Pulse Echo and Doppler

Pulse Echo and Doppler

 The Doppler Effect: 

When a source, such as a sound wave, moves towards an observer, the observed frequency increases and the wavelength decreases. Similarly, when a source moves away from an observers, the observed frequency decreases and the wavelength increases. This is known as the Doppler effect and can be explained in terms of the wave crests. As the wave source moves towards the observer, each wave crest that is emitted is closer to the observer than the previous wave. This means the wave takes less time to reach the observer than the previous wave. Therefore the time between waves arriving at the observer is decreased, causing an increase in frequency.

When the wave is travelling away from the observer, each wave is emitted from a distance greater than the previous wave. This causes an increase in time interval between the waves, reducing the frequency. This causes the change in pitch when a sound passes an observer.

 Pulse Echo: 

Pulse echo technique refers to the emission of a sound wave to an object an unknown distance away. An example of this is being on a boat and wanting to calculate the distance to the ocean floor below you. First, you would send an ultrasound pulse into the water, which would eventually hit the bottom and reflect back upwards to the boat. A detector on the boat could then record the time taken for the wave to complete the trip. Knowing the speed of sound to be 340m/s, you could use s=d/t in order to calculate the distance. This distance is the complete cycle and so to determine the distance from the boat to the ocean floor we must divide by 2.