This effect becomes more pronounced when the frequency of the transmitted sinusoid is high (short wavelength). Then the effect of the receive wavelength being shorted or lengthened due to the Doppler effect is more noticeable. Therefore, Doppler frequency shifts are more easily detected when using higher frequency waves, as the percentage. Doppler effect is the apparent change in the frequency of a wave caused by the relative motion between the observer and the wave’s source. In simple terms, if either the source of the sound, or an observer, or both, are in motion with respect to each other, then the frequency of sound at its origin will be different from the frequency at the.

Have you ever noticed a siren of an ambulance, seemingly changing pitch as it passes by you on the road? Or, have you ever wondered why the universe is believed to be expanding? The phenomenon responsible for both observations is called the Doppler Effect. Doppler effect is the apparent change in the frequency of a wave caused by the relative motion between the observer and the wave’s source. In simple terms, if either the source of the sound, or an observer, or both, are in motion with respect to each other, then the frequency of sound at its origin will be different from the frequency at the point where it is being observed. It is important to note that frequency is not changed by the source but because of the relative motion between observer and source. When the source of the wave is moving towards the observer, each successive wave emitted by the source takes less time than the previous one to reach the observer. This decrement in time causes an increase in the frequency (ν = 1/t). Conversely, if the source of waves is moving away from the observer, each wave is emitted from a position farther from the observer than the previous wave, so the arrival time between successive waves is increased, thereby, reducing the frequency.

Doppler proposed this effect in 1842 in his research paper titled “Über das farbige Licht der Doppelsterne” (“Concerning the coloured light of the double stars and certain other stars of the heavens”) while he was studying the binary stars in the sky. After three years, this hypothesis was tested for sound waves by dutch meteorologists named Buys Ballot.

Formulation of Doppler Effect

The change in the apparent frequency of the sound wave is given by

The mathematical formulation of Doppler effect for an object moving with the velocity much less than that of light is pretty straight forward. In the above figure, { f }_{0} is the actual frequency of the sound, whereas f is the apparent frequency of the sound. The apparent frequency (f) will be the product of the ratio of change in velocity and the original frequency ({ f }_{0}).

Let’s examine some of the real-life examples of Doppler Effect.

1. Sirens

We are all familiar with the sliding pitch of a moving siren, be it an ambulance, a police siren, or a fire truck bell. As explained above, this apparent change in the pitch is due to the Doppler effect. But in real life, the observation is not that obvious. We hear the siren of an emergency vehicle non uniformly increasing as it approaches us and non uniformly decreasing as it recedes. It is because of the angular resolution of the wave. In simple terms, we are not directly standing in the path of approaching or receding source. The wavefront from the source hits us on an angle; therefore, we consider the radial component of its velocity.

{ v }_{ radial } = { v }_{ s } • cosθ

where θ is the angle between the object’s forward velocity and the line of sight from the object to the observer.

2. Acoustic Doppler Current Profiler (ADCP)

An Acoustic Doppler Current Profiler is a SONAR-like device that is used to measure water velocities over a depth range, using the Doppler effect of sound waves scattered back from particles within the water column. ADCPs contain a piezoelectric transducer, which transmits or receives sound signals. The ADCP works by transmitting “pings” of sound at a constant frequency (above the range of human senses) into the water. As the sound waves travel, they bounce off particles suspended in the moving water and reflect back to the instrument. Due to the Doppler effect, sound waves bounced back from a particle moving away from ADCP have a slightly lowered frequency when they return. Particles moving toward the instrument send back higher frequency waves. ADCPs use this shift to calculate how fast the particle and the water around it are moving. It works within the frequency range of 38KHz to a few MHz. A device named Sodar (sonic detection and ranging), which is used to profile air currents, works on the same underlying principle.

3. Police Radar Guns

You might have wondered how a little handy speed gun can calculate your vehicle’s speed so precisely. A speed gun is a device used by law enforcement officers to keep a check on over-speeding vehicles. It is basically made up of a radio transmitter and a receiver. A speed gun transmits a radio wave signal, and it receives the same signal back after it bounces off the target object. When the object is approaching the radar, the frequency of the return waves is higher than the transmitted waves, and when the object is moving away, the frequency is lower. From that difference, the radar speed gun can calculate the speed of the object from which the waves have been bounced back.

4. Pulse Doppler Radar

Perhaps the greatest organizational achievement of the Second World War, after the atomic bomb, was the radio proximity fuse. It was a massive breakthrough in technology and had changed the course of the War. The proximity fuse was a radio transmitter and receiver that was used to detect the path of an object using the Doppler effect. When the object was close enough, about 30 feet, the fuse would go off, thereby destroying the target. Before the proximity fuse, it was difficult to precisely calculate the trajectory of fast-moving targets and set conventional fuses to hit them. The proximity fuse was such a valuable secret that it was forbidden to be used over enemy territory until late in the war, in case the enemy possibly reproduce the technology or develop a countermeasure.

