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CBSE Class 8 Science Question 6 of 13

Sound — Question 6

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Question 6

The sound from a mosquito is produced when it vibrates its wings at an average rate of 500 vibrations per second. What is the time period of the vibration?

Answer

Given,

Total number of vibrations = 500

Total time taken = 1 s

We know,

Time period = 1Frequency\dfrac{1}{\text{Frequency}} and

Frequency = Total number of oscillationsTotal time taken\dfrac{\text{Total number of oscillations}}{\text{Total time taken}}

Hence,

Time period = Total time takenTotal number of oscillations \dfrac{\text{Total time taken}}{\text{Total number of oscillations }}

Substituting we get,

Time period = 1500\dfrac{1}{500} = 0.002 s

So, time period = 0.002 s.

Chapter Overview: Sound

This chapter covers how sound is produced by vibrating objects, how it travels through media (solid > liquid > gas), and its characteristics: frequency (pitch), amplitude (loudness), and time period. The audible range, ultrasound applications, human ear anatomy, and noise pollution are important exam topics.

Board Exam Weightage: 5-7 marks | Difficulty: Moderate to High

Key Formulas & Concepts

ConceptDetails
Frequencyf = number of oscillations per second; Unit: Hertz (Hz)
Time PeriodT = 1/f; Time for one complete oscillation; Unit: seconds
Audible Range20 Hz to 20,000 Hz (for humans)
Sound SpeedSolid (~5000 m/s) > Liquid (~1500 m/s) > Gas (~340 m/s in air at 20°C)

Must-Know Concepts

  • Sound is produced by vibrating objects and needs a medium to travel (cannot travel in vacuum)
  • Loudness depends on amplitude; pitch depends on frequency
  • Men have deeper voices because their vocal cords are longer (lower frequency)
  • Ultrasound (>20,000 Hz): used in medical imaging (ultrasonography), SONAR, cleaning
  • Infrasound (<20 Hz): produced by earthquakes, elephants, whales
  • Noise pollution causes hearing loss, stress, high blood pressure; controlled by silencers, green belts, ear protection

Loudness vs Pitch

PropertyLoudnessPitch
Depends onAmplitudeFrequency
Greater → Louder soundHigher (shriller) sound
UnitDecibel (dB)Hertz (Hz)
ExampleShouting vs whisperingWoman's voice vs man's voice

Common Mistakes to Avoid

  • Confusing loudness (amplitude) with pitch (frequency)
  • Thinking sound can travel through vacuum
  • Errors in T = 1/f calculations (they are inversely related)
  • Mixing up ultrasound (>20,000 Hz) and infrasound (<20 Hz)

Scoring Tips

  • Practice T = 1/f problems: if f = 500 Hz, T = 0.002 s
  • Draw and label the human ear with all parts
  • Learn 3 applications each of ultrasound and noise prevention
  • Remember: solid > liquid > gas for speed of sound

Frequently Asked Questions

Q: How many marks does this chapter carry in the exam?
A: Approximately 5-7 marks in the annual exam.

Q: What question types are commonly asked?
A: MCQs (1 mark), Very Short Answer (2 marks), Short Answer (3 marks), and Long Answer / Diagram (5 marks).

Q: Is this chapter important for competitive exams?
A: Yes, concepts from this chapter appear in NTSE, Olympiad, and other science competitions.