultrasonic wave is reflected when it strikes an interface between materials with different speeds of sound (acoustic impedance). Furthermore, an interface between materials with a larger difference in acoustic impedance reflects ultrasonic waves more strongly and that with a smaller difference in acoustic impedance reflects them less strongly and lets part of them travel through.
For example, the human body consists of a variety of cells and tissues with their acoustic impedance different from each other. Ultrasonic waves striking these cells or tissues are reflected differently. Using the difference in energy level of the echoes makes it possible to image the internal tissues of the body.
Ultrasonic waves are gradually attenuated to become weaker while travelling through the medium. Those with a higher frequency show a higher attenuation factor.
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