NCERT Solutions for Class 9th Science Chapter 10 Sound Waves: Characteristics and Applications
Updated on 2026-09-19
About this chapter
Sound is produced by vibrating objects. Stop the vibration and the sound stops with it; the vibrating object is the source . Sound travels as a longitudinal mechanical wave — alternating compressions (higher density) and rarefactions (lower density) that move forward while the particles only oscillate about their mean positions. It cannot travel through vacuum. A sound wave is described by wavelength λ (m), frequency ν (Hz), time period T (s) and amplitude. They are linked by ν = 1/T and v = ν × λ . Speed depends on the medium, not on the source: fastest in solids (steel ≈ 5000 m s⁻¹), slower in liquids (water ≈ 1500 m s⁻¹), slowest in gases (air ≈ 340 m s⁻¹), and it rises with temperature and humidity. Reflection of sound gives echo (needs a gap of at least 0.1 s, so a reflector at least
- Chapter opener
- Chapter introduction
- Let us explore
- .1 Production of Sound
- .1.1 Tuning fork
- .2 Propagation of Sound
- Let us investigate
- .2.1 Sound needs a medium to propagate
- Let us observe
- .3 Sound Waves
Quick revision
| Term | What it means | Formula / typical value | Where it appears |
|---|---|---|---|
| Vibration | Periodic to-and-fro motion (oscillation) of an object | — | Section 10.1, Activity 10.1 |
| Compression (C) | Region of the medium with density above the average density | Crest on the density–distance graph | Fig. 10.9b, Fig. 10.16 |
| Rarefaction (R) | Region of the medium with density below the average density | Trough on the density–distance graph | Fig. 10.9c, Fig. 10.16 |
| Wavelength (λ) | Distance between two consecutive crests or two consecutive troughs | SI unit: metre (m) | Section 10.6.1, Fig. 10.18 |
| Frequency (ν) | Number of density oscillations at a fixed point per unit time | ν = 1/T; SI unit hertz (Hz = s⁻¹) | Eq. 10.1, Example 10.1 |
| Time period (T) | Time for one complete density oscillation at a fixed point | T = 1/ν; SI unit second (s) | Section 10.6.1 |
| Amplitude | Maximum change in air density in a compression (or rarefaction) from the average density | Larger amplitude → more energy | Section 10.6.2, Fig. 10.20 |
| Intensity | Sound energy passing per unit area per unit time, perpendicular to the direction of propagation | Falls as distance from the source grows | Section 10.6.2, Fig. 10.21 |
| Speed of sound (v) | Distance a crest (or trough) travels in unit time | v = ν × λ; air 331 m s⁻¹ at 0 ºC, 344 m s⁻¹ at 22 ºC | Eq. 10.2, Section 10.6.3 |
| Pitch / Loudness | How we perceive frequency / amplitude | Loudness measured in decibel (dB) | Section 10.6.4 |
| Echo | A reflected sound heard separately from the original | Needs ≥ 0.1 s gap → reflector ≥ 17 m away | Section 10.7.1, Example 10.5 |
| Reverberation | Persistence of sound due to multiple reflections in a hall | Reflections arriving within 0.05 s | Section 10.7.2 |
| Ultrasonic / Infrasonic | Sound above 20 kHz / below 20 Hz | Human audible range 20 Hz – 20 kHz | Section 10.8, Fig. 10.27, 10.28 |
Exercises
- Chapter opener — Think It Over Page 184
- Chapter introduction — In-text Questions Page 184
- Let us explore — Activity 10.1 Page 185
- .1 Production of Sound — In-text Questions Page 18510
- Let us explore — Activity 10.2 Page 186
- .1.1 Tuning fork — Pause and Ponder Page 18610
- .2 Propagation of Sound — In-text Questions Page 18610
- Let us investigate — Activity 10.3 Page 186 – 187
- Let us investigate — Activity 10.4 Page 187
- .2 Propagation of Sound — In-text Questions Page 18710
- .2.1 Sound needs a medium to propagate — Pause and Ponder Page 18810
- Let us observe — Activity 10.5 Page 188
- .3 Sound Waves — In-text Questions Page 19010
- .3 Sound Waves — Pause and Ponder Page 19110
- Let us experiment — Activity 10.6 Page 191 – 192
- .4 Energy of Sound Waves — Pause and Ponder Page 19210
- .5 Graphical Representation of a Sound Wave — Pause and Ponder Page 19310
- Let us experiment (demonstration activity) — Activity 10.7 Page 194
- .6.1 Wavelength, frequency and time period — Pause and Ponder Page 19510
- .6.3 Speed of Sound — What if … Page 19610
- .6.3 Speed of Sound — Pause and Ponder Page 19710
- .6.3 Speed of Sound — Pause and Ponder Page 19810
- Let us experiment (demonstration activity) — Activity 10.8 Page 198
- .6.4 Human perception of sound — In-text Questions Page 19910
- .7.1 Echo — Pause and Ponder Page 20110
- .8 Ultrasonic and Infrasonic Waves, and their Applications — What if … Page 20210
- .8.1 Echolocation — Pause and Ponder Page 20310
- End-of-chapter questions — Revise, Reflect, Refine Page 204 – 206
- Project work — The Journey Beyond Page 207