We have heard stalwart vocalists like Lata Mangeshkar, Balagandharva, Bade Gulam Ali Khan, Asha Bhosale, Kishori Amonkar, Begam Akhtar, Vasantrao Deshpande, Amir Khan, Bheemasen Joshi singing for years on either Organs and/or Harmoniums tuned to the 'Equitempered' (ET) scale, for several years.
We found that there singing was very pleasant, or surel.
If they sang on ET scale, how do we explain this ?
Were all these great artists, and those who heard them for decades, wrong ?
This is one of the most important issues needing proper understanding.
1. How does an ear perceive what is 'Pleasant', or, what is Surel (in tune)' and what is 'Besur' (out of tune)? :
For making this judgement, an ear requires a 'Base' note to be compared to the 'Sung' note. If the sung note is 'in tune' with the Base note, the brain perceives the same as 'Pleasant' or 'Surel'.
2. The singer is forced to sing notes on an accompanying Harmonium:
When an artist sings with an accompaniment of an Equitempered (ET) tuned instrument such as an Organ or Harmonium, he/she has no choice, but to 'align' his/her sung notes with the notes tuned in the ET instrument.
The notes in the instrument therefore, act as the 'Base' notes, and the 'Surel' ness of voice is judged on the alignment of 'Sung' notes with ET notes in the instrument which the ear regards as 'Base' notes.
Hence, whenever the alignment is 'absolute', (which was always the case with all the above great artists) the ear judges the performance as 'Surel (or in tune)', although in reality, the positions of the notes in the Equitempered scale are not those of the Indian Natural Scale of 22 Shrutis!
The ear is merely saying that the 'Sung' note is 'In tune' with the 'Base' note.
The ear is unable to say that the note is not any of the 22 Shrutis, as none of the 22 Shrutis is available to the ear for comparison, at that time. In other words, the ear is simply telling the listener that the 'Sung' note and the 'Base ET note' is identical.
3. We all can experience the feeling that ET note is OK:
Anyone can carry out an experiment to witness this truth on his/her own.
Take an Equitempered Harmonium. Start singing say, Bihag. Align your 'Sung' Gandhar with the 'Base' Equitempered Gandhar very accurately.
Your ear will tell you that both notes are matching and the experience is 'pleasant' or 'surel'(in tune).
This happens in spite of the fact that 'Equitempered' Gandhar is at 26 % distance from Shadja, than the 'Natural Gandhar's true and natural place of 25 %. (It is this Gandhar that comes out naturally from a well-tuned Tanpura.
It is claimed that some harmonists use the control of air flow through the bellows and pressure on the keys, to bring the tempered notes in the instrument closer to the highly precise natural notes.
However, the actual measurements on the Electronic Musical Analyzer shows the reality that any amount of the control of air-flow or finger-pressure cannot reduce the frequency of a vibrating reed from 26 % to 25 %.
4. Few artists never accepted ET scale :
There have been many vocalists with a very sharp and sensitive ear, not taking Equitempered Harmonium for their accompaniment, and preferring Sarangi or Violin instead.
While there was no guarantee that the Sarangi or Violin player would play the 22 Shrutis accurately, they still relied on them than the Equitempered Harmonium (which was guaranteed to give wrong notes).
5. The great artists also had no choice:
When I analyzed the actual frequencies sung by some of the above stalwarts by a Computer Software and also my Electronic Musical Analyzer, I found that they were singing the 'Equitempered' notes inadvertently because, Equitempered harmonium was taken for accompaniment ! Although, they still sounded 'Surel (in tune)', they were actually singing 'out of tune' notes of the Equitempered tuning.
These are the reasons why singing by Indian Stalwarts in the tempered scale was appreciated for over a century.
Now, if any artist takes a 22-Shruti Harmonium for accompaniment, he/she will be facilitated to sing the 'real' Shrutis and the 'surelness' (natural melodic content) of the concert will increase multifold.