NCERT Solutions for Class 9 Science Chapter 11 Sound
NCERT Solutions for Class 9 Science Chapter 11 Sound covers the production and propagation of sound, mechanical longitudinal waves, compressions and rarefactions, characteristics of sound waves (frequency, amplitude, wavelength, speed), reflection of sound, echo, reverberation, audible range, applications of ultrasound, SONAR, and human ear structure.
Overview of Topics & Subtopics (Click to Jump)
1. Production & Propagation of Sound Waves
Short Note:
• Production of Sound: Sound is produced by vibrating objects. Vibration refers to rapid to-and-fro motion of an object.
• Propagation of Sound: Sound needs a material medium (solid, liquid, or gas) to travel. It cannot travel in a vacuum.
• Nature of Sound Waves: Sound waves are mechanical longitudinal waves. Medium particles vibrate parallel to the direction of wave propagation.
• Compressions & Rarefactions:
- Compression (C): Region of high pressure and high density in the medium.
- Rarefaction (R): Region of low pressure and low density in the medium.
2. Characteristics of Sound: Pitch, Loudness & Speed
Short Note:
• Wavelength (λ): Distance between two consecutive compressions or two consecutive rarefactions. SI Unit: Metre (m).
• Frequency (ν): Number of complete oscillations or vibrations per second. SI Unit: Hertz (Hz).
• Time Period (T): Time taken by two consecutive compressions or rarefactions to cross a fixed point. Formula: T = 1 / ν. SI Unit: Second (s).
• Pitch: How the brain interprets the frequency of a sound. Higher frequency = higher pitch (shrill); lower frequency = lower pitch (deep/grave).
• Loudness: Measure of response of the ear to sound. Depends on amplitude (Loudness ∝ Amplitude²).
• Wave Velocity Formula: Speed (v) = Wavelength (λ) × Frequency (ν) or v = λ / T
• Speed of Sound: Maximum in solids, lower in liquids, lowest in gases. Increases with temperature increase in air (Speed in air ≈ 344 m/s at 22°C).
3. Reflection of Sound: Echo & Reverberation
Short Note:
• Laws of Reflection of Sound: Angle of incidence equals angle of reflection; incident wave, reflected wave, and normal all lie in the same plane.
• Echo: The repetition of sound caused by reflection from a distant hard surface (like a wall, mountain, or cliff).
- Persistence of hearing in humans: 0.1 seconds.
- Minimum distance to hear a distinct echo in air at 22°C: d = (v × t) / 2 = (344 × 0.1) / 2 = 17.2 metres.
• Reverberation: Multiple reflections causing persistent, prolonged sound in big halls or auditoriums.
- Reduced by using sound-absorbent materials like curtains, compressed fiberboards, or cushioned seats.
4. Range of Hearing, Ultrasound & SONAR
Short Note:
• Audible Range of Humans: 20 Hz to 20,000 Hz (20 kHz).
- Infrasonic / Infrasound: Sound frequencies below 20 Hz (produced by elephants, whales, earthquakes).
- Ultrasonic / Ultrasound: Sound frequencies above 20 kHz (produced by bats, dolphins, porpoises).
• Applications of Ultrasound: Cleaning hard-to-reach parts, detecting cracks/flaws in metal blocks, Echocardiography (heart imaging), Ultrasonography (internal body organ imaging), breaking kidney stones.
• SONAR (Sound Navigation and Ranging): Technique using ultrasound waves to measure depth of sea, detect underwater sunken ships, submarines, or icebergs.
- Formula: 2d = v × t (where d = depth, v = speed of sound in water, t = time taken for echo return).
Intext Questions - Page Number: 132
Intext Questions - Page Number: 135
• When a vibrating object moves forward, it pushes and compresses the air in front of it, creating a region of high pressure and density called a compression (C).
• When the vibrating object moves backwards, it leaves a region of low pressure and density called a rarefaction (R).
As the object moves back and forth rapidly, a series of compressions and rarefactions are produced and propagated through air.
Intext Questions - Page Number: 138
(a) Loudness is determined by the amplitude of the sound wave (Loudness ∝ Amplitude²).
(b) Pitch is determined by the frequency of the sound wave.
Given:
• Frequency (ν) = 2 kHz = 2000 Hz
• Wavelength (λ) = 35 cm = 0.35 m
• Speed of sound (v) = ν × λ = 2000 × 0.35 = 700 m/s
• Distance (s) = 1.5 km = 1500 m
• Time taken (t) = Distance / Speed = 1500 / 700 = 2.14 seconds
Textbook Chapter End Exercise Questions
Given:
• Speed of sound (v) = 342 m/s
• Time taken for echo return (t) = 3 s
• Total distance traveled by sound = v × t = 342 × 3 = 1026 m
• Distance of reflecting surface (d) = Total distance / 2 = 1026 / 2 = 513 metres
Principle: It works on the principle of echo reflection. Ultrasonic sound pulses transmitted from a boat travel through water, strike underwater objects or sea bed, get reflected back, and are received by a detector.
Read More & Chapter Resources
These short notes are designed for quick revision. Read the main points once, then use the related material below to practise the same chapter. Revising a concept and checking it with questions can make your preparation more focused.