# Can a pilot hear engine > mach 1?

## Main Question or Discussion Point

Can a pilot hear engine > mach 1????

Can a the pilot of an airplane hear the engine when he is traveling greater then the speed of sound?

part 2:

Can the pilot of airplane 1 hear the engine of airplane 2 which is traveling right behind him, both traveling at the same speed, and both greater then the speed of sound?

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Borek
Mentor
Is the pilot inside, or outside of the plane?

Is the pilot inside, or outside of the plane?
We are debating this...can you describe both examples..

rcgldr
Homework Helper
Can a the pilot of an airplane hear the engine when he is traveling greater then the speed of sound?
The pilot mostly hears the engine via vibrations through the aircraft's body, so it doesn't matter what the speed is. From what I've read, there's no abrupt change in the sound of the air going past the air craft, but note that a shock wave is generated at the leading edge (or point) of the aircraft, and that the cockipt is within the boundaries of this shock wave. The shock wave accelerates the air, so much of the relative air flow with the aircraft within the shock wave boundaries is sub-sonic.

Can the pilot of airplane 1 hear the engine of airplane 2 which is traveling right behind him, both traveling at the same speed, and both greater then the speed of sound?
If aircraft 2 was close and within the shockwave cone generated by aircraft 1, it's possible that the pilot of aircraft 1 could hear aircraft 2, because the air is accelerated enough by aircraft 1's shockwave that relative air speed is sub-sonic. If the two aircraft were flying side by side, outside of each others shockwave cone, they would not be able to hear each other. Note, it would probably be dangerous to fly close and within the shockwave zone of another aircraft.

[\QUOTE]If aircraft 2 was close and within the shockwave cone generated by aircraft 1, it's possible that the pilot of aircraft 1 could hear aircraft 2, because the air is accelerated enough by aircraft 1's shockwave that relative air speed is sub-sonic. If the two aircraft were flying side by side, outside of each others shockwave cone, they would not be able to hear each other. Note, it would probably be dangerous to fly close and within the shockwave zone of another aircraft.[/QUOTE]

We are talking about when both aircraft are already above the speed of sound..

russ_watters
Mentor
We are talking about when both aircraft are already above the speed of sound..
He knows. What he said was that airplanes going above the speed of sound pull a bubble of air along with them that is traveling less than the speed of sound relative to them. So one plane inside the other's mach cone may experience air that is subsonic and thus can hear the engines of the plane next to him.

He knows. What he said was that airplanes going above the speed of sound pull a bubble of air along with them that is traveling less than the speed of sound relative to them. So one plane inside the other's mach cone may experience air that is subsonic and thus can hear the engines of the plane next to him.
ok I understand this...i'll have to read up a bit more. But thanks for the responce.

rcgldr
Homework Helper
After thinking about this, the bubble of air is proably too small too maintain relative sub-sonic flow for any significant distance behind a supersonic jet. One exception could be a very small aircraft following within the shock cone wake of a very large supersonic aircraft. So the lead pilot wouldn't hear the aft jet's engine except for unusual cases. It turns out that for sonic boom level testing, planes do follow within the shock wave cone of other aircraft.

For a typical aircraft, there are multiple shock zones, one at the nose, and again at the wing root, even though the wing root is within the shock wave cone from the nose (assuming it's not a blunt nose). The sharp nose shock wave only accelerates a relatively small volume of air, and it's not enough to prevent super-sonic flow at the wider and further back wing root.

A video of a F14 doing two flybys. In the first pass, the shockwave at the root is strong enough to cause condensation and make the wave visible. In the second pass you can see the shock wave impact on the water surface below, and you'll hear a crack (not a boom) as the F14 flies by. Shock waves sound like a very loud crack, until the listener is far enough away that the shock wave dissipates into a normal like sound wave and become a boom like sound.

f14flyby.wmv

http://en.wikipedia.org/wiki/Shock_wave

http://www.nasa.gov/centers/dryden/news/DTRS/1996/HTML/DRC-95-32/index.html

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good stuff thanks - this whole thing started as a "mind experiment"

I was discussing with a co-worker if you could actually hear yourself yelling if you were free-falling at super sonic speeds. (lets say you were really high up and there wasn't a ton of air so your terminal velocity was really high)

My notion was that you would "outrun" your own yelling b/c you would be falling faster then your voice could travel and your voice could never catch up to your ear. This is b/c the medium through which your voice was traveling is the air (which is moving supersonic relative to your body). So its not like your voice is traveling at your velocity + the speed of sound to catch up to your ear. B/c your voice has to travel through the air, which is not moving (relative to the earth)....

make sense???.... a picture would help here..

Its sort of an interesting problem b/c once your voice leaves your mouth it needs to travel though a medium that is not moving (relative to the earth). and you can in essence "out run" it

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rcgldr
Homework Helper
I was discussing with a co-worker if you could actually hear yourself yelling if you were free-falling at super sonic speeds.
You'd still be able to hear the sound waves that travel internally from your vocal chords to you inner ear.

A better example would be two people falling. If falling side by side they couldn't hear each other. If one person was above the other, then you'd have the same drafting (just like Nascar) situation, and the air speed relative to the higher person would be sub-sonic if the higher person was close enough. Also because of the draft, the higher person would fall onto the lower person if in the lower person's draft. Another draft effect would be the fact that both persons would speed up as the higher person closed in on the lower person because the higher person would occupy the "void" left above the lower person reducing the pressure decrease and drag on the lower person. This is why two cars in line at a high speed oval track at Nascar are faster than one car, and why 3 are faster than 2.

Mythbusters did an experiment to see if two skydivers could hear each other at normal free fall speed (120mph); and they couldn't because of the very loud wind noise. Foam or similar ear plugs to filter out wind noise probably would have worked, but this was to bust the myths in the movie "Point Break" where ear plugs weren't used.

Getting back to your original question, sound can't travel forward (or any direction) faster than the speed of sound relative to the speed of the air near the objects in question. The draft or shockwave cone affects affect the speed of the air near the objects, so in this case it depends if the relative air speed from the sensing (hearing) point to the target object is sub-sonic. In the case of the two jets, it would depend if the trailing jets engine noise could travel to the leading jets fuselage and cause enough vibration in the fuselage for the lead pilot to hear it. For a simpler example, everyone inside a Concorde traveling at Mach 2 will hear each other just fine.

Regarding super-sonic free fall, it's only been done on purpose once, but there will be another attempt to break the old record later this year:

http://en.wikipedia.org/wiki/Joseph_Kittinger

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Impressive, he sounds quite fearless that guy Kittinger, and maybe ever so slightly crazy :)
Nah, fearless, I think...
Reminds me of that guy who took to the air in his favorite chair bound to helium balloons :)
And survived the experience too. Controlling his height by his trusty shotgun if I remember right :)

But as for sound and that shockwave cone. Isn't that behind the airplane when the aircraft is moving faster than sound. I agree to the sound/vibrations of the engines mowing inside the molecules of matter consisting of the aircraft.
When the aircraft pressure itself through the air can't we look at that as an innumerable array of 'planes' meeting said aircraft, and the reaction to its passing taking action behind it as sound will obey the slower speed?

rcgldr
Homework Helper
As for sound and that shockwave cone. Isn't that behind the airplane when the aircraft is moving faster than sound.
Picture of the Thrust SSC breaking the sound barrier, where it's clear the shock wave originates from the nose:

thrust1.jpg

A cool photo.
The lighted area is equivalent to the soundbarrier then?
And it stretches quite a bit out at the sides too.

How does it do that whilst obeying the speed of sound?

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Could this photo perhaps be just before breaking the sound barrier?

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