A(la) 440 Hz

Today, I learned about a frequency of babies cry.

The frequency is always 440 Hz, which is the music node A (also called la).

Surprisingly, this law can be applied for not only human babies, but also babies of every mammal.

The reason why the frequency is constant has not been revealed yet, but there are some hypotheses.

According to the one hypothesis, the frequency 440 Hz is stimulate us, and babies seem to try to wake us.

By the way, orchestras tune their instruments using the music node A(440 Hz).

Also, the international standard pitch has been determined to 440 Hz at the conference in London.

• Today, I learned about a frequency of babies cry.
• Today, I learned about the frequency that babies cry [at].

  According to physics, sound propagates "at a frequency". So, instead of "a frequency of babies cry", you should say "the frequency that babies cry at".

• The frequency is always 440 Hz, which is the music node A (also called la).
• This sentence is perfect! No correction needed!

• Surprisingly, this law can be applied for not only human babies, but also babies of every mammal.
• Surprisingly, this law can be applied to not only human babies, but also to babies of every mammal.

  In English, we say "apply/applied to" instead of "apply/applied for".

• The reason why the frequency is constant has not been revealed yet, but there are some hypotheses.
• The reason why the frequency stays constant [between mammals] has not been discovered yet, but there are some hypotheses.

  You don't need to say "between mammals", but I prefer to say it. As for "is constant", "stays constant" is much better sounding.
  
  Also, "revealed" makes it sound like the reason for the frequency's constancy is being hidden from us. But the fact is that we just don't know it yet, so we're waiting for it to be "discovered"!

• According to the one hypothesis, the frequency 440 Hz is stimulate us, and babies seem to try to wake us.
• According to the one hypothesis, the frequency 440 Hz is stimulates us, and babies seem to try to wake us.

  Watch your tense here.

• By the way, orchestras tune their instruments using the music node A(440 Hz).
• This sentence is perfect! No correction needed!

• Also, the international standard pitch has been determined to 440 Hz at the conference in London.
• Also, the international standard pitch has been agreed upon to be 440 Hz at the conference in London.

  For a conference's decision, it's best that the decision for the frequency of the international standard pitch "be agreed upon to be".

Overall, good job! It's really interesting that 440Hz seems to be such a universal frequency XD.

• Today, I learned about a frequency of babies cry.
• Today, I learned about athe frequency of a babiesbaby's cry.

  As this journal is about one specific frequency, you should use the definite article "the" instead of the indefinite article "a".

• The frequency is always 440 Hz, which is the music node A (also called la).
• The frequency is always 440 Hz, which is the music nodenote A (also called la).

• Surprisingly, this law can be applied for not only human babies, but also babies of every mammal.
• Surprisingly, this law can be applied forto not only human babies, but also babies of every mammal.

• According to the one hypothesis, the frequency 440 Hz is stimulate us, and babies seem to try to wake us.
• According to the one hypothesis, the frequency 440 Hz is stimulatestimulates us, and babies seem to trybe trying to wake us.

• By the way, orchestras tune their instruments using the music node A(440 Hz).
• By the way, orchestras tune their instruments using the music nodenote A(440 Hz).