Jet sound contribution

zoki85 · Mar 16, 2007

Typical supersonic airplane in supersonic flight creates 2 shock waves (2 Mach cones):
"N-shaped" shock wave.
How much of the "sound" energy near the shock waves region is contributed by the sound emmited from jet engines work?

minger · Mar 20, 2007

I believe not much at all. The biggest sound in a jet engine (the screech as it goes over you) is from unsteady disturbances on the stators (or something like that, as I've been told).

The shock is merely a discontinuity in the flow, and is caused by the rapid pressure drop,raise,drop. Someone correct me if I'm wrong, but that N shock is considered one shock as well.

On a side note, there are various different sounds emitted from a jet engine, and they all produce characteristically different sounds.

zoki85 · Mar 22, 2007

"N shock" is constituted of two Mach cones separated aproximately by a plane lenght (say 10..20 m in aircraft's vicinity).
Imagine theoretical aircraft with completely silent engines propagating supersonic.It creates two Mach cones.
Real aircrats are also source of a sound created by the work of its' propulsion engines.If you ever heard stationary work of such engines they can be source of a VERY POWERFUL SOUND!
Considering shock wave from the aircraft's nose they don't contribute nothing at all.Considering the shock from tail they can contribute and I think they may contribute quite a lot!
Another thinking is what is contribution in between two Mach cones.
Are my questions clear?

minger · Mar 22, 2007

I forget the exact physics of the particular sounds, but there are two major "sounds" that are emitted by the engine. There is the "screech" you hear as a plane passes directly over you (I'm talking subsonic right now, but it should apply). If you've ever sat near an airport, you know what I'm talking about.

The properties of these unsteady waves allow it to spread at an almost right angle when it leaves the engine. Now, after a couple of seconds, you hear the low rumble. This is a completely different family of pressure waves. Those are higher amplitude, but much lower frequency.

I may be completely wrong, but I think at 50,000ft, the enormous pressure discontinuity would overwhelm effects from the jet engine.

FredGarvin · Mar 22, 2007

The two main contributors to jet engine noise are fan noise and exhaust noise. Fan noise is usually dominated by a single frequency component that is a function of things like tip speed, blade geometry and number of blades. Exhaust noise is highly random and is pretty much as broad band frequency as you can get.

zoki85 · Mar 29, 2007

FredGarvin said: ↑

The two main contributors to jet engine noise are fan noise and exhaust noise. Fan noise is usually dominated by a single frequency component that is a function of things like tip speed, blade geometry and number of blades. Exhaust noise is highly random and is pretty much as broad band frequency as you can get.

Thank you for the reply.
Note that my question wasn't about mechanisms and types of sounds emmited by the engine.
It's about to what extent that sound magnify effect of the shock wave of supersonic aircraft.