Cambridge IGCSE Physics 0625

Topic: 3.4 Sound

Objective: Describe how changes in amplitude and frequency affect the loudness and pitch of sound waves

This section explores the relationship between the physical properties of sound waves – amplitude and frequency – and how these properties are perceived as loudness and pitch, respectively.

Amplitude and Loudness

The amplitude of a sound wave is the maximum displacement of the air molecules from their rest position. This displacement corresponds to the intensity of the sound wave.

Loudness is our perception of the intensity of a sound wave. A sound wave with a larger amplitude has a greater intensity and is therefore perceived as louder. The relationship between amplitude and loudness is approximately logarithmic.

• Larger amplitude means greater intensity means louder sound.
• Smaller amplitude means lower intensity means quieter sound.

Frequency and Pitch

The frequency of a sound wave is the number of complete oscillations per second, measured in Hertz (Hz). Frequency determines the perceived pitch of a sound.

Pitch is our perception of how high or low a sound is. A sound wave with a higher frequency is perceived as having a higher pitch, while a sound wave with a lower frequency is perceived as having a lower pitch. The relationship between frequency and pitch is linear.

• Higher frequency means higher pitch.
• Lower frequency means lower pitch.

Summary Table

Property	Effect on Sound
Amplitude	Determines Loudness (Intensity)
Frequency	Determines Pitch

Mathematical Relationship

The intensity (I) of a sound wave is related to its amplitude (A) by the following equation:

$$I = A^2$$

This shows that the intensity is proportional to the square of the amplitude.

The frequency (f) of a sound wave is related to its period (T) by the following equation:

$$f = \frac{1}{T}$$

Examples

1. A loud sound has a large amplitude.
2. A high-pitched sound has a high frequency.
3. A low-pitched sound has a low frequency.
4. A quiet sound has a small amplitude.