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What Is A Sonic Boom? Why Breaking The Sound Barrier Creates A Sonic Boom

Breaking the sound barrier and creating a sonic boom tends to be associated with fighter

jets travelling at incredibly fast speeds, but what is a sonic boom, and why does going

faster than the speed of sound create a boom?

Firstly, you need to understand what sound is and how it propagates.

Sound waves travel fastest through solid materials, then liquids and then gasses.

Unlike light, sound waves are mechanical waves whereas light is a transverse electromagnetic

wave which means that sound needs a medium to travel though.

On average, the speed of sound at sea level is 340 metres per second, in water it is 1,433

metres per second and in a solid such as diamond the speed of sound is 12,000 metres per second

- 35 times faster than the speed of sound in dry air.

The speed of sound in a medium also varies with temperature as the change in temperature

affects the density of a medium.

There are two properties which affect the velocity of a sound wave: elastic properties

and density, with this relationship being described by this equation.

Sound travels faster through steel than rubber, as steel has higher elastic properties.

The elastic properties relate to the tendency of a material to not deform when a force is

applied to it and to also maintain its shape.

At the particle level, rigid materials have molecules with strong forces of attraction

for each other.

The greater the elasticity and the lower the density of a medium, the faster sound will

travel through that medium.

Although solids such as steel are far denser than air, steels elastic properties are much

greater meaning that sound transverses through a solid medium such as steel faster than a

gaseous medium such as air.

When an object is moving through the air for example, it creates pressure waves at the

speed of sound around the object – such as how a boat does when it travels through

water as it creates bow and stern waves.

When such object travels faster and faster, the pressure waves begin to not be able to

get out of the way of each other, causing them to build up, compress and eventually

merge into a shockwave.

Essentially when something breaks the sound barrier it creates compressions faster than

the compressions can move away from it and they end up piling on top of each other.

It is a common misconception that when you go supersonic that you only hear one boom

from the transition from subsonic to supersonic, but the boom is continuous for as long as

you are going supersonic.

This means that if an aircraft is approaching you at supersonic speeds you will not hear

any sound from the aircraft as it is travelling faster than the sound it can produce, meaning

you will only hear the sound after the aircraft has travelled overtop of you.

Sustaining supersonic flight does come with its issues with aerodynamic heating occurring

meaning that aircraft need to be designed with materials such as stainless steel or

titanium to sustain supersonic flight as some parts of the aircraft can hit hundreds of

degrees.

For those not into guns, bullets break the sound barrier with boring regularity.

The sound difference between a supersonic and subsonic bullet is noticeable as you’ll

hear now.

When bullets enter the transonic region, the shifting of the centre of pressure causes

the amplification of static and dynamic instability causing the angle of attack and yaw of the

bullet to potentially dramatically change which is impossible to compensate for, and

can cause bullets to impact on their sides instead of at the tip and decrease overall

accuracy.

Bullets can however enter into the transonic region with no problem, but the ability of

such a bullet to do so is quite unpredictable.

It might sound as though breaking the speed of sound is unreachable unless you have a

supersonic plane or a gun around you, but with just the flick of a whip – you too

can break the speed of sound.

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