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For my summer duty, I was looking at the sound barrier effects with a glimmer of hope to understand them:

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"A supersonic aircraft is one that can travel faster that the speed of sound. As the plane approaches the speed of sound, it catches up with the sound waves traveling in front of it and pushes them up against each other. The result is a barrier of squashed air in front of the aircraft." Source

Is it theoretically possible to design an aircraft totally silent or at least quiet?

In that case would the problems associated with transonic/supersonic speeds disappear, or at least are reduced?

As the plane approaches the speed of sound, it catches up with the sound waves traveling in front of it and pushes them up against each other.

Or is there a silence barrier too?

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    $\begingroup$ An airplane makes sound just by running into air. Much of the noise you associate with a jet plane is not the engine noise, but the noise of the air being disturbed. A glider at high speed makes a sound not altogether unlike that of a jet. $\endgroup$
    – Wayne Conrad
    Jul 17, 2017 at 21:11
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    $\begingroup$ Related: Would a perfectly silent supersonic aircraft create a sonic boom? $\endgroup$
    – Ralph J ♦
    Jul 17, 2017 at 22:03
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    $\begingroup$ An airplane which disturbs no air at all when it moves will be totally silent. $\endgroup$
    – kevin
    Jul 18, 2017 at 11:18
  • $\begingroup$ That graphic seems to reinforce the myth that the sonic boom only occurs at Mach 1.0. It would be nice if you included a graphic showing a 'boom carpet' instead. $\endgroup$
    – Sanchises
    Jul 18, 2017 at 16:39

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The sound barrier has nothing to do with the noise the aircraft makes. It has to do with the fact that if it is going fast enough the air can't get out of the way. This creates a shock wave where the aircraft is pushing the air aside. A shock wave is where the speed of the air transitions from faster than sound to slower than sound and involves the release of considerable energy. You can control the shockwave to some extent by careful shaping of the surfaces, but a plane going fast enough can't be silent.

You can make very quiet sub-sonic aircraft, but they tend to go slowly so that the noise of the air flowing over the craft is minimized.

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    $\begingroup$ I wanted to add here that the shuttle produced sonic booms despite having no power during reentry $\endgroup$
    – Eugene Styer
    Jul 17, 2017 at 22:48
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I'll try to keep this really simple, as OP's question leads me to believe they are not familiar with the physics involved. As indicated by others here, the sound barrier is not directly related to noise, but rather to the nature of sound itself:

  • Sound is just a pressure wave moving through the air.

  • In turn, the speed of sound is the speed at which a disturbance (pressure wave) can propagate through air.

  • Us hearing these disturbances (or not, there is a lot of sound that escapes us) is just an evolutionary adaptation.

  • For air to "move aside" to let an object like a plane fly through, it must sense it aproaching. Aircraft disturb air a good distance in front of them, this may sound a bit odd the first time you hear it.

  • If the plane is faster than the speed of sound, the air cannot "move aside" in the usual manner, as the aircraft outruns the pressure wave that would have moved the air out of the way in time. This is why fast aircraft measure their speed using the Mach number to show how fast they are in relation to sound, as this becomes the most important factor once you approach $M_{\infty}=1.0$

  • The end effect is that a supersonic aircraft must then force the air out of its way in a more forceful manner, compressing and accelerating it to its own speed. This phenomenon is commonly called the sound barrier.

  • As an added point of interest, a similar phenomenon happens at the rear of an aircraft flying over the speed of sound, the airflow cannot adapt to the end of the body fast enough, creating a low pressure area.

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  • $\begingroup$ Air doesn't really "sense" anything. What is happening is a pressure wave is created in front of the aircraft as it pushes the air out of the way. This is called a "bow shock" and is the same principle as a boat pushing water out of the way ahead of the boat. $\endgroup$
    – Ron Beyer
    Jul 18, 2017 at 15:47
  • $\begingroup$ @Ron Anthropomorphic explanations are usually perfectly acceptable. Air does not sense, but it is disturbed by pressure propogating at the speed of sound - much like you do not sense sound but your eardrums are displaced by the pressure. $\endgroup$
    – Sanchises
    Jul 18, 2017 at 16:43
  • $\begingroup$ As both aircraft displacement and aircraft noise create a pressure wave which propagate at the "speed of sound", I imagine both participate to the creation of the shock wave. What is missing in your answer is in which proportion, and how much the shock wave energy would be reduced if the aircraft sound was reduced. This may be not significant, but that what the question is about. $\endgroup$
    – mins
    Jul 18, 2017 at 18:17

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