Sound barrier doesn't make sense to me!

Sound barrier doesn’t make sense to me! If I correctly remember my childhood’s
experience, breaking the sound barrier makes one big boom. Why is it not a
continuing sound even if the jet continues flying supersonic? You can hear
the sound of electricity, but can you amplify the sound of light to be heard?

It’s one big boom to you standing at one point on the ground.

The sonic “boom” is actually a shock-wave, created by the air which the aircraft is trying to push side, well, faster than air molecules generally move (which is what sound is.) When this shock-wave (which is a moving front, rather than a wall of moving air, much like a wave of water in the sea) intersects with your spatial position, boom!!

It is a continuing sound which is why when a plane going super sonic passes over head more than just one person hears the boom along the flight path - there’s a whole wiki article and everything on it

My armchair explanation (to myself) of how a sonic boom operates is as follows;

Picture that an airplane is approaching from a distance at less than the speed of sound; you’d hear it approaching, faintly at first and then louder as it got nearer. Let’s say that takes one minute. In the case of an airplane travelling at the speed of sound, you don’t hear it coming initially because the airplane is approaching as fast as the sound; you don’t hear anything until the airplane arrives, at which point you hear the entire one minute’s worth of sound all at once.

The effect is enhanced by the fact that a sonic-speed aircraft may require a heftier and thus louder engine.

As others have correctly answered the first question, I’ll have a go at the second one.

When you say “you can hear the sound of electricity”, you are probably referring to one of two things:

  • a low-frequency hum associated with the frequency of AC current;
  • the crackling sound associated with an electric spark.

In the case of the hum, this is vibrations being set up in the air by the mechanical vibration of the windings in a motor or transformer as the current reverses flow 60 (or 50) times per second. Other audible frequencies can be set up from other processes, such as the high-pitched whine from the flyback in a CRT display.

In the case of a spark (including thunder, which is the sound of a really big spark), the spark crossing the gap causes rapid heating and expansion of the air along its path, which is heard as a percussive sound (effectively white noise over a very wide range of audible and non-audible frequencies).

For light, or anything else, to be “heard”, you would need a similar mechanism for compression waves to be set up in the air, in the frequency range which we experience as sound (approx 20 Hz to 20 kHz).

Sorry, what does this bit mean?

ETA I see **hibernicus **has had a go at this!

Possibly he means the corona discharge from a HV transmission line (more audible in damp conditions)

Humans, at sea level, can hear sounds with frequencies between 20 and 20000 Hz. If you wanted to hear anything else you would have to map those higher frequencies to audible ones. For example visible light is roughly between 400000000000000 and 790000000000000 Hz so you could simply set 400 Hz for Red light (400 THz) and 790 Hz for violet (790 THz).

But as the plane passes me, why I don’t hear the sound after some hundred meters - you know - a wavelike phenomenon.

I’m still not clear. How is the sound concentrated?

Is a sonic boom the same sound you would hear if the plane were hovering overhead?


When the sound source moves faster than sound, the wavefronts of each generated sound arrive together.

Have you ever poked your finger in a pool and watched the ripples?
Now do that but move your finger between consecutive pokes. Watch the relationship of the consecutive waves. Do this faster and faster.
Eventually you will see the wavefronts form 2 lines, trailing above and below and away from your direction of motion. This is the shockwave; analogous to the wavefront of the sonic boom.

A sonic boom will be heard even when there is no engine. It’s not caused by the “concentration” of sounds made by the aircraft. The problem is that the aircraft is advancing faster than the air can easily get out of the way, and so the air ends up compressing into a shock wave that moves away at some angle from the aircraft. The space shuttle, during its unpowered supersonic descent, presents a hefty sonic boom despite a complete lack of engine noise. After the sonic boom has gone by, if the aircraft itself is making noise (e.g. engines), you’ll hear that, but if there’s no noise being made (e.g. the space shuttle), you won’t hear much of anything after the shock wave has passed.

The sonic boom typically presents as a double-thump. see video of supersonic aircraft, first at full speed, then in slow motion. The double-thump is more apparent in the slow motion section; click here for an explanation of why a double-thump happens. The space shuttle is longer, hence the greater temporal spacing between the double-thumps. See video at 0:40; note the lack of aircraft noise after the shuttle’s sonic boom has passed.

You do. The wave front is cone stretching out behing the aircraft. You hear the sonic boom where it touches the ground. If the plane is a high altitude this can be several miles back from where the plane is when the sound reaches you.

See the explanation here.

I always think of sonic booms in terms of boat wakes. If a boat is moving faster than the speed of a wave on the water, then it will create a wake that trails out behind it in a “V” shape. Now think of what happens from the point of view of a person on the shore.

If something is coming at you more slowly than the wave speed, then you will be able to detect it BEFORE it gets to you from the ripples it’s putting out - those ripples travel faster than the boat is coming so get to you first.

But if the boat is approaching faster than the speed of water waves, you will not see any ripples - the first thing you’ll detect is the boat’s wake hitting you after it passes by.

THis makes sense.

Why is it significant that the aircraft passes the speed of sound before it generates a sonic boom?

ETA - is it because the shock wave doesn’t register as sound until it is moving that fast?


Sound is, essentially, the product of air molecules vibrating. There is a characteristic speed (varying slightly for density, humidity, etc.) at which that sound propagates – e.g., you see lightning before you ear the associated thunderclap, or you see a pyrotechnic explosion (as in fireworks) before hearing the sound they make. When a solid object is moving at a speed greater than that which sound propagates, it is moving air out of its way faster than its normal speed of motion, resulting in the shock wave effect that produces the sonic boom.

How to produce a small sonic boom without access to supersonic aircraft? Quite simply, get and crack a whip – the tip of the whip is moving at supersonic speed, resulting in the characteristic ‘crack’, which is actually a localized sonic boom.

Think of sound as a pressure wave. Like dropping a pebble in a pond produces circles of waves emanating out from the center sound does the same thing.

Now imagine moving in a direction while continuously dropping pebbles. Each new pebble drop will be close to the crest of a wave that is moving away from the previous drop. If you match the speed these waves will pile up on each other.

All that may be confusing so here is a picture illustrating the point.

The plane does make sound prior to the speed of sound. You’d hear a “whoosh” from something like the Space Shuttle if you were close enough. However, when the shuttle is supersonic all that noise piles up on itself till you get a wall (so to speak) of all that noise piled on itself. Sort of concentrated noise if you will. It is this wall you hear as a boom.

Thanks very much for the picture - I think I get it now.

Another question, if you are so inclined - if the “sound barrier” a physical phenomenon, or just a figure of speech? Your diagram makes it appear that the supersonic plane has to pass thru its own sound waves - does that matter?


It’s both, actually. That name originated due to the difficulty of progressing through the transonic range. It was assumed by many that any plane that broke the speed of sound would kill its pilot given the difficulties at sustained speeds just shy of Mach 1. It was always well known that things could travel faster than sound, however. Bullets had been doing it for decades.

Once the speed of sound had been achieved by humans it was seen that there was no such barrier, just aerodynamic conditions that had not been fully understood.

The sounds barrier was a term coined to flag issues WWII aircraft had as they approached Mach 1. Their engines and control surfaces simply couldn’t deal with the increase pressures very well. Queue the late 40s, early 50s and you had planes with jet engines and swept wings routinely cracking the “barrier”.

I guess you could say we now have a hypersonic barrier where our engines and aircraft design aren’t suited for those speeds.