Frequencies are measured in Hertz. One Hertz is equal to one cycle of compression and rarefaction, per second. High sounds have high frequencies and low sounds have low frequencies. Thunder has a frequency of only 50 Hertz, while a whistle can have a frequency of 1000 Hertz. The human ear is able to hear frequencies of 20 to 20,000 Hertz.
Some animals can hear sounds at even higher frequencies.
The reason we cannot hear dog s whistles, while they can, is because the frequency of the whistle is too high be processed by our ears. Sounds that are too high for us to hear are called ultrasonic . Ultrasonic waves have many uses. In nature, bats emit ultrasonic waves and listen to the echoes to help them know where walls are or to find prey. Captains of submarines and other boats use special machines that send out and receive ultrasonic waves.