How do i detect an impact?

I'm trying to make a physics-based bridge with a func_physbox, and i want it to play a sound when it hits the ground. How would i do that?

I tried using a trigger_once, but the trigger activates way before the impact (Screenshot, please excuse the horrible textures, it was just a test map)

I even tried using a filter_activator_name to see if that would fix the trigger, but that didn't help either.

I literally can't find anything online about triggers activating too early, could it be because the bridge is a group, which messes up the collisions?

Any help would be appreciated!

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EDIT: I'm making a Garry's Mod map, in case that helps.

SOLVED: The fix was to just make the trigger extremely small. Thank you for all the help, and thank you u/DimasDSF for solving it!