Music Theory/Indian/Ṣaḍja

Ṣaḍja/Sadja/Shadja is the 1st Svara from the 7 svaras of Hindustani music and Carnatic music. Sadja is the long form of the syllable सा. For simplicity in pronouncing while singing the syllable, Sadja is pronounced as Sa (notation - S). It is also called as षड्ज.

More information and importance of Sadja

Following are the information and importance of Sadja in the Indian classical music :

Sadja is the first svara in an octave or Saptak.

Sadja is also the first Svara of a Raga. ( Not always but mostly it is the first svara.)

The svara that is played on the Tanpura for the singer is Sadja. It is played to know that one is singing on the right pitch and octave.

Shadja is the base or basic svara.

It is fascinating to know that after one raga is sung or played, and when one more raga is to be performed, then the svara Sa is played so that there is no confusion in mixing of svaras in both the ragas that are performed. (Specifically in ragas of the same svaras but different Chalan or movement.)

The svara of Sadja is never Komal or Tivra, it is always Shuddha in any given raga.

It is said that Sadja is the basic svara from which all the other 6 svaras are produced. When we break the word Sadja then we get , Shad And Ja. It means that Shad is 6 and ja is 'giving birth' in marathi. So basically the translation is :

 षड् - 6 , ज -जन्म . Therefore it collectively means giving birth to the other 6 notes of the music.

The frequency of Shadja is 240 Hz. The frequencies of the 7 svaras are also given below: Sa 240 Hz, Re 270 Hz, Ga 300 Hz, Ma 320 Hz, Pa 360 Hz, Dha 400 Hz, and Ni 450 Hz, Sa 480 Hz (Taar Saptak) ........ (and so on).

Consequently the Sa after the Ni of 450Hz has a frequency of 480 Hz i.e. the double of the Lower octave Sa.

There are 4 Shruti's of Sadja. Previously the main Shruti not only for Sa but for all the other svaras was on the last Shruti but now it is considered to be on the 1st Shurti.

For example if these are the 4 Shruti's of Sa then,

                      Previously this was the position of the main Shruti of Sa.
                       ^ 
             1   2  3  4
             ^
             But now this position has become the main Shruti of Sa.

All the other svaras except Sadja(Sa) and Pancham (Pa) can be Komal or Tivra svaras but Sa and Pa are always Shuddha svaras. And hence svaras Sa and Pa are called Achal Svaras , since these svaras don't move from their original position. Svaras Ra, Ga, Ma, Dha, Ni are called Chal Svaras, since these svaras move from their original position.

    Sa, Re, Ga, Ma, Pa, Dha, Ni - Shuddha Svaras
   
    Re, Ga, Dha, Ni - Komal Svaras 
  
    Ma - Tivra Svaras

It will be fascinating for you to know that Sa is the only svara which cannot be omitted in a particular raga. So Sa cannot be Varjit in any of the ragas. Hence Sa is the most important svara in the Saptak or in the Indian classical music.
For some ragas where Sa is the Vadi or Samvadi svara, Sa is played repeatedly. But for ragas where Sa is non, Vadi or the Samvadi svara, it is still played repeatedly since it the main svara.
Hypothetically speaking, Sadja is said to be the Sarkar, Sarkar as in the three main gods, Bhrama, Vishnu and Shiva. Sa is made the acronym of Sarkar for showing the importance of the syllable Sa.
Sadja is said to be sourced from the cry of a peacock.