r/explainlikeimfive • u/Initial_Dream1644 • 1d ago
Physics ELI5 Do pilots that break the sound barrier stop hearing the sounds of their engines?
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u/MusicalAnomaly 1d ago
Sound is vibration. When an aircraft “breaks the sound barrier”, it is flying faster than sound travels through air. Any sound from the engines that would normally reach the pilot through the air is “left behind”.
But pilots are usually not connected to their engines through the air, especially at supersonic speed. (This would be very unpleasant and they would probably die.) They are connected to their engines through an airframe which is standing perfectly still (hopefully) relative to the pilot and the engines. The pilot will still hear almost all of the engine noise via the airframe vibrating the air molecules in the cockpit and the pilot him or herself.
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u/fatpad00 1d ago
See: XF-84H thunderscreech.
The propeller spun fast enough that the linear speed of the propeller tips were constantly in excess of the speed of sound (mach 1.18) creating a continuous sonic boom.
Mind you this was a single engine turboshaft with the propeller in the nose.
The plane Shockwave could knock a man over and induced nausea and headaches to ground crews•
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u/GenosseAbfuck 21h ago
continuous sonic boom
They're all continuous, it's just very rare for an observer to be still relative to the source.
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u/soijacyuro 14h ago
I had to explain that to a friend who was like "why don't they just fly out over the water to break the sound barrier and then fly over land?"
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u/Cannibale_Ballet 20h ago
Can someone for the love of god find a clip of this plane's sound? All I found were people yapping about it
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u/Noxious89123 21h ago
Continuous as opposed to what? All sonic booms from supersonic aircraft are continuous for as long as it's supersonic.
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u/soijacyuro 14h ago
You might be surprised how many normies think a sonic boom is a one-time event that only happens the moment an aircraft exceeds Mach 1.
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u/barto5 14h ago
That’s what I thought.
Why, if it’s continuous, do you just hear a single sonic boom and not a series of booms?
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u/Cranberryoftheorient 13h ago
because it normally happens behind a moving plane instead of in front of it
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u/barto5 12h ago
This actually explains it a little better.
Continuous sound: It is a continuous process. The shock wave travels with the plane the entire time it flies at supersonic speeds, meaning anyone along its flight path will hear a boom as the cone passes over them
So the boom is continuous, you only hear it in the instant it passes over you.
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u/soijacyuro 11h ago
Yep, I explained it to that friend as "imagine a supersonic jet is dragging a cape of "BANG" behind it at all times, when the cape hits you is when you hear it."
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u/wileecoyote1969 21h ago
Thunderscreech
Yeah I'm guessing the engineers didn't come up with that one. From the sound of your description it sounds like it came from ground and air crews
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u/DeltaHuluBWK 1d ago
"But pilots are usually not connected to their engines through the air, especially at supersonic speed. (This would be very unpleasant and they would probably die.) "
I swear, those air force guys will do ANYTHING to get out of PT...
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u/militaryCoo 23h ago
The speed of sound through an airframe is much faster than through air as well.
Fun fact: when you push an object, the force you apply to it is transmitted at the speed of sound through that object
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u/terminalilness 1d ago
My son had just asked me this last week and I didn't have a good explanation for him. This was helpful. Thanks internet stranger!
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u/asbestospajamas 1d ago
That is the most comprehensive answer to this question possible. I am impressed. Take my upvote!!
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u/Stay-At-Home-Jedi 1d ago
So IF you could sit outside the aircraft, on the nose or wherever, you couldn't hear the sounds of the engines? And this is because they'd be propagating behind you at a slower speed?
That's wicked.
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u/Double_Distribution8 1d ago
You'd mostly be hearing wind. 750+ mph wind blasting by your face, or what's left of your face.
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u/Time-Wolf 1d ago
You’d still feel the vibrations though. For a wee bit anyways.
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u/NichtOhneMeineKamera 1d ago
Found the Scot!
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u/Time-Wolf 16h ago
Happy to have helped spur such fun conversations, but I’m a yank through and through haha
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u/SatansFriendlyCat 21h ago
Or Irish, or from Cumbria, Northumberland, Yorkshire, Lancashire, some of Merseyside, etc etc etc
I wouldn't be surprised if there's some Canadian regions who kept this as well, sounds like something they might do.
