Absorption panels

Panels are often sold as “soundproofing”, which causes more disappointment than anything else in this industry. Absorption and isolation are different problems, measured differently and solved differently. A panel reduces the sound reflecting around inside a room: the echo, the harshness, the difficulty of following speech when the room is full. Stopping sound passing between two spaces is a matter of mass, separation and sealing, and belongs to the structure.

Both may be needed in the same room. A boardroom that echoes and can also be heard from the corridor needs treatment on its surfaces and work on its partitions and door. Deciding which is which is the first thing we do on site, because spending the budget on the wrong one produces a room that is no better and a client who believes acoustics does not work.

Sound entering a porous absorber sets the air in its pores moving. Friction against the fibres converts that motion into a small amount of heat, and what returns to the room is quieter. How well this happens at a given frequency depends chiefly on the thickness of the absorber, and on how far it stands off the wall behind it.

• Thickness — the deeper the absorber, the further down in frequency it works. Thin panels tame hiss and sibilance; low frequencies need depth.
• Air gap — spacing a panel off the wall extends its reach downwards without adding material.
• Density — too light and the panel is transparent to sound; too dense and sound reflects off its face instead of entering.
• Facing — fabric, perforated board or slats must be open enough to let sound reach the core.
• Placement — panels in the right position do several times the work of the same area spread evenly.

Panels are made to suit the room and the interior rather than pulled from a single catalogue. The usual constructions are below; all can be made to size, and to a shape where the architecture asks for one.

The quantity is arithmetic, not estimation. We measure the reverberation in the room as it stands, agree a target for what the room is for, and calculate the absorption needed to get from one to the other. That gives an area, a construction and a placement — and the same measurement at the end shows whether the room met the target.

1. Measure the room as built, across frequency.
2. Agree the target: speech clarity, music, or a compromise between them.
3. Calculate the absorption required, by frequency band.
4. Place it where it does the most work, and draw it against the interior.
5. Measure again on completion, and report both figures.

Numbers before adjectives. A panel layout is arithmetic against a target, not a guess at how much wall to cover.

• For speech — the first reflection points either side of the talker, and the ceiling above the listeners.
• For music — the rear wall, to stop a slap returning to the stage, and the corners, where low frequencies gather.
• Open-plan offices — over the desks rather than on distant walls, so speech is absorbed before it travels.
• Classrooms — ceiling first; it is the largest surface and the least likely to be damaged.
• Restaurants and lobbies — high on the walls and across the soffit, out of reach of furniture and people.

Finish is chosen with the interior: fabric colour and weave, timber species, paint, or printed faces. Where a panel sits in a public building, the fire performance of the facing and the core are specified with the construction, not chosen afterwards. In gyms, corridors and schools we use reinforced facings, because a panel that is damaged in a year is not economical however well it performed on day one.

• Room dimensions, including height, and what the surfaces are made of.
• What the room is used for, and by how many people at once.
• A drawing or photographs, if you have them.
• Any finish the interior is already committed to.
• Whether the room is in use, and when work can be carried out.

Panels are one part of the picture. If sound is arriving from elsewhere, see doors, windows and partitions and vibration control; if the room has no ceiling to work with, see acoustic beams.