SoundCopy
Cheat Sheet: Sound and Ultrasound (O Level / IGCSE Physics)
1. Production of Sound
- Sound is produced by vibrating sources
- E.g. guitar strings, vocal cords, tuning forks
- Vibrations cause surrounding particles to oscillate → sound wave
2. Nature of Sound Waves
- Sound waves are longitudinal
- Vibrations are parallel to direction of wave travel
- Made up of:
- Compressions: regions of high pressure (particles close)
- Rarefactions: regions of low pressure (particles far apart)
3. Audible Frequency Range
- Human hearing range: 20 Hz to 20,000 Hz (20 kHz)
4. Sound Cannot Travel in a Vacuum
- Sound requires particles to carry vibrations
- Vacuum = no particles → no sound transmission
Experiment:
- Place a ringing bell in a sealed bell jar connected to a vacuum pump
- As air is removed, sound becomes fainter and eventually stops
5. Amplitude and Frequency Effects
| Property | Affects | Description |
|---|---|---|
| Amplitude | Loudness | Larger amplitude = louder sound |
| Frequency | Pitch | Higher frequency = higher pitch |
6. Timbre (Sound Quality)
- Different sources have different wave shapes on an oscilloscope
- Timbre = quality of sound, depends on waveform
- Pure tone = smooth sine wave
- Musical instrument = complex wave
7. Echo
- Echo: sound wave reflected off a surface and heard again
- Delay depends on distance and speed of sound
- Must be ≥0.1 s apart from original sound to be distinguishable
8. Experiment for Reflection of Sound
- Clap hands near a wall and measure delay of echo
- Or use two microphones and an oscilloscope to detect reflected sound
9. Measuring Speed of Sound in Air
Method 1: Echo method
- Measure time taken for echo to return after sound is made
- Measure distance to reflecting surface
- Speed = 2 × distance / time
Method 2: Two Microphones
- Place microphones a known distance apart
- Clap near one and measure time delay between signals
- Speed = distance / time delay
10. Approximate Speed of Sound
- In air: 330–350 m/s (varies slightly with temperature and humidity)
11. Speed of Sound in Different Media
- Fastest in solids
- Slower in liquids
- Slowest in gases
12. Ultrasound
- Ultrasound = sound with frequency > 20 kHz
- Inaudible to humans
- Used in various scientific and medical applications
13. Uses of Ultrasound
| Application | Description |
|---|---|
| Cleaning | Vibrations dislodge dirt from delicate objects (e.g. jewellery, instruments) |
| Prenatal scanning | Reflected waves create image of fetus |
| Medical scanning | Imaging soft tissues, organs |
| SONAR | Detect depth/distance underwater (submarines, fish finders) |
- Depth = (speed × time) / 2 (divide by 2 for round trip)