Sound Speed in Media
Air vs water vs steel vs diamond.
Key Notes
Sound speed depends on the MEDIUM. Generally: solid > liquid > gas.
Air at 20°C: ~343 m/s. Water: ~1500 m/s. Steel: ~5000 m/s. Glass: ~5000 m/s. Rubber: ~60 m/s.
Speed in a medium: v = √(B/ρ) for fluids, √(Y/ρ) for thin rod in solid, √(γP/ρ) for ideal gas.
Sound in solids can be longitudinal OR transverse (shear). Sound in fluids only longitudinal.
In gas: v ∝ √T (Kelvin), independent of pressure. Air at 0°C: 331 m/s.
Diatomic gas (air ~80% N₂): γ = 1.4. Monatomic (helium): γ = 5/3.
Sound speed in helium is 3× that in air at same T (lighter gas, lower M) — high-pitched 'Donald Duck' voice.
Underwater communication uses sound because water's slow attenuation (vs radio waves).
Formulas
Sound in fluid
B = bulk modulus.
Sound in solid (1D rod)
Y = Young's modulus.
Sound in ideal gas
Independent of P; depends on T, γ, M.
Ratio of speeds (gas vs gas)
Useful for comparing gases at different T or M.
Important Points
Solids > liquids > gases in sound speed due to stiffness/density ratios.
Hotter gas = faster sound (~0.6 m/s per °C in air).
Lighter gas = faster sound (helium 3× air at same T).
Pressure has no effect on sound speed in ideal gas — only T.
Echoes: useful to measure distance/depth (sonar, echolocation).
Whales communicate at 20 Hz over hundreds of km — very low f penetrates far in water.
Sound Speed in Media notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 11 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.