Transmission of Sound Waves (AQA GCSE Physics)

• Sound waves are longitudinal waves
  • They transfer energy by the molecules vibrating and knocking into neighbouring molecules

• The more molecules that are present the faster the wave can transfer energy, therefore:
  • Sound waves travel fastest in solids
  • Sound waves travel slowest in gases

• When sound waves move from one medium to another, there will be changes to its:
  • Wave speed
  • Frequency
  • Wavelength

• The relationship between the wave speed, wavelength and frequency can be determined using the wave equation
• This change in velocity can also result in a change of direction of the sound wave
  • This phenomenon is also known as refraction

Refraction of Sound

• When sound moves from a denser medium to a less dense medium:
  • The wavelength of the sound wave decreases
  • The frequency of the sound wave stays the same
  • The velocity of the sound wave decreases
    

• When sound moves from a less dense medium to a denser medium:
  • The wavelength of the sound wave increases
  • The frequency of the sound wave stays the same
  • The velocity of the sound wave also increases

• The speed of sound in air is also affected by temperature
  • On warm days, air molecules move faster, hence they carry sound waves faster, increasing the speed of sound
  • On cold days, air molecules move at a slower pace, so they carry sound waves at a slower pace, decreasing the speed of sound

Step 1: Identify the difference in conditions between the day and the night

• • During the day, it is warmer near the ground and cooler in the atmosphere
  • At night, it is cooler near the ground and warmer in the atmosphere

Step 2: Consider the effect of sound waves travelling between the two air temperatures

• • Sound waves can travel faster in hot air and slower in cold air
  • This change in temperature will cause the sound waves to refract

Step 3: Explain the motion of the waves during the day and the night

• • During the day, the sound wave will travel faster in the warm air and refract towards the sky as the wave slows down
  • At night, the sound wave will travel slower in the cool air and refract towards the ground as the wave speeds up

Step 4: Relate the explanation back to the question

• • Sound travels further at night because its speed increases as it moves from a denser medium (cold air) to a less dense medium (warm air)