Ultrasonic Range Finding.

First Principles.

Measurement of relatively short distances has traditionally been carried out using a tape measure made of wood, metal or paper etc. However, in recent years another method of measuring distances has become popular, that of using sound, ultrasonic sound to be exact.

The word Ultrasonic means ‘above sound’, the above part referring to above the human hearing range which is approx 300Hz to 14KHz. Therefore any frequency that is above the human hearing wavelength and below the low frequency RF wavelength may be considered as ultrasonic or ‘ultrasound’.

Nature has used sound as a method of distance sensing for tens of millions of years without a single semiconductor. Bats, Dolphins and to a lesser extent, a few fish, use ultrasound as a form of sight, allowing them to see where they’re going and to catch prey on the darkest night or in the muddiest water. And in the dolphins case, it can also increase the amplitude of its ultrasonic transmitter, and use it as a form of stun gun. This has also recently been found true for some breeds of Bat.

Even when ultrasound is not used as a sixth sense, many mammals have a much higher upper limit to their hearing, so ultrasound to them could start as high as 20KHz. This is the principle behind the dog whistle. When blown, we humans do not hear the high frequency vibrations, but a dog hears it as if it were a referee’s whistle. However, I’m straying from our objective a little, so lets get back on track.

Ultrasonic ranging is performed by transmitting a pulse of high frequency sound, then counting how long it takes for its echo to be detected. Because sound through a given medium (liquid or air) is a known quantity, it can be considered a constant, the length of time taken between the transmitted pulse and the received echo can be converted into distance. This is called Time of Flight (TOF).