NCERT Solutions for Class 9th Science Chapter 10 Sound Waves: Characteristics and Applications

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