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Year 11 Science

Sound Waves

Investigate how sound travels as longitudinal waves, exploring frequency, pitch, loudness, the speed of sound in different media and the phenomenon of resonance.

Sound as a Longitudinal Wave

Sound is a mechanical wave that requires a medium (solid, liquid or gas) to travel through. Unlike transverse waves (like light), sound is a longitudinal wave -- the particles vibrate parallel to the direction of energy transfer, creating alternating regions of compression and rarefaction.

Compressions and Rarefactions

C

R

C

R

C

C = Compression (particles close together, high pressure)  |  R = Rarefaction (particles spread apart, low pressure)

One wavelength (λ) = distance from one compression to the next

Key equation: v = f × λ  where v = speed (m/s), f = frequency (Hz) and λ = wavelength (m). This applies to all waves.

Properties of Sound

The way we perceive sound depends on its physical properties: frequency determines pitch, and amplitude determines loudness.

Frequency & Pitch

Frequency is the number of complete vibrations per second, measured in hertz (Hz).

Higher frequency = higher pitch. Humans hear from about 20 Hz to 20,000 Hz. Below 20 Hz is infrasound; above 20 kHz is ultrasound.

Amplitude & Loudness

Amplitude is the maximum displacement of particles from their rest position.

Greater amplitude = louder sound. Loudness is measured in decibels (dB). A whisper is ~30 dB; a rock concert can exceed 110 dB.

Speed of sound depends on the medium: Sound travels at ~340 m/s in air (at 20 °C), ~1500 m/s in water, and ~5000 m/s in steel. Sound travels fastest through solids because particles are closest together, allowing vibrations to transfer more quickly.

Resonance and the Doppler Effect

Resonance occurs when an object is forced to vibrate at its natural frequency, causing the amplitude of vibration to increase dramatically. This is why a singer can shatter a glass -- the sound wave frequency matches the glass's natural frequency.

Resonance

Every object has a natural frequency. When an external force vibrates the object at that frequency, energy builds up and the amplitude grows. Musical instruments use resonance -- e.g. the body of a guitar amplifies sound by resonating with the strings.

Doppler Effect

When a sound source moves towards you, compressions are pushed closer together (higher frequency = higher pitch). When it moves away, compressions spread out (lower frequency = lower pitch). This explains the pitch change of a passing siren.

Sound Cannot Travel Through a Vacuum

Since sound is a mechanical wave, it requires particles to transfer vibrations. In space (a vacuum), there are no particles, so sound cannot travel. This is a fundamental difference from electromagnetic waves (like light) which can travel through empty space.

Key Vocabulary

Longitudinal Wave

A wave where particles vibrate parallel to the direction of energy transfer, creating compressions and rarefactions.

Frequency (Hz)

The number of complete wave cycles per second. Measured in hertz (Hz). Determines pitch for sound waves.

Resonance

The dramatic increase in amplitude when an object vibrates at its natural frequency due to an external driving force.

Doppler Effect

The apparent change in frequency (and pitch) of a wave when the source and observer are in relative motion.

Worked Examples

1

A sound wave has a frequency of 440 Hz and travels at 340 m/s in air. Calculate its wavelength.

Step 1: Use v = f × λ, rearranged to λ = v / f.

Step 2: λ = 340 / 440.

Answer: λ = 0.773 m (approximately 77 cm).

2

Explain why sound travels faster in water than in air.

Step 1: In water, molecules are much closer together than in air (higher density and stronger intermolecular forces).

Step 2: When molecules are closer, vibrations transfer from one molecule to the next more rapidly.

Answer: The closer spacing of particles in liquids allows compressions to propagate faster (~1500 m/s vs ~340 m/s).

3

An ambulance siren sounds higher-pitched as it approaches you and lower-pitched as it moves away. Explain this using the Doppler effect.

Step 1: As the ambulance approaches, each successive compression is emitted from a position closer to you, so compressions arrive more frequently.

Step 2: This increases the observed frequency, which you perceive as a higher pitch.

Step 3: As it moves away, compressions are emitted from further away, spreading them out. Lower observed frequency = lower pitch. The actual frequency emitted by the siren does not change.

Knowledge Check

Select the correct answer for each question. Click "Check Answer" to see if you are right.

Question 1

Sound is classified as which type of wave?

Question 2

A sound wave with wavelength 0.5 m travels at 340 m/s. What is its frequency?

Question 3

Sound cannot travel through:

Question 4

A louder sound, compared to a quieter sound of the same pitch, has:

Question 5

The Doppler effect explains why:

Key Concepts Summary

Year 11: Rates of Chemical Reactions Year 11: Light and EM Waves