Sound is a longitudinal wave caused by vibrating objects. The vibrations create compressions and rarefactions in the surrounding medium (air, water, or solid).
How sound is produced and transmitted
- A source vibrates (speaker cone, vocal cords, tuning fork).
- Vibrations push air molecules together (compression) then apart (rarefaction).
- This pattern of compressions and rarefactions travels outward through the air.
- When it reaches the ear, the compressions and rarefactions cause the eardrum to vibrate.
Sound needs a medium
Sound cannot travel through a vacuum because there are no particles to vibrate. The classic demonstration: a ringing bell in a jar gradually becomes inaudible as air is pumped out.
Speed of sound
| Medium | Approximate speed |
|---|---|
| Air (at room temperature) | 340 m/s |
| Water | 1500 m/s |
| Steel | 5000 m/s |
Sound travels fastest in solids (particles closest together) and slowest in gases (particles furthest apart).
Loudness and pitch
| Property | Depends on | Increase means |
|---|---|---|
| Loudness | Amplitude | Louder sound |
| Pitch | Frequency | Higher-pitched sound |
Audible range
The human ear can detect frequencies from about 20 Hz to 20 000 Hz (20 kHz).
- Below 20 Hz: infrasound (cannot be heard).
- Above 20 000 Hz: ultrasound (cannot be heard).
Common errors and how to correct them
Saying “sound travels in waves through a vacuum.” Sound requires a medium. It is electromagnetic waves (light, radio) that travel through a vacuum.
Confusing loudness and pitch. Loudness depends on amplitude; pitch depends on frequency. A loud, low sound has large amplitude but low frequency.
How to apply this in an exam
When explaining sound, state it is a longitudinal wave, name the medium, and describe compressions and rarefactions. For the vacuum demonstration, explain that removing air removes the particles needed to transmit the vibrations.
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