Low-pass filter
Also said as LPF, High-cut, Muffling
A filter that lets low frequencies through and cuts high ones above a chosen point, the cutoff. Close it and a sound goes dull, as if heard through a wall.
It is the cheapest way to say "this is far away, behind something, or underwater." Moving it over time also changes a sound's energy without touching the volume.
Say it
“When the player pauses, slide a low-pass down to 800 Hz over 200 ms, so the world goes muffled and the menu sounds sit on top of it.”
Where you’ve seen it
Hitman's sound propagation, described at GDC Europe in 2015, treats rooms and portals as a graph and applies a low-pass filter to sounds heard through them.
Not to be confused with volume
Turning down makes everything quieter equally. A low-pass removes the brightness and leaves the weight, which is why a muffled explosion still shakes.