madyama shruti

What is Madyama Shruti ?
There are two main types of sruthi, the regular panchama sruthi which denotes the swara 'pa', and the madyama sruthi (ma).

Singing without the pa, so that the tambura or shruti box is set to play only sa, ma, and Sa (instead of s P S). This can be done with raagas that have no pa but which have a ma in their scales. In the sruthi box , you have the adjustments for individual volumes of sa,pa and Sa in Panchama Shruti or sa ma Sa for madhyama shruti.
Choosing 'madyamam' in the sruthi box as a basic note instead of Sa is termed as madyama sruthi.

Which raagas are sung in Madyama sruthi ?
Some songs especially which has the madayama swara sound better if sung in madyama sruthi. To name a few, raagas like Nadanamakriya, Shenjuruti, Navroj and Kurunji are sung in madhyama sruthi.
The range of these ragas is limited and they are mainly panchamantya, dhaivatantya or Nishadantya raagas, i.e the sancharas do not go beyond these swaras. If sung in the regular panchama sruti, the pitch will sound very low and will have less of an impact.
Of late, other ragas like Manirangu, Sindhu bhairavi and Yaman Kalyan are also being sung in madhyama sruti just to add some "flavour" & "effect".