An engineering student investigates how the thickness of a sound-absorbing polyurethane foam panel affects the noise level reduction of an acoustic enclosure.
She tests different thicknesses of foam panels placed between a speaker emitting a constant-intensity tone and a sound level meter. For each test, she records the source sound level at the speaker and the transmitted sound level at the meter.
The table below shows her results:
| Thickness of foam (mm) | Source sound level (dB) | Transmitted sound level (dB) | Sound level reduction (dB) |
|---|---|---|---|
| 5 | 95 | 83 | 12 |
| 10 | 98 | 82 | 16 |
| 15 | 96 | 76 | 20 |
| 20 | 94 | 70 | 24 |
| 25 | 97 | 69 | 28 |
| 30 | 95 | 90 |
Calculate the sound level reduction when the thickness of the foam is 30 mm30\text{ mm}30 mm.
The result when the thickness of the foam is 30 mm30\text{ mm}30 mm is anomalous. Suggest a reason why this result appears to be anomalous, referring to the trend shown by the other results.
Describe how the sound level reduction of the enclosure is affected as the thickness of the polyurethane foam increases.