Sound — Drills · Grade 12 Science

Chapter: Waves, Sound & Optics

What these drills covers

By the end of Sound you should be able to:

6 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: longitudinal wave, pitch, loudness, medium, Doppler effect, resonance, standing wave.

Problem 1

Unlike light, a sound wave requires…
  1. a source of light
  2. a vacuum
  3. a medium (matter to travel through)
  4. a magnetic field
Answer and explanation

a medium (matter to travel through)

Sound is a mechanical wave and needs a medium of particles; it cannot travel through a vacuum.

Problem 2

Increasing the frequency of a sound wave produces a…
  1. louder sound
  2. lower pitch
  3. softer sound
  4. higher pitch
Answer and explanation

higher pitch

Frequency sets pitch — a higher frequency means a higher pitch.

Problem 3

Using the speed of sound as 343 m/s, how far away is a storm if thunder is heard 6 seconds after the lightning flash?
Answer and explanation

2058 m

distance = speed × time = 343 × 6 = 2,058 m.

Problem 4

A source approaches you at 40 m/s while emitting a 450 Hz tone. Using the speed of sound as 340 m/s, what frequency do you hear? (f′ = f × v ÷ (v − vₛ))
Answer and explanation

510 Hz

f′ = f × v ÷ (v − vₛ) = 450 × 340 ÷ (340 − 40) = 450 × 340 ÷ 300 = 450 × 1.13333 = 510 Hz.

Problem 5

Explain the Doppler effect using a passing ambulance siren as your example.
Answer and explanation

As the ambulance approaches, its sound waves bunch together in front of it, shortening the wavelength and raising the frequency, so you hear a higher pitch. As it passes and moves away, the waves stretch out, lowering the frequency, so the pitch drops. That falling pitch as it goes by is the Doppler effect.

Problem 6

A sound wave in air has a wavelength of 0.25 m. Using the speed of sound as 340 m/s, find its frequency.
Answer and explanation

1360 Hz

f = v ÷ λ = 340 ÷ 0.25 = 1,360 Hz.

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