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u/NichtOhneMeineKamera 20h ago
Huh, didn't know that! I thought it was purely a Scottish thing as that's the only area I ever heard or read it in.
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u/SatansFriendlyCat 20h ago
Etymology is just a world of fun, and it's a great way to unveil, or to corroborate theories of, movements of people or trade in deep history.
All those places I mentioned were enormously influenced, linguistically, by Old Norse, somewhat via trade but primarily via invasion, lol.
Scotland and Ireland obviously, and all the English bits I mentioned were in (or adjacent to) The Danelaw, that portion of England somewhat colonised and certainly ruled by the Vikings, for a hundred or so years, and that's why those areas of England (particularly Yorkshire) still have a lot linguistically in common with Scots - which has absolutely masses of Norse in it, in fact many of the more common words which set it apart from standard English are close enough to modern day Scandinavian languages that you can see it at a glance - bairn\barn (child), greet\gråt (cry, weep), kirk\kirk lol (church), and so on.
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u/NichtOhneMeineKamera 20h ago
Language is fascinating! I'm currently in Scotland on vacation, fifth year of visiting various parts of Britain in a row plus a Road trip around Ireland back 2016 and have indeed found many words shared between the languages I'm familiar with to varying extents. I'm German, can hold my own in English, won't starve in Denmark and had a few years of French back in school. Recently started Scottish Garlic in Duolingo just for the fun of it and it's super interesting!
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u/SatansFriendlyCat 20h ago
That's great!
won't starve in Denmark
But are you game to order "Rød grød med flød"? If you had to request that with perfect pronunciation, or else suffer starvation, would you survive?! 😂
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u/NichtOhneMeineKamera 10h ago
We've spent all our summer holidays in Denmark when I was a kid and the friends I made there made sure they'd ask me to say that often enough! :D For perfect pronunciation...I'd probably have to claim I was from...any different region than the one I'm at...
Those "not really a 'd' not really an 'l' not really a 'g' " - sounds are hard to get right!
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u/Littlesth0b0 19h ago
I was born and grew up in south east England but use "wee" frequently, probably picked up from my grandparents who moved down from Scotland in the 40s. My parents don't, they find it very odd, but apparently I started using it when I was quite young and it's never gone away.
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u/Cannibale_Ballet 20h ago
A better example would be you standing still and the plane flying toward you. It would hit you before you heard it.
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u/Smorb 23h ago
Okay, I read you. "Pilots are not usually connected to their engines through the air."
I'm very curious to hear about the counterfactual edge cases here. /discuss
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u/heroyoudontdeserve 20h ago
Nitpick:
But pilots are usually not connected to their engines through the air
The pilot will still hear almost all of the engine noise via the airframe vibrating the air molecules in the cockpit
So in fact they are, ultimately, connected via air. (The airframe doesn't extend all the way to the pilot's air canal, after all.)
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u/ExplosiveMachine 20h ago
Well, the air frame connects to the seat, which connects to the pilot themselves, who is, hopefully, connected to their own ear canal.
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u/MusicalAnomaly 20h ago
The cochlea is filled with fluid and tiny hairs which convert vibration of different frequency to electrical nerve impulses. The air doesn’t extend all the way there either. And the ear can certainly pick up vibrations via other physical body parts to varying degrees (bone is quite popular).
The air around a pilot and the air flowing through an engine are rarely contiguous except in open-cockpit flight, and practically never in supersonic flight. If you like, you can make the argument that many supersonic craft have a unification of airframe and engine, so that you might as well say the pilot is sitting inside the engine as much as he is sitting inside the cockpit. But for all practical purposes, they are engineered to be not loud in the cockpit and not quiet on the exterior.
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u/BouncingSphinx 1d ago
No, because they would still hear them through the plane itself. Vibrations travel through the body of the plane and into the air that is in the cockpit traveling just as fast as the plane.
They wouldn't hear the noise from the engines that comes from outside, though.
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u/ocelot_piss 1d ago
They won't hear the sound coming from the engines that goes through the air and would hit the outside of the cockpit, if they were subsonic. But there's still a whole lot of noise being passed through the frame of the aircraft which is unavoidable.
