NCERT Class 9 Science Chapter 11 – Sound

NCERT Class 9 Science Chapter 11 explains sound as a form of energy produced by vibrating objects and propagated through a material medium. NCERT Class 9 Science Chapter 11 builds strong conceptual clarity about wave motion, characteristics of sound waves and reflection of sound, which are essential for higher physics.

The chapter explains that sound waves are longitudinal waves consisting of compressions and rarefactions. It introduces important wave quantities such as frequency (Hz), wavelength (λ), time period (T), amplitude and speed of sound. The wave relation is expressed as v = fλ.

NCERT Class 9 Science Chapter 11 also provides important scientific values such as the speed of sound in air at 20°C ≈ 343 m/s, audible range of humans (20 Hz to 20,000 Hz), and the minimum distance required for hearing a distinct echo (approximately 17.2 metres).

The chapter further explains reflection of sound, echo, reverberation and applications of ultrasound in medicine and SONAR technology.

For structured preparation of NCERT Class 9–12 for UPSC, BPSC and State PCS examinations, strengthen your basics with our complete NCERT Book Notes PDF for Class 9-12, available inside the NCERT foundation course level-2.

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1. Introduction

• Sound is a form of energy that produces sensation of hearing.
• Sound is produced by vibrating objects.
• Sound requires a material medium to propagate and cannot travel through vacuum.
• Examples of vibrating sources include tuning fork, guitar string, drum membrane and vocal cords.
• Sound waves are mechanical waves because they require medium.

2. Production of sound

• Sound is produced when an object vibrates.
• Vibrations create compressions and rarefactions in the surrounding medium.
• Compressions are regions of high pressure and density.
• Rarefactions are regions of low pressure and density.
• These alternating regions travel as a longitudinal wave.

3. Propagation of sound

• Sound travels through solids, liquids and gases.
• It travels fastest in solids, slower in liquids, slowest in gases.
• Sound travels through medium due to interaction between particles.
• Sound cannot travel in vacuum due to absence of particles.

4. Longitudinal wave

• In longitudinal waves, particles vibrate parallel to direction of wave propagation.
• Sound waves in air are longitudinal waves.
• Compressions and rarefactions move through medium.
• Distance between two consecutive compressions is wavelength.

5. Characteristics of sound wave

• Wavelength (λ) is distance between two consecutive compressions or rarefactions.
• Frequency (f) is number of vibrations per second.
• SI unit of frequency is Hertz (Hz).
• Time period (T) is time taken for one complete vibration.
• T = 1/f.
• Speed (v) of wave = fλ.

6. Speed of sound

• Speed depends on medium and temperature.
• Speed of sound in air at 0°C = 331 m/s.
• At 20°C, speed ≈ 343 m/s.
• In water, speed ≈ 1500 m/s.
• In steel, speed ≈ 5000 m/s.
• Speed increases with temperature.

7. Frequency and pitch

• Frequency determines pitch of sound.
• Higher frequency produces higher pitch.
• Human audible range = 20 Hz to 20,000 Hz.
• Sound below 20 Hz is infrasonic.
• Sound above 20,000 Hz is ultrasonic.

8. Amplitude and loudness

• Amplitude is maximum displacement of particle from mean position.
• Greater amplitude produces louder sound.
• Loudness measured in decibel (dB).
• Normal conversation ≈ 60 dB.
• Sound above 80 dB may cause hearing damage.

9. Quality or timbre

• Quality distinguishes sounds of same pitch and loudness.
• Depends on waveform.
• Example: Flute and violin produce different sound quality.

10. Reflection of sound

• Sound waves reflect from hard surfaces.
• Laws of reflection of sound same as reflection of light.
• Angle of incidence equals angle of reflection.
• Echo is repetition of sound due to reflection.
• Minimum distance for distinct echo = 17.2 m at 20°C.

11. Echo

• Echo occurs when reflected sound reaches listener after 0.1 second.
• Speed of sound used to calculate echo distance.
• Time interval must be at least 0.1 s for distinct echo.
• Used in SONAR technology.

12. Reverberation

• Persistence of sound due to multiple reflections.
• Excessive reverberation distorts sound.
• Controlled in auditoriums using sound-absorbing materials.

13. Uses of ultrasound

• Ultrasound frequency above 20,000 Hz.
• Used in medical imaging (ultrasonography).
• Used in detecting cracks in metal blocks.
• Used in cleaning delicate parts.
• Used in SONAR for measuring sea depth.

14. SONAR

• SONAR stands for Sound Navigation and Ranging.
• Used to measure depth of sea and detect submarines.
• Time interval between transmission and reception gives distance.
• Distance = (Speed × Time) / 2.

15. Numericals

• If frequency = 500 Hz and wavelength = 0.68 m, speed = 340 m/s.
• If echo heard after 2 s, distance = 340 m.
• Time period for 1000 Hz wave = 0.001 s.
• If wavelength = 2 m and speed = 340 m/s, frequency = 170 Hz.

16. Human ear structure

• Outer ear collects sound.
• Middle ear amplifies vibrations through ossicles.
• Inner ear converts vibrations to nerve impulses.
• Cochlea contains sensory cells.
• Hearing range limited to 20–20,000 Hz.

17. Conclusion

• Sound is mechanical longitudinal wave.
• Speed depends on medium and temperature.
• Frequency determines pitch.
• Reflection produces echo.
• Ultrasound has many practical uses.

18. Exam oriented facts

• Speed of sound in air at 0°C – 331 m/s.
• Audible range – 20 Hz to 20,000 Hz.
• Ultrasound – Above 20,000 Hz.
• Echo time gap – Minimum 0.1 s.
• SONAR formula – Distance = (vt)/2.
• Frequency unit – Hertz (Hz).
• Wave speed formula – v = fλ.
• Decibel (dB) – Unit of loudness.

Understanding NCERT Class 9 Science Chapter 11 is essential for developing clarity about wave motion and sound propagation.

This chapter forms the foundation for advanced topics such as wave optics, acoustics and communication systems in senior secondary classes.

For school examinations, students must focus on characteristics of sound waves, formula v = fλ, reflection of sound, echo conditions and applications of ultrasound.

For competitive examinations like JEE and NDA, NCERT Class 9 Science Chapter 11 provides fundamental understanding required for solving numerical problems related to waves and sound.

Continue reading NCERT Class 9 Science Chapter 12 – Improvement in Food Resources to understand agricultural practices and food production systems.

FAQs

Q1. What is NCERT Class 9 Science Chapter 11 about?
It explains sound as a mechanical longitudinal wave, its characteristics such as frequency and wavelength, speed of sound and reflection of sound.

Q2. What is the formula for speed of sound?
The speed of sound is given by the wave equation v = fλ, where f is frequency and λ is wavelength.

Q3. What is the audible range of humans?
The human audible range is approximately 20 Hz to 20,000 Hz.

Q4. What is echo?
Echo is the repetition of sound due to reflection from a distant surface, heard when the time gap between original and reflected sound is at least 0.1 second.

Q5. Why is NCERT Class 9 Science Chapter 11 important for competitive exams?
It builds the foundation of wave motion and sound propagation, which are essential topics in physics for engineering and defence entrance examinations.

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