Sound — AP Board Class 8 Physical Science
1. How is Sound Produced?
'Sound is produced by VIBRATIONS — the rapid back-and-forth motion of an object.'
Demonstrations
- Pluck a guitar string: The string VIBRATES and produces sound. Touch it — you can FEEL the vibration.
- Strike a tuning fork: The prongs vibrate. Bring it near a suspended table tennis ball — the ball is THROWN AWAY as the vibrating prong hits it.
- Speak while touching your throat: You can FEEL your vocal cords vibrating.
Key Concept
'If there is no vibration, there is NO SOUND. A stationary object does NOT produce sound.'
2. Propagation of Sound — Needs a Medium
'Sound needs a MEDIUM (solid, liquid, or gas) to travel. It CANNOT travel in a VACUUM.'
Bell Jar Experiment
- An electric bell is placed inside a glass bell jar connected to a vacuum pump.
- Initially, the bell is heard RINGING clearly.
- As air is slowly pumped OUT, the sound becomes FAINTER and FAINTER.
- When the jar is nearly a vacuum, NO sound is heard — though the hammer is still striking the bell.
'This proves that sound NEEDS a material medium to travel. Without air (or any other medium), sound cannot reach our ears.'
How Sound Travels
Sound travels as a LONGITUDINAL WAVE — particles of the medium vibrate BACK AND FORTH along the direction of the wave.
- Compressions: Regions where particles are CLOSE together (high pressure).
- Rarefactions: Regions where particles are FAR apart (low pressure).
3. Speed of Sound in Different Media
'Sound travels FASTEST in solids, SLOWER in liquids, and SLOWEST in gases.'
| Medium | Speed (m/s) |
|---|---|
| Solids (steel) | ~5,000 |
| Solids (glass) | ~4,500 |
| Liquids (water) | ~1,500 |
| Gases (air at 20°C) | ~343 |
| Gases (air at 0°C) | ~330 |
Why Does Speed Vary?
'Particles are CLOSEST in solids, so vibrations are transferred FASTER. In gases, particles are FAR APART, so it takes LONGER for vibrations to travel from one particle to the next.'
Application
'In AP's coastal regions like Visakhapatnam, during thunderstorms, we see LIGHTNING first and hear THUNDER later. This is because light travels MUCH faster (3 × 10⁸ m/s) than sound (343 m/s).'
The time gap between seeing lightning and hearing thunder can be used to estimate the distance of the lightning strike:
[ \text{Distance (m)} = \text{Time gap (s)} \times 343 ]
4. Amplitude, Time Period, and Frequency
Amplitude
'The maximum displacement of a vibrating particle from its MEAN (equilibrium) position.'
- Large amplitude → LOUDER sound
- Small amplitude → SOFTER sound
- Unit: metres (m)
Time Period (T)
'The time taken to complete ONE full vibration (oscillation).'
[ \text{T} = \frac{\text{Total time taken}}{\text{Number of oscillations}} ]
- Unit: seconds (s)
Frequency (ν or f)
'The number of COMPLETE oscillations per second.'
[ \nu = \frac{1}{T} \quad \text{or} \quad T = \frac{1}{\nu} ]
- Unit: hertz (Hz)
- 1 Hz = 1 vibration per second
Relationship
| Quantity | Symbol | Definition | Unit |
|---|---|---|---|
| Amplitude | A | Maximum displacement from mean position | m |
| Time Period | T | Time for one complete oscillation | s |
| Frequency | ν | Number of oscillations per second | Hz (s⁻¹) |
[ \nu = \frac{1}{T} ]
'High frequency → HIGH pitch (shrill sound). Low frequency → LOW pitch (bass sound).'
Worked Example 1
Problem: A pendulum takes 20 seconds to complete 10 oscillations. Find its time period and frequency.
Solution: [ T = \frac{20}{10} = 2 \text{ s} ] [ \nu = \frac{1}{T} = \frac{1}{2} = 0.5 \text{ Hz} ]
Answer: Time period = 2 s, Frequency = 0.5 Hz.
Worked Example 2
Problem: A source vibrates with a frequency of 200 Hz. What is its time period?
Solution: [ T = \frac{1}{\nu} = \frac{1}{200} = 0.005 \text{ s} ]
5. Loudness and Pitch
Loudness
'The SENSATION of the intensity of sound — determined by the AMPLITUDE of the vibration.'
- Larger amplitude → LOUDER sound
- Measured in decibels (dB)
- 'Whisper: ~20 dB. Normal conversation: ~60 dB. Jet engine: ~140 dB (painful).'
Pitch (or Shrillness)
'Pitch depends on the FREQUENCY of vibration.'
| Pitch | Frequency | Source | Sound Characteristic |
|---|---|---|---|
| HIGH | High frequency | Small birds, baby crying, violin | Shrill, sharp |
| LOW | Low frequency | Lion roar, drum beat, male voice | Deep, bass |
'Women generally have a HIGHER-pitched voice than men because their vocal cords are SHORTER and vibrate at a FASTER rate.'
Table: Loudness vs Pitch
| Feature | Loudness | Pitch |
|---|---|---|
| Depends on | Amplitude | Frequency |
| Effect | How LOUD or soft | How HIGH or low |
| Unit | Decibel (dB) | Hertz (Hz) |
| Changes with | Force of vibration | Speed of vibration |
6. Audible Range of Sound
'The human ear can hear sounds in the frequency range of 20 Hz to 20,000 Hz (20 kHz).'
