Naad

The term Naad in itself has a very vast meaning as it refers to all the pleasing, musical sounds that can or cannot be perceived through human ears. But when it comes to music, we talk of the sound that our ears are capable of hearing and understanding.
So here we get familiar to another term that is called "Shruti". Concept of Swaraas, Saptak, Pitch or Frequency all find their derivation from Shruti.