Josh's Ultrasonic Range Finder Cache

Principles of Ultrasonic Range Finders

Ultrasonic waves share the common property of being mechanical waves with a frequency higher than the upper limit of the human hearing range. The only difference in physical properties between ultrasonic and sub-ultrasonic waves is that ultrasonic waves cannot be heard by humans while sub-ultrasonic waves can.

The limit varies from person to person, with upper cut-off frequency in the average adult being around 20 kilohertz. (20,000 Hz) Ultrasound devices operate at frequencies from the kilohertz range to the gigahertz range.

Applications of ultrasound include distance measurement, object detection, imaging, non-destructive testing, cleaning, mixing and chemical process acceleration.

Distance measurement and object detection using ultrasound is employed in ultrasonic range finders. An ultrasonic pulse is generated in a specific direction. Objects in the path of the wave reflect the pulse back to the transmitter as an echo. A mounted receiver measures the properties of the reflected sound wave and the time it took for the wave to bounce back. These reflected waves properties give information on the distance, topography, size and composition of the object.

Medical imaging using ultrasound is called medical sonography. The higher the frequency of the ultrasound wave, the clearer the produced image becomes. Frequencies above 2 megahertz are commonly used in imaging applications. Although the frequency of the sound waves is high, the power density is generally lower than 1 watt per centimeter to prevent tissue damage.

Bats use a variety of ultrasonic ranging (echolocation) techniques to detect their prey. They can detect frequencies beyond 100 kHz, possibly up to 200 kHz.

Many insects have good ultrasonic hearing and most of these are nocturnal insects listening for echolocating bats. This includes many groups of moths, beetles, praying mantids and lacewings. Upon hearing a bat, some insects will make evasive manoeuvres to escape being caught.Ultrasonic frequencies trigger a reflex action in the noctuid moth that cause it to drop slightly in its flight to evade attack. Tiger moths also emit clicks which may disturb bats' echolocation, but may also in other cases evade being eaten by advertising the fact that they are poisonous by emitting sound.

Dogs and cats' hearing range extends into the ultrasound; the top end of a dog's hearing range is about 45 kHz, while a cat's is 64 kHz. The wild ancestors of cats and dogs evolved this higher hearing range to hear high-frequency sounds made by their preferred prey, small rodents. A dog whistle is a whistle that emits ultrasound, used for training and calling dogs. The frequency of most dog whistles is within the range of 23 to 54 kHz.