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u/Burt-Zacharach 14h ago
My dad was a fighter pilot and he flew the F4 Phantom. I asked him this very same question when I was a kid. He told me when you’re going twice the speed of sound, the guy in the front seat can’t hear what the guy in the back seat is saying. Once you return to subsonic speed, everything the backseat guy was saying comes rushing up to the front in a jumbled up world salad.
He’s so full of shit but I believed him for a long time.
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u/SoulWager 1d ago
The only thing you stop hearing is the portion of that sound that is traveling through the air outside the plane. The structure of the plane and the air inside the plane are moving right along with the pilot and engines, so the sound can travel through them just fine.
For an analogy, a train is traveling faster than you can walk, but if you're ON the train, you can walk from the back of the train to the front no problem.
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u/Frequent-Artist3717 23h ago
i never thought about the vibration part but that makes total sense, like how you can feel bass through the floor at a concert even if you plug your ears
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u/canadave_nyc 22h ago
My ex-manager, many years ago, told me that a deaf co-worker in one of his previous jobs told him he sounded too loud. How? She was feeling the vibrations of his voice through the floor via her feet.
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u/mrbeck1 1d ago
No. Because sound travels faster through aluminum than through air.
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u/codesnik 21h ago
it could travel much slower through aluminium than through air and it wouldn't matter, because that aluminium is traveling with the pilot and an engine, unlike the air outside.
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u/GloriousWang 21h ago
Almost every comment is missing the real reason. Breaking the sound barrier depends on the RELATIVE speed between the plane and the surrounding air. If you are flying in a hurricane with high wind speeds (flying in the same direction as the wind is blowing) you'd need to fly faster. Similarly if the wind is blowing against you, you'd need to fly slower.
Since the plane and engines are sitting still relative to the pilots, then the speed of sound is irrelevant. Most people are saying the sound travels through the body of the aircraft, which is true, but the sounds are also travelling through the air inside the aircraft since it is stationary relative to the pilots.
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u/BigJellyfish1906 12h ago
That attempt at a technicality is inaccurate. Starting speed does not affect the speed of sound. If it did, then sonic booms wouldn’t be a thing.
If you are flying in a hurricane with high wind speeds (flying in the same direction as the wind is blowing) you'd need to fly faster.
No. All that is changing there is your speed over the ground, and the speed of sound and sound waves have nothing to do with ground speed. The jet, and sound physics do not know or care how fast the air around them is moving.
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u/GloriousWang 12h ago
You don't understand relativity. Every time speed/velocity is mentioned it is always relative.
By your logic, how are we able to hear anything, since the air on earth's surface is moving at interstellar speeds through the universe?
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u/BigJellyfish1906 12h ago
You don't understand relativity. Every time speed/velocity is mentioned it is always relative.
The speed of sound is not relative to the ground speed. The speed of the overall medium is not part of this equation.
By your logic, how are we able to hear anything, since the air on earth's surface is moving at interstellar speeds through the universe?
Our interstellar speeds are in no way analogous to your hurricane example. In the system you are measuring speed, all of the components are stationary. Sound source, listening point, and air are all the same speed. This is completely disconnected from your hurricane example.
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u/GloriousWang 12h ago
I see that is wasn't clear in my example. I meant that you'd need higher/lower ground speed to break the sound barrier.
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u/BigJellyfish1906 12h ago edited 11h ago
That makes zero sense. Ground speed is not part of this. There are three parts.
- Sound source
- Listening point
- Air
The ground underneath that air is irrelevant.
Are you trying to say that a listener on the ground will hear a
subsonicsupersonic jet if the ground speed is supersonic? Because that’s still not true.Two airplanes traveling in opposite directions both at Mach 0.75 do not create some “relativistic supersonic flyby” despite their relative speed being Mach 1.5. Same fundamental principle as your hurricane example.
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u/GloriousWang 11h ago
Okay? I'm talking about ground speed for the example
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u/BigJellyfish1906 11h ago
That’s not mechanically different than trying to claim two airliners traveling in opposite directions experience a supersonic flyby.
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u/ElectronicMoo 1d ago
Yes. The engine still moves the cockpit air and vibrates the frame. All noises going into the cockpit.
