# Eli5 What is the so called ‘sound barrier’?

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What is the so called ‘sound barrier’? And why is there something visible when an Aircraft ‘breaks through’ it? What is it we see? I dont get it at all

In: Physics

It’s not a physical barrier, it’s just a speed, the speed of sound, we say it’s like a barrier because many things change about the airflow once it goes supersonic, a lot of turbulence is created and a loud boom noise can be heard as a supersonic object files past you.

This sonic boom it’s not just when you “break the sound barrier”, it’s constant as the plane flies over new people, because the sound form the plane can’t get ahead of it, when it flies over you it all hits you at once making a boom sound

Imagine pushing a box through sand. The sand has to move out of the way of the box as you push it. Due to friction, density, and probably other things, the speed that the sand can move out of the way of the box is limited. Meaning, at a certain point, the box will reach a speed faster than the sand can move out of the way, and it begins to pile up. Eventually, the sand will get packed together and make a wall, and if the box isn’t strong enough, it will break. That’s the barrier in sand. The sound barrier is called what it is because that speed is the same as the speed of sound, because sound is how fast a wave moves through the air naturally.

The sound “barrier” is just the speed of sound, specifically when it pertains to an object accelerating to and past it.

The air suddenly can’t get pushed out the way in the same way any more. It’s way easier to explain with a gif than in words:
https://imgur.com/MZf0mot

What you see (sometimes, and it can also happen without breaking the sound “barrier”), is water vapor condensing out the air due to the pressure drop immediately behind the shockwave.

As for it being a barrier, drag increases sharply as you approach the speed of sound, and drops off after passing it (although never back to where it was when you where doing only 80% or so the speed of sound. This means that while aircraft might struggle to reach the speed of sound, they can frequently accelerate after passing it. There is also some incorrect maths you can do where the speed of sound has infinite drag, it’s wrong, but it created a bit of a reputation before anyone either knew why it was wrong, or had actually travelled faster.

The sound barrier is measured as 1.00 Mach, or the exact speed of sound. Most commercial aircraft fly around 0.81 Mach, essentially 81% the speed of sound. Supersonic aircraft (for example, the Concorde, which flew at 2.02 Mach) break the “sound barrier” once they cross 1.00 Mach. Depending on how fast this speed is reached, as the aircraft reaches the speed of sound (1.00 Mach), the pressure waves (sound waves) become visible, because the object is now flying faster than the sound waves within the air. In short, all sound travels through air, so when an aircraft abruptly reaches the sound barrier (unlike the Concorde, which gradually reached the sound barrier), the aircraft is now flying faster than the sound waves traveling through the air, thus creating a sonic boom and visible atmospheric pressure form around the aircraft. Essentially, the aircraft is flying faster than the speed of its own sound, thus resulting in an abrupt sound (sonic boom) and visible waves (the actual sound waves being pushed out of the way by the aircraft). Hope this answers your question!

“Sound barrier” is the speed of sound (in this case in air). Sound can only travel as fast as a longitudinal wave can vibrate the material it is teavelling in. This corresponds to 761mph or 1100 ft/s (1225 kph or 335 m/s) in stp air (sea level, dry air, at ~75 F or 25C). The reason you see something is there is no more room to compress the air in front of the vehicle, so a sudden ring of steam (or condensed water really, like a cloud) is formed from the pressure change when the object “breaks” the sound barrier.