Forum: aviation.stackexchange.com
Tags: <aerodynamics><jet-engine><fighter><supersonic><noise>
Title: Is an afterburner louder than the same jet engine without it?
Question: I would speculate that an afterburner increases the noise because it is nearer to the exhaust. But it certainly causes so many changes in the exhaust flow that it could be less loud as well. It may also depend on the shape of the nozzle.  That an afterburner increases the spacial volume of exhaust gases does not necessarily mean that the sound volume increases as well, because it strongly depends on the level of turbulence.  Does activating an afterburner make a jet engine louder? And why?

Answers:
sents: text: This PDF indicates an increase by ~10 dB for an F-8K in afterburner versus the same aircraft in 100% dry thrust.
text: This PDF indicates smaller increases: +5 dB for an F-15 +4 dB for F-22 and F-35
answer_details: author: aviation.stackexchange.com/users/11395/Hobbes
score: 29
sents: text: It has been around 20 years since I've been on a carrier deck, but I recall that it wasn't as dramatic of an increase as you might think.
text: It may have gotten a little bit louder, but what I remember more is that the tone changed.
text: The sound was more "full" when the afterburner was engaged.
text: I realize this is a rather subjective answer.
answer_details: author: aviation.stackexchange.com/users/22182/Michael Hall
score: 24
sents: text: I would say definitely yes, because of all the extra energy added to the exhaust flow and it's obvious to anyone who attended enough military airshows.
text: Watch an F-16 depart with reheat on, then reduce thrust to military power (max thrust with reheat off) on the climb out, and it almost sounds like the engine flamed out.
text: The flow out the nozzle may be just over mach at military power, but will be well over Mach 2 with reheat on.
text: The nozzle changes shape with afterburner to manage all the extra energy and pressure, from a straight convergent duct to a convergent/divergent duct, like a rocket exhaust bell.
text: Although he doesn't cover the noise issue, this video explains why the nozzle has to change shape to control the mass flow through the engine because of all the heat energy added (math warning to those put in a catatonic state by arcane formulae).
text: The extra sound that results is a given.
answer_details: author: aviation.stackexchange.com/users/30504/John K
score: 14
sents: text: To explain if the afterburner makes the engine louder, you must understand what the afterburner does.
text: In the afterburner, the exhaust gases are re-heated by injecting fuel in the afterburner duct.
text: The left oxygen is used to burn the fuel, which results in an increased exhaust gas flow.
text: Note that the engine itself will not spool up faster: this is done by opening the exhaust nozzle; without opening the nozzle, the pressure would be too high and the fan would stall.
text: The extra gases leaving the engine produce a higher velocity jet stream; more mass and more velocity will yield more thrust, which is the purpose of afterburning fuel.
text: If you look at jet noise modelling , you will find that the formula to calculate the jet noise includes the exhaust velocity (to the power of 8), so increasing the fuel flow and increasing the velocity will also increase the noise production.
text: The referenced model has been implemented in our gas turbine simulation program and verified against the noise measurements of a fighter aircraft to find that this model perfectly agrees with the measurements.
answer_details: author: aviation.stackexchange.com/users/29952/0scar
score: 10
sents: text: From an energy standpoint, the engine produces heat, thrust, and less significantly, sound.
text: Ignore the afterburner for a second and just consider throttling up, whether a jet or your car.
text: The engine gets louder.
text: That's not a law of physics, that's just what happens.
text: There's no theoretical reason why the extra waste energy can't go 101% into heat, and -1% into sound (think of noise-cancelling headphones) or into sound at frequencies inaudible to the human ear.
text: But practical combustion engines get louder (both overall and to the human ear) as they burn fuel faster.
text: You would be surprised if throttling up made your engine quieter.
text: The same goes for increasing fuel burn via afterburner, only more so.
text: For one thing the afterburner is inefficient, so there is a higher proportion of waste energy to dissipate.
text: For another the afterburner noise happens later and is less controllable, even if there was any desire to do so.
answer_details: author: aviation.stackexchange.com/users/45327/Justin
score: 1

Summary of the answers:
S0: I would say definitely yes, because of all the extra energy added to the exhaust flow and it's obvious to anyone who attended enough military airshows. It may get a little bit louder, but what is more noticeable is that the tone changes. If you look at  jet noise modelling , you will find that the formula to calculate the jet noise includes the exhaust velocity (to the power of 8), so increasing the fuel flow and increasing the velocity will also increase the noise production.   
Scientifically speaking, the extra energy due to added fuel flow should produce extra noise. However, in practice that is subjective, and  it can be argued that there is only a small increase; the sound mainly changes in pitch.