When a sound wave is reflected we call it an echo. Echoes of ultrasound are used by bats and sonar, to detect the position of things.
A sound wave can be represented as a sine curve; by altering its amplitude and frequency, we can change the sound.
If the amplitude is increased, the sound gets lounder - so a higher volume. If the frequency is increased, you get a higher pitched sound. Humans can only hear sounds that are in a certain range of frequencies, this is generally 20 to 20 000 Hz, but perception of sounds at the upper end reduced with age. Sounds that are above 20 000Hz are said to be ultrasonic.
Humans can hear within the 20 to 20 000Hz range. Other animals have much higher ranges of hearing: dogs can hear up to 45 000 Hz, cats up to 64 000Hz and the bat up to 110 000 Hz. Some animals also have ranges of hearing much smaller than humans: the chicken has a range of 125 - 2000Hz.
Ultrasound has many uses, and here we will look at SONAR and medical imaging.
Sonar stands for 'sound navigation and ranging'. In it's simplest form, it can be used to calculate the depth of the sea. This is done by emitting an ultrasonic pulse, this bounces off of the sea bed as an echo, and the time taken for it to return is recorded. Using, speed = distance/time it is possible to calculate the distance, take the example below.
Ultrasound is also used in medicine for ultrasound imaging, this works by using echoes of ultrasound to create a picture of the inside of the body. It is preferable to other methods because it doesn't result in any radiation that could damage the cells, like x rays do. Below is an example of an ultrasound image from medical imaging, below is a picture of a fetus.
N.B. sound that is below the range of human hearing is called infrasound. It is the range of frequencies detected by seismographs in detecting earthquakes, these waves can cover long distances with little obstacle.