Absorption, diffusion and isolation
"Acoustic treatment" covers three different jobs that are often confused. Each needs different materials, and using the wrong one wastes money.
1. Absorption
What it does: removes energy from sound inside the room, which shortens the reverberation and reduces reflections.
How: porous materials such as mineral wool, foam and fabrics, and resonant absorbers such as membranes and perforated panels, see the absorber sizing tool.
What it does not do: stop sound from passing through walls. Lining a room with absorbers lowers the sound level inside by only a few decibels, since the direct sound remains.
2. Diffusion
What it does: scatters reflected sound in many directions, so that the room sounds lively and spacious without strong individual reflections or echoes.
How: surfaces with a pattern of varying depth, such as quadratic residue diffusers, or irregular shapes, see the QRD diffuser designer. The structure must be comparable in size to the wavelength it is meant to scatter.
What it does not do: reduce the total sound energy. A diffuser reflects almost all of it, only in different directions.
3. Isolation (sound insulation)
What it does: reduces the sound that passes from one space to another.
How:
- Mass. Heavier walls transmit less. By the mass law, the insulation rises by about 6 dB for each doubling of mass per area, and of frequency.
- Decoupling. Separating the two sides of the wall, with double leaf construction and resilient mounts, stops vibrations from travelling through.
- Sealing. A small gap lets sound through much more easily than the wall allows, so doors, joints and ducts matter.
- Absorption in the cavity of a double wall damps the resonance.
What it does not do: improve the sound inside the room.
Comparison
| Goal | Absorption | Diffusion | Isolation |
|---|---|---|---|
| Shorter reverberation | yes | slightly | no |
| Fewer strong reflections | yes | yes | no |
| Keep a lively sound | no, reduces it | yes | no |
| Keep neighbours happy | no | no | yes |
| Cost per effect at bass | high (thick) | high | high (heavy) |
Typical mix-ups
- "I put foam on the wall to soundproof my studio." Foam is absorption.
- "The room echoes, so I will add a heavy wall." Mass is for isolation, and the echo stays inside.
- "I will make the room completely dead." Over-absorbing sounds unnatural, see Acoustic treatment myths.
Order of work
For a new studio, plan isolation first, since it is built into the structure. Then deal with room shape and modes, see Standing waves: from strings to rooms, then absorption and finally diffusion to taste.
Draft: written by an AI assistant, awaiting technical review.
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