RWC

The RWC (Real World Computing) Music Database is a copyright-cleared music database (DB) that is available to researchers as a common foundation for research. It contains around 100 complete songs with manually labeled section boundaries. For the 50 instruments, individual sounds at half-tone intervals were captured with several variations of playing styles, dynamics, instrument manufacturers and musicians.

Source: https://staff.aist.go.jp/m.goto/RWC-MDB/ Image Source: http://www.cs.tut.fi/sgn/arg/matti/demos/basstrans/