The bass ratio is the relation between the reverberation energy in the frequency range of 125 Hz to 250 Hz compared to the range of 500Hz to 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.