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Loudness & Pitch

Key idea: O Level sound: relate loudness to amplitude and pitch to frequency, interpret wave diagrams, and use v = fλ to link frequency and wavelength.

  • SEC G3 Physics 2027
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Learning objectives

  • Describe wave generation by vibrating sources, ropes and springs
  • Describe ripple-tank waves using wavefronts
  • Explain that waves transfer energy
  • Explain that wave energy transfer does not transfer matter
  • Use amplitude, frequency and wavelength to describe wave motion
  • Define and use wave speed and period and interpret wave graphs
  • Recall and apply wave speed = frequency × wavelength
  • Compare transverse and longitudinal waves and give examples
  • Explain sound production by vibration and the need for a medium
  • Describe sound using compressions and rarefactions
  • Relate sound loudness to amplitude and pitch to frequency
  • Explain reflected-sound echoes and use them to measure distance
  • Explain ultrasound use in sonar and soft-tissue scanning
  • Use the normal, angle of incidence and angle of reflection
  • Apply the law of reflection in constructions, measurements and calculations
  • Use the normal, angle of incidence and angle of refraction
  • Apply sin i divided by sin r as a constant for a fixed pair of media
  • Define refractive index as vacuum light speed divided by medium light speed
  • Explain the critical angle
  • Explain the conditions for total internal reflection
  • Apply total internal reflection to optical fibres and state advantages
  • Describe how a thin converging lens acts on a light beam
  • Define the focal length of a converging lens
  • Construct real and virtual image ray diagrams for a thin converging lens
  • Describe lens images as real or virtual, magnified or diminished, and upright or inverted

1. Definition

A. Loudness

Loudness is how loud or soft a sound seems. For otherwise identical conditions, a larger sound-wave amplitude produces a louder sound.

B. Pitch

Pitch is how high or low a sound seems. It is related to the frequency of the sound wave.

2. Key Ideas

  • Larger amplitude → louder sound.
  • Higher frequency → higher pitch.
  • If the wave speed is fixed (same medium), higher frequency means shorter wavelength because v = fλ.
Comparing loudness and pitch from sound tracesFour pressure-time traces compare soft and loud sounds at the same frequency, then low- and high-pitched sounds at the same amplitude.Same frequency: compare amplitudeSofterLoudersmaller amplitudelarger amplitudeSame amplitude: compare cycles in the same timeLower pitchHigher pitchfewer cyclesmore cycles
On pressure–time traces with identical scales, amplitude compares loudness and cycles per second compare pitch. These curves represent changing pressure; sound itself remains longitudinal.

3. Detailed Explanations

A. Loudness depends on amplitude

Amplitude is the maximum variation from the equilibrium position (for sound, this relates to how large the pressure variations are).

  • bigger amplitude → larger vibrations → sound is louder
  • smaller amplitude → smaller vibrations → sound is softer

B. Pitch depends on frequency

Frequency is the number of complete oscillations per second.

  • higher frequency → higher pitch (a “higher note”)
  • lower frequency → lower pitch (a “lower note”)
What you need for this course

You should be able to relate loudness ↔ amplitude and pitch ↔ frequency.

C. Using wave diagrams

If you are given two wave diagrams:

  • compare the height (amplitude) to decide loudness
  • on a displacement- or pressure–time graph, compare the number of cycles in the same time to decide frequency and pitch
  • on a distance graph, spacing gives wavelength; infer pitch from wavelength only when the waves travel at the same speed

The drawn curve is a graph of pressure or displacement; it is not a picture of sound moving up and down. Sound remains longitudinal.

4. Common Mistakes

  • Saying loudness depends on frequency (loudness is mainly about amplitude).
  • Saying pitch depends on amplitude (pitch is mainly about frequency).
  • Comparing two diagrams but forgetting the axes scale might be different.
  • Treating a pressure–time trace as the physical shape of a transverse wave.

5. Exam Tips

  • Write short, direct exam phrases:
    • “Wave A is louder because it has a larger amplitude.”
    • “Wave B has a higher pitch because it has a higher frequency.”
  • If the question gives wavelength instead of frequency, use v = fλ (same medium → same v).

6. Worked Examples

Modelled example 1

Loudness from amplitude

Core

Problem

Two sound waves have the same frequency, but wave A has a larger amplitude than wave B. Which wave sounds louder?
Study the worked solution

  1. Select the relevant wave quantity

    Method

    Compare amplitudes, not frequencies.

