Sound

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Sound —

⭐ Speed of Sound in Different Mediums

  • Slowest in gases
  • Faster in liquids
  • Fastest in solids
  • Speed increases with temperature
  • Speed increases with humidity
  • Speed of sound in air at 22°C = 344 m/s
  • Speed of light is faster than speed of sound

🔁 Reflection of Sound

When sound waves hit a hard surface and bounce back → reflection of sound

Laws of Reflection:

  1. The incident wave, reflected wave, and normal lie in the same plane
  2. Angle of incidence = Angle of reflection

🔊 Echo

  • Echo = the repetition of sound due to reflection
  • Requires hard surfaces (walls, mountains) for clear echo
  • Soft surfaces absorb sound, so no echo
  • Echo is heard only if there is at least 0.1 sec time gap

Minimum distance for hearing echo:

  • Total distance = 344 × 0.1 = 34.4 m
  • Listener ↔ wall distance = 34.4 / 2 = 17.2 m

👉 Minimum distance to hear echo = 17.2 m

🌩️ Rolling of Thunder

  • Occurs due to multiple reflections of thunder sound
  • Between clouds and earth surfaces
  • Causes thunder to sound prolonged and rolling

🏛️ Reverberation

  • Persistence of sound in a hall due to repeated reflections
  • From walls, ceiling, and floor
  • If too long → sound becomes:
    • Blurred
    • Distorted
    • Confusing

🛠️ Methods to Reduce Reverberation

  1. Use sound-absorbing panels/fiber boards on walls & ceiling
  2. Use heavy curtains on windows & doors
  3. Lay carpets on floor
  4. Use sound-absorbing seats

🔍 Difference Between Echo and Reverberation

Echo Reverberation
Single reflection Multiple reflections
Heard clearly Becomes blurred & confusing
Happens in open spaces Happens in closed halls
Requires 0.1 sec time gap No clear gap — overlaps with original sound
Listener hears a distinct repeated sound Listener hears prolonged continuous sound

🔥 Sonic Boom

  • Objects moving faster than sound have supersonic speed
  • These create shock waves
  • Shock waves cause loud explosive noise called sonic boom
  • Sonic boom can break glass windows due to strong sound energy

🎤 Applications of Reflection of Sound

1️⃣ Megaphone / Horn / Loudspeaker / Trumpet

  • Funnel shape
  • Sound repeatedly reflects inside
  • Sound directed forward
  • Increases loudness

2️⃣ Stethoscope

  • Used by doctors
  • Heart and lung sounds travel through rubber tube
  • Reach doctor’s ears by multiple reflections

3️⃣ Sound Board

  • Curved wooden board placed behind speaker
  • Directs sound towards audience
  • Works using multiple reflections

4️⃣ Curved Ceilings in Concert Halls

  • Sound reflects evenly
  • Ensures sound reaches all listeners equally

🔹 Important Questions & Answers

Q1. What is an echo?
A1. When a sound wave reflects from a hard surface and returns to the listener so it is heard again separately, it is called an echo.

Q2. What is the minimum time gap required to hear an echo?
A2. A minimum time gap of 0.1 seconds is required between the original sound and the reflected sound to hear an echo.

Q3. What is the minimum distance required between the listener and a reflecting surface to hear an echo at 22°C?
A3. The minimum distance required is 17.2 metres.

Q4. Define reverberation.
A4. Reverberation is the persistence of sound in a hall or room due to multiple reflections from walls, floor, and ceiling.

Q5. How is reverberation reduced in halls and auditoriums?
A5. Reverberation is reduced by using sound-absorbing materials like carpets, curtains, acoustic panels, and cushioned seats.

Q6. What causes the rolling effect in thunder sound?
A6. Thunder reflects repeatedly between clouds and the ground due to multiple reflections, which produces a prolonged rolling sound.

Q7. What is a sonic boom?
A7. When an object moves faster than the speed of sound, it creates shock waves that produce a loud explosive noise called a sonic boom.

Q8. Write the two laws of reflection of sound.
A8.

  1. The incident wave, reflected wave, and normal all lie in the same plane.
  2. The angle of incidence is equal to the angle of reflection.

Q9. How does a megaphone increase sound?
A9. The funnel shape causes repeated reflections of sound waves inside, directing them in one direction and increasing loudness.

Q10. How does a stethoscope work on the principle of reflection of sound?
A10. The heartbeat sound undergoes multiple reflections inside the rubber tube of the stethoscope, allowing the doctor to hear it clearly.

