SPL ANS BRACING

[ZUaudio 0]

why does box bracing make the box quite?

[dopey 0]

Yes.

[JimJ 11]

Figuring how out something will affect an SPL score is a pretty complex task...anything that lowers a panel's resonance or obstructs airflow could change it...who knows until you get on the meter. It could be that the box's resonance is reinforcing the rear wave in just such a way to contribute to a higher score.

Same thing happens with adding sound deadener - sometimes you see a gain, sometimes you don't.