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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?

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  • $\begingroup$ Keep in mind military jets with afterburners are already insanely loud. They're not LEAP-X's. $\endgroup$ – Harper - Reinstate Monica Nov 25 '19 at 16:33
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This PDF indicates an increase by ~10 dB for an F-8K in afterburner versus the same aircraft in 100% dry thrust.

This PDF indicates smaller increases:

  • +5 dB for an F-15
  • +4 dB for F-22 and F-35
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    $\begingroup$ Worth noting that the decibel is a logarithmic measurement, and that human hearing has a more or less logarithmic response. So, an increase of 3dB indicates a sound of twice the intensity (ie. the actual energy in the sound), while an increase of 10dB will sound "twice as loud" to the human ear. $\endgroup$ – anaximander Nov 25 '19 at 11:41
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    $\begingroup$ @anaximander yep - and also that the human ear's responsiveness varies with frequency. So as per Michael Hall's answer, its possible for louder to merely sound "different" as the pitch/frequency changes. $\endgroup$ – Criggie Nov 26 '19 at 8:04
  • $\begingroup$ @anaximander Yes, hearing is logarithmic. We can hear sounds over a large range of intensity. What is interesting is that we are talking about jet engines that are so loud that they are outside that range as experienced otherwise. I expect, somehow, that hearing two different engines from nearby can not differ in how loud they are because the human sense of hearing goes into saturation, to the maximal perceived loudness. Maybe that is when the ear get's mechanically destroyed. A difference can be always perceived from mechanical effects, of course. $\endgroup$ – Volker Siegel Nov 28 '19 at 0:20
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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. It may have gotten a little bit louder, but what I remember more is that the tone changed. The sound was more "full" when the afterburner was engaged. I realize this is a rather subjective answer.

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    $\begingroup$ That's not subjective, and interesting! I did not anticipate that it could be different with the same volume. It means it is louder at some frequencies, and quieter in others. So the volume difference depends on the method of measurement. $\endgroup$ – Volker Siegel Nov 24 '19 at 22:13
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    $\begingroup$ on a carrier deck, everyone wears a helmet which provides noise reduction - but this reduction is frequency-dependent. That will have influenced your experience. $\endgroup$ – Hobbes Nov 25 '19 at 9:20
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    $\begingroup$ @Hobbes Usually earplugs plus the helmet. The combination provides about 40-50dB attenuation from 100-2000Hz, and about 60-65dB attenuation above about 2500Hz. That certainly distorts the perception of the sound. This paper studied the F-22 and found about a 5dB increase in sound pressure with reheat on. I suspect the F-22 is probably quieter than older generation aircraft - certainly at airshows I've often noticed the afterburner being considerably louder than mil thrust - especially when facing the business end. $\endgroup$ – J... Nov 25 '19 at 15:31
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    $\begingroup$ @J... Your comment could be an answer. It provides a clear answer (yes, by 5dB for the F22) with link to support it and for further reading. $\endgroup$ – Manu H Nov 26 '19 at 7:01
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    $\begingroup$ @VolkerSiegel Nitpick: This is the textbook definition of subjective. $\endgroup$ – JollyJoker Nov 26 '19 at 8:54
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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. 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.

The flow out the nozzle may be just over mach at military power, but will be well over Mach 2 with reheat on. 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.

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). The extra sound that results is a given.

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  • $\begingroup$ I used to live near Farnborough - under the flight path in fact. I always got a good show and can definitely confirm that it's significantly louder. $\endgroup$ – Baldrickk Nov 25 '19 at 12:01
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To explain if the afterburner makes the engine louder, you must understand what the afterburner does. In the afterburner, the exhaust gases are re-heated by injecting fuel in the afterburner duct. The left oxygen is used to burn the fuel, which results in an increased exhaust gas flow. 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.

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. 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.

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.

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    $\begingroup$ Nice first post! Welcome to Aviation.SE, I hope you stick around. $\endgroup$ – AEhere supports Monica Nov 26 '19 at 8:13
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From an energy standpoint, the engine produces heat, thrust, and less significantly, sound. Ignore the afterburner for a second and just consider throttling up, whether a jet or your car. The engine gets louder. That's not a law of physics, that's just what happens. 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. But practical combustion engines get louder (both overall and to the human ear) as they burn fuel faster.

You would be surprised if throttling up made your engine quieter.

The same goes for increasing fuel burn via afterburner, only more so. For one thing the afterburner is inefficient, so there is a higher proportion of waste energy to dissipate. For another the afterburner noise happens later and is less controllable, even if there was any desire to do so.

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  • $\begingroup$ An ICE may not be a good comparison since much of the noise is caused by mechanical movements of the engine (pistons, camshafts, lifters, belts, etc.). However, the reason more energy => more sound for a jet is entropy. A cloud of combusting gases cannot be expected to form a noise-cancelling system, nor is it reasonable to expect that such a system is feasible. The sound volume is directly related to the velocity of the exhaust stream as well as vibrations induced in the mechanical parts of the engine (compressors, turbines, etc.). $\endgroup$ – Lawnmower Man Nov 26 '19 at 2:12
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If you are asking if the engine is louder, no it is not. The afterburner and the diversion of the exhaust is what makes it louder. Just like in a car motor running straight pipe to gain horsepower. You can take the same motor and put an exhaust system on it and it will give that motor a different tone.

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    $\begingroup$ Isn't an afterburner part of the engine? It's just a second fuel injection. The nozzle is after it, and a relevant part of the engine. $\endgroup$ – Volker Siegel Nov 25 '19 at 8:09
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    $\begingroup$ Hi and welcome to Aviation.SE! Your exhaust pipe analogy is not really applicable to an afterburner because exhaust pipes are a net energy sink while afterburners add energy to the system by working as a rocket engine of sorts. $\endgroup$ – AEhere supports Monica Nov 25 '19 at 8:27

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