Unit 9: Sound

Unit 9: Sound

1. Describe how waves are produced by vibrating sources

   • Sound waves are produced when a vibrating surface pushes and pulls on the surrounding substance.

2. Recall and describe the longitudinal nature of sound waves

   • Sound waves are longitudinal waves which means they vibrate along the direction in which the wave travels.

3. Explain what compressions and rarefractions are

   • compressions
     • part of a sound wave where air molecules are pushed together
   • rarefractions
     • part of a sound wave where air molecules are spaced further apart than when they are undisturbed

4. Recall that a medium is required to transmit sound waves and that sound waves cannot pass through a vacuum

   • Sound waves can travel through solids, liquids and gases
   • Sound waves cannot travel through a vacuum

5. Recall the range of frequencies that can be heard by the human ear

   • Young people can usually hear sound frequencies from about 20 Hz to about 18,000 Hz.

6. Describe how to measure the speed of sound in air

   • You and a friend should stand on opposite sides of a field at a measured distance apart. You should be as far away from each other as possible but within sight of each other. If your friend bangs two cymbals together, you will see them crash together straightaway but you will not hear them straightaway. This is because the 'bang' will be delayed as sound travels much slower than light. Use a stopwatch to time the interval between seeing the impact and hearing the sound. Work out the speed of sound in air using the formula: speed = distance / time.

7. Recall 'order of magnitude' values for the speed of sound in air, in liquids and in solids

   • The speed of sound in air is about 340 m/s
   • The speed of sound in water is about 1400 m/s
   • The speed of sound in many solids is more than 3000 m/s

8. Explain how echo sounding can be used to measure distances

   • d = vt / 2, where d=distance, v=speed of sound in water (substance), t=time taken by pulse to travel distance

9. Relate the loudness and pitch of a sound to the amplitude and frequency of the sound waves

   • The loudness of a note increases if the amplitude of the sound waves increases
   • The pitch of a note increases if the frequency of the sound waves increases

10. Compare the loudness and pitch of different sounds from their waveforms

    • comparing A, B and C waveforms from picture
    • A and B have the same amplitude and therefore the same loudness, however C has a smaller amplitude so C is not as loud as A or B.
    • C has more waves shown than A or B, which means C has a higher frequency (i.e. pitch) than A, which also has a higher frequency than B.

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