    Reason

    Loudness is linked to the size of the pressure or displacement variation.

    Working

    Wave A: larger amplitude → louder sound.

  2. Keep pitch separate

    Method

    State that both sounds have the same pitch.

    Reason

    The frequencies are equal even though the amplitudes differ.

    Working

    Same frequency → same pitch.

Guided practice 2

Pitch from frequency

About 4 min

Problem

Two sound waves have the same amplitude, but wave A has a higher frequency than wave B. Which wave has the higher pitch, and what can be concluded about loudness?

Match each perception to its wave quantity

Higher-pitch wave

Hints

Hint 1: focus on pitch
Pitch depends on frequency.
Hint 2: treat loudness separately
Equal amplitude means neither wave is louder under the stated conditions.
View solution step by step

  1. Compare pitch

    Method

    Select wave A as the higher-pitch sound.

    Reason

    Wave A has the higher frequency.

    Working

    Higher frequency → higher pitch.

  2. Compare loudness

    Method

    State that the sounds have the same loudness under otherwise identical conditions.

    Reason

    Their amplitudes are equal.

    Working

    Same amplitude → same loudness.

Common misconception 3

Wavelength comparison (same medium)

Find and correct the mistake

Learner response

Two notes travel through the same air. Note A has frequency 200 Hz and note B has frequency 400 Hz. A student says B has the longer wavelength because it completes more cycles each second. Diagnose the claim.

Hold wave speed constant

Shorter wavelength

View solution step by step

  1. Fix the medium's wave speed

    Method

    Use the same sound speed for both notes.

    Reason

    Both waves travel through the same air under the stated conditions.

    Working

    v = fλ

  2. Apply the inverse relationship

    Method

    Select note B as having the shorter wavelength.

    Reason

    At fixed speed, wavelength is inversely proportional to frequency.

    Working

    Frequency doubles: 200 → 400 Hz, so wavelength halves.

Examiner practice 4

Pitch from wavelength (same medium)

3 marks

Examination question

Two sound waves travel through the same air. Wave A has wavelength 0.85 m and wave B has wavelength 0.42 m. State which has the higher pitch and explain. [3 marks]

State the comparison and full reasoning chain

View solution step by step

  1. Relate wavelength to frequency

    2 marks

    Method

    Keep speed constant and infer that wave B has the higher frequency.

    Reason

    For the same medium, v is the same and f = v/λ.

    Working

    Shorter wavelength → higher frequency.

  2. Relate frequency to pitch

    1 mark

    Method

    Conclude that wave B has the higher pitch.

    Reason

    Pitch increases with frequency.

    Working

    Wave B: higher frequency → higher pitch.

Challenge 5

Frequency change and wavelength

Minimal support

Changed-source transfer

A note in air changes from 250 Hz to 500 Hz while the medium stays the same. Predict what happens to both pitch and wavelength.

Track the perceptual and wave changes separately

Hints

Hint 1: first decide pitch
The frequency doubles.
Hint 2: then hold speed fixed
In unchanged air, use λ = v/f.
View solution step by step

  1. Predict pitch

    Method

    State that the pitch becomes higher.

    Reason

    Frequency increases from 250 to 500 Hz.

    Working

    Higher frequency → higher pitch.

  2. Predict wavelength

    Method

    State that the wavelength halves.

    Reason

    The sound speed remains constant in the same medium, so λ varies inversely with f.

    Working

    λ_new = v/2f = (1/2)λ_old

7. Mind Stretchers

Mind stretcher 1: Same pitch, different loudnessExtension

Two tuning forks have the same frequency but are struck with different force. What changes about the sound and what stays the same?

Show Answer

The pitch stays the same (frequency unchanged), but the loudness changes because the amplitude is larger when struck harder.

Mind stretcher 2: Same loudness, different pitchExtension

A person can sing the same loudness but at different notes. What changes in the sound wave when the note changes?

Show Answer

The frequency changes (so the pitch changes). If the loudness is kept the same, the amplitude is roughly unchanged.

8. Practice and next step

Draw four sound traces that isolate amplitude and frequency changes, then annotate loudness and pitch separately. Continue to echoes and speed of sound.

Continue with the next resource in this course.

Course and syllabus information
Course
SEC G3 Physics
Edition
SEC G3 Physics 2027