Chapter at a glance
How sound is produced and travels as a longitudinal mechanical wave, its characteristics (wavelength, frequency, time period, amplitude, intensity, speed), and reflection: echoes, reverberation, infrasound and ultrasound.
What this chapter covers
- 10.1 Production of Sound
- 10.2 Propagation of Sound
- 10.3 Sound Waves
- 10.4 Energy of Sound Waves
- 10.5 Graphical Representation of a Sound Wave
- 10.6 Characteristics of a Sound Wave
- 10.7 Reflection of Sound
Key points
- Sound needs a medium (solid, liquid or gas); the density disturbance travels, not the particles.
- Frequency is the number of oscillations per unit time; the time period is the time for one oscillation.
- Echoes and reverberation come from reflected sound.
- Below 20 Hz is infrasonic; above 20 kHz is ultrasonic.
Read the NCERT chapter
Read this chapter in the official NCERT textbook Exploration (2026-27 edition) for free on ncert.nic.in.
Source: NCERT, Exploration, Grade 9 (2026-27). NCERT chapter PDF. Spotted a mistake? Email edura.class9.yt@gmail.com. Last updated 11 October 2026.