Imagine that you came on a concert and you realized that something is wrong with the sound.

It all sounds a little bit messy, like the instruments don't get along with each other. Like they were all tuned differently. It is just a small difference, but since your ears work fine so they can hear it it becomes so bothering. And if you don't hear the difference, you can feel that something weird is going on.
To prevent this kind of things, International Organization of Standardization brought the decision that the tone A above the middle C should be tuned on frequency of 440 Hz (Hertz). This standard was brought in 1939, so before that, A was only few Hertz around 440 Hz. Before a proper standardzation, every village in the world had their own way of tuning. In the middle of 19th century, French already came up with a law that A should be 435 Hz. Today, in US and UK, all music was strict tuned on A440. In the rest of the world, standard tuning is between 432 and 444 Hz. Depends on musicians, authors, the atmosphere in the song.

Now you will tell me that you can't hear the difference in those few Hz...

Well, this little difference doesn't change the song completely, but it change the feeling that the song causes to you.

Try yourself, can you hear the difference? There is a little Youtube video just below with few beautiful examples played on the piano.

Now imagine how 8 Hz of difference can change the atmosphere in a whole song.

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Many musicians claim that A above middle C should be tuned on frequency of 432 Hz (A432).

That tuning standard is also called "Verdi's A" after Giuseppe Verdi, a composer who wrote music using A432 tuning. If the A above middle C is 432 Hz, then middle C is approximately 256 Hz and that is multiple of 8 Hz, and just because of that you can hear a lot of conspiracy theories related to tuning standardization.
They say that A432 is closer to the natural vibration of the universe. The frequency of 432 Hz is considered as the frequency of the heartbeat of planet Earth. Some also claim that A440 was created by Nazis to brainwash the people. This all is probably a pile of crap, but some studies proved that music recorded in A432 standard ease blood pressure, the pulse and anxiety. The rest are only theories and you can't be sure about them. But still, I find music made in A432 standard much more live, emotional an true than A440.

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Thank you!!!