12
$\begingroup$

Assume that you have an aircraft that is so amazingly stealthy that it is perfectly quiet: no engine noise at all, nothing. And, it's going supersonic. (Maybe in a steep dive with the motor shut off.) Will it make a sonic boom?

Would the sonic boom be louder if the aircraft was making more noise but all else remained constant?

The (excellent) answer to this question discusses aircraft size in relation to the strength of the sonic boom, but no mention of aircraft noise. So would the amount of noise the aircraft makes matter?

Edit: by "silent" I'm not trying to imply "displaces no air" or creates no sound by its passage. The point is simply that the aircraft itself doesn't make any noise from an engine or otherwise. It's silent, like a bullet, or a glider.

| improve this question | |
$\endgroup$
  • 7
    $\begingroup$ A totally silent aircraft is probably so unphysical that this question has no real basis for an answer (it's like asking about transonic control authority on a magic carpet) $\endgroup$ – cpast Jul 28 '15 at 23:11
  • 6
    $\begingroup$ Does a supersonic bullet create a sonic boom? (Hint: yes, it does, although it's more of a click you hear before the report of the gun being fired.) (I don't recommend getting into a position where you can observe this first-hand, if you can avoid it!) So: does the sonic boom get louder as the bullet/aircraft/whatever gets louder? $\endgroup$ – Ralph J Jul 28 '15 at 23:18
  • 3
    $\begingroup$ Depends on what you mean by "silent" and "aircraft". The "Busemann's Biplane" airfoil configuration, for example, doesn't produce a sonic boom, but it doesn't produce lift, either. $\endgroup$ – Mark Jul 28 '15 at 23:42
  • 1
    $\begingroup$ But subsonic bullets aren't silent. If it is perfectly silent, then by definition there is no pressure wave created. That is a totally unphysical assumption, and makes analogies to any real object highly questionable. $\endgroup$ – cpast Jul 28 '15 at 23:50
  • 5
    $\begingroup$ You need to send this question over to XKCD, in his "What If" section. Very very interesting hypothetical question. Not sure how you'd make a soundless craft but... Still. $\endgroup$ – Jay Carr Jul 28 '15 at 23:52
31
$\begingroup$

Sonic booms aren't created by the aircraft's engine noise. They're created by the aircraft moving through the air. The aircraft has a non-zero volume (an aircraft with zero volume is like the emperor's new clothes; you can argue all you want how much design and engineering work it took but all anyone will see is an empty spot on the flight line), meaning it must displace air as the plane passes through air. This displaced air equalizes with the surrounding air in an expanding pressure wave, which moves at the speed of sound in all directions.

As your hypothetical silent aircraft approaches the speed of sound, the air being pushed aside can no longer equalize by moving out directly in front of the aircraft. This causes a single strong compression wave to build in front of the aircraft. Similar waves have already started to build over the top of the wing surfaces, as the airflow over the leading edge of the wing (by design intended to make the air move faster thus lowering its pressure and producing lift) is temporarily supersonic before friction causes it to settle back below sonic speed; at this transition point between super- and sub-sonic, air still moving supersonically "backs up" against the subsonic airflow, creating a wall of relatively high-density air.

At this point, an observer would already hear a small sonic boom from some combination of these compression waves. However, the boom becomes much more pronounced as the entire aircraft transitions beyond Mach 1. At this point the "wall" in front of the aircraft becomes more of a "cone" as the air can no longer build up directly in front of the jet and is instead violently pushed aside at any local "disconnect" in the angle presented to the oncoming airflow. This sudden compression produces a strong shockwave "wake" which is what is heard as a sonic boom.

As a perfect example of this, there actually is (was) a vehicle that spends a very critical portion of its flight under zero engine power, effectively silent in that regard, at not only supersonic but hypersonic speeds. This vehicle, as shown in the linked YouTube, produces a characteristic "double sonic boom", proof that even a "silent" aircraft isn't silent when it's supersonic. Can you guess what it is?

The noise of an aircraft's engines could, conceivably, add a little extra to the sonic boom; the biggest difference is that immediately following the sonic boom, if you were close enough, you'd also get the engine noise, which can be deafening all by itself at full throttle up to 100 feet away.

| improve this answer | |
$\endgroup$
  • 1
    $\begingroup$ Keep in mind, the question is extremely hypothetical... Pushing air aside would create pressure waves, which are (by definition) what sound waves are. So a noiseless craft would have to (somehow, and no I don't know how) somehow it would have to not displace the air. Otherwise sound would be produced. So, while I agree that your scenario is likely (if we follow those silly laws of physics), if you somehow created a noiseless aircraft (one that doesn't displace air), I'm not sure this would apply. :) $\endgroup$ – Jay Carr Jul 28 '15 at 23:55
  • 2
    $\begingroup$ By "silent" I was thinking of a hypothetical machine that could fly under power without engine noise or any related thrust turbulence noises. Gliders would be the only such craft. I was going to say no glider could be supersonic, but there actually is a very famous vehicle that glides with zero thrust power at supersonic speeds that is a perfect example. $\endgroup$ – KeithS Jul 29 '15 at 0:01
  • $\begingroup$ So, would the sonic boom be louder (or otherwise different) if the aircraft instead of having no engine noise had a whole lot of engine noise? $\endgroup$ – Ralph J Jul 29 '15 at 0:07
  • $\begingroup$ Ah, and the question was clarified as well to say that your reading of it was the correct one. So, yes, you'd be right. $\endgroup$ – Jay Carr Jul 29 '15 at 0:07
  • 4
    $\begingroup$ @Simon In order to answer this extremely hypothetical question, one has to come up with a definition of "silent". Keith has chosen to define "silent" as meaning that the only sound generated by the aircraft is from its movement through the air, i.e., that the propulsion mechanism makes no sound. A glider is clearly silent in that sense. $\endgroup$ – David Richerby Jul 29 '15 at 9:21

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.