Hard reflective surfaces: painted concrete alpha ~ 0.02; glass ~ 0.03; brick ~ 0.04. Soft: heavy curtains ~ 0.5, 100 mm rockwool ~ 0.9+.
Pressure-doubling at a rigid wall: incident + reflected pressure stack = up to +6 dB SPL right at the surface.
Boundary gain (loading): 1 boundary (floor) = +3 dB, 2 boundaries (floor+wall) = +6 dB, 3 boundaries (corner) = +9 dB for bass.
Standing wave (axial room mode): f = 343 / (2 x L), where L is the distance in metres between two parallel walls.
Example mode: 5 m wall spacing -> 343 / 10 = 34.3 Hz fundamental, with harmonics at 68.6, 102.9 Hz etc.
Phase cancellation comb filter: first dip at f = c / (2 x d), where d is the path-length difference to a reflecting surface in metres.
How it works
Wave travels out from the source until it meets a boundary (wall, floor, speaker mouth).
At the boundary energy splits three ways: reflected back, absorbed as heat, transmitted through.
Hard dense surface = high reflection (low alpha); soft porous surface = high absorption (high alpha).
Reflected waves overlap the direct sound, adding (louder) or cancelling (comb filtering).
Between parallel hard walls, reflections stack into standing waves at specific room-mode frequencies.
Corners load bass from 3 boundaries at once, piling up low-end build-up.
Real examples
Clap in an empty tiled bathroom: hard walls (alpha ~0.02) reflect everything, long ringy echo.
Same clap in a carpeted, curtained lounge: soft surfaces soak it, dead and dry.
Sub shoved into a room corner: +9 dB boundary gain, bass suddenly boomy and uneven.
Mic 1 m from a glass window: reflection combs the vocal, hollow phasey tone.
5 m x 4 m room: 34 Hz and 43 Hz modes make bass loud in some seats, gone in others.
How it helps in live sound
Pull subs OUT of corners unless you WANT +6 to +9 dB of boom; corner-load only if you need cheap SPL.
Keep mics and monitors away from hard reflective surfaces (glass, bare walls) to kill comb filtering.
Hang heavy drapes / deploy absorption (alpha 0.5+) on rear and side walls to tame slap-back echo.
Aim speakers so primary energy hits absorptive areas (people, drapes), not bare parallel walls.
Watch RT60: target ~0.8-1.2 s for speech clarity in a function room; tiled venues run way longer.
Use room-mode math (343 / 2L) to predict bass nulls, then move sub or seating off the null.
Everyday analogy
Like a ball rolling across a room: it bounces hard off a wall, dies in a cushion, or rolls out a doorway, and the edge decides which.
Watch out
Myth: a boundary just makes sound 'echo'. Reality: it splits energy into reflected + absorbed + transmitted, and a rigid wall can ADD up to +6 dB and create standing waves, not merely echo.
Fun fact
Stand right against a hard wall and bass gets up to +6 dB louder for free: incident and reflected pressure waves stack at the surface, which is why DJ subs against a back wall always sound bigger.
Key takeaways
A boundary splits wave energy: reflect + absorb + transmit, always summing to the incident energy.
Hard = reflect (low alpha), soft = absorb (high alpha); alpha runs 0 to 1.
Rigid walls double pressure: up to +6 dB SPL right at the surface.