Sound from generators outside a parked van can be heard inside the van. The sound enters the van through a metal side panel.
One generator produces a sound with a frequency of 120 Hz.
State the frequency of the sound heard inside the van. Give a reason for your answer.
Two other generators, A and B, are tested one at a time. The table shows the vibration amplitude of the panel for each generator.
| Generator | Panel vibration amplitude in μm\mu\text{m}μm |
|---|---|
| A | 12 |
| B | 30 |
Explain which generator produces the sound with the greater intensity inside the van. Use data from the table in your answer.