<< Chapter < Page Chapter >> Page >

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

Light with wavelength of 160nm is shone onto a metal plate with a work function of 8,5×10^-19J Determine 1.thr frequency of the light 2.the threhold frequency of the metal 3.Kenetic energy of the released electrons
Desire Reply
An 800 kg car travelling at 15 m.s down a 30° hill needs to stop within 100 m to avoid an accident. using energy calculations only determine the magnitude of the average force that must be applied to the brakes over the 100m.assume that the surface is frictionless
Mihlali Reply
What is Projectile?
Nontobeko Reply
how to convert cm to nanometr?
Renier
When two objects collide in the absence of friction which is observed
Ronny Reply
how to calculate impulse
Smanga Reply
why do esters with higher molecular weight not have have strong fragrances
Prudence Reply
what's a relationship between velocity and momentum?
Eliya Reply
The greater the velocity the greater the momentum
Thato
How to calculate velocity
Ibanathi Reply
distance/time
AJITHKUMAR
v=s/t units is metres per second/km per hour
Zodwa
v=displacement/change in time= v=Xf-Xi/t
LUTHO
Thanks
Eliya
what happen when you add water on an ester
Rose Reply
 present to produce ethanoic acid and ethano
Renier
whats a functional isomer
Andile
methypropanone
Teekayairs
what's a momentum
Teekayairs Reply
Momentum is the product of mass and velocity P=m×v
Thato
thanks
Teekayairs
this is all about physics only or chemistry can be asked?
Teekayairs
Physics only
Thato
how to calculate the change in momentum of a rocket is initially for 250s at rest with a mass of 2.55×10^5kg of which 1.8×10^5 is fuel.then the rocket engine is fired up for 250s during which 1.2×10^5kg of fuel is consumed.the speed of a rocket after firing is 2.78×10^9 in a forward direction
Teekayairs
wich equation should I use if I'm given the momentum and the change in time..
Alakhe Reply
how to do IUPAC naming ?
Nomsakk Reply
u must first master the functional group of your parent chain
Vunene
how to calculate a single resistor
TEBATSO Reply
one function of diode
TEBATSO Reply
to allow current to flow through one direction
Zodwa
one function of variable resistor
TEBATSO
how to calculate speed
Mahlatsi Reply
Distance /time
Loretta

Get the best Siyavula textbooks: gr... course in your pocket!





Source:  OpenStax, Siyavula textbooks: grade 12 physical science. OpenStax CNX. Aug 03, 2011 Download for free at http://cnx.org/content/col11244/1.2
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Siyavula textbooks: grade 12 physical science' conversation and receive update notifications?

Ask