1
$\begingroup$

In general, jet engines on passenger aircraft make so much noise that in a long-duration flight it becomes really tiresome and uncomfortable. Nowadays, technology is much more advanced than before. Manufacturers have added many features to control the noise. Then, why do jet engines still produce so much noise?

$\endgroup$
6
  • 7
    $\begingroup$ Does this answer your question? What is the major source of engine noise for non-propeller aircraft? $\endgroup$
    – user14897
    Sep 29, 2021 at 9:29
  • 1
    $\begingroup$ You may be interested in this article which not only explains what are the causes and what was done, but also what is currently researched to improve the situation, known to be unsatisfactory (not the noise within the aircraft, but the noise for overflown inhabitants - but solving this problem would benefit to passengers too). Introduction: "noise suppression has become one of the most important fields of research due to airport regulations and aircraft noise certification requirements." $\endgroup$
    – mins
    Sep 29, 2021 at 14:20
  • 7
    $\begingroup$ Because physics. Understanding the physical causes of the noise is the only way to address this question. If a qualitative explanation is not enough to satisfy you, the next step would be to quantitatively model these causes and see what levels of noise they produce. If you don't want to dig that deep and still feel you don't have a satisfactory answer, then you would just be arguing against physics. $\endgroup$
    – sdenham
    Sep 29, 2021 at 15:06
  • 2
    $\begingroup$ I actually find modern aircraft engines (A380 and newer) to be TOO quiet. You can hear a little kid carrying on 10 rows back - give me a smooth engine roar any day. $\endgroup$
    – Ben
    Sep 30, 2021 at 0:11
  • 2
    $\begingroup$ Respectfully, "because physics" sounds to me like a demeaning insult to the OP, and it also seems to imply that the problem of noise on an aircraft is an unsolvable problem, similar to "Why can't we travel faster than light? Because physics." It might be impossible from preventing a fast-moving jet aircraft from being noisy on the outside "because physics", but the noise on the inside could surely be reduced by adding more insulation, although doing so might not be economical. But "to add more sound insulation wouldn't be economical" isn't covered by "because physics". $\endgroup$
    – rclocher3
    Sep 30, 2021 at 23:10

2 Answers 2

3
$\begingroup$

Briefly: inside a jet engine's core or within its fan shroud, huge quantities of air get sucked through while their flow directions get rapidly changed up. This causes parcels of air to bounce off the walls of the engine passages and off their neighbors, a process that generates random (roaring) noise.

In addition, in the center of the engine's core, flammable fuel is continuously injected and vigorously mixed with air, and then set on fire. This too creates a roaring and hissing noise.

The compressor and turbine blades tend to shed sound waves as long as they are spinning around. For slow speeds (like a 2-bladed helicopter rotor, for example) each full rotation of the rotor sends two sound waves your way and you hear a whap-whap-whap noise. For lots of blades spinning at very high speeds, a screaming-siren noise is produced instead. On top of this, the fan on a high-bypass turbofan will emit a groaning moan when it comes up to full speed.

Then, at the exhaust port of the engine, the bypass, core, and ambient air flows get violently mixed which adds to the hiss and roar of the engine.

$\endgroup$
4
  • $\begingroup$ Did you mean two sound waves as broadband and rotational? $\endgroup$
    – Auberron
    Sep 30, 2021 at 4:48
  • 1
    $\begingroup$ A large part of the engine noise perceptible outside the engine is not created in the engine, but at the exhaust by the vortexes generated when the two engine flows are released in the free stream. All velocities and temperatures are different, air cannot mix smoothly. This is why some manufacturers pre-mix the 3 flows using serrated ducts. What is heard outside the engine is first the noise of turbulent air. $\endgroup$
    – mins
    Sep 30, 2021 at 16:59
  • 1
    $\begingroup$ @mins, thanks, will edit. -NN $\endgroup$ Sep 30, 2021 at 17:04
  • $\begingroup$ See also this article with nice pictures. $\endgroup$
    – mins
    Sep 30, 2021 at 17:09
-3
$\begingroup$

A turbine rotor fan spins at 10,000 RPM. With a diameter fan of 123 inches, it has a circumference (Pi x diameter) of 386 inches meaning it spins so fast on it's outer circumference it goes thru the air at 3,600+ miles per hour (Mach 4.8) , that's a huge amount air displacement and sound comes from air displacement

$\endgroup$
5
  • 4
    $\begingroup$ Mach 58!? Rule of thumb: if you get a result > Mach 10, something is wrong with your calculation or this belongs on Space.SE. I get 3655 mph for your numbers. Converting that to a Mach number requires knowing the temperature (since speed of sound is temperature dependent). $\endgroup$
    – Bianfable
    Sep 30, 2021 at 7:04
  • 3
    $\begingroup$ Your calculation went terribly wrong somewhere along the way, I'd blame it on the funny units. When the fan has a diameter 312 cm, it has a circumference of 9.81 m, meaning it spins so fast that the tips move at 1636 m/s, which is approximately Mach 4.77 at sea level conditions (speed of sound 343 m/s). $\endgroup$
    – DeltaLima
    Sep 30, 2021 at 7:15
  • $\begingroup$ My mistake i forgot the decimals and forgot to divide 10,000 rpm by minutes. Pi x Diameter = circumference π x 123 inches of a GE90 engine fan. circumference is 386.4 inches (32.2 feet) with a revolution of 10,000 rpm (166 per second) to complete a revolution of 32.2 feet 166 times. $\endgroup$
    – LazyReader
    Sep 30, 2021 at 7:27
  • 5
    $\begingroup$ Your RPM is wrong for that diameter. Pick any random type certificate for a turbofan, and you'll find the fan blade tip at max RPM at around Mach 1.2 after you use realistic numbers. $\endgroup$
    – user14897
    Sep 30, 2021 at 8:00
  • $\begingroup$ Also don't forget a turbofan doesn't run at the max permissible RPM or even 100% N1 for the majority of a flight, and the sound absorbing role of the inlet. $\endgroup$
    – user14897
    Sep 30, 2021 at 9:39

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .