If you are a guitarist then today is the day joyously to play D# – string 4, fret 1. The reason to celebrate the 155th birthday of Heinrich Rudolph Hertz (1857-94), the man after whom the calibration of pitch frequency is named. That D# (the one below Middle C) is the nearest note to 155 Hz.
1 Hz = 1 cycle per second – that is to say that the D# in question makes the air vibrate 155 times per second.
Orchestras in the west tune to A = 440 Hz, with the exception of the French who prefer 444 Hz.
But the situation has not always been stable. Harpsichords in the Baroque were tuned to A = 415 Hz; some organs were tuned to A = 465 Hz. The French preferred 398 Hz.
Human hearing range is 20 Hz to 20,000 Hz (20 kHz).
The range of a piano is 27.5 Hz (no point in going much lower) to 4186 Hz (we could perceive notes which are much higher than this, but would we enjoy them?)