Answer the following question given this paragraph: 

The frequency of sound waves is measured in hertz (Hz), or the number of waves that pass a fixed point in a second. Human beings can normally hear sounds with a frequency between about 20 Hz and 20,000 Hz. Sounds with frequencies below 20 hertz are called infrasound . Infrasound is too low-pitched for humans to hear. Sounds with frequencies above 20,000 hertz are called ultrasound . Ultrasound is too high-pitched for humans to hear.


Q: The frequency of sound waves is measured in what unit?

 Choices:

- watt

- rads

- decibel

- hertz

A:
hertz
