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

3.4 Sound

🔊 Amplitude and Loudness

The amplitude of a sound wave is the maximum displacement of particles from their rest position. A larger amplitude means the particles move more, which carries more energy.

• Loudness is how we perceive the energy of a sound.
• Loudness increases with amplitude.
• In physics, the intensity (power per unit area) of a wave is proportional to the square of its amplitude:

$$I \propto A^{2}$$

Where:

• $I$ = intensity (W/m²)
• $A$ = amplitude (m)

Doubling the amplitude makes the sound about four times more intense, which we hear as noticeably louder.

🎵 Frequency and Pitch

The frequency of a sound wave is the number of complete vibrations (cycles) per second, measured in hertz (Hz).

• Pitch is how high or low a sound seems.
• Pitch increases with frequency.
• Human hearing ranges roughly from $20\,\text{Hz}$ to $20{,}000\,\text{Hz}$.

For example:

1. A tuning fork at $256\,\text{Hz}$ produces a middle‑C note (low pitch).
2. A whistle at $4{,}000\,\text{Hz}$ produces a sharp, high‑pitched sound.

📊 Summary Table

?? Quick Check

1. If you strike a drum harder, what happens to the amplitude and loudness?
2. What change in frequency would make a sound appear higher‑pitched?
3. True or False: Doubling the amplitude makes the sound twice as loud.

Answers:

• Amplitude increases → sound becomes louder.
• Increase the frequency (more cycles per second).
• False – loudness roughly follows the square of amplitude, so it becomes about four times louder.