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?"
Answer
There is a comprehensive article on absorption of ultrasonic waves in air in the article Absorption of ultrasonic waves in air by A. Vladišauskas and L. Jakevičius.
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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