Of the the 0.2% to 1% of energy that is generated as sound, most of the sound is generated well after the exhaust gases have fulfilled their purpose of thrust:
As a crude analogy, imagine being on roller skates, then taking a bowling ball and throwing it. You will roll backwards on the skates. This is thrust.
Next, a moment later, the bowling ball slams into the ground, generating lots of sound waves. Those sound waves won’t have taken away from the thrust generated earlier.
Similarly, exhaust gases slamming into the atmospheric gases will generate lots of sound, albeit by different mechanisms (outlined in the jet noise Wikipedia article linked above).
Now I do know that there are also tremendous sound waves that are generated inside the bodies of rockets that can damage the rocket and cause it to explode, and they must be mitigated (using fins, baffling, etc.). These internal sound waves are waste energy, but they are a relatively small fraction of the total sound generated.
NASA's launch pad sound suppression system uses water jets to convert sound energy into heat energy:
“This sound energy is sufficient to kill a person or damage the surroundings.
Thus NASA’s launch pads consists of large water tanks. As the rocket is launched these water tanks dumps one million lbs or water onto the launch pad in just forty seconds. When the sound waves meet water, the bubbles of air absorb them. Further these bubbles contract and heat up, hence converting the sound energy into heat energy.”
Geez this question is extremely complex. Sorry I didn’t save links to these sources.
- one source mentions that it’s not possible to distinguish chaotic “turbulent flow” from “acoustics”. Instead, they give up and instead they just measure sound on the outside of the jet flow.
- be wary of different measurement units sound dbA, dbW, or the familiar db.
- SPL vs SL
- The only example I could piece together is that Saturn V consumed up to 200 gigawatts of thermal power. Another source mentions that Saturn V rocket measured 204 dB, which is a supersonic shock wave above the 195dB “sound” limit. More on this later, but this calculation works out to a (temporary) 2 gigawatts of acoustic power, presumably during a detonation event. In other words, 2 GW / 200GW = 1% of the power.
- conflagration vs detonation