Key Concepts: Intensity & Loudness

Sound Intensity

• Intensity is the power per unit area: I = P/A. SI unit: W/m².
• Intensity follows an inverse-square law: I ∝ 1/r² (for a point source).
• Threshold of hearing: ~10⁻¹² W/m². Threshold of pain: ~1 W/m².

Decibel Scale

• Sound level in decibels: β = 10 log₁₀(I/I₀), where I₀ = 10⁻¹² W/m².
• Every increase of 10 dB = 10× intensity. Every increase of 3 dB ≈ 2× intensity.
• Normal conversation: ~60 dB. Rock concert: ~110 dB. Damage threshold: ~85 dB (prolonged).

Loudness vs Intensity

• Intensity is a physical measurement (objective).
• Loudness is a perception (subjective) — depends on frequency, duration, and the listener's hearing.