Warmth is a term used to describe a cozy smoothness to the music. Its counterpart may be considered to be brilliance, which refers to a bright, clear, ringing sound. Either one of these qualities is desirable in moderation. If a sound field is too warm, the hall can be undesirably "dark." With too much brilliance, the sound is harsh, brittle, and metallic sounding.
Acousticians have determined that balancing warmth and brilliance is achieved by balancing the ratio of low frequency Reverberation Time (RT) to high frequency RT. Thus, the Bass Ratio (BR) has been suggested as an objective measure of warmth:
where the numerator is an average of RT's measured in the 125 Hz and 250 Hz octave bands, and the denominator is an average of RT's measured in the 500 Hz and 1000 Hz octave bands.
A bass ratio between 1.1 and 1.25 is desirable in halls with a high RT, and a bass ratio between 1.1 and 1.45 is recommended for any hall with an RT of 1.8 sec or less.
To increase the warmth, one should increase the low frequency RT while maintaining or decreasing mid to high frequency RT. One way to do this is to add materials in the space which absorb energy at high frequencies better than at low frequencies. However, the balance between warmth and brilliance should be kept in mind; excessive high frequency absorption will reduce brilliance unfavorably.