The Doppler effect

The Doppler effect

It is a well-known phenomenon that the sound of an approaching sound source is higher than that of a receding one.

Physics

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Scenes

The Doppler effect

  • wavefronts of the emitted sound
  • observer
  • sound of the approaching vehicle
  • sound of the receding vehicle

The Doppler effect

The sound of an approaching car is different from that of a receding one. When the car is approaching, the sound we hear is higher pitched, and when the car is receding, it is lower pitched than the sound of a stationary car. This phenomenon is called the Doppler effect.

Explanation

  • f₀ = 200 Hz
  • f₂ = 200 Hz
  • f₁ = 200 Hz
  • λ₀ = 1.65 m (5.41 ft)
  • λ₂ = 1.65 m (5.41 ft)
  • λ₁ = 1.65 m (5.41 ft)
  • f₀ = 200 Hz - The approximate frequency of the vehicle's sound. In reality, this sound may have more than one component.
  • f₂ = 208 Hz - When the vehicle is approaching, the observer hears a higher frequency sound.
  • f₁ = 192 Hz - When the vehicle is moving away, the observer hears a lower frequency sound.
  • λ₂ = 1.58 m (5.18 ft) - The wavefronts pile up in front of the vehicle, the wavelength decreases, the frequency increases, the wave speed does not change.
  • λ₁ = 1.72 m (5.64 ft) - The wavefronts 'spread apart' behind the vehicle, the wavelength increases, the frequency decreases, the wave speed does not change.
  • f₀ = 200 Hz - The approximate frequency of the vehicle's sound. In reality, this sound may have more than one component.
  • f₂ = 218 Hz - When the vehicle is approaching, the observer hears a higher frequency sound.
  • f₁ = 184 Hz - When the vehicle is moving away, the observer hears a lower frequency sound.
  • λ₂ = 1.51 m (4.95 ft) - The wavefronts pile up in front of the vehicle, the wavelength decreases, the frequency increases, the wave speed does not change.
  • λ₁ = 1.78 m (5.84 ft) - The wavefronts 'spread apart' behind the vehicle, the wavelength increases, the frequency decreases, the wave speed does not change.
  • f₀ = 200 Hz - The approximate frequency of the vehicle's sound. In reality, it does not just produce a sound of a single frequency value.
  • f₂ = 240 Hz - When the vehicle is approaching, the observer hears a higher frequency sound.
  • f₁ = 171 Hz - When the vehicle is moving away, the observer hears a lower frequency sound.
  • λ₂ = 1.37 m (4.49 ft) - The wavefronts pile up in front of the vehicle, the wavelength decreases, the frequency increases, the wave speed does not change.
  • λ₁ = 1.92 m (6.3 ft) - The wavefronts 'spread apart' behind the vehicle, the wavelength increases, the frequency decreases, the wave speed does not change.

Explanation

The Doppler effect occurs because the speed of the sound waves is independent of the speed of the source.

Be it a stationary or moving vehicle, the sound waves it emits travel at a constant speed that is characteristic of the medium they are traveling through. Therefore, when the sound source is moving, wavefronts pile up in front of it and spread apart behind it.

As a result of the piling up of the wavefronts, the wavelength of the sound waves decreases. Since the product of the wavelength and the frequency equals the speed of the waves, which is constant, the frequency of the waves increases, producing a higher pitched sound. The exact opposite happens behind the sound source: the wavelength increases and the frequency decreases, resulting in a lower pitched sound.

The same phenomenon occurs when it is the observer that moves, not the source.

Sonic boom

  • Mach cone - When the sound source is traveling faster than the speed of sound, the wavefronts form a cone. Here the amplitude of the sound becomes very large and the observer, over whom the Mach cone passes, can hear a sonic boom.
  • hyperbola - Where the Mach-cone intersects the water surface, it forms a hyperbola.
  • sonic boom

Sonic boom

If the sound source, for example, an aircraft moves so fast that it reaches the speed of sound characteristic of the particular medium it is traveling through, the wavefronts form a cone, which is known as the Mach cone.

The Mach cone moves together with the aircraft. The sound waves are amplified along the cone surface, so an observer, over whom the cone surface passes, can hear a sonic boom.

Contrary to popular belief, it does not occur only at the moment when the aircraft exceeds the speed of sound. It is continuously occurring during supersonic flight but is not heard everywhere.

