Children under the age of five have the ability to hear up to

Sound waves are longitudinal mechanical waves, and frequency is the number of wave cycles that occur in a second. Ultrasound refers to sound waves with frequencies greater than the upper audible limit of human hearing.

Children under the age of five and some animals, such as dogs can hear up to 25kHz (1kHz =1000 Hz).As people grow older their ears become less sensitive to higher frequencies.

The audible range of sound for human beings extends from about 20Hz to 20000Hz (one Hz = one cycle/s).

Hence, Children under the age of five have the ability to hear up to 25 Hz.