The human ear can detect sound waves in the frequency range from about 20Hz to about 20,000Hz. Sound waves above the frequency of the human ear are called Ultrasonic waves.
The uses of ultrasonics:
Ultrasonic scanners are used to produced images of organs in the body or of a baby in the womb. A scanner consists of a probe placed on the body surface, a control unit and a display screen. The probe produces and detects pulses of ultrasonic waves.
For each pulse the probe sends out, it detects reflected pulses from the different boundaries in the path of the transmitted pulse. The probe is moved slowly over the body surface to build up an image of internal tissue boundaries on the screen.
The advantages of using ultrasonic waves instead of X-rays for medical scanning are that ultrasonic waves are:
- Non-ionising, and therefore harmless when used for scanning.
- Reflected at boundaries between different types of tissue, so they can be used to scan organs
They can also be used to detect flaws in metal castings. A flaw such as an internal crack is a boundary inside the metal. The ultrasonic waves are partly reflected from the boundary.
An ultrasonic transmitter on the metal surface sends pulses of ultrasonic waves into the metal object. A detector is placed on the surface next to the transmitter. The detected pulses are displayed on an oscilloscope screen or on a computer monitor.
The further away a boundary is from the transmitter, the longer a reflected pulse takes to return.