Categories of Sound
| Category | Frequency Range | Can Humans Hear? |
|---|---|---|
| Infrasonic (infrasound) | Below 20 Hz | NO |
| Audible (sonic) | 20 Hz — 20,000 Hz | YES |
| Ultrasonic (ultrasound) | Above 20,000 Hz | NO |
Animals and Sound Ranges
- Dogs: Can hear up to ~40,000 Hz (ultrasonic). 'This is why DOG WHISTLES work — people cannot hear them but dogs can.'
- Dolphins: Can hear up to ~150,000 Hz. 'Dolphins in the Bay of Bengal use ULTRASOUND for echolocation — navigating and finding prey.'
- Bats: Can hear up to ~120,000 Hz. Bats use echolocation to fly in the dark.
- Elephants: Communicate using INFRASOUND (below 20 Hz) that travels long distances through the ground.
AP Board Context
'Dolphins are frequently spotted off the AP coastline near Visakhapatnam. Scientists study their use of ultrasound for communication and navigation. This is a popular topic in AP Board science exams with connections to animal behaviour.'
7. Noise Pollution
'NOISE is unwanted or unpleasant sound. Noise POLLUTION is excessive noise that harms human health and the environment.'
Sources of Noise Pollution
- Traffic: Vehicles, horns, trains, aeroplanes — major sources in cities like Visakhapatnam, Vijayawada, and Guntur.
- Industrial: Factory machinery, generators, construction equipment.
- Celebrations: Loudspeakers, firecrackers during festivals (especially Ganesh Chaturthi, Diwali).
- Household: TV at high volume, kitchen appliances, vacuum cleaners.
Harmful Effects
| Effect | Description |
|---|---|
| Hearing loss | Prolonged exposure to >85 dB can damage eardrums |
| Insomnia | Loud noise disrupts sleep patterns |
| High blood pressure | Stress caused by constant noise affects heart health |
| Reduced concentration | Noise interferes with learning and work performance |
| Tinnitus | Ringing sensation in the ears due to nerve damage |
Noise Control Measures
- Silent zones: Areas around schools, hospitals, and courts where honking is banned.
- Soundproofing: Installing sound-absorbing materials in buildings.
- Reducing source noise: Silencers in vehicles, earplugs for workers.
- Legislation: Noise Pollution (Regulation and Control) Rules, 2000.
- Planting trees: Trees and vegetation absorb sound.
AP Board Context
'AP has designated SILENT ZONES near the Tirumala Tirupati Devasthanams (TTD) and major hospitals. The AP Pollution Control Board sets permissible noise limits for residential, commercial, and industrial areas.'
8. Common Mistakes and Fixes
| Mistake | Why It's Wrong | Correction |
|---|---|---|
| "Sound travels fastest in air" | Sound travels fastest in SOLIDS because particles are closest | Speed order: solids > liquids > gases |
| "Frequency and pitch are different" | Frequency DETERMINES pitch | Higher frequency = higher pitch |
| "Amplitude determines pitch" | Amplitude determines LOUDNESS, not pitch | Amplitude → loudness; Frequency → pitch |
| "Sound can travel through vacuum" | Sound needs a MATERIAL medium to travel | Sound CANNOT travel in a vacuum |
| "All animals hear the same range as humans" | Different animals have different hearing ranges | Dogs hear ultrasound; elephants hear infrasound |
9. AP Exam Focus and Marks Distribution
| Topic | Likely Marks | Question Type |
|---|---|---|
| Production and propagation of sound | 2 | Very short answer |
| Bell jar experiment (needs medium) | 2 | Short answer |
| Amplitude, time period, frequency | 3 | Short answer / Numerical |
| Loudness vs pitch | 2 | Short answer / Table |
| Audible range and ultrasound applications | 2 | Short answer |
| Noise pollution — sources, effects, control | 3 | Short answer / Long answer |
| Total | 14 marks |
10. Quick Self-Test
Q1: What produces sound?
Q2: A drum is beaten. You hear the sound. Describe how the sound reaches your ear.
Q3: What is the frequency range of human hearing?
Q4: A pendulum completes 15 oscillations in 30 seconds. Calculate its time period and frequency.
Q5: Differentiate between loudness and pitch.
Q6: Why do we see lightning before we hear thunder?
Q7: What is the unit of loudness?
Q8: Give TWO ways to reduce noise pollution.
Answers
A1: Sound is produced by VIBRATIONS — the rapid back-and-forth motion of an object.
A2: When the drum skin is beaten, it VIBRATES. These vibrations create COMPRESSIONS and RAREFACTIONS in the surrounding air. These pressure variations travel through the air as a longitudinal wave and reach your ear. The eardrum vibrates, and the brain interprets these signals as sound.
A3: The human ear can hear frequencies from 20 Hz to 20,000 Hz (20 kHz).
A4: [ T = \frac{30}{15} = 2 \text{ s} ] [ \nu = \frac{1}{2} = 0.5 \text{ Hz} ]
A5: Loudness depends on AMPLITUDE and is measured in DECIBELS (dB). Pitch depends on FREQUENCY and is measured in HERTZ (Hz). Loudness tells us how LOUD or soft a sound is; pitch tells us how HIGH or low it is.
A6: Light travels at a speed of (3 \times 10^8) m/s, which is MUCH FASTER than the speed of sound (343 m/s in air). So the light from the lightning reaches our eyes almost instantly, while the sound from the thunder takes several seconds to reach our ears.
A7: The unit of loudness is the decibel (dB).
A8: (i) Creating SILENT ZONES around schools and hospitals where honking is banned. (ii) Using SOUNDPROOFING materials in buildings and keeping the volume of TVs and music systems at reasonable levels. (Any two appropriate measures.)
