Sound
1. Production of Sound
Sound is produced by VIBRATIONS of an object.
Examples:
- Guitar string: STRINGS vibrate
- Drum: SKIN (membrane) vibrates
- Human voice: VOCAL CORDS vibrate
- Tuning fork: PRONGS vibrate
'If an object is VIBRATING, it is producing sound. If sound is being produced, something MUST be vibrating.'
2. Propagation of Sound
Sound needs a MEDIUM to travel. It CANNOT travel through a VACUUM.
Bell Jar Experiment
- An electric bell is placed inside a bell jar.
- As air is PUMPED OUT, the sound becomes FAINTER.
- When the jar is ALMOST vacuum, SOUND IS NOT HEARD (though the hammer is seen striking).
- Conclusion: Sound NEEDS a material medium for propagation.
How Sound Travels
Sound travels through a medium by COMPRESSION and RAREFACTION of particles.
- The particles VIBRATE about their mean positions
- Energy is PASSED from one particle to the next
- The particles themselves do NOT move forward
3. Speed of Sound
| Medium | Speed (m/s) | Relative Speed |
|---|---|---|
| Solids (steel) | ~5000 | FASTEST |
| Liquids (water) | ~1500 | MEDIUM |
| Gases (air) | ~330–350 | SLOWEST |
Sound vs Light:
- Speed of sound in air: ~340 m/s
- Speed of light: 3 × 10⁸ m/s
'We see lightning FIRST, then hear thunder LATER — because light travels much FASTER than sound.'
4. Characteristics of Sound
Loudness
- Determined by AMPLITUDE of vibration
- Unit: DECIBEL (dB)
- LARGER amplitude → LOUDER sound
Threshold of hearing: 0 dB Normal conversation: 60 dB Pain threshold: 120 dB
Pitch (Frequency)
- Determined by FREQUENCY of vibration (number of vibrations per second)
- Unit: Hertz (Hz)
- HIGHER frequency → HIGHER pitch
| Female voice | Higher pitch (higher frequency) |
|---|---|
| Male voice | Lower pitch (lower frequency) |
| Humming bee | Lower pitch |
| Mosquito buzz | Higher pitch |
Quality (Timbre)
- The characteristic that DISTINGUISHES two sounds of the SAME loudness and pitch
- Depends on the WAVEFORM (harmonics/overtones)
- Allows us to tell the DIFFERENCE between a piano and a violin playing the same note
5. Audible and Inaudible Sound
| Type | Frequency | Heard by |
|---|---|---|
| Infrasonic | Below 20 Hz | Some animals (elephants, whales) |
| Audible | 20 Hz to 20,000 Hz | HUMAN beings |
| Ultrasonic | Above 20,000 Hz | Dogs, bats, dolphins |
6. Reflection of Sound — Echo
An ECHO is the REPETITION of sound due to reflection from a hard surface.
Condition for hearing an echo: The reflecting surface must be at LEAST 17 m away (because the human ear can distinguish two sounds only if they are at least 0.1 seconds apart).
Minimum distance = (Speed × Time)/2 = (340 × 0.1)/2 = 17 m
7. Noise Pollution
| Aspect | Music | Noise |
|---|---|---|
| Definition | PLEASANT sound | UNPLEASANT sound |
| Effect | Relaxes the mind | Causes STRESS |
| Examples | Melodious song, instrument | Honking, drilling, loud speakers |
Effects of Noise Pollution
- Hearing LOSS (temporary or permanent)
- HIGH blood pressure
- SLEEP disturbance
- Reduced CONCENTRATION
- IRRITABILITY
Control Measures
- Plant TREES along roads (absorb sound)
- Use SILENCERS in vehicles
- Maintain SOUNDPROOFING in factories
- Follow NOISE regulations
- Use EARPLUGS in noisy environments
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| 'Sound can travel through vacuum' | Sound NEEDS a medium. There IS NO sound in space |
| 'Loudness and pitch are the same' | Loudness depends on AMPLITUDE. Pitch depends on FREQUENCY. They are DIFFERENT |
| 'Sound travels fastest in gases' | Sound travels FASTEST in SOLIDS (particles closest). Speed increases with density |
| 'Ultrasonic sound is very loud' | Ultrasonic means HIGH FREQUENCY (above 20,000 Hz), not necessarily high amplitude |
ICSE Exam Focus (5–6 marks)
- 2-mark questions: How is sound produced? Define amplitude, frequency
- 3-mark questions: Distinguish between loudness and pitch
- 4-mark questions: Why does sound need a medium? Bell jar experiment
- 5-mark questions: Noise pollution — causes, effects, and control measures
Self-Test
Q1. What is the range of audible frequencies for humans? A1. 20 Hz to 20,000 Hz.
Q2. Why do we hear thunder after seeing lightning? A2. Light travels at 3 × 10⁸ m/s, while sound travels at only ~340 m/s in air. Light reaches us ALMOST INSTANTLY, but sound takes time.
Q3. What is an echo? What is the minimum distance for hearing an echo? A3. An echo is the REPETITION of sound due to REFLECTION from a hard surface. Minimum distance = 17 m (for 0.1 s persistence of hearing at 340 m/s).
Q4. Differentiate between loudness and pitch. A4. Loudness depends on AMPLITUDE (unit: decibel). Pitch depends on FREQUENCY (unit: hertz). A high-pitched sound can be soft, and a low-pitched sound can be loud.
Q5. Give two effects and two control measures of noise pollution. A5. Effects: Hearing loss, high blood pressure, sleep disturbance. Control: Planting trees, using silencers, soundproofing, earplugs.
Q6. What is ultrasonic sound? Give one use. A6. Sound with frequency ABOVE 20,000 Hz is ultrasonic. Used by bats for echolocation, medical ultrasound imaging, and SONAR.