5. Doppler Echocardiogram

An Echocardiogram is a device used to identify the direction and velocity of blood flow. The principle of the Doppler effect can be applied to ultrasound waves to determine the velocity and direction of moving blood. Just like the speed of a moving vehicle is determined by the radar gun, in an echocardiogram, we can determine the velocity of blood cells by measuring the magnitude of the frequency shift between the transmitted and the received signal. Furthermore, we can determine the direction of the flowing blood, depending on whether the Doppler shift is positive or negative.

6. Laser Doppler Anemometer

To investigate the fluid dynamics in liquids and gases, laser doppler anemometers are widely used all over the world. The directional sensitivity, contactless and precise measurements of LDA makes it useful for applications with reversing flow, chemically reacting or high-temperature media, and rotating machinery, where physical sensors might be difficult or impossible to use. In general, the LDA sends a monochromatic laser beam toward the target and collects the reflected radiation. According to the Doppler effect, the change in wavelength of the reflected radiation depends on the targeted object’s relative velocity. Therefore, the velocity of an object can be found by measuring the change in wavelength of the reflected laser light. This is done by forming an interference fringe pattern (pattern of light and dark stripes) by superimposing the original and reflected signals.

7. Audio Applications

Sound is a minute fluctuation in air pressure. The earliest known use of the Doppler effect in the music domain was a Leslie speaker associated with a Hammond organ. In the realm of digital music production, the Doppler effect is used to enhance the music quality. There are numerous plugins and effects that are based on the Doppler effect. In practice, music composers use such effects to channel the particular beat to a specific target environment by changing the mono and stereo audio formats to a multi-channel format. In order to make the effect as real as possible, the start and endpoint of the effect, the curve of the track, centre time, and tone colour controls of the Doppler plug-in must be given careful attention.

8. Satellites

The Doppler effect is routinely measured in the frequency of the signals received by ground receiving stations when tracking satellites. The increasing or decreasing distance between the satellite and the ground station may be caused by a combination of the satellite’s trajectory, its orbit around a planet, Earth’s revolution about the sun, and Earth’s daily rotation on its axis. A satellite approaching Earth will add a positive frequency bias to the received signal. However, the received Doppler bias will become zero as it passes Earth and then, it becomes negative as the satellite moves away from Earth. The frequency shift produced by the Doppler effect is proportional to the relative velocity between the transmitter and receiver or, more accurately, the relative phase velocity, which is the relative velocity modified by the propagation medium. Keeping all these parameters into account, the ground stations on Earth observe and track the navigation of satellites.

9. Astronomy

Although the example of the sound is generally experienced, the Doppler effect of light or electromagnetic waves was suggested by an Austrian Physicist Christian Doppler, as an attempt to explain the colouration observed in the stars. The Doppler effect can’t be observed visually in the case of electromagnetic light waves. Though with the help of proper spectroscopic devices, a displacement of the lines in the spectra of distant galaxies toward the red region of the visible light, i.e., toward longer wavelengths, can be observed. This phenomenon is known as Doppler Red-Shift. Light from moving objects will appear to have different wavelengths depending on the relative motion of the source and the observer. Observers, looking at an object moving away from them, see a light that has a longer wavelength than it had when it was emitted (a redshift), while observers looking at an approaching source see a light that is shifted to a shorter wavelength (a blueshift). In 1929, the astronomer Edwin Hubble measured the velocities of a large selection of galaxies using the Doppler Effect. He discovered that almost all galaxies are moving away from us. He also found that their velocity was always proportional to their distance. The ratio of the two became the famous ‘Hubble constant’ and represents the expansion rate of the universe.

10. Developmental Biology

One thing that you, me, and many other animals have in common is the segmentation clock. During our embryo development, spatial and temporal cues are integrated to form a specific number of embryonic segments that, later on, gives rise to corresponding ribs and vertebrae. The underlying mechanism was thought to operate like a conventional clock that ticks with a precise period: one tick of the clock equals one new segment. There is something called a gene expression wave that travels from the posterior towards the anterior of the animal (from the tip of the tail towards the head). As they do, the embryo develops, changing its shape, and the tissue in which the waves travel shortens. This leads to a relative motion of the anterior end (the observer) of the tissue, where the new segments form, towards the posterior (the source). This motion of the observer into travelling gene expression waves leads to a Doppler effect in the developing embryo. Since this timing, as mentioned above, determines the number and size of the body segments, it affects the number and size of the developing ribs and vertebrae.

11. Sonic Boom

A sonic boom is an acoustic disturbance caused by supersonic flow over an aircraft’s surface. Supersonic flow creates a discontinuous shock boundary that emanates from the aircraft’s surface, and the shock wave propagates behind the aircraft with a large amount of energy.

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