Editing Rocket engine (section)

===Exhaust noise===
{{Main|acoustic signature}}
For all but the very smallest sizes, rocket exhaust compared to other engines is generally very noisy. As the [[hypersonic]] exhaust mixes with the ambient air, [[shock wave]]s are formed. The [[Space Shuttle]] generated over 200 [[dB(A)]] of noise around its base. To reduce this, and the risk of payload damage or injury to the crew atop the stack, the [[mobile launcher platform]] was fitted with a [[Sound Suppression System]] that sprayed {{convert|1.1|e6L|USgal}} of water around the base of the rocket in 41 seconds at launch time. Using this system kept sound levels within the payload bay to 142&nbsp;dB.<ref>{{Cite web
  |title=Sound Suppression System
  |publisher=NASA
  |url=https://www.nasa.gov/mission_pages/shuttle/launch/sound-suppression-system.html
  |access-date=2017-02-09
  |archive-date=2020-08-10
  |archive-url=https://web.archive.org/web/20200810203904/https://www.nasa.gov/mission_pages/shuttle/launch/sound-suppression-system.html
  |url-status=dead
  }}</ref>

The [[sound intensity]] from the shock waves generated depends on the size of the rocket and on the exhaust velocity. Such shock waves seem to account for the characteristic crackling and popping sounds produced by large rocket engines when heard live. These noise peaks typically overload microphones and audio electronics, and so are generally weakened or entirely absent in recorded or broadcast audio reproductions. For large rockets at close range, the acoustic effects could actually kill.<ref name="CR566">R.C. Potter and M.J. Crocker (1966). [https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19660030602_1966030602.pdf NASA CR-566, Acoustic Prediction Methods For Rocket Engines, Including The Effects Of Clustered Engines And Deflected Flow] From website of the National Aeronautics and Space Administration Langley (NASA Langley)</ref>

More worryingly for space agencies, such sound levels can also damage the launch structure, or worse, be reflected back at the comparatively delicate rocket above. This is why so much water is typically used at launches. The water spray changes the acoustic qualities of the air and reduces or deflects the sound energy away from the rocket.

Generally speaking, noise is most intense when a rocket is close to the ground, since the noise from the engines radiates up away from the jet, as well as reflecting off the ground. Also, when the vehicle is moving slowly, little of the chemical energy input to the engine can go into increasing the kinetic energy of the rocket (since useful power ''P'' transmitted to the vehicle is <math>P = F*V</math> for thrust ''F'' and speed ''V''). Then the largest portion of the energy is dissipated in the exhaust's interaction with the ambient air, producing noise. This noise can be reduced somewhat by flame trenches with roofs, by water injection around the jet and by deflecting the jet at an angle.