but many older people can hear the signals, which may be because of the considerable variation of age-related deterioration in the upper hearing threshold.

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Industrially, ultrasound is used for cleaning, mixing, and to accelerate chemical processes.

Animals such as bats and porpoises use ultrasound for locating prey and obstacles.

A popular consumer application of ultrasonic ranging was the Polaroid SX-70 camera which included a light-weight transducer system to focus the camera automatically.

Polaroid later licensed this ultrasound technology and it became the basis of a variety of ultrasonic products.

A common ultrasound application is an automatic door opener, where an ultrasonic sensor detects a person's approach and opens the door.

Ultrasonic sensors are also used to detect intruders; the ultrasound can cover a wide area from a single point.After each burst, the electronics looks for a return signal within a small window of time corresponding to the time it takes for the energy to pass through the vessel.Only a signal received during this window will qualify for additional signal processing.The flow in pipes or open channels can be measured by ultrasonic flowmeters, which measure the average velocity of flowing liquid.In rheology, an acoustic rheometer relies on the principle of ultrasound.Acoustics, the science of sound, starts as far back as Pythagoras in the 6th century BC, who wrote on the mathematical properties of stringed instruments.