Sound Waves Characteristics and Applications — Class 9 Science NCERT Solutions (Free)
Free step-by-step NCERT solutions for Class 9 Science chapter "Sound Waves Characteristics and Applications" — 8 important questions with detailed answers for CBSE board exam preparation.
TL;DR: Free step-by-step NCERT solutions for Class 9 Science chapter "Sound Waves Characteristics and Applications" — 8 important questions with detailed ans…
Written & reviewed by the Syllab.in Academic Team (CBSE/NCERT subject experts) · Updated
NCERT Solutions for Class 9 Science — Sound Waves Characteristics and Applications. Step-by-step answers to all 8 textbook questions from this chapter, written for CBSE board preparation and free to use.
Sound Waves Characteristics and Applications — All 8 Questions Solved
Q1. Define sound and explain how sound is produced and propagated.
Sound = Mechanical wave produced by vibrating object; requires medium (air, water, solid) to propagate.
Production: Vibrating object causes surrounding medium particles to oscillate, creating disturbance.
Propagation: Disturbance travels through medium as longitudinal wave (particles oscillate parallel to direction of wave motion).
Example: Tuning fork vibrates → air molecules vibrate → sound propagates
Sound CANNOT travel in vacuum (no medium to vibrate)
Q2. Define and explain pitch. How is pitch related to frequency?
Pitch = Characteristic of sound that makes it "high" or "low" to listener.
Pitch depends on frequency (f) of vibrating source:
• Higher frequency → Higher pitch (shrill sound, like whistle)
• Lower frequency → Lower pitch (deep sound, like bass)
Frequency (f) = Number of vibrations per second (Hz)
Human hearing range: 20 Hz to 20,000 Hz (20 kHz)
Sounds below 20 Hz = Infrasound; above 20 kHz = Ultrasound
Example: Violin (high pitch) has higher frequency than drum (low pitch)
Q3. Define and explain loudness. What factors affect loudness?
Loudness = Characteristic of sound that makes it "loud" or "soft" to listener; depends on intensity/amplitude.
Intensity (I) = Power per unit area = Energy per unit time per unit area (W/m²)
Amplitude (A) = Maximum displacement of particle from mean position
Factors affecting loudness:
(1) Amplitude: Greater amplitude → Louder sound
(2) Distance from source: Closer → Louder sound
(3) Area of vibrating object: Larger area → Louder sound
Loudness measured in decibels (dB)
0 dB = Threshold of hearing, 120 dB = Threshold of pain
Q4. Define reflection of sound. What are the laws of reflection of sound?
Reflection of sound = Sound wave bounces back from hard surface without passing through.
Laws of reflection of sound (same as light):
(1) Angle of incidence = Angle of reflection (measured from normal)
(2) Incident ray, reflected ray, normal all lie in same plane
Conditions for reflection:
• Surface must be hard and rigid (not absorbing)
• Size of surface >> wavelength of sound
Applications: Echo, reverberation, design of concert halls, stethoscope
Q5. Define echo and reverberation. Distinguish between them.
Echo = Repeated sound heard distinctly after original sound; reflection of sound from distant hard surface.
Condition for echo: Time gap between original and reflected sound ≥ 0.1 s
Distance for echo = (v × t)/2 ≥ (343 × 0.1)/2 = 17.15 m (in air at room temp)
Reverberation = Prolonged persistence of sound due to multiple reflections from various surfaces.
Sound heard as continuous buzz (reflections arrive rapidly, <0.1 s intervals).
Distinction: Echo = distinct delayed sound; Reverberation = continuous blended sound
Example: Clapping in empty room → Reverberation; in canyon → Echo
Q6. What is SONAR? How does it work? Give applications.
SONAR = Sound Navigation And Ranging; uses sound waves to locate and measure distance of underwater objects.
Working principle: (1) Transmit ultrasound pulse into water
(2) Pulse reflects from object (ship, submarine, sea bed)
(3) Receiver detects reflected signal
(4) Distance = (v × t)/2, where t = time for round trip, v = speed in water (≈1500 m/s)
Applications:
• Detecting submarines and ships
• Finding depth of sea
• Fishing (detecting fish schools)
• Navigation underwater
Q7. What is ultrasound? List its characteristics and applications.
Ultrasound = Sound with frequency > 20,000 Hz (20 kHz); inaudible to humans.
Characteristics:
(1) Frequency > 20 kHz
(2) Higher energy than audible sound
(3) Can travel long distances through medium
(4) Travels in straight line (like light)
(5) Undergoes reflection, refraction, diffraction
Applications:
• Medical imaging: Ultrasound scan (fetal imaging, organ scan)
• SONAR: Underwater detection
• Cleaning: Removing dirt from delicate items
• Breaking kidney stones
• Pest control (insect repellent)
Q8. A sonar pulse sent from a ship takes 4 seconds to return. Calculate depth of sea. (Speed of sound in water = 1500 m/s)
Given: Total time for round trip (down and up) = 4 s
Speed of sound in water (v) = 1500 m/s
Using formula: Distance = (v × t)/2 = (1500 × 4)/2 = 3000 m
Explanation: Pulse travels DOWN to sea bed (time = t/2 = 2 s) and UP back (time = 2 s)
Depth of sea = Distance traveled downward = 3000 m = 3 km
Alternatively: Distance down = v × (t/2) = 1500 × 2 = 3000 m
How to Use These Solutions
Attempt each question yourself first and only then compare with the worked answer. Marks in the CBSE board exam are awarded for the METHOD as much as the final result, so reproduce the steps rather than memorising the last line. Where a solution states a law, a formula or a definition, learn that wording — examiners look for it.
Related practice for this chapter: chapter MCQs, previous-year questions, revision notes and sample papers.