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3.3.1Waves

Sound Waves

5 min read 5 parts Lesson 16 of 29 2 quizzes

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Section 1 of 5The Nature of Sound

3.3.1 Sound Waves
Longitudinal wave
  • Sound waves consist of compressions and rarefactions, with particles vibrating parallel to the direction of travel.
Medium
  • Sound needs particles to pass on the vibration, so it cannot travel through a vacuum.
Hearing range
  • Humans can detect frequencies between 20 Hz and 20 000 Hz, known as the range of human hearing.
Pressure variations
  • Compressions and rarefactions cause small changes in pressure that vary with time, which is why sound is a pressure wave.

Section 2 of 5Pitch and Loudness

Pitch and Loudness
Pitch
  • Pitch depends on frequencyhigh-frequency sounds have a high pitch and a short wavelength.
Loudness
  • Loudness depends on amplitudea larger amplitude gives a louder sound.
Speed
  • The speed of sound depends on the medium and temperature, not on the frequency of the sound.
Diffraction
  • Sound diffracts through gaps and around objects, so it can be heard even when the source is out of sight.

Section 3 of 5Echo Sounding

1
A ship emits a short pulse of sound downwards from its hull.
2
The pulse reflects from the seabed and returns to a detector on the ship.
3
The time between emission and return is measured accurately.
4
Depth is found from distance = speed × time, then halved, because the sound travels down and back.

Section 4 of 5Worked Example

Question

  • An echo sounder sends a pulse into the sea, and the echo returns after 0.80 s.
  • The speed of sound in sea water is 1500 m/s.
  • Calculate the depth of the sea at that point.

  1. Total distance travelled = speed × time = 1500 × 0.80 = 1200 m
  2. The pulse travels down to the seabed and back up, so depth = 1200 ÷ 2
  3. Depth = 600 m

Answer The depth is 600 m.

Section 5 of 5Exam Checklist & Tips

3.3.1 – Key Points for the Exam

  • Compression A region of higher particle density in a sound wave.
  • Rarefaction A region of lower particle density in a sound wave.
  • Pitch How high or low a sound is perceived, determined by its frequency.
  • Echo A sound wave reflected back to the listener or detector from a surface.
  • Amplitude The maximum displacement of a wave, which determines the loudness of a sound.

Common Mistakes

  • Forgetting to halve the distance in echo calculations, since the sound travels to the boundary and back.
  • Saying the frequency of a sound sets its loudness, when amplitude does.
  • Stating that sound travels through a vacuum.
  • Describing compressions as particles simply bunched up, rather than regions of higher density.
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