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 hotel lobby, step by step.
Step 1: the room
A typical hotel lobby of 60 × 40 ft with a 20 ft ceiling has a volume of about 48,000 ft³. With hard and bare surfaces (tile, concrete, glass and little soft furniture), its echo time is about 5.35 seconds. The target for hotel lobbies is 1.00 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 1.00 s, the room needs about 2,355 ft² sabins of absorption in total. It already has about 440 from its surfaces, so the panels must add the rest.
Step 3: what one panel adds
A 60 in round round ceiling cloud, 2 in thick has about 19.6 ft² of face. At NRC 1.00, and allowing for the surface it covers, it adds about 28.0 sabins, including the extra absorption from its back face and edges when it hangs in the air.
Step 4: the answer
About 69 panels bring the room from 5.35 s to 0.99 s.
| Thickness | NRC | Panels needed | Echo time after |
|---|---|---|---|
| 1 in | 0.8 | 87 | 1.00 s |
| 2 in | 1 | 69 | 0.99 s |
| 4 in | 1.15 | 60 | 0.99 s |
| Room | Size | Volume | Echo time now | Panels needed |
|---|---|---|---|---|
| Small | 45 × 30 × 20 ft | 27,000 ft³ | 4.65 s | 38 |
| Typical | 60 × 40 × 20 ft | 48,000 ft³ | 5.35 s | 69 |
| Large | 90 × 60 × 22 ft | 118,800 ft³ | 6.70 s | 178 |
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.