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  Ultrasounds
are sounds of frequency beyond 20 KHz and they cannot be heard by the
human ear because the Corti organ lacks the appropriate receptors for
such high frequency sounds.
Ultrasounds can be divided into:
low frequency ultrasounds (20-100 KHz) which are used for
industrial purposes (such as cleaning of metals and glass);
intermediate frequency ultrasounds (100 KHz-1MHz) which are used
for treatment because they induce hyperthermia;
high frequency ultrasounds (1-10 MHz) which are employed for
medical puposes (such as echographia and Doppler test).
Low frequency ultrasounds may induce different effects depending on
whether they are transmitted to the body through the hand (local warming
and mechanical damage to the epidermal surface; bony surface degeneration
in the case of intense and prolonged exposure) or through the respiratory
route, characterized by general symptoms with asthenia, nausea, vomiting,
dizziness and psychological effects.
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