The number of panels depends on three things: the room's volume, how much its surfaces already absorb, and how short you want the echo to be. Here is the calculation for a typical library, step by step.
Step 1: the room
A typical library of 60 × 40 ft with a 12 ft ceiling has a volume of about 28,800 ft³. With typical surfaces (drywall, a rug and some furniture), its echo time is about 1.96 seconds. The target for libraries is 0.60 seconds.
Step 2: how much absorption is missing
Sabine's formula says RT60 = 0.161 × V ÷ A, with V in cubic meters and A, the total absorption, in square-meter sabins. To reach 0.60 s, the room needs about 2,355 ft² sabins of absorption in total. It already has about 720 from its surfaces, so the panels must add the rest.
Step 3: what one panel adds
A 48 in round round ceiling cloud, 2 in thick has about 12.6 ft² of face. At NRC 1.00, and allowing for the surface it covers, it adds about 17.0 sabins, including the extra absorption from its back face and edges when it hangs in the air.
Step 4: the answer
About 97 panels bring the room from 1.96 s to 0.60 s.
| Thickness | NRC | Panels needed | Echo time after |
|---|---|---|---|
| 1 in | 0.8 | 124 | 0.60 s |
| 2 in | 1 | 97 | 0.60 s |
| 4 in | 1.15 | 83 | 0.60 s |
| Room | Size | Volume | Echo time now | Panels needed |
|---|---|---|---|---|
| Small | 45 × 30 × 12 ft | 16,200 ft³ | 1.77 s | 52 |
| Typical | 60 × 40 × 12 ft | 28,800 ft³ | 1.96 s | 97 |
| Large | 90 × 60 × 14 ft | 75,600 ft³ | 2.47 s | 277 |
These are estimates for speech frequencies (500–1,000 Hz). Use the room calculator with your own measurements, then load the result into the designer.