There are two standard tone vibration frequencies: the first international pitch a1 is 440hz, and the second international pitch a1 is 435hz, which is used by a few countries such as France and French colonies.

The height of each sound level in the music system has certain standards. The standard height of the sound has varied throughout the ages. The internationally accepted standard height (the first international height) is a sound with a mechanical wave frequency of 440Hz and a wavelength of 78cm. That is, the a in small letters is the "standard sound".

The pitch of the standard A in the past, Renaissance (High Renaissance): 460Hz.

Baroque: 415Hz, Romantic: 430Hz.

Helmholtz: 431Hz, Old Philharmonic: 452Hz.

New Philharmonic: 439Hz, Concert Pitch/First International Pitch (Concert): 440Hz.

There is a unified pitch standard internationally, which facilitates music theory research, musical instrument production and cultural exchanges. The mechanical wave is about 261.6Hz and the C sound with a wavelength of 1.3m is called middle C in the musical sound system, which is at the top of the basic tone level.