📌 Range of Hearing

⭐ Human Hearing Range

  • Humans can hear frequencies from 20 Hz to 20,000 Hz
  • Children below 5 years and dogs can hear up to 25 kHz

⭐ Infrasonic Sounds (Below 20 Hz)

  • Sounds less than 20 Hz are called infrasonic
  • Produced by:
    ✔ simple pendulum
    ✔ rhinoceroses (communication at 5 Hz)
    ✔ elephants
    ✔ whales
    ✔ earthquakes (before shock waves – animals get disturbed)

⭐ Ultrasonic Sounds (Above 20 kHz)

  • Sounds above 20,000 Hz are called ultrasonic
  • Heard by: dogs, dolphins, bats, porpoises, rats
  • Bats and rats can also produce ultrasonic sounds

🎧 Hearing Aid

  • A hearing aid is a battery-operated device used by people with weak hearing.

Working:

  1. Microphone converts sound → electrical signals
  2. Amplifier increases the strength of signal
  3. Speaker converts amplified signals → sound
  4. Sends sound clearly into the ear

⚙️ Applications of Ultrasound

  1. Used to detect cracks in metal blocks (without damaging them)
  2. Used to clean complicated machine parts
  3. Used to scan body organs: liver, kidneys, uterus, heart
  4. Echocardiography → ultrasound forms image of beating heart
  5. Ultrasonography → pictures of internal organs using ultrasound echoes
  6. Used to break kidney stones into small pieces

🌊 SONAR (Sound Navigation And Ranging)

Definition:

  • SONAR is a device used to measure the distance, direction and speed of underwater objects.

Components:

  • Transmitter – sends ultrasonic waves
  • Detector/Receiver – receives reflected waves

Working:

  • Ultrasonic waves travel through water
  • Hit underwater object
  • Reflect back
  • Time of echo is used to calculate distance

Q1. What is the range of hearing in normal humans?
Ans. Humans can hear frequencies from 20 Hz to 20,000 Hz.

Q2. What are infrasonic sounds? Give two examples.
Ans. Sounds below 20 Hz are called infrasonic sounds.
Examples: elephants and rhinoceroses use infrasonic sounds for communication.

Q3. What are ultrasonic sounds? Name two animals that can hear them.
Ans. Sounds higher than 20,000 Hz are called ultrasonic sounds.
Dogs and bats can hear ultrasonic sounds.

Q4. How does a hearing aid work?
Ans. A hearing aid:

  1. Microphone converts sound into electrical signals
  2. Amplifier increases the signal strength
  3. Speaker converts signals back to sound
  4. Sends clear sound to the ear

Q5. Name two applications of ultrasound in medicine.
Ans.

  1. Ultrasonography – imaging internal organs
  2. Echocardiography – imaging the working heart

Q6. How does ultrasound help in industries?
Ans. Ultrasound is used to detect cracks in metal blocks and to clean hard-to-reach machine parts.

Q7. What is the full form of SONAR?
Ans. SONAR stands for Sound Navigation And Ranging.

Q8. Where is SONAR used and for what purpose?
Ans. SONAR is used in ships and submarines to detect underwater objects, and to measure their distance, speed, and direction.

Q9. Why do some animals behave unusually before earthquakes?
Ans. Because earthquakes produce infrasonic waves, which some animals can hear, making them disturbed before the shock waves arrive.

Q10. Name two animals that can produce ultrasonic sounds.
Ans. Bats and rats can produce ultrasonic sounds.

🔟 Important Questions & Answers

Q1. What are the three main parts of the human ear?
A. The three main parts of the ear are: Outer ear, Middle ear, and Inner ear.

Q2. What is the function of the pinna?
A. The pinna collects sound from the surroundings and directs it into the auditory canal.

Q3. What is the ear drum and what does it do?
A. The ear drum (tympanic membrane) is a thin membrane that vibrates when struck by sound waves.

Q4. Name the three small bones of the middle ear.
A. Hammer (Malleus), Anvil (Incus), and Stirrup (Stapes).

Q5. What is the role of the middle ear bones?
A. They amplify the vibrations of the ear drum and transmit them to the inner ear.

Q6. What is the cochlea?
A. The cochlea is a coiled fluid-filled structure in the inner ear containing nerve cells that convert vibrations into electrical signals.

Q7. What is the function of the auditory nerve?
A. The auditory nerve carries the electrical signals from the cochlea to the brain.

Q8. What does the brain do in the hearing process?
A. The brain interprets the electrical signals as sound.

Q9. What is the function of the Eustachian tube?
A. The Eustachian tube maintains pressure balance on both sides of the ear drum.

Q10. What happens during compression and rarefaction of sound waves at the ear drum?
A. During compression, the ear drum moves inward; during rarefaction, it moves outward. This creates vibrations.



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