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Doppler Effect LO5

History

❖ Doppler effect is named after the Austrian physicist Christian Doppler

❖ Christian Andreas Doppler (29 November 1803 – 17 March 1853)

What is the Doppler Effect ?

❖ The Doppler Effect is an increase or decrease in the frequency of sound waves as the sound-producing object moves toward or away from an observer.

Doppler Effect Example: Ambulance Siren

❖ As an ambulance approaches, the siren's sound waves become more frequent (compressed together).

❖ As the ambulance drives away, the siren's sound waves become less frequent (stretched apart).

Uses of Doppler EffectDoppler Radar Gun - Police Speed Detector

❖ A radar speed gun (also radar gun and speed gun) is a device used to measure the speed of moving objects. It is used in law-enforcement to measure the speed of moving vehicles and is often used in professional spectator sport, for things such as the measurement of bowling speeds in cricket, speed of pitched baseballs, athletes and tennis serves.

❖ It measures the speed of the objects at which it is pointed by detecting a change in frequency of the returned radar signal caused by the Doppler effect, whereby the frequency of the returned signal is increased in proportion to the object's speed of approach if the object is approaching, and lowered if the object is receding.

❖ The Doppler effect can be observed for any type of wave - water wave, sound wave, light wave, etc. We are most familiar with the Doppler effect because of our experiences with sound waves.

❖ Perhaps you recall an instance in which a police car or emergency vehicle was traveling towards you on the highway. As the car approached with its siren blasting, the pitch of the siren sound (a measure of the siren's frequency) was high; and then suddenly after the car passed by, the pitch of the siren sound was low. That was the Doppler effect - an apparent shift in frequency for a sound wave produced by a moving source.

Formula

❖ The equation that describes this effect can be used whether the source is approaching or receding from the observer

❖ perceived frequency = actual frequency(speed of sound +speed of observer/speed of sound-speed of source)

❖ or f = fo(V+Vo)/(V-Vs)

❖ Here, fo refers to the actual frequency being emitted by an object, while f is the frequency heard by the observer as the source approaches or recedes. If a source approaches, the perceived frequency will be higher than the actual frequency. If a source recedes, the perceived frequency is lower than the actual frequency.

❖ In order for this equation to work properly, there is a standard convention to which you must adhere whenever solving Doppler exercises.

❖ Vo is (+) if the observer moves toward the source.

❖ Vo is (-) if the observer moves away from the source.

❖ Vs is (+) if the source moves toward the observer.

❖ Vs is (-) if the source moves away from the observer.

Example Question

❖ Sitting on the beach, Sunny finds herself beneath the flight path of the airplanes. What frequency will Sunny hear as a jet, whose engines emit sound at a frequency of 1000. Hz, flies toward her at a speed of 100.0 m/s?

❖ Solution: First draw a diagram of the situation. Notice in the calculation below that Sunny is sitting at rest and the plane is approaching. Therefore, the source is moving toward the observer. The observer remains stationary.

❖ In the previous example, what frequency will Sunny observe as the jet travels away from her at the same speed?

❖ Solution: Again, draw a diagram of the situation. This time, the source is moving away from the observer, so the value for vs must be negative.

Example Question

❖ The following figure shows pressure vs. time for a sound wave when the source is

stationary with respect to the observer.

❖ Which of the following best represents the sound wave if the source is moving towards the observer?

Answer: d