As stated elsewhere, the soundwaves formed by a rocket launch are powerful enough to do structural damage to the spacecraft during launch. How are astronauts protected against the soundwaves, or why is the crew capsule not affected by the extreme sound, especially at subsonic speeds?
There are several barriers between the engines and the astronauts.
- The capsule walls. The pressure vessel is covered in insulation on both sides. This is mainly heat insulation, but provides a barrier to sound too. This could easily provide 30 dB of reduction.
- The space suit helmet. Again, a hard shell with insulation and padding inside. Another 20-30 dB of reduction.
- They may have noise-insulating headphones (in-ear), which can provide another 20 dB of reduction. Not sure about this, I'll try and find a reference later.
The noise level at the top of the rocket is around 130 dB at launch, dropping quickly as the rocket takes off and the amount of sound reflected by the ground reduces. So layers 1 and 2 are already enough to get to a non-dangerous noise level.
NASA has standards for noise levels (NASA-STD-3001 : Acoustic Limits for Launch, Entry, and Abort Phase) in the cabin during a mission. They indicate a maximum noise level during launch of 105 dBA continuous, 140 dB peak, 150 dB for infrasonic noise (1-20 Hz).
Sound level near the engines is in the region of 200 dB. With the water deluge system in use, sound level in the payload bay peaks at 142 dB.
Noise level at the Shuttle cabin level (outside) is 12 dB lower than at the base of the stack (page 9).