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:
- A tuning fork at \$256\,\text{Hz}\$ produces a middle‑C note (low pitch).
- A whistle at \$4{,}000\,\text{Hz}\$ produces a sharp, high‑pitched sound.
📊 Summary Table
| Property | What it Changes | Effect on Perception |
|---|
| Amplitude (\$A\$) | Energy / Intensity (\$I \propto A^{2}\$) | Loudness (louder → larger \$A\$) |
| Frequency (\$f\$) | Number of cycles per second (Hz) | Pitch (higher pitch → larger \$f\$) |
??
Quick Check
- If you strike a drum harder, what happens to the amplitude and loudness?
- What change in frequency would make a sound appear higher‑pitched?
- 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.