Room acoustics
First reflection point
The spot on a wall, floor, or ceiling where sound from a speaker bounces once and reaches the listener, arriving just after the direct sound.
A first reflection point is where a single bounce off a boundary lands at the listening position. The sidewalls, ceiling, and floor each have one per front speaker, and the bounce arrives a few milliseconds after the direct sound.
Reflections arriving within about 20 ms combine with the direct sound and smear imaging, so dialogue loses focus and the stereo stage widens uncontrollably. Treating these points is the highest-value change after bass control.
Find them with the mirror trick: sit in the main seat while someone slides a mirror along the sidewall and mark wherever a front speaker appears. Cover those spots with 2 inch or thicker absorption or with diffusion; in a 3 m wide room they usually fall about 1 m ahead of the seats.