room acoustics

you almost never hear a sound. you hear the sound plus the room — thousands of copies of it arriving late off every surface. the room is most of what you think a voice sounds like.

— the sound that goes straight to you— the first bounce off each wallclick to move yourself around the room

the same room, from the side

the plan above can't show the two nearest surfaces. in a normal room the floor and ceiling are both closer than any wall, so one of them is almost always the first reflection to land — which one depends on whether the ceiling is lower over your head than the floor is under your feet.

— direct— off the floor— off the ceilingsource at 1.5 m, ears at 1.6 m

what a clap looks like here

every spike is one arrival. the tall one is the direct sound; the rest got there late by bouncing. after a few dozen they stop being countable and become a decaying hiss.

RT60

0.45 s

volume

54 m³

first bounce — off the ceiling

2.4 ms

surfaces

furnished

a normal living room: enough tail to sound alive, not enough to smear words

at 2 ms the bounce arrives inside the window where your ear merges it with the direct sound rather than hearing two events. you don't notice an echo; you just notice the room sounds bigger.

width — 5.0 mdepth — 4.0 mheight — 2.7 mabsorption — 0.22

the clap is run through an impulse response built from this exact room — the reflections above, plus a tail decaying at the rate sabine predicts. change the walls and the sound changes with them.

why bathrooms make everyone a singer

tile absorbs almost nothing — about 3% of the energy per bounce, so sound rattles around for a second or more before it dies. that tail arrives just late enough to blur the wobble in your pitch and glue your notes to each other. carpet and curtains swallow closer to half per bounce, which is why a living room is honest about how you sound.

the floor and ceiling are the reflections nobody thinks about and everybody hears. they land a millisecond or two behind the direct sound — far too close to register as an echo, but close enough that the two copies add and cancel frequency by frequency, notching holes right through the tone. that comb of holes is a lot of why a voice sounds different on tile than on carpet, and it's why a studio gets an absorber on the floor between the speakers before anything else in the room is touched.

sabine worked this out around 1900 by hauling seat cushions into a lecture hall and timing the decay by ear with a stopwatch and an organ pipe. the constant 0.161 is what fell out. it's the same number in the readout above.