Room size and absorption both govern how long reverberant sound takes to build up.

Evidence

The buildup of the reverberant field level: This is the initial portion of the reverberant field and is affected by the room size, which affects the time between reflections and therefore the time it takes the reverberant field to build up. The amount of absorption in the room also affects the time that it takes the sound to get to its steady-state level. … As the rate at which sound builds up depends on the time between reflections and the absorption, the reverberant sound level will take more time to reach a louder level than to reach a smaller one. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1740

Categorization

• DEFINE: Defines the ‘buildup’ phase of the reverberant field, one of three temporal phases of room sound.