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 classroom, step by step.
Step 1: the room
A typical classroom of 30 × 25 ft with a 9 ft ceiling has a volume of about 6,750 ft³. With hard and bare surfaces (tile, concrete, glass and little soft furniture), its echo time is about 2.66 seconds. The target for classrooms 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 552 ft² sabins of absorption in total. It already has about 125 from its surfaces, so the panels must add the rest.
Step 3: what one panel adds
A 48 × 48 in rectangular panel, 1 in thick has about 16.0 ft² of face. At NRC 0.80, and allowing for the surface it covers, it adds about 12.0 sabins.
Step 4: the answer
About 36 panels bring the room from 2.66 s to 0.60 s.
| Thickness | NRC | Panels needed | Echo time after |
|---|---|---|---|
| 1 in | 0.8 | 36 | 0.60 s |
| 2 in | 1 | 29 | 0.59 s |
| 4 in | 1.15 | 25 | 0.59 s |
| Room | Size | Volume | Echo time now | Panels needed |
|---|---|---|---|---|
| Small | 23 × 19 × 9 ft | 3,933 ft³ | 2.37 s | 21 |
| Typical | 30 × 25 × 9 ft | 6,750 ft³ | 2.66 s | 36 |
| Large | 45 × 38 × 11 ft | 18,810 ft³ | 3.52 s | 107 |
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.