As you may already know, the SI unit for frequency is Hertz (Hz); an event that repeats once per second has a frequency of 1 Hz.

According to most sources, the human hearing (frequency) range is between 20 Hz and 20,000 Hz (bats can "hear" up to 100,000 Hz!). What does that mean? Sound is a mechanical wave that propagates through a medium (air, water), and the frequency of a sound wave determines its tone and pitch. For example, the middle-C note on a standard 88-key piano has a frequency of approximately 261.6 Hz.

Below is a link to a website with a frequency test (from 22,000 Hz to 8,000 Hz) -- try it out! Note: they use "kHz" units instead of "Hz" (1 kHz = 1000 Hz).

So the human ear has the ability to translate vibrations in molecules (usually air molecules) into the sensation of sound. A single vibration in an air molecule can be thought of like a wave where the molecule goes up, then down then back to where it started. The frequency is the number of vibrations in a second and is given the unit Hertz or Hz. High frequency sounds are usually shrill like a whistle whereas low frequency sounds are more like the deep hum of an amplified bass instrument.

The highest frequency that most humans can hear is 20,000 cycles per second or 20,000 Hz. This limit gets lower as humans age. Older people generally don’t have as large of a hearing range as younger people. The lowest frequency a human can hear is only 20 Hz, but we can feel sounds lower than that such as for an earthquake.

Humans can hear from about 20 - 20,000 Hz, under normal conditions. In extreme silence this may go down to as low as 12 Hz. This upper limit of 20,000 Hz drops as you age, however. Adults slowly lose the ability to hear higher frequencies.

Fun trick: If you have a teacher who is middle aged or older, you can get cell phone ringtones that ring at around 20,000Hz. You and your classmates will be able to hear it, but your teacher or parents won't. It is a cool biology trick.

Humans can hear frequencies up to 20,000 Hz. Hertz is the unit for frequency, and it refers to the number of oscillations of the sound wave in a second. Check out this cool youtube video that plays from low frequency (20 Hz) to high frequency (20,000 Hz) to see how much of it you can hear!

Humans can hear sounds up to about twenty kilo-Hertz (20,000 Hz).