Frequencies are most commonly measured in Hertz (Hz), where 1 Hz is equal to one cycle wave per second. For humans, the audible range of frequencies is usually said to be between 20 Hz and 20 kHz (20,000 Hz).
However, there is considerable variation between individuals especially at the high frequency end, which is also affected by a gradual decline with age. And this app is a quick test to determine your Hearing Age, your age according to your ability to hear high-frequency sounds.