🌱 1. Production of Sound
✅ Key Definition:
Sound is a form of energy that produces the sensation of hearing in our ears.
🌱 Explanation:
• Sound is produced when an object vibrates.
• Vibrations are to and fro or back and forth movements of an object.
• When a vibrating object moves, it disturbs the particles of the medium around it.
• These disturbances travel through the medium in the form of sound waves.
📚 Subtopics:
• Vibrations
• Sound needs a medium to travel
🖊 Diagram Description:
Draw a tuning fork and show it vibrating. Draw air particles around it showing compressions and rarefactions.
🧠 Memory Trick:
🔥 “No Vibration = No Sound”
🌱 2. Propagation of Sound
✅ Key Definition:
Propagation of sound means the travel of sound waves from one place to another through a medium.
🌱 Explanation:
• Sound travels through solids, liquids, and gases.
• Sound cannot travel through vacuum (no particles).
• Sound travels as a longitudinal wave.
• In longitudinal waves, particles move parallel to the direction of wave travel.
📚 Subtopics:
• Compressions (C) – region of high pressure
• Rarefactions (R) – region of low pressure
🖊 Diagram Description:
Draw a straight line showing C and R alternately. Label one compression and one rarefaction.
🧠 Memory Trick:
🔥 CRCR = Compression → Rarefaction → Compression → Rarefaction
🌱 3. Sound Waves and Their Characteristics
(a) Amplitude
✅ Key Definition:
Amplitude is the maximum displacement of a vibrating particle from its mean position.
🌱 Explanation:
• Larger amplitude → Louder sound
• Smaller amplitude → Softer sound
• Measured in metres (m)
(b) Time Period
✅ Key Definition:
Time taken to complete one vibration is called time period.
🌱 Explanation:
• Denoted by T
• Unit = second (s)
(c) Frequency
✅ Key Definition:
Number of vibrations per second is called frequency.
🌱 Explanation:
• Denoted by ν (nu)
• Unit = Hertz (Hz)
• Frequency = 1 / Time period
• Higher frequency → Shrill sound
(d) Wavelength
✅ Key Definition:
Distance between two consecutive compressions or two rarefactions is called wavelength.
🌱 Explanation:
• Denoted by λ (lambda)
• Unit = metre (m)
(e) Speed of Sound
✅ Key Definition:
Distance travelled by sound per unit time.
🌱 Explanation:
• Speed = Frequency × Wavelength
• v = νλ
• Sound travels fastest in solids, slowest in gases.
📊 Comparison of Speed of Sound
| Medium | Speed of Sound |
|---|---|
| Solids | Highest |
| Liquids | Medium |
| Gases | Lowest |
🧠 Memory Trick:
🔥 “S L G” → Solids > Liquids > Gases
🌱 4. Reflection of Sound
✅ Key Definition:
Bouncing back of sound when it strikes a hard surface is called reflection of sound.
🌱 Explanation:
• Follows laws of reflection.
• Angle of incidence = Angle of reflection.
• Sound reflects from walls, mountains, buildings.
📚 Subtopics:
• Echo
• Reverberation
(a) Echo
✅ Key Definition:
Repetition of sound heard after reflection is called echo.
🌱 Explanation:
• Time gap must be at least 0.1 second to hear echo.
• Distance of reflecting surface should be about 17 m or more.
(b) Reverberation
✅ Key Definition:
Persistence of sound due to repeated reflections in a closed space.
🌱 Explanation:
• Occurs in large halls.
• Reduced by using curtains, carpets, acoustic panels.
🧠 Memory Trick:
🔥 Echo = One clear repeat
🔥 Reverberation = Many mixed repeats
🌱 5. Range of Hearing
✅ Key Definition:
The audible range of sound for humans is 20 Hz to 20,000 Hz.
📊 Types of Sound Based on Frequency
| Type of Sound | Frequency Range |
|---|---|
| Infrasonic | Less than 20 Hz |
| Audible | 20 Hz – 20,000 Hz |
| Ultrasonic | More than 20,000 Hz |
🌱 Explanation:
• Humans hear only audible sound.
• Some animals hear infrasonic or ultrasonic sounds.
🧠 Memory Trick:
🔥 “20 to 20k = Human Hearing”
🌱 6. Applications of Ultrasound
✅ Key Definition:
Ultrasound is sound with frequency more than 20,000 Hz.
🌱 Explanation:
• Used in medical imaging (sonography).
• Used to detect cracks in metal blocks.
• Used in cleaning delicate parts.
🌱 7. Structure of the Human Ear
✅ Key Definition:
Human ear is the organ responsible for hearing sound.
🌱 Parts of Ear:
• Outer ear
• Middle ear
• Inner ear
🌱 Working:
• Sound waves enter through ear pinna.
• Eardrum vibrates.
• Vibrations pass through three bones (ossicles).
• Cochlea converts vibrations into electrical signals.
• Brain interprets these signals as sound.
🖊 Diagram Description:
Draw outer ear, ear canal, eardrum, three bones (hammer, anvil, stirrup), cochlea. Label all parts clearly.
🧠 Memory Trick:
🔥 P-E-O-C-B → Pinna → Eardrum → Ossicles → Cochlea → Brain
🌟 Quick Revision Summary
• Sound is produced by vibrations.
• Sound travels through a medium as longitudinal waves.
• Compression and rarefaction are parts of sound waves.
• Frequency determines pitch.
• Amplitude determines loudness.
• v = νλ
• Sound reflects and produces echo.
• Human hearing range: 20 Hz to 20,000 Hz.
• Ultrasound has frequency above 20,000 Hz.
• Ear converts sound vibrations into electrical signals for brain.
