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Why do you only hear engines and other noise from a jet going supersonic after the plane has passed? A lot of resources use this picture to illustrate :

enter image description here

This is cool and all, but say you changed the geometry of the nose of the aircraft in a way (presumably changing the flow turning angle) that made the shock at a sharper angle. That would make it so you heard the aircraft later after it passed compared to the one with the standard nose geometry, despite the same speed of both aircraft. (I think)

Basically I'm asking why you only hear the plane after the Mach cone passes.

Or you could also do a theoretical scenario, where an infinitely thin plate (at 0 AoA) was going supersonic through the air, and it had something like a speaker on it. When would you hear the speaker after the plate had passed? There is no shockwave so you wouldn't hear that.

((I understand that sound won't travel upstream when going supersonic, but I'm curious as to why the shock forms at the exact point where you can hear sound from said object))

Some of the sources I was looking at :

If the plane accelerates to the speed of sound or faster, then the sound waves can no longer move fast enough to get out of the way of the aircraft. The waves bunch up and combine to form a shock wave. This then trails behind the aircraft in a V-shape. Something similar is seen for the boat on water.

from here. (this one didn't seem to descriptive, but it provided an interesting way to look at it)

Also, the Wikipedia article on shockwaves has some interesting points

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  • $\begingroup$ Downvoter, what on this page did I not follow correctly? $\endgroup$
    – Wyatt
    Apr 2 at 1:12
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    $\begingroup$ I didn't downvote this, but there is a certain amount of repetitive similarity in your questions. Have you ever seen a boat wake, or been rocked by the waves? Do the waves hit you after the boat has passed, or when is it directly abeam you? Think about it... this one seems quite obvious. $\endgroup$ Apr 2 at 1:39
  • $\begingroup$ @MichaelHall yeah good points, but what still confuses me is if you change the shape of the nose of an aircraft, that will change the shock angle, presumably changing when you hear the plane. All of this at the same speed, which is the really confusing part. $\endgroup$
    – Wyatt
    Apr 2 at 1:55
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    $\begingroup$ "if you change the shape of the nose of an aircraft, that will change the shock angle" -- not true, the angle depends entirely on the speed of sound and the speed of the aircraft. $\endgroup$ Apr 2 at 2:02
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    $\begingroup$ The shockwave itself is just noise from the engine. If the Mach cone hasn't hit you yet, the sound has not had enough time to travel from the source to the observer. The question seems to be asking why you can't hear a sound wave before it reaches you? $\endgroup$ Apr 2 at 15:50

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Why do you only hear engines and other noise from a jet going supersonic after the plane has passed?

If the object making noise is far enough, then you always hear it after it has passed, no matter if it is super or subsonic.

Looking for example at the picture in your question, an observer on the ground would hear the airplane only when the biggest "sound bubble" reaches it and this always happens when the airplane has already travelled forward a bit in the meantime, it's just a geometrical matter and it has nothing to do with being supersonic. So, no matter the speed, an observer on the ground will always hear the airplane with a delay, delay which is obviously bigger the faster the airplane is.


Anyway I agree that some of your sources are definitely misleading since they call the cone on the right part of your picture "shock wave". When the sound bubbles coalesce together behind a supersonic airplane they form a "Mach cone", not a shockwave. A shockwave is a completely different phenomenon which depends, among other things, also on the shape of the aircraft. The angle of the Mach cone depends instead only on the speed of the airplane according to the following equation $\alpha=arcsin(1/Ma)$ (source Wikipedia):

enter image description here

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You seem to have such a grasp on this question that it seems impossible that you don't actually understand. I'd really like to understand what is missing between what you know and what you're trying to understand.

The sound of the aircraft (from engines, whatever) travels at the speed of sound. This is also the fundamental sound that all pressure disturbances travel at. The speed of sound is through the medium -- i.e. through the air. Which means the sound travels the same speed no matter what creates it (or how fast the noise source moves).

A supersonic aircraft (one that flies faster than sound) will fly faster than the sound it makes.

Imagine that the aircraft was 5000 ft (about a mile) away from you -- but headed directly at you. The noise it makes right now will take about 5 seconds to reach you (speed of sound is about 1000 ft/s). If the aircraft is traveling at Mach 1.5, then it will take 3.3 seconds for the aircraft to reach you.

Of course, the aircraft is constantly making noise -- but it is traveling faster than all of that noise.

If the aircraft is pointed directly at you -- such that it crashes into you -- then at zero ft. of separation, it would make noise -- and the noise and the aircraft would hit you at the same instant.

