Chapter 10 — Sound Waves: Characteristics and Applications

Sound Waves: Characteristics and Applications (Class 9 Science, Chapter 10) covers how sound is produced by vibrating objects, travels as a longitudinal mechanical wave through compressions and rarefactions, and finds applications ranging from echo and sonar to ultrasonic medical imaging.

Chapter 10 of the NCERT Class 9 Science textbook 'Exploration' (2026-27 edition) explains that sound is produced by vibrations and propagates as a longitudinal mechanical wave requiring a material medium. Key wave characteristics — wavelength (λ), frequency (ν), time period (T), amplitude, and speed — are defined and related by v = λ × ν and ν = 1/T. The human audible range spans 20 Hz to 20,000 Hz; sounds below 20 Hz are infrasonic and above 20 kHz are ultrasonic. The chapter also covers reflection of sound (echo and reverberation) and applications such as sonar and echolocation by bats.

1. 01Sound is produced by vibrating objects and propagates as alternating compressions and rarefactions in a medium; it cannot travel through vacuum.
2. 02Sound is a longitudinal mechanical wave — particles of the medium oscillate parallel to the direction of wave propagation, but do not travel with the wave.
3. 03Key wave quantities: wavelength (λ, SI unit m), frequency (ν, SI unit Hz), time period T, with the relations ν = 1/T and wave speed v = λ × ν.
4. 04Speed of sound depends on the medium: approximately 340 m s⁻¹ in air at room temperature, ~1500 m s⁻¹ in water, and ~5000 m s⁻¹ in steel; it increases with temperature.
5. 05The minimum distance from a reflecting surface to hear an echo is 17 m (based on the 0.1 s minimum separation time at 340 m s⁻¹ speed of sound).
6. 06Ultrasonic waves (above 20 kHz) are used in sonar, echolocation by bats and dolphins, ultrasonography, breaking kidney stones, and industrial flaw detection; infrasonic waves (below 20 Hz) are used to detect earthquakes and severe storms.