# Sounds of supersonic objects

Would you, as a stationary observer, hear a sound in reverse when the source of the sound travels with twice the speed of sound? Of course, he wouldn't hear anything at all before the airplane passed by and the sound reaches him, but what about after? Does it reverse due to the Doppler effect?

• Is the airplane far overhead? Or is it travelling directly toward the observer. If the latter, I think they have more important and imminent concerns than what it will sound like – Jim Jul 3 '15 at 15:47
• Curiously, this was asked yesterday by another user. – Kyle Kanos Jul 3 '15 at 15:54

Yes, the sound can be reversed. Thanks to JiK, we have this animation (Python source code) of a supersonic jet moving forwards that can illuminate what is going on:

The red circle represents the first sound produced by the object, the blue circle the second sound produced by the object and the remaining (black) circles representing the sounds produced elsewhen along the simulation. Two things to look for here:

1. The generation of the Mach wave (or Mach cone)--this is what produces the sonic boom that the observer first hears (which is actually generated from compression waves made after the first two in this case).
2. The blue wave clearly hits the stationary observer first, so the sounds played by the moving object would be heard in reverse.

Obviously point #2 is key to OPs question.

However, sound intensity follows an inverse square law: $$I\propto r^{-2}$$ which means that the larger circles are "quieter" than the smaller circles. Thus, the sounds that you would hear as backwards are muffled in comparison to the normally-directed sounds. So while there is reversed sounds, it would be very unlikely that you would actually distinguish them from the normal-directed sounds.

The image below is a still from a slightly modified version of JiK's code (added an elif (t_i > 7) condition). This shows a bit more clearly the details of the above paragraph, that the normal sounds (in orange and received in the normal direction) would be louder than the reversed sounds (because smaller is louder).

As you stated, the observer would not hear anything as the plane approached him. If fact, he still would not hear anything until the plane had past by since it takes some time for the sound to travel from the plane to the observer. In answer to your question; no, you would not hear the sound backwards since you still hear the sound in the same sequence as it was made (although the pitch may differ).

The image is drawn incorrectly. The sound waves propagate outward in circles, never ovals.

If the plane passes very close to the observer, he would hear the sound in reverse, but he would also hear the sound of the receding plane (forward) since it is still making noise. Since the sound of the receding plane is louder (since the circle is smaller) than the sound of the plane approaching, you may not be able to distinguish the sound.

• In the lower right picture, the sound represented by the biggest circle is the "first" emitted sound, right? Let's assume the listener is places at the tip of the cone. he would hear the smallest circles sound before the bigger ones. thus he hears the sound emitted before the plane passed by in reverse. or am i completely wrong? – Mystery Jul 3 '15 at 15:59
• Then the observer would need to move at nearly the same speed as the plane. For a stationary observer, the largest sound wave would pass by him before the smaller sound waves. – LDC3 Jul 3 '15 at 16:27
• I meant, the observer is currently at the cones tip, but stationary, while the plane is moving. The smallest wave would hit him before the biggest. right? – Mystery Jul 3 '15 at 17:15
• I see what you mean. Yes, he would hear the sound in reverse, but he would also hear the sound of the receding plane (forward) since it is still making noise. Since the sound of the receding plane is louder (since the circle is smaller) than the sound of the plane approaching, you may not be able to distinguish the sound. – LDC3 Jul 3 '15 at 17:58
• @LDC3: I think you are saying your original answer was incorrect. You might want to edit it. – Mike Dunlavey Jul 4 '15 at 0:02

BUT: One important question is (in case of faster than light), the actions of the object who created the rays, are already done by the object, and they can not be changed even if we move faster than the light (images of the object). Example:

A starship with hull1 is moving from pos a to pos b, but we move faster than light. if the starhip is changing its hull in time=a+1 to hull2 , we see still its hull at pos a/time a, but inbetween - as said - the hull is changed physically. If the starhip changes at time/pos=a+1 its hull to a completely different hull9, we see the starship still its hull2. And if we are transmitting to the ship it change its hull meanwhile to hull6 what we will see at faster than light. I mean what we see is the (so far theoretical) what happened in past, but can we physically change the images that we see movin faster than light

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• Is the meant to answer the question? I think it's a new question or a comment, in which case it should not be entered as an answer. – Brick Jul 16 at 17:38