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I am not a physicist but I am extremely interested in this area.

The simple version of my question is:

"What is the maximum range of an ultrasonic wave traveling through air?" Now, I know it depends on the exact frequency, air temperature, humidity, ... I don't need to know exact number. I just need to know the order of magnitude. Is it ~1m? ~10m? ~1000m?

I also know that this range is finite due to wave's attenuation and attenuation is higher as frequency is higher. So this question could possibly be translated into these two: "What is the minimum ultrasonic wave amplitude detectable by a commercially available sensor?" + "What is the maximum ultrasonic wave amplitude that can be emitted by commercially available ultrasound emitter?"

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A 'ultrasonic' wave can travel arbitrarily far, just like a regular sound wave, it depends on the amplitude of the signal. Regarding your 'two' questions: you'd probably need to look it up... – zhermes Jan 26 at 4:18
The reason I'm asking this is the following: everytime I see a discussion about measuring distance using ultrasonic waves I see proximity sensors e.g. In cars, in card readers... I am interested in measuring distance in range 1m-100m indoors using ultrasound. Required precision would be +- 5cm. – ancajic Jan 26 at 12:05

1 Answer

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There is a comprehensive article on absorption of ultrasonic waves in air at http://www.ktu.lt/ultra/journal/pdf_50_1/50-2004-Vol.1_09-A.Vladisauskas.pdf.

We're not supposed to just quote links in articles, but it seems silly to reproduce the entire paper here. The conclusions are that absorption is strongly affected by air temperature and pressure (though I'd guess you're only interested in normal pressure) and also by frequency. At low frequencies and temperatures (around zero C) the absorption is very small. However as the temperature increases the absorption rises with temperature. For low frequencies (c. 100KHz) the absorption peaks around 50C at about 5 dB/m. For frequencies around 1MHz the absorption is several hundred dB/m and doesn't show a peak below 100C.

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Thank you for reply. The link is very helpful, but I guess I was hoping for something a little bit more from industry's standpoint. As I said in the comment below the question, everywhere I look, I allways find ultrasonic devices that detect proximity in range of up to ~5m. And I am wondering why hasn't anybody invented an ultrasonic device that would consist of emitter on one side and receiver on over 100m distance that could measure the correct distance without lasers or other stuff? – ancajic Jan 26 at 16:52

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