What are Sruti or Shruti in Hindustani Classical Music: History Origin and Facts

What are Shruti/Sruti?
To understand what Sruti is, first you need to understand how the octaves are defined. When you sing the Saragam (Sa Re Ga Ma Pa Dha Ni SA), the first Sa and the second SA always have a relation in their frequencies, that is, the frequency of SA (the last note) is exactly double of that of the frequency of Sa (this first note). So, an Octave is actually the frequency band between Sa and SA.

Now considering the capabilities of human biology, human Ears cannot differentiate clearly between very small changes in the frequencies and hence to make a clear difference in the sounds, there must be a minimum difference between two sounds, and this is how Srutis are defined. Srutis divide the frequencies in an Octave into 22 distinct parts and each Sruti represents a relation between its previous Sruti and the next Sruti.

It is important to understand that Srutis represent a relation between the previous and next Sruti and hence no matter which frequency an Octave starts the Srutis are going to divide it into 22 relative parts.

Also, Srutis are not equidistant, which means two Srutis can have a variable frequency difference but the proportion in each octave is supposed to be same.

Here is the list of divisions of Srutis in Hindustani Classical Music.

Sa Re Ga Ma Pa Dha Ni
4 4 3 2 4 3 2

Here is a full list of all the Srutis and their Carnatic Music names.

Sruti Name ​Sign Carnatic Name
22 SHADJA TARA S Shadja Tara
21 Tivra Ni N+
20 SHUDDHA NI N Kakali Ni
19 Komal Ni n Kaisiki Ni
18 Ati Komal Ni n- 
17 SHUDDHA DHA D Chatuhasruti Dha 
16 Trisruti Dha D-
15 Komal Dha d SHUDDHA DHA
14 Atikomal Dha d
12 Tivratara Ma m+
11 Tivra Ma m Prati Ma
10 Ekasruti Ma M+
09 SHUDDHA MA M Shuddha Ma
08 Tivra Ga G+
07 SHUDDHA GA G Antara Ga
06 Komal Ga g Sadharana Ga
05 Atikomal Ga g-
04 SHUDDHA RI R Chatuhsri Ri
03 Madhya Ri R-
02 Komal Ri r Suddha Ri
01 Atikomal Ri r-
0 SHADJA S Shadja Madhya 

There is a significant similarity between Hindustani Classical Music and the ancient Greek Music culture. But a major difference between Indian and Greek musicology is that in Greece the Octave was divided into 24 Srutis so the proportion were also different.