The bass ratio is the relation between the reverberation energy in the frequency range up to 125Hz and in the frequency range between 500Hz and 1000Hz.
bass ratio formula |
bass ratio range 125Hz/250Hz + 500Hz/1000Hz |
For music the bass ratio should be between 1,0 and 1,3 and for speech it should be between 0,9 amd 1,0.
See: Reverberation Time and Concert Hall Acoustics.