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Shock waves and sonic booms

Now we know that the waves move away from the source at the speed of sound. What happens if the source moves at the same time as emitting sounds? Once a sound wave has been emitted it is no longer connected to the source so if the source moves it doesn't change the way the sound wave is propagating through the medium. This means a source can actually catch up to the sound waves it has emitted.

The speed of sound is very fast in air, about 340 m · s - 1 , so if we want to talk about a source catching up to sound waves then the source has to be able to move very fast. A good source of sound waves to discuss is a jet aircraft. Fighter jets can move very fast and they are very noisy so they are a good source of sound for our discussion. Here are the speeds for a selection of aircraft that can fly faster than the speed of sound.

Aircraft speed at altitude ( km · h - 1 ) speed at altitude ( m · s - 1 )
Concorde 2 330 647
Gripen 2 410 669
Mirage F1 2 573 715
Mig 27 1 885 524
F 15 2 660 739
F 16 2 414 671

Subsonic flight

Subsonic

Subsonic refers to speeds slower than the speed of sound.

When a source emits sound waves and is moving slower than the speed of sound you get the situation in this picture. Notice that the source moving means that the wavefronts, and therefore peaks in the wave, are actually closer together in the one direction and further apart in the other.

If you measure the waves on the side where the peaks are closer together you'll measure a different wavelength than on the other side of the source. This means that the noise from the source will sound different on the different sides. This is called the Doppler Effect .

Doppler Effect

when the wavelength and frequency measured by an observer are different to those emitted by the source due to movement of the source or observer.

Supersonic flight

Supersonic

Supersonic refers to speeds faster than the speed of sound.

If a plane flies at exactly the speed of sound then the waves that it emits in the direction it is flying won't be able to get away from the plane. It also means that the next sound wave emitted will be exactly on top of the previous one, look at this picture to see what the wavefronts would look like:

Sometimes we use the speed of sound as a reference to describe the speed of the object (aircraft in our discussion).

Mach Number

The Mach Number is the ratio of the speed of an object to the speed of sound in the surrounding medium.

Mach number is tells you how many times faster than sound the aircraft is moving.

  • Mach Number < 1 : aircraft moving slower than the speed of sound
  • Mach Number = 1 : aircraft moving at the speed of sound
  • Mach Number > 1 : aircraft moving faster than the speed of sound

To work out the Mach Number divide the speed of the aircraft by the speed of sound.

Mach Number = v aircraft v sound

Remember: the units must be the same before you divide.

If the aircraft is moving faster than the speed of sound then the wavefronts look like this:

If the source moves faster than the speed of sound, a cone of wave fronts is created. This is called a Mach cone. From constructive interference, we know that two peaks that add together form a larger peak. In a Mach cone many, many peaks add together to form a very large peak. This is a sound wave so the large peak is a very, very loud sound wave. This sounds like a huge "boom" and we call the noise a sonic boom .

Questions & Answers

what does organic mean
ntuthuko Reply
it means "from living thing"
Maqaqatu
What is a successful collision?
Refiloe Reply
It's a collision that results to a reaction
Tshepo
what is the factor and reaction rate of 20%HCL and 5%HCL
Kleng Reply
concentration and 0.05M/s
Tokozan
how you reaching with apartment depends on what each of you have
Edelita Reply
how with 4looseyellowprotons in your left hand stars are mostly hydrogen so this is your fuel you will also need one six so did die
Edelita
besides the concentration what are the two conditions for the hydrogen half cell to function under standard conditions
Malwandla Reply
forces of non-concervative
Marlet Reply
What is the following process is likely to involve carbon burning?
Sofie Reply
Combustion
Luyanda
Which Statement is part of the cell theory?
Hananiya Reply
What is the second formal statement in the cell theory
Hananiya
🤔
Bontle
what is meant by ohmic
nthabiseng Reply
when someone is referred to as ohmic, it basically means that it obeys ohm's law
Tanaka
*something
Tanaka
what is photosynthesis
Sihle Reply
is process whereby green plants to convert light energy into chemical energy.
Marcelo
what is an isomer?
Kiba Reply
organic compounds with the same molecular formula but different structural formula
liphelo
how is frequency directly proportional to kinetic energy max. and what is the relationship between the wavelength and the energy of a photon. eg if a photon with a greater wavelength and a photon with less frequency. what would be the energies. as in which one would be greater and why
Pearl Reply
relationship wave depend n enrgy greadt reflect of energy n the form of tex shape
thulare
what is Cell capacity?
liphelo Reply
what are hydrocarbons
Nyiko Reply
compounds that are made up of hydrogen and carbon only
Patience
They are compounds that consist only with hydrogen &carbon elements.
Duncan
how do i calculate the work done on an object by a frictional force
Emmelda Reply
@ Emmelda it's W = FxCos180 Where 180° is the angle between Frictional force and where the object is going, it's always 180° because it opposes the motion. Then F is the magnitude of the force and x is the displacement
Vukona

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Source:  OpenStax, Siyavula textbooks: grade 12 physical science. OpenStax CNX. Aug 03, 2011 Download for free at http://cnx.org/content/col11244/1.2
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