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PHYSICS · Stage 7 · Chapter 3: Sound

Physics 3.4 Chapter 3 Review: Sound

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3.4 Chapter 3 Review: Sound — Alhussan International Academy — Physics (Stages 7–9) notes: Revise how sound travels, its speed in different media, the ear, microphones and loudspeakers, pitch and loudness, and echo and sonar calculations.Sound — it is a wave of vibrations that ne…

  • Order of speeds
  • Comparisons
  • Range of hearing
  • Ultrasound
  • Distance given
  • Transducer

Section 1 of 5Speed of sound in different media

  • Sound
    • it is a wave of vibrations that needs a material (a medium) to travel through.
  • Vacuum
    • sound cannot travel through a vacuum because there are no particles to pass on the vibration.
  • Order of speeds
    • sound is fastest in solids, slower in liquids and slowest in gases,
    • because the particles are closer together in a solid so each passes the vibration on more quickly.
MediumApproximate speed of sound
Air330 m/s
Water1500 m/s
SteelAbout 5000 m/s
  • Comparisons
    • the speed of sound in water is greater than in air because the particles are closer together in water.
    • The speed in air is smaller than in steel because the particles are closer together, and held more firmly, in steel.

Section 2 of 5Pitch, loudness and range of hearing

  • Frequency
    • the number of vibrations each second, measured in hertz (Hz), which determines pitch.
    • A higher frequency gives a higher pitch.
  • Amplitude
    • the size of the vibration, which determines loudness.
    • A bigger amplitude gives a louder sound.
  • Range of hearing
    • humans can typically hear from about 20 Hz to 20 000 Hz.
  • Ultrasound
    • sound above 20 000 Hz, used in sonar and in scanning.

Section 3 of 5Sonar and echo calculations

  • Sonar
    • it sends a pulse of ultrasound and detects its reflection.
  • Echo time
    • the recorded time is for the journey to the object and back.
    • distance to object = speed × (echo time ÷ 2).
    • echo time = 2 × distance ÷ speed.
  • Uses
    • a fisherman uses sonar to find a shoal of fish by timing the reflection, and a dolphin finds a fish by making a click and listening for its echo.

Section 4 of 5Method for echo questions

  1. Write down the speed and the time given and check the units are m/s and s.
  2. If the time is the echo time, divide by 2 to get the one-way time.
  3. Use distance = speed × time.
  4. State the answer with a unit.
  • Distance given
    • if a question gives the distance and asks for the time, first double the distance (there and back), then use time = distance ÷ speed.

Section 5 of 5Sound in the body and in devices

  • The ear
    • sound waves reach the outer ear, make the eardrum vibrate and pass through the ossicles to the cochlea,
      • where hair cells send signals along the auditory nerve to the brain.
  • Microphone
    • a transducer that converts sound into an electrical signal.
  • Loudspeaker
    • a transducer that converts an electrical signal into sound.

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