sound waves that have a frequency that is higher than what humans can hear (i.e. greater than 20,000 Hz). Bats and dolphins use these high frequency sounds for communication and navigation.

sound vibrations that have frequencies above the range of human hearing

the unit used to measure the intensity of an underwater sound. Underwater dB is a phrase that refers to decibels referenced to 1 microPascal, which is abbreviated as dB re 1 µPa. To be able to compare relative intensities given in dB to one another, a standard reference intensity or reference pressure must always be used. Scientists have agreed to use 1 microPascal (µPa) as the reference pressure for underwater sound. In air, however, scientists have agreed to use a higher reference pressure of 20 microPascals. It is important to remember that sound intensity given in underwater dB is not the same as sound intensity given in air dB.