Why an empty room echoes and a furnished one doesn't

Quick answer: Sound bounces off hard, smooth surfaces and keeps bouncing until something soaks it up. Rugs, curtains, upholstery and people soak it up. Bare drywall, glass and concrete don't. Fix the echo by adding soft surfaces, not by hanging things on the walls.

The first night in the new house

Every buyer knows it. The movers leave, you say something to your wife from the other end of the living room, and your own voice comes back at you. Footsteps sound like a drum line. Two weeks and one couch later, the room is quiet and nobody remembers it was ever a problem.

Nothing about the walls changed. The room just got a lot better at swallowing sound.

What sound is doing in there

Sound is a pressure wave moving through air at about 1,125 feet per second at room temperature. When it hits a wall, most of the energy bounces back. A little passes into the wall, and a little turns to heat. Hard, dense, smooth surfaces like painted drywall, tile, glass and poured concrete send nearly all of it back into the room.

So the wave keeps going. It hits the opposite wall, the ceiling, the floor, and comes back again. Each trip loses a small slice of energy. In a bare room that slice is tiny, so the sound hangs around for a long time.

An echo and a reverb aren't the same thing

People use the words as if they meant one thing. They don't.

An echo is a distinct repeat. Your ear merges reflections that arrive within a few dozen milliseconds of the original, so they just sound like the same sound being a bit fuller. Once a reflection lags by roughly 50 milliseconds, you hear it as a separate event. At 1,125 feet per second, that means the reflected path has to be about 56 feet longer than the direct one. That's a wall roughly 28 feet away. You get true echoes in canyons, big empty halls and stadiums.

Reverberation is what an ordinary room does. Thousands of reflections arrive so close together that they smear into one fading wash. That's the "echoey" sound in your empty living room. It isn't a clean repeat. It's a tail.

Engineers measure that tail with a number called RT60: the time it takes a sound to drop by 60 decibels, which is roughly from loud to inaudible. Short is dry and clear. Long is wet and boomy.

The man with the seat cushions

Around 1900, Harvard asked a young physicist named Wallace Clement Sabine to fix a lecture hall where speakers couldn't be understood. The reverberation was the culprit. Sabine borrowed seat cushions from a nearby theater, carried them in and out of the room, and timed how long an organ pipe's tone lingered. He did it at night, with a stopwatch, because the streetcars outside ruined his daytime measurements.

What he found became the basic rule of room acoustics. Reverberation time goes up with the size of the room and down with the amount of absorption in it. In American units:

RT60 ≈ 0.049 × volume (cubic feet) ÷ total absorption (sabins)

One sabin is one square foot of a surface that absorbs everything that hits it, such as an open window. Everything else is counted as a fraction of that.

A worked example, with round numbers

These coefficients are approximations for mid-range frequencies, and real materials vary. But the arithmetic shows why furniture matters.

Take a 20 by 15 foot room with 8 foot ceilings. That's 2,400 cubic feet of air and about 1,160 square feet of surface. Bare painted drywall and a hard floor absorb about 5 percent of the sound that hits them. Total absorption: about 58 sabins. RT60 comes out around 2 seconds. That's a gym.

Now lay carpet with a pad over the 300 square foot floor, absorbing perhaps 30 percent. Absorption jumps to about 133 sabins, and RT60 falls to under a second. One purchase cut the ringing by more than half.

Add a couch, curtains and a few people, who are surprisingly good absorbers because clothing and bodies are soft, and you're in comfortable territory. Which is why a restaurant sounds fine at 5 p.m. and like a train station once it's full. Wait, that's backwards. Crowds absorb, but they also make noise, and everyone raises their voice to be heard over everyone else's raised voice. Hard ceilings and bare floors in many restaurants do the rest of the damage. If you've ever tried to hold a conversation with your family at a loud table, you've met that problem.

Why soft things work

Sound doesn't vanish into a curtain. It pushes air back and forth through the tiny gaps between fibers. Friction against those fibers turns a very small amount of the wave's energy into heat. A porous material is a trap for moving air.

That has a consequence people miss. Absorption depends on frequency. High-pitched sound has short wavelengths and gets caught in a thin layer of fabric or foam. Bass has long ones. A 100 Hz note is over 11 feet from crest to crest, and a half-inch of foam barely touches it. That's why the thin foam panels sold online fix the clatter and leave the boom. Real bass control takes thick material, often several inches, usually placed in corners.

Why a hallway buzzes and a bathroom flatters you

Stand between two bare parallel walls and clap. You'll hear a fast buzzing or ringing, like a spring. That's a flutter echo: the sound bouncing back and forth between flat, parallel surfaces dozens of times a second. The fix is easy. Put anything irregular on one of the walls. A bookshelf full of mismatched books works well. Odd-shaped surfaces scatter sound instead of returning it in a straight line.

The shower is a different effect. A small hard room has a short reverb, but it also has resonances. A room of a given length boosts the note whose half wavelength fits that length. For a 10 foot dimension, that's about 56 Hz. Sing near the right pitch and the room amplifies you. That's why you sound great in there, and why you stop sounding great the second you step out.

Radio waves do something similar in cities. Signals bounce off glass towers and arrive late, which is part of why your phone's blue dot jumps downtown. Resonance, the same principle that makes a shower flatter you, is also what bridge engineers spend careers designing against. The bridge piece covers the stakes.

What to do about a room that rings

Start with the big, cheap surfaces.

• A thick rug with a pad underneath, as large as the room allows. Bare floor is usually the biggest reflector.
• Heavy curtains, lined if you can, hung with some fullness rather than stretched flat.
• Upholstered furniture over leather or wood where you can choose. Leather reflects more than most people expect.
• A full bookshelf on a wall that faces another bare wall. It absorbs a little and scatters a lot.
• Soft goods in the kids' space. A playroom with a hard floor and bare walls gets loud fast, and all those toys on tile don't help.

Skip the picture frames. A few frames on a wall do almost nothing.

A garage or basement shop with a concrete floor and bare walls is the toughest case. Pegboard and a rug under your workbench help. So do shelves of cardboard boxes, which are decent diffusers.

Clap once in the middle of the room and listen to how long it hangs on. Then move a rug in and clap again. You just ran Sabine's experiment, minus the streetcars.