PROTON+ Experimenters Notebook
Ultrasonic Range Finding.
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 partreferring 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 havea 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 ofhigh 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).
The Speed of Sound.
For an ideal gas; the speed of sound ismainly a function of temperature. Luckily for us on earth, the behaviour of air is very close to that of an ideal gas unless the temperature or pressure is very high or very low compared to standard sea level conditions, or my office. Therefore the speed of sound c for an ideal gas, in our case air, is: c= γRT
PROTON+ Experimenters Notebook Page 1
PROTON+ Experimenters Notebook
where c γ R T= = = = Speed of sound in metres per second Ratio of specific heats. For dry air γ = 1.4 (non-dimensional) Gas constant. For dry air, R = 286.9 N⋅m/(kg⋅K) Absolute temperature (Kelvin), where 0°C = 273.16 K
For example, the speed of sound at room temperature (22°C, 71.6°F) is: c= 1.4 * (22 + 273.16) * 286.9 = 344.31 metres per second
The speed of sound also depends on the type of gas.Suppose we want to operate a sonar range finder on Mars! How can we determine the speed of sound there? The atmosphere on Mars is approximately 95.3 % carbon dioxide (CO2). For CO2, the ratio of specific heat γ equals 1.29, and the gas constant R equals 188.9 N⋅m/(kg⋅K). Assuming a pure CO2 atmosphere, the speed of sound at room temperature, which on Mars is considered a hot day, is as follows: c= 1.29* (295.16) * 188.9 = 268 metres per second
Notice that neither pressure nor density appear in this equation. Even though surface pressure on Mars is only a tiny fraction of that on Earth, the low pressure has essentially no effect on the speed of sound in a gas. Aptly named ‘Echo Ranging’ or SONAR does not exclusively require ultrasound, ‘during the war’, submarine detection was carried out bytransmitting a relatively low frequency, but high amplitude ‘ping’ in the order of 2KHz. Any submarines in the proximity of the sound wave will reflect a portion of the sound back to the receiver. By moving the transmit/receive device named a transducer, the submarine’s bearing and approximate distance could be ascertained. However, as electronics gained more sophistication, it became possible...
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