Of course, real aircraft are flying overhead (not directly at us). So, we're listening to the sound as it propagates down to us on the ground. We also don't have as good of a perception of when the aircraft is directly overhead.

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  • $\begingroup$ I see. The part I am a little confused about is why the flow turning (that changes with speed) and therefore determines the shock angle, determines when you’d hear the aircraft. Couldn’t you have a shock whose angle is such that you hear the shock before you hear the sound? Like there would be a delay from when you heard the shock and the sound of the aircraft. $\endgroup$
    – Wyatt
    Apr 2 at 15:49
  • $\begingroup$ A shock can not possibly form at that angle. Look at the oblique shock chart en.wikipedia.org/wiki/Oblique_shock#/media/… The greatest a shock angle can be is 90 degrees -- which is a normal shock. If that shock propagated to the ground, you would hear it at the same time as the disturbance passed. However, normal shocks cause zero turning (but the downstream flow is subsonic). Because of all this, normal shocks usually only exist near the body and are curved to form oblique shocks away from a body. $\endgroup$ Apr 2 at 16:23
  • $\begingroup$ "Couldn’t you have a shock whose angle is such that you hear the shock before you hear the sound?" You always hear the shock before you hear the sound... because the shock is the sound. $\endgroup$ Apr 2 at 17:23
  • $\begingroup$ Ah I see. What else confuses me is the reason why changing the nose cone shape wouldn't change the shock angle. As said here, the oblique shock angle is determined by the Mach number and the amount of turning needed. Wouldn't the amount of turning needed change with a different nose cone, and therefore change the shock angle? $\endgroup$
    – Wyatt
    Apr 2 at 18:50
  • $\begingroup$ Yes, absolutely. Look at the oblique shock chart again. Changing the shape of the nose will change the shock angle. $\endgroup$ Apr 2 at 19:46
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if you change the shape of the nose of the aircraft ...

It seems that is what NASA is doing here.

So now one is reducing shock wave amplitude (how loud it is), not the shape of the cone, which depends on Mach number.

This becomes more obvious when one changes the frame of reference to an observer on the ground. Since sound will propagate at, well, the speed of sound, the shape of the cone depends entirely on the aircraft speed.

One may reason this by geometry from the fixed point of the observer a fixed distance away from the aircraft when it passes.

Where will the aircraft be at Mach 1.2 compared with Mach 2.4 when it's "sound" reaches you? The angle from the observer to the plane is the angle of the shock wave.

This concept is not restricted to supersonic flight. The change of frequency of sound waves emanating from a passing object is known as the Doppler effect.

curious why the shock wave forms at the exact point where you can hear the sound.

The shock wave is the sound. Our eardrums perceive pressure change of the wave as noise.

when would one hear the noise of the speaker after the plate had passed?

Well it depends what your carrier medium is. Vibrations made in air are restricted to the speed of sound just as the propagation of the whine of the jet engine is. But radio or light waves travel much faster. Thankfully, vision and radar enable us to see and communicate ahead even at speeds greater than Mach 1.

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  • $\begingroup$ Fun fact: people flying at Mach 2 in the Concorde can speak to each other fine. But their voices cannot carry ahead of the aircraft in flight. A bit of a mind bender, but not unlike a glider with a "rate of descent" less than the upward vertical wind speed in a thermal. $\endgroup$ Apr 2 at 12:38
  • $\begingroup$ @Wyatt based on your recent questions: could the sound (drag) "barrier" be partly the result of shock waves causing turbulence in the boundary layer of the aircraft? $\endgroup$ Apr 2 at 12:41
  • $\begingroup$ I think I see what you mean. So the amount of flow turning needed is less, (for the X-59) but the shock angle stays the same? $\endgroup$
    – Wyatt
    Apr 2 at 15:12
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    $\begingroup$ "curious why the shock wave forms at the exact point where you can hear the sound." "The shock wave is the sound." Upvoting for this clear, concise, (and should be obvious...) statement. It isn't just a coincidence Wyatt! $\endgroup$ Apr 2 at 16:12
  • $\begingroup$ @Wyatt I read the article about flow turning. This may be a local phenomenon applied to (supersonic) air intakes. Any sound propagated from any angle cannot be faster than the lead shockwave, regardless of shape. I worry that the Quesst designers are in for a nasty surprise when they scale up to airliner size as lift creation (larger wings) may create a much bigger boom. $\endgroup$ Apr 2 at 16:49

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