The human hearing range is between 20 Hz and 20 kHz. Higher sound frequencies that humans can no longer hear are referred to as ultrasound. Ultrasound can have frequencies from 20 kHz up to 1 GHz. Sound frequencies above this are referred to as hypersonic.
With ultrasound, the sound waves propagate as material vibrations, unlike electromagnetic waves that propagate in free space.
There are various areas of application that use ultrasound. These include material testing and medical examinations, but ultrasound techniques are also used in geophysics, cleaning technology, driver assistance systems and remote control systems. Various ultrasound techniques use reflection methods to determine a localized unevenness such as that found in cracks in materials or in fetal examinations.
This technique is similar to time domain reflectometry with outgoing and reflective vibration trains. In this technique, an ultrasonic sensor sends an ultrasonic pulse that is applied to the material. This pulse is reflected at irregularities and the reflected ultrasonic signal is detected by the ultrasonic sensor and then evaluated. The local resolution depends on the level of the ultrasonic frequency.