bass register; bass part
Refers to the low-pitched range in music, or the part written for bass instruments or voices.
The cello handles the bass part in this piece.
In the choir, there are few people who sing the bass part.
中音部 is the middle register, between 低音部 and 高音部.
Compound of 低音 (ていおん, 'low sound') + 部 (ぶ, 'part, section').