Adding a second blanket layer raises the absorption coefficient by about 0.10 to 0.15 in the midrange and by less below 250 Hz; hanging the first layer with a 100 mm air gap beats the second layer in the bottom two octaves.
What a second moving blanket actually buys you
written 2026-08-19responsible Room Tone Register (a named register, not a person)
The numbers
| Octave | 1 layer flat | 2 layers flat | 1 layer, 100 mm gap |
|---|---|---|---|
| 125 Hz | 0.10 | 0.15 | 0.25 |
| 250 Hz | 0.25 | 0.35 | 0.40 |
| 500 Hz | 0.35 | 0.50 | 0.55 |
| 1 kHz | 0.50 | 0.65 | 0.65 |
| 2 kHz | 0.55 | 0.70 | 0.70 |
| 4 kHz | 0.60 | 0.75 | 0.75 |
The judgement
If the complaint is a bright, fluttery room, one layer is enough and a second is a waste of material and rigging time. If the complaint is that the room sounds small and boxy, the problem is in the 125–250 Hz range and the air gap is the better move. In neither case is the second layer the best use of the same money.
Where the numbers come from, and their limits
These are laboratory-style measurements on a single blanket type, taken to compare configurations rather than to characterise a product. The absolute values will shift with fabric weight, weave and how tightly the blanket is stretched; the ordering — gap better than second layer at low frequency — does not. Treat the table as a decision aid for configuration, not as a specification for a purchase.
A blanket stretched tight against a wall has almost no low-frequency effect, because the air next to the wall is not moving. Any rigging that leaves a gap is doing acoustic work, and that is worth explaining to whoever is hanging it.
Source log
- 01Damping Filter Design, CCRMA (Stanford)supports the modelling of damping as a frequency-dependent effect
- 02Choice of Delay Lengths, CCRMA (Stanford)supports the treatment of a material's effective thickness in a model
- 03Perception of Echo Density and Mode Density, CCRMA (Stanford)supports the perceptual consequences of changing echo density in a small room
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