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In reading an answer here: How close can I get to a rocket launch and not get killed?, the comment is made that noise from the launch can be tremendous. This isn't a big surprise to anyone who's watched a launch on video, or especially has had the privilege of watching one live. So, fundamentally, why are rocket launches so loud? What components of a rocket motor firing contribute to the tremendous noise? I understand that a tremendous amount of energy is being expended in a very short timeframe and concentrated area (rocket nozzle).

This article indicates that a typical nozzle velocity is 2802 m/s, which is around Mach 8. I'm sure the shock wave of that nozzle velocity (and the 2.8 km/sec escape exhaust velocity) is a significant contributor to the noise, but are there other significant contributors to launch noise?

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  • $\begingroup$ At some level this is simply an inevitable result of the second law of thermodynamics. A rocket engine is a type of heat engine, and the second law implies a strict limit on the efficiency of heat engines. That means some energy must be wasted. If that waste energy is generated, where is it going to go? Sound waves efficiently and rapidly transport energy away. $\endgroup$ – Ben Crowell Aug 26 '16 at 3:51
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From Prediction of Acoustic Loads Generated by Propulsion Systems

The primary noise source during rocket engine testing is the jet noise generated by the rocket exhaust plume . The jet noise is produced directly from the formation, propagation and dissipation of vortices or eddies that are formed in the shear layer of the plume. These eddies are very small in size near the nozzle exit where they originally form and as they propagate along the plume they become larger until they eventually dissipate. Once the sound waves that are created from the eddies begin to propagate through the air and spread out over an area they create pressure changes that will then be picked up by the microphones. Since these sound waves vary in frequency, a spectrum of varying sound pressure levels will be generated along the length of the plume.

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