Ultrasound is acoustic (sound) energy in the form of waves having a frequency above the human hearing range. The highest frequency that the human ear can detect is approximately 20 thousand cycles per second (20,000 Hz). This is where the sonic range ends, and where the ultrasonic range begins. Ultrasound is used in electronic, navigational, industrial, and security applications. It is also used in medicine to view internal organs of the body. Ultrasound can be used to locate objects by means similar to the principle by which radar works. High-frequency acoustic waves reflect from objects, even comparatively small ones, because of the shortwavelength. The distance to an object can be determined by measuring the delay between the transmission of an ultrasound pulse and the return of the echo. This is the well-known means by which bats navigate in darkness. It is also believed to be used underwater by cetaceans such as dolphins and whales. Ultrasound can be used in sonar systems to determine the depth of the water in a location, to find schools of fish, to locate submarines, and to detect the presence of SCUBA divers. In an ultrasonic intrusion detection system, a constant, high-frequency acoustic signal is transmitted by a group of transducers. The ultrasound waves flood the protected area. Receiving transducers monitor the ultrasound reflected by objects in the protected zone. If anything moves, it produces a change in the phase of some of the reflected waves. This phase change is detected by sensitive electronic circuits, which send signals to an alarm or dispatch center. Ultrasonic security systems are also popular among automobile owners. These devices detect motion in the immediate vicinity of a vehicle. In ultrasonic medical imaging, high-frequency acoustic energy is transmitted into the human body using a set of transducers attached to the skin. The ultrasound waves reflect from boundaries between organs and surrounding fluid, and between regions of differing tissue density. This technique has been used to observe the condition and behavior of fetuses prior to birth. It has also been used to locate tumors, and to observe the condition of the human muscles and bones. Ultrasound is used in industry to analyze the uniformity and purity of liquids and solids. It can also be used for cleaning purposes. Subminiature ultrasonic cleaning instruments are used by some dentists during routine examinations