Sound Wave Propagation
Concentric wavefronts from a point source.
Key Notes
Sound is a LONGITUDINAL pressure wave — alternating compressions and rarefactions of the medium.
Speed in air at 20°C: v ≈ 343 m/s. In water: ~1500 m/s. In steel: ~5000 m/s.
Sound requires a MEDIUM — cannot propagate in vacuum (unlike EM waves).
Speed in fluid: v = √(B/ρ), with B = bulk modulus, ρ = density.
In gas: v = √(γRT/M) — depends on T (not P or ρ separately).
Speed of sound rises ~0.6 m/s per °C in air. Doubling T (Kelvin) increases v by √2.
Audible range: 20 Hz - 20 kHz. Below: infrasound. Above: ultrasound.
Sound intensity: power per unit area, measured in W/m² or in decibels (logarithmic).
Formulas
Sound speed in fluid
Bulk modulus and density determine v.
Sound speed in gas
Temperature dominant; pressure cancels in ideal gas.
Temperature dependence
Useful approximation for air.
Decibel scale
I₀ = 10⁻¹² W/m² (threshold of hearing).
Important Points
Sound needs a medium — cannot propagate through vacuum.
Hotter air = faster sound (v ∝ √T).
Solids > liquids > gases in sound speed because of stiffness/density ratios.
Audible: 20 Hz - 20 kHz. Dogs: up to ~45 kHz. Bats: up to ~200 kHz.
Sound intensity I is in W/m²; loudness in decibels uses a logarithmic scale.
Doubling sound intensity ⇒ +3 dB. Tenfold increase ⇒ +10 dB.
Sound Wave Propagation 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.