Copy-pasted from another thread about this:
Regarding the sound issue, the transition from air to the body (which, for simplification, we'll call "fat") actually reflects a significant amount of sound.
The formula for transmission of soundwaves from one medium to another is:
T= 1- ((Z2-Z1)/(Z2+Z1))^2
Where Z2 is the acoustic impedance of the medium the sound is trying to enter, and Z1 is the impedance where it's coming from. The impedances are defined as the speed of sound in the medium multiplied by their density.
The acoustic impedance for air at 20 degrees celsius is ~413.3 Nsm^-3 and for fat it's around 1.3 million Nsm^-3 (much much higher).
Plugging these into the equation gives: T= ~0.0013. Meaning 0.13% of the sound gets transmitted into the body and 99.87% is reflected.
This is a reduction of approximately a factor of 770, which equals approximately 29dB. For comparison, a good set of hearing protection is around 30 dB noise reduction rating (NRR).
This doesn't even factor in the absorption of the sound waves by the tissue itself, just the reflection from the transition from air to body.
Whether this means the sound is safe for the baby or not is not something I will speculate on, but I wanted to correct some of the misinformation that has been posted about how the sound will be transmitted to the womb. The sound will be significantly less intense when it reaches the baby's ears, not more.