Sounds with a frequency of 20 kHz and higher are referred to as
ultrasound (or ultrasonic sound). High frequency sound is sound of
which the frequency lies between 8 and 20 kHz. High frequency sound
with a frequency over 16 kHz can hardly be heard but it is not
completely inaudible. High frequency sound and even ultrasound in
the lower frequency zone (up to 24 kHz) can be audible if the sound
level is high enough. The sound threshold (the sound level where
sound can be perceived) rises sharply once the frequency (and
therefore the tone) becomes higher. Younger persons hear high
frequency sound better and their hearing range is greater toward
the high frequencies.