Loudness & Pitch

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.

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”)

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

Example 1: Loudness from amplitudeCore

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

Show Answer

Wave A sounds louder because it has a larger amplitude.

Example 2: Pitch from frequencyCore

Two sound waves have the same amplitude, but wave A has a higher frequency than wave B. Which wave has the higher pitch?

Show Answer

Wave A has the higher pitch because it has a higher frequency.

Example 3: Wavelength comparison (same medium)Core

Two notes travel through the same air. Note A has frequency 200 Hz and note B has frequency 400 Hz. Which has the shorter wavelength?

Show Answer

In the same medium, v is the same and v = fλ. Higher f means smaller λ, so note B has the shorter wavelength.

Example 4: Pitch from wavelength (same medium)Core

Two sound waves travel through the same air. Wave A has wavelength 0.85 m and wave B has wavelength 0.42 m. Which has the higher pitch?

Show Answer

In the same medium, wave speed v is the same and v = fλ, so fpropto 1/λ. The shorter wavelength means higher frequency (higher pitch), so wave B has the higher pitch.

Example 5: Frequency change and wavelengthCore

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

Show Answer

Pitch increases because frequency increases. Since v stays the same in the same medium and v = fλ, doubling f halves λ.

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.