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Leighton, T. G. (2007). What is ultrasound? Progress in Biophysics and Molecular Biology, 93(1–3), 3–83.   
Last edited by: Mark Grimshaw-Aagaard 06/04/2022, 12:16
"Foetal ultrasonic scanning is now so established in industrialised nations that it would now be difficult to find a control group for epidemiological studies."
"Given the paucity of information on the safe levels for human exposure to ultrasound in air (Section 6), and the lack of traceability for the measurement of such fields (see Section 2.3), this could be a safety issue."
"The limiting feature for the use of ultrasound in air is the severe absorption which rapidly reduces the amplitude of the field, as it propagates away from the source, to levels which are too low for most processing activities, or even to provide sufficient signal-to-noise ratios (SNRs) for many diagnostic applications. There is however one exception, the manifestation of which illustrates a key point which must be appreciated in the assessment of the safety of ultrasound in air.
"The human ear is an extremely sensitive sensor for acoustic waves. Intensities which are low by the standards used for ultrasonic diagnostic technology, and certainly for ultrasonic processing, are generally very much higher than the maximum intensities which the human ear can sustain at audio frequencies without damage. Therefore when ultrasound is used to generate signals to which the ear can responds [sic] (which may not necessarily be restricted to audio frequencies—see Section 6), whilst the resulting intensities may be thought of as ‘low’ from the perspective of many ultrasonic technologies, they may be ‘high’ from the perspective of the ear. This point is discussed further in Section 7."
Leighton suggests three categories of exposure to ultrasound production, the first of which he classifies as ultrasonic noise exposure:
  1. ultrasound generated by some device as a byproduct of its operation (unintended exposure)
  2. ultrasound generated explicitly by some device and fundamental to its operation (unintended exposure e.g., ultrasonic rangefinder, automatic door openers)
  3. ultrasound generated in order to have an effect on humans and/or to change their behaviour (intended exposure).
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