Reflection of Sound

Sound waves, like light w+aves, can be reflected when they hit a large, hard surface (e.g., wall, building, cliff).

• The laws of reflection apply to sound as well:
  • The angle of incidence = the angle of reflection.
  • Incident wave, reflected wave, and normal all lie in the same plane.
• Applications:
  • Megaphones and horns use reflection to direct sound in one direction.
  • Concert halls use reflectors to distribute sound evenly.
  • Stethoscopes use reflection of sound in tubes.

Echo

An echo is a reflected sound heard after the original sound when it bounces off a large surface such as a wall, mountain, or building.

• For an echo to be heard clearly:
  • The reflecting surface must be at least 17 m away (because the human ear can only distinguish two sounds if the time gap is at least 0.1 s).
  • Speed of sound in air ≈ 340 m/s → in 0.1 s, sound travels about 34 m → so the wall must be half that distance (17 m away).
• Formula for echo method:
  

  \( v = \dfrac{2d}{t} \)

  • \(v\) = speed of sound (m/s)
  • \(d\) = distance to wall (m)
  • \(t\) = time taken for echo (s)

Applications of Echoes

• Measuring speed of sound (echo method).
• SONAR (Sound Navigation and Ranging) used by ships and submarines to detect depth of sea and obstacles.
• Ultrasound scanning in medicine to see internal organs.
• Animals like bats and dolphins use echoes for navigation (echolocation).