In a previous blog post we discussed diffraction, a phenomenon that prevents beams from being focused into arbitrarily small spots.
Another factor that might limit distance is air absorption. The energy levels of different technologies decrease by different amounts when travelling through air.
Absorption particularly impacts ultrasound technologies. For example, in standard air conditions (temperature of +25°C, 50% relative humidity, at sea level), over 40% of a 60 kHz ultrasound beam is absorbed per each meter of propagation.
If Ultrasound loses 40% after each meter, then only 60% is left. After 3 meters, the remaining energy is (0.6)^3 Which is 21.6%,
An absorption model and an absorption calculator can be found at the website of the UK National Physical Laboratory. See http://resource.npl.co.uk/acoustics/techguides/absorption/ .
In contrast to ultrasound, both RF and IR travel well over distance.Tags: absorption, diffraction, safety.RF, ultrasound