Refraction, Diffraction and Resonance – Practice


1) A hiker stands on one side of a valley and wants to know how far it is to the other side. He shouts across the valley and times how long it takes to hear his echo. He records a time of 4.2 seconds. Calculate the distance across the valley: (speed of sound in air = 340 m/s)

2) A ship is using sonar to determine the depth of a section of the Pacific ocean. It sends out a pulse of sound and records a time of 0.84 seconds. Calculate the depth of the ocean at this position: (speed of sound in seawater = 1500 m/s)

3) Bats can detect prey by sending out sound waves with a wavelength that corresponds to the size of the prey and detecting the echo. The typical frequency of the sound emitted by bats is 50 kHz. How big will an insect be that causes an echo to reflect back to the bat? (speed of sound in air = 340 m/s)

4) A driver in a car notices that as they drive along the car starts shaking at 75 km/hr. Any faster or slower and the vibration stops. Explain why this is happening:


Quick Answers:

1) 714m

2) 630m

3) 6.8mm

4) The shaking is caused by the driving frequency of the vehicle at 75 km/hr matching the resonant frequency of the source of vibration. The source of the vibration could be the engine, air resistance or the effective wind at that speed. Faster or slower than 75 km/hr results in a resonant frequency different to the driving frequency of the source

Worked Answers:

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