Pa public system. Broadcast system. Pa refers to live mixing.

Ob refers to the field signals sent to the control room for relay. Pa is responsible for external sound reinforcement, and monitor is responsible for reverse sound transmission on the stage. The human body can absorb sound, and the smooth walls in an empty room will make the sound sound different.

An empty room without an audience (without sound field treatment) may hum or sound a little mean. Especially drums and bass, so sometimes the reflection problem can't be solved when auditioning. Don't worry too much. When the audience comes, the problem is solved.