A museum conservator uses a pulse-echo instrument on a laminated wooden panel made of two sections, A and B. The instrument sends an ultrasound pulse from the outer face of section A. The pulse crosses section A and then section B, reflects from the far face of section B, and returns to the instrument along the same path.
| Section | Thickness | Speed of ultrasound |
|---|---|---|
| A | 12 mm | 3000 m/s |
| B | unknown | 4000 m/s |
The echo is detected 20 μs20\ \mu\mathrm{s}20 μs after the pulse is sent.
What is the thickness of section B?
18 mm18\ \mathrm{mm}18 mm
24 mm24\ \mathrm{mm}24 mm
40 mm40\ \mathrm{mm}40 mm
48 mm48\ \mathrm{mm}48 mm