Where the Mach cone intersects the surface of Earth, it forms a hyperbola, known as the boom carpet. The sonic boom spreads across this curve, which follows the flight path. It can cause severe damage: windows can break and unstable rocks may fall to the ground.

Sonic booms are produced mostly by supersonic fighter aircraft, but the snapping of a whip also generates a minor sonic boom.

Astrophysics

  • receding galaxy
  • redshift - In the case of light waves, the Doppler effect causes a shift in the spectral lines. These lines shift towards the red end of the spectrum if an object is receding.
  • observer

Astrophysics

Similarly to sound waves, light waves also demonstrate the Doppler effect when the light source is approaching or moving away from the observer.

Light emitted by an approaching source has a shorter wavelength, that is, it will be perceived bluish, and light emitted by a receding source will be perceived reddish.

It has been observed that the light of galaxies is continuously shifting towards the red end of the spectrum, that is, they are moving away from each other and from Earth, and the further away they are, the faster they move. This phenomenon is called redshift. The universally accepted theory of the expanding Universe is based on this observation.

Heart ultrasound

  • heart
  • Doppler ultrasound device - When the ultrasound is reflected, its frequency changes. Using this, the device can provide information on the movements of the internal organs and on the flow of blood in blood vessels.
  • emitted wave
  • reflected wave

Heart ultrasound

Another characteristic application of the Doppler effect is echocardiography. Ultrasound machines are used to examine the structure of the internal organs. By examining the wavelength of the reflected ultrasound, the speed of movements within internal organs can also be determined.

Therefore, not only the structure of the organs is detected by the Doppler ultrasound device, but also the amount and speed of blood flow in the arteries and veins, for example. This may give information about the blood supply of the scanned organ or tumor, or the condition of an obstructed blood vessel.

A Doppler ultrasound machine is also used in hospitals to monitor the fetus's heartbeat during pregnancy and the labor process.

Speedcams

  • Speed ​​measurement with reflected radar waves
  • speed camera - Due to the Doppler effect, the wavelength of the wave reflected by the moving object changes and the device detects this.
  • emitted wave
  • refleced wave

Speed camera

One of the applications of the Doppler effect is the measuring of the speed of moving vehicles with a speed camera.

The device emits a radio wave which is reflected by the vehicle that is being targeted. The wavelength of this reflected wave, however, changes due to the Doppler effect. The speed of the vehicle can be calculated from the wavelength of the reflected radio wave.

Unlike the operation of speed cameras, the operation of laser speed guns is not based on the Doppler effect but the precise measuring of the reflection time. This can be used to first calculate the vehicle's distance at different time intervals and then its speed.

Animation

  • 0 km/h (0 mph)
  • 50 km/h (31 mph)
  • 100 km/h (62 mph)
  • 200 km/h (124 mph)

Narration

The sound of an approaching car is different from that of a receding one. When the car is approaching, the sound we hear is higher pitched, and when the car is receding, it is lower pitched than the sound of a stationary car. This phenomenon is called the Doppler effect.

The Doppler effect occurs because the speed of the sound waves is independent of the speed of the source.

Be it a stationary or moving vehicle, the sound waves it emits travel at a constant speed that is characteristic of the medium they are traveling through. Therefore, when the sound source is moving, wavefronts pile up in front of it and spread apart behind it.

As a result of the piling up of the wavefronts, the wavelength of the sound waves decreases. Since the product of the wavelength and the frequency equals the speed of the waves, which is constant, the frequency of the waves increases, producing a higher pitched sound. The exact opposite happens behind the sound source: the wavelength increases and the frequency decreases, resulting in a lower pitched sound.

The same phenomenon occurs when it is the observer that moves, not the source.

If the sound source, for example, an aircraft moves so fast that it reaches the speed of sound characteristic of the particular medium it is traveling through, the wavefronts form a cone, which is known as the Mach cone.

The Mach cone moves together with the aircraft. The sound waves are amplified along the cone surface, so an observer, over whom the cone surface passes, can hear a sonic boom.

Contrary to popular belief, it does not occur only at the moment when the aircraft exceeds the speed of sound. It is continuously occurring during supersonic flight but is not heard everywhere.

Where the Mach cone intersects the surface of Earth, it forms a hyperbola, known as the boom carpet. The sonic boom spreads across this curve, which follows the flight path. It can cause severe damage: windows can break and unstable rocks may fall to the ground.

Sonic booms are produced mostly by supersonic fighter aircraft, but the snapping of a whip also generates a minor sonic boom.

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