An ultrasonic transducer is a device that converts energy into ultrasound, or sound waves above the normal range of hearing for humans. While technically a dog whistle is an ultrasonic transducer that converts mechanical energy in the form of air pressure into ultrasonic sound waves, the term is more apt to be used to refer to piezoelectric transducers that convert electrical energy into sound. Piezoelectric crystals have the property of changing size when an electric voltage is applied, thus applying an alternating voltage (AC) can cause them to oscillate at very high frequencies producing very high frequency sound waves. Since piezoelectric crystals generate a voltage when force is applied to them the same crystal can be used as an ultrasonic detector. Alternative methods for creating and detecting ultrasound include magnetostriction and capacitive actuation. Ultrasonic transducers are used in many applications including sonar, medical ultrasonography, burglar alarms, and non-destructive testing.