Sound waves require a "transport medium", this is why sound cannot be heard in a vacuum. They pass through matter as a series of "compressions" and "rarefactions" and the speed of sound varies according to the medium it is passing through.



The picture above is that of an F-18 Super Hornet fighter jet, this jet has a maximum speed of 1190 miles an hour at an altitude of 40,000 feet. The speed of sound in air is 343 m/s which equates to 767 miles an hour (usually quoted as 760mph) so you can see that this plane has the capability to travel faster than sound.


Consider this jet flying forward in a straight line at 400 miles an hour. The sound emitted by the jet engines will travel in front of it at 760 miles an hour because this is the speed of sound in air. If the plane breaks the sound barrier and flies faster than the speed of sound, it produces a sonic boom when it flies past. The boom is the "wake" of the plane's sound waves. All of the sound waves that would have normally propagated ahead of the plane are combined together so at first you hear nothing, and then you hear the boom they create.


If you're finding it difficult to grasp this, try to think of it another way. The sound emitted by the plane is travelling in front of the plane, at 760 miles an hour relative to the ground but at about 360 miles an hour relative to the plane. As the plane accelerates the difference in speed decreases until the plane is effectively "flying in a bubble of its own sound". At this point the sonic boom occurs.


Sound waves, as we know by now are compression waves and so they need a medium to transport themselves through. The compression and rarefaction of particles of air (if we're talking about air) cause the sound to be propagated. The speed of sound is faster in liquids, and even faster still in solids because in liquids and solids there are more particles available per unit volume than there are in air. 


More particles means the transmission of the compression wave can occur much faster......



Take a look at the animation.This shows dominoes being used to simulate particles of air (gas) liquids and solids, and will hopefully make you understand why the speed of sound increases as the medium becomes more dense.


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