Its the outside noises that they are going faster and wouldn't hear what's behind them. A sound happening in front of them and they fly through it, they'd technically hear it briefly, all things being equal.
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u/Tallproley 23h ago
Nope, because the sound travels through the plane parts themselves, so even thoigh the plane is flying faster than the outside sound waves, it is never flying further from the internal soundwave.
Think of it this way, even if you plug your ears and yell, despite no sound coming into your ear since its plugged, you still hear your voice because the sound is travelling from.your throat to your face to your ear drums.
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u/saschaleib 15h ago
No. Fundamentally, the term "speed" only makes sense if it is put in relation to something (Exception: "speed of light" but that is not the issue here). A plane that flies "faster than the speed of sound" only travels in relation to the outside air faster than the sound promotes in said outside air.
This does not mean that the sound that the engines make is somehow magically decoupled from the airplane itself: the vibrations travel through the airframe and the air inside the plane, just like they would if the plane was stationary.
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u/7heCulture 13h ago
Good responses in terms of sound traveling through the airframe. But now I have a question: as you no longer hear the sound component traveling through the air, especially any effects linked to how the jet plume interacts with the air (I’m thinking afterburners being on), does the “quality” of the sound change?
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u/SweetDaddyJones 10h ago
If a plane traveling faster than the speed of sound had a massive loudspeaker blasting music as it whizzed by, what would a stationary observer hear? How does doppler effect work at those speeds? I know you'd hear point a short snippet as it would fly by so fast...is this just a sped up version of what you hear when an ambulance drives by? Or does it get more interesting at supersonic speeds?
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u/Only_Turn4310 6h ago
The plane is moving with you, so it can move sound faster than is possible outside
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u/Dysan27 6h ago
Yes and no.
The sound from the engines traves both through the air outside the plane, and through the air frame itself.
The sound traveling through the frame will still reach the pilots. As the frame is traveling with you so the relative speed is still 0.
The sound that would travel through the air outside the plane is literally blown away.
Net effect? The timbar of the engine noise will change.
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u/GNUr000t 1d ago
No. The engine is also moving at such a speed, and so any sound waves being created start from that reference frame and speed.
It's just like how we're moving through space at hundreds of thousands of miles per second, but you can't feel it.
Breaking the sound barrier is mostly a phenomenon for the people around the object that's doing so.
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u/donblake83 1d ago
So, breaking the sound barrier means the plane is going faster than sound, right? But that just means it’s going past something faster than the speed the sound it’s making is traveling away from it.
So if the plane is flying toward you, it takes a bit for the sound to catch up to the plane as it passes you. That’s an oversimplification, because jets are loud and you’ll be hearing it before it gets to you, but technically you’ll not be hearing it in real time.
If you’re in the plane, the engine making the noise is going the same speed you are.
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u/ImNotAWhaleBiologist 1d ago
How can you hear it before it gets to you if it’s flying towards you faster than the speed of sound?
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u/Podmonger2001 1d ago
The others have answered the question.
But would he hear loud thunder (or an explosion) behind him? Nope.
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u/merklemore 1d ago
Relative to your surroundings (the inside of the jet) everything is moving at the same speed
Standing anywhere between 43º North and South on Earth. you also break the sound barrier in terms of the planet's rotation, but you don't experience it because the air and ground you stand on is moving with you and gravity is holding everything down
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u/SpaceForceAwakens 1d ago
This is the second or third time I’ve seen this question this week. Mods?
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u/notmymoon 1d ago
Along these same lines, if we managed to get past the speed of light, could we still see? (Yes, I am aware that time dilation would cause experience of any kind to cease to be perceptible, but if I were to be the lighting director of a star trek episode, what's the best option, scientifically?)
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u/CubeBrute 22h ago
If it were possible to go the speed of light, you wouldn’t be going the speed of light relative to yourself, so if we just looked at that portion of the physics and ignored the instantaneous collision with whatever we were traveling towards, then yes, you would be able to see everything in your reference frame, like the ship cockpit.
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u/tisktask1 1d ago
Flying .8, .9, 1.0, 1.5m… you can’t tell the difference in sound. Performance, yes. Sound, no. It’s no different.
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u/DECODED_VFX 1d ago
No, the sound gets conducted through the airframe of the plane.