Sound
1. What Is Sound?
Sound is a form of ENERGY produced by VIBRATING objects. It travels through a medium as a MECHANICAL WAVE.
How Sound Is Produced
- Sound is produced when an object VIBRATES (moves back and forth rapidly).
- These vibrations are passed to neighbouring particles in the medium.
- The vibrations travel as a WAVE through the medium.
Examples of Sound Production
| Object | What Vibrates |
|---|---|
| Human voice | Vocal cords |
| Guitar | Strings |
| Drum | Stretched membrane |
| Tuning fork | Prongs (two metal rods) |
| Bell | Metal body |
2. Propagation of Sound
Sound needs a MEDIUM to travel. It CANNOT travel through VACUUM.
The Bell Jar Experiment
- An electric bell is placed inside a glass jar.
- Air is gradually removed with a vacuum pump.
- The sound becomes FAINTER and finally INAUDIBLE.
- When air is let back in, the sound becomes AUDIBLE again.
Conclusion: Sound requires a MATERIAL MEDIUM for propagation.
Speed of Sound in Different Media
| Medium | Approximate Speed (m/s) |
|---|---|
| Air (at 0°C) | 330 |
| Air (at 20°C) | 340 |
| Water | 1500 |
| Iron/Steel | 5000 |
Sound travels FASTEST in SOLIDS, SLOWER in LIQUIDS, and SLOWEST in GASES.
3. Longitudinal Waves
Sound travels as a LONGITUDINAL WAVE through air.
- Compression: Region where particles are CLOSE together (high pressure).
- Rarefaction: Region where particles are FAR APART (low pressure).
The wave moves FORWARD, but the particles oscillate BACK AND FORTH about their mean position.
4. Characteristics of Sound
Pitch (Frequency)
- Pitch is how HIGH or LOW a sound is.
- Depends on FREQUENCY (number of vibrations per second).
- Unit of frequency: HERTZ (Hz).
- HIGH frequency → HIGH pitch (shrill sound).
- LOW frequency → LOW pitch (deep sound).
| Sound | Frequency Range | Pitch |
|---|---|---|
| Woman's voice | 250-500 Hz | Higher |
| Man's voice | 100-250 Hz | Lower |
| Birds chirping | ~2000 Hz | Very high |
| Lion roar | ~100 Hz | Very low |
Loudness (Amplitude)
- Loudness is how LOUD or SOFT a sound is.
- Depends on AMPLITUDE (maximum displacement from mean position).
- Larger amplitude → LOUDER sound.
- Measured in DECIBELS (dB).
| Sound | Loudness (dB) |
|---|---|
| Whisper | 20-30 |
| Normal conversation | 50-60 |
| Traffic noise | 70-80 |
| Jet engine | 120-140 |
| Threshold of pain | 120+ |
Quality (Timbre)
- Quality distinguishes sounds of same pitch and loudness from different sources.
- Two musical instruments playing the same note sound DIFFERENT due to quality.
- Depends on WAVEFORM (presence of overtones/harmonics).
5. Audible and Inaudible Sounds
| Range | Frequency | Can Humans Hear? |
|---|---|---|
| Infrasonic | Below 20 Hz | NO (elephants, whales can) |
| Audible | 20 Hz to 20,000 Hz (20 kHz) | YES |
| Ultrasonic | Above 20,000 Hz (20 kHz) | NO (dogs, bats, dolphins can) |
Applications of Ultrasound
- Echo sounding (SONAR) to detect submarines and objects underwater.
- Medical imaging (ultrasound scans for pregnancy, organ examination).
- Cleaning delicate objects (jewellery,精密 instruments).
- Detection of cracks in metal blocks.
6. Noise Pollution
Musical Sound vs Noise
| Feature | Musical Sound | Noise |
|---|---|---|
| Waveform | Regular, periodic | Irregular, non-periodic |
| Feeling | Pleasant | Unpleasant |
| Examples | Music, singing | Honking, drilling |
Effects of Noise Pollution
- Hearing loss (prolonged exposure to loud noise).
- Increased stress and blood pressure.
- Difficulty in concentration and sleep.
- Interference with communication.
Measures to Reduce Noise Pollution
- Use SILENCERS in vehicles and machinery.
- Plant TREES along roads (absorb sound).
- Keep volume of music systems LOW.
- Follow NOISE regulations and restrictions.
- Use EARPLUGS in noisy environments.
7. ICSE Exam Focus
| Topic | Marks | Frequency |
|---|---|---|
| Production and propagation of sound | 2-3 marks | Very High |
| Characteristics of sound (pitch, loudness, quality) | 3 marks | Very High |
| Speed of sound in different media | 2 marks | High |
| Audible range and ultrasound | 2 marks | Medium |
| Noise pollution | 2 marks | Medium |
Common Mistakes
- Thinking sound can travel through vacuum (it CANNOT).
- Confusing pitch and loudness (pitch = frequency, loudness = amplitude).
- Writing 'the speed of sound is 330 m/s at 0°C in air' without specifying the medium.
- Saying 'ultrasound is used for sonar' without clarifying it is ultrasonic waves.
Self-Test (5 Questions)
Q1. In which medium does sound travel the fastest? (1 mark)
- A) Air
- B) Water
- C) Iron
- D) Vacuum
Q2. What is the audible range for humans? (1 mark)
Q3. 'What determines the loudness of a sound?' (1 mark)
- A) Frequency
- B) Amplitude
- C) Wavelength
- D) Speed
Q4. 'A woman's voice has a higher ___ than a man's voice.' (1 mark)
Q5. 'Why can we not hear sound on the Moon?' (2 marks)
Answers
A1. C) Iron (sound travels fastest in solids). A2. 20 Hz to 20,000 Hz (20 kHz). A3. B) Amplitude (larger amplitude = louder sound). A4. Pitch (or frequency). A5. The Moon has NO atmosphere (no medium for sound to travel through). Sound cannot travel through vacuum.
