An ultrasonic probe is a type of sensor that uses ultrasound, which refers to sounds at frequencies higher than the human ear can detect. Due to the structure of the ear, a healthy young adult human can perceive sounds with frequencies of up to about 20 kilohertz (kHz), though the precise threshold varies from person to person. Ultrasound is useful because it can penetrate through a medium, such as a person's body, and provide information about the medium's internal structure. Ultrasonic probes are used for a number of different areas, such as diagnostic imaging in medicine and quality testing in industry.
When a sound wave passes from one material to another material with a different density, part of the sound is reflected back. By measuring the precise times at which sound waves from the ultrasonic probe echo back, the probe can determine the depth of different materials within whatever is being scanned. The frequencies used by an ultrasonic probe can vary enormously depending on its application, ranging from as low as 50 kHz to as high as 50 megahertz.
The best-known use of ultrasound is in medicine. An ultrasonic probe can produce images of internal structures, such as muscles and soft tissue, as well as the structure of organs. This can be used while diagnosing a patient to examine tissues and organs for damage or abnormalities, to detect the state of arteries and the flow of blood through them, and to identify structures such as tumors and kidney stones. Ultrasound is also frequently used to produce images of a fetus while it is in the womb. In many situations, ultrasound is preferred over other methods because it is a noninvasive way to see the inside of the body that does not involve involve electromagnetic radiation and so does not create the risk of chromosome damage posed by methods of examination such as x-rays.
An ultrasonic probe does have some disadvantages. Ultrasound can provide images of the exterior surface of bones, but ultrasonic waves do not penetrate bone well and so are not very effective as a way of probing the interior of bones or examining the brain through the skull. Ultrasonic waves also do not propagate well through gas. This makes ultrasonic imaging of the lungs nearly impossible, with very limited exceptions, and greatly impedes ultrasonic imaging of the pancreas due to interference caused by gas in the nearby gastrointestinal tract. Ultrasound also has limited penetration, which can make ultrasonic imaging problematic when trying to probe deep in the body or when a patient is significantly overweight.
Ultrasound is also used for industrial testing. Similar to ultrasound in medicine, an ultrasonic probe can be used to provide imaging of an object's internal structure. This is an extremely valuable capability because it can identify internal abnormalities and defects that would be invisible to an exterior inspection. It can also reveal a material's microstructural properties, such as grain size and porosity, and provide information about a material's mechanical characteristics.
Industrial ultrasound is most commonly used for metals and metal alloys, though it can also be applied to other materials, such as ceramics. It is commonly used to check manufactured objects for manufacturing defects and to check equipment for damage suffered from mechanical stress, corrosion, or other sources during use. The use of ultrasonic probes for testing is extremely common in transportation-related industries, such as railroads and aerospace, where subtle manufacturing defects or accumulated microstructural damage to materials can have catastrophic consequences if unnoticed.