Class 9 Sound Waves Notes

Class 9 Sound Waves Notes

🔊 1. What is Sound?

Sound is a form of energy produced by vibrations of objects and is heard when it travels through a medium.

👉 Sound always needs a medium (solid, liquid, or gas) to travel.

🌊 2. Sound Waves

Sound travels in the form of mechanical waves called longitudinal waves.

📌 Key Feature:

Particles of the medium vibrate parallel to the direction of wave motion.

📊 3. Characteristics of Sound Waves

🔹 (A) Amplitude

  • Maximum displacement of particles
  • Determines loudness

👉 Higher amplitude = louder sound

🔹 (B) Frequency

  • Number of vibrations per second
  • Unit: Hertz (Hz)

👉 Determines pitch

  • High frequency → high pitch (shrill sound)
  • Low frequency → low pitch (deep sound)

🔹 (C) Wavelength

  • Distance between two consecutive compressions or rarefactions

🔹 (D) Time Period

  • Time taken for one complete vibration

🔹 (E) Speed

  • Distance travelled per unit time

🔁 4. Relation Between Speed, Frequency & Wavelength

📌 Formula:

v = f × λ

Where:

  • v = speed of sound
  • f = frequency
  • λ = wavelength

🔊 5. Loudness, Pitch & Quality

🔹 Loudness

  • Depends on amplitude
  • Measured in decibel (dB)

🔹 Pitch

  • Depends on frequency
  • High pitch = high frequency

🔹 Quality (Timbre)

  • Helps distinguish different sounds even with same pitch and loudness

🌍 6. Speed of Sound in Different Mediums

  • Fastest in solids
  • Slower in liquids
  • Slowest in gases

👉 Reason: Particle arrangement

🔁 7. Reflection of Sound

Sound can bounce back after hitting a surface.

Applications:

  • Echo
  • SONAR
  • Megaphones
  • Hearing aids

📡 8. Applications of Sound Waves

  • Medical ultrasound
  • SONAR for underwater detection
  • Communication systems
  • Musical instruments
  • Echolocation (bats, dolphins)

🧠 Key Takeaways

  • Sound is produced by vibrations
  • Sound travels as longitudinal waves
  • Amplitude → loudness
  • Frequency → pitch
  • v = f × λ is key formula
  • Sound reflects and is widely used in technology

SECTION A: MCQs (1–60)

Choose the correct option

  1. Sound is a form of ________
    (a) matter (b) energy (c) force (d) light
    Ans: (b)
  2. Sound needs a ________ to travel
    (a) vacuum (b) medium (c) space (d) field
    Ans: (b)
  3. Sound waves are ________ waves
    (a) transverse (b) longitudinal (c) surface (d) light
    Ans: (b)
  4. SI unit of frequency is
    (a) Joule (b) Hertz (c) Newton (d) Watt
    Ans: (b)
  5. Pitch depends on ________
    (a) amplitude (b) frequency (c) speed (d) wavelength
    Ans: (b)
  6. Loudness depends on ________
    (a) frequency (b) amplitude (c) time (d) speed
    Ans: (b)
  7. Sound travels fastest in
    (a) gas (b) liquid (c) solid (d) vacuum
    Ans: (c)
  8. Sound cannot travel in
    (a) air (b) water (c) solid (d) vacuum
    Ans: (d)
  9. Formula of wave speed is
    (a) v = f + λ (b) v = f × λ (c) v = f/λ (d) v = λ/f
    Ans: (b)
  10. Echo is due to ________
    (a) refraction (b) reflection (c) absorption (d) diffraction
    Ans: (b)
  1. Human audible range is
    (a) 0–20 Hz (b) 20–20000 Hz (c) 200–20000 Hz (d) above 20000 Hz
    Ans: (b)
  2. Ultrasound frequency is
    (a) below 20 Hz (b) above 20000 Hz (c) 100 Hz (d) 1000 Hz
    Ans: (b)
  3. Infrasound is below
    (a) 20 Hz (b) 200 Hz (c) 2000 Hz (d) 20000 Hz
    Ans: (a)
  4. Sound is produced by
    (a) heat (b) vibration (c) light (d) motion only
    Ans: (b)
  5. SONAR stands for
    (a) Sound Navigation and Ranging
    (b) Sound Network and Radar
    Ans: (a)

SECTION B: ASSERTION–REASON (61–80)

Choose:
A) Both true, R explains A
B) Both true, R not explanation
C) A true, R false
D) A false, R true

  1. A: Sound is a mechanical wave
    R: It needs a medium to travel
    Ans: A
  2. A: Pitch depends on amplitude
    R: Amplitude controls loudness
    Ans: C
  3. A: Sound travels faster in solids
    R: Particles are tightly packed
    Ans: A
  4. A: Echo is reflection of sound
    R: Sound bounces back
    Ans: A
  5. A: Vacuum carries sound
    R: Sound needs medium
    Ans: D
  6. A: Frequency determines pitch
    R: Higher frequency → higher pitch
    Ans: A
  7. A: Loudness depends on amplitude
    R: Higher amplitude increases energy
    Ans: A
  8. A: Ultrasound is above 20000 Hz
    R: Humans cannot hear it
    Ans: A
  9. A: Sound is transverse wave
    R: Particle motion is perpendicular
    Ans: C
  10. A: SONAR uses sound waves
    R: Used for underwater detection
    Ans: A

📊 SECTION C: NUMERICALS (Wave-based)

Formula: v = f × λ

  1. v = 340 m/s, f = 170 Hz → λ = ?
    Ans: 2 m
  2. λ = 1 m, f = 200 Hz → v = ?
    Ans: 200 m/s
  3. v = 300 m/s, λ = 0.5 m → f = ?
    Ans: 600 Hz
  4. f = 50 Hz, λ = 4 m → v = ?
    Ans: 200 m/s
  5. v = 330 m/s, f = 110 Hz → λ = ?
    Ans: 3 m
  6. λ = 0.34 m, v = 340 m/s → f = ?
    Ans: 1000 Hz
  7. Sound travels 680 m in 2 s → speed?
    Ans: 340 m/s
  8. λ = 2 m, v = 500 m/s → f = ?
    Ans: 250 Hz
  9. Echo distance 170 m in 1 s → speed?
    Ans: 340 m/s
  10. f = 20 Hz, v = 340 m/s → λ = ?
    Ans: 17 m

📄 SECTION D: FILL IN THE BLANKS (Compact)

  1. Sound is a form of ________ → energy
  2. Sound needs a ________ → medium
  3. Sound waves are ________ waves → longitudinal
  4. SI unit of frequency → ________ → Hertz
  5. Pitch depends on ________ → frequency
  6. Loudness depends on ________ → amplitude
  7. v = f × ________ → wavelength
  8. Echo is due to ________ → reflection
  9. Sound travels fastest in ________ → solids
  10. Sound cannot travel in ________ → vacuum
  11. Ultrasound > ________ Hz → 20000
  12. Infrasound < ________ Hz → 20
  13. SONAR used for ________ → underwater detection
  14. Compression has high ________ → pressure
  15. Rarefaction has low ________ → pressure