Soundproofing a home office: seal the door before buying foam

Foam panels do almost nothing for the sound your neighbour hears, and almost everyone buys them first. The leak is at the door and it costs a tenth as much to fix.

The foam did almost nothing

Hana bought six foam panels before she bought a door seal, because the panels were visible and the seal was not. The panels changed the sound of her own voice.

Her neighbour’s television was exactly as loud after the panels went up. The panels absorbed reflections inside the room and left the wall untouched.

She had treated the room she could see instead of the path the sound took. That path was a 14 mm gap under the door.

Absorption is not isolation

Absorption removes energy from sound already in the room. Isolation stops energy crossing a barrier, and the two jobs use different materials.

Foam is light and porous, which makes it good at the first and useless at the second. A heavy curtain does more for the second and less for the first.

Hana needed both treatments and had bought only one of them. The cheaper of the two was the one she had skipped entirely.

The gap under the door

A door with a 14 mm gap along the bottom leaks more sound than the door itself passes. The gap is a slot the width of the doorway.

Hana measured hers with a piece of card and found 16 mm at the hinge side. The door had been trimmed to clear a carpet that was gone.

A brush strip and a threshold seal closed it in an hour. That hour did more than the six panels and cost a fifth as much.

Measuring the leak with a phone

Play a steady tone on one side of the door and record it on the other with a phone. Move the phone along the frame and watch where the level rises.

Hana used a 500 Hz tone and found two hot spots at the top corners. The frame had shrunk away from the wall on one side.

The measurement took ten minutes and told her exactly where to spend. Without it she would have sealed the bottom and stopped there.

The door itself, hollow or solid

A hollow core door is two thin skins over a cardboard honeycomb and weighs about 15 kg. A solid core door of the same size weighs twice that.

Weight is the proxy for isolation in a door, and the difference is audible at speech level. Hana’s door was hollow and rang when knocked.

She replaced it with a solid core slab and kept the frame. The door cost less than the panels she had already bought.

Sealing the frame, not just the bottom

Sound leaves through the perimeter as much as the base, especially where a frame meets old plaster. A self-adhesive seal around the stop closes that line.

Hana ran a seal on the stop and a bead of acoustic sealant where the frame met the wall. The top corners stopped whistling.

The door then became hard to close, which is the sign of a seal working. She adjusted the strike plate and it latched again.

Why mass beats softness

Transmission loss rises with surface mass, so a heavy barrier stops more than a thick soft one. This is why foam fails and a bookcase works.

Hana put a full bookcase against the shared wall and gained more than the panels had given. The books added mass where sound was leaving.

The panels still hang on the opposite wall, where they reduce the slap of her own voice. They are in the right place for the wrong reason.

The wall that carried the sound

The shared wall was a plastered brick partition, better than stud and worse than concrete. Sound crossed it as vibration and re-radiated in her room.

She could hear low frequencies from the neighbour’s kitchen and nothing above them. Low sound travels through structure, and no seal touches it.

That part was not solvable from her side. She moved her desk to the far wall and put the bookcase where the desk had been.

What the foam is still good for

The panels do reduce the flutter between two parallel plastered walls, which makes calls sound less hollow. That is a real gain and worth keeping.

Hana’s callers noticed the difference weeks before her neighbour did. Her microphone had stopped picking up the ring of the empty room.

Treating a room for recording and treating a door for privacy are separate projects. Most people start with the second and buy the first.

Ventilation after sealing

A sealed door turns a room into a box, and a box with a person in it needs air. Hana’s carbon dioxide reading passed 1,500 after an hour.

She opened the window for ten minutes every two hours and the reading halved. The seal stayed and the window did the ventilation.

Soundproofing and ventilation pull against each other, and the window is where that compromise happens. A sealed room needs a ventilation schedule.

The neighbour conversation

Hana spoke to the neighbour after the door was replaced and learned the television sat against the shared wall. He moved it 40 cm and the bass dropped.

That conversation did more than any material she could have hung. It also cost nothing and took four minutes in a hallway.

Sound problems are described as building problems and are often furniture problems. The other side of that wall has furniture in it too.

The order she would do it in

Next time she would measure the gaps, seal the door and replace the door before buying anything soft. Materials follow the leak and not the reverse.

The panels would come last and only for the microphone. That order would have saved her two months and most of the spend.

The room is quiet enough now that she forgets the neighbour exists. What she remembers is six foam panels in a cupboard.

Before you buy another foam panel, have you put your hand along the bottom of the door and felt the air moving through?