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Physics - Sound Waves (AQA Syllabus A)

In GCSE Science students will spend some time looking at both electromagnetic and mechanical waves. This is the third of six quizzes on waves and it looks at one particular type of longitudinal wave - sound.

All waves are one of two different types - transverse or longitudinal. When something vibrates, it creates a sound wave. Sound waves are longitudinal waves and the energy they carry causes vibrations in a medium, so they are mechanical waves. Like all mechanical waves, sound waves can only travel in places where there are particles that are sufficienty close to make the next-door particles vibrate. This limits sound to being heard only in solids, liquids and gases. Science fiction films nearly always show sounds in space. This is technically wrong but let's face it, without the sounds these films would be dull and boring!

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The pitch describes how high or low the wave sounds. This depends on how many times it vibrates every second - its frequency. Sounds that are pitched higher than about 20,000 Hz are too high for most human ears to detect, but some animals have much more sensitive hearing than humans and can hear these sound waves. A device that is used by walkers and cyclists to keep aggressive dogs away from themselves uses sound above 20,000 Hz - that irritates the dog and it moves away to a distance where the sound is no longer annoying. Sound below about 20 Hz is too low for most people to hear but is used by whales to communicate with one another. These low pitched sound waves can travel for hundreds of kilometres through the oceans.

The volume of a sound depends on the amplitude of the vibrations. A loud sound has a large amplitude and vice versa. Large amplitude loud sounds carry more energy than small amplitude quiet sounds. When you listen to loud music, for example at a party or disco, or if you are using noisy machinery, the loud sounds carry enough energy to damage your ears. It is generally only a problem to people regularly exposed to these levels of noise as your ears have a defence mechanism. When exposed to loud sounds, muscles tighten the eardrum so that it vibrates less. It takes a while for the muscles to relax after the loud sound which is why your hearing seems a bit 'muffled' for a while.

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  1. Sound is a longitudinal wave. This means that the vibrations travel in which direction?
    The vibrations are in fact backwards and forwards in time with the vibration causing the sound
  2. Which of the following pieces of equipment is not required to see a sound wave?
    You would also need a signal generator and amplifier to make the loudspeaker make a sound and to control the frequency of sound produced
  3. As a sound wave passes, the particles of the medium are alternately compressed and uncompressed. Areas where the particles are not compressed are called what?
    The areas where the particles are compressed by the vibrations of the wave are unsurprisingly called compressions!
  4. Using an oscilloscope, if you heard two waves of identical pitch but one was louder than the other, what would you see?
    The CRO displays sound waves as transverse waveforms. Louder waves carry more energy and therefore the crests will be taller than the quieter sound
  5. A student looked up the speed of sound for several different media and noticed a pattern - it was fastest in solids and slowest in gases. Which of the following statements is the correct reason for this?
    Sound waves travel as vibrations so if the particles are closer and fixed in place, they can pass on the vibrations faster. Gases have a slower speed of sound as their particles are much further apart
  6. Sound of a short wavelength makes what kind of noise?
    A short wavelength means it is vibrating very quickly (has a high frequency)
  7. If you count the seconds between a flash of lightning and the rumble of thunder, then divide the time by three, you get the approximate distance in kilometers to the lightning. Why do you divide by three?
    In three seconds sound will travel 990 metres, which is about a kilometer so dividing the total number of seconds by three gives you the approximate distance the sound will have travelled since it was made. On the surface of the Earth, you can regard light as travelling instantaneously (it could go right round the Earth's equator 7 times in a second)
  8. The Apollo mission astronauts on the surface of the Moon needed radios to talk to each other, even when they were standing beside one another. Why?
    Sound waves require particles to travel from one place to another
  9. Which of the following statements is true of an echo?
    Hard, flat, smooth surfaces create the best echoes
  10. Sound waves diffract. What does this mean?
    Diffraction happens to all waves - light, sound, infrared, gamma etc...

Author: Kev Woodward

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