Sound Intensity

Sound intensity is the acoustic power passing through a unit area perpendicular to the direction of propagation, measured in watts per square metre (W/m²). Unlike sound pressure, intensity is a vector quantity with both magnitude and direction, making it valuable for identifying noise sources and transmission paths. Sound intensity level is expressed in decibels relative to 10⁻¹² W/m². The intensity method, standardized in ISO 9614 (parts 1–3), enables sound power determination in situ without requiring a special test environment. In building acoustics, intensity measurements can map flanking transmission paths and identify weak points in partitions. The relationship between intensity and pressure in a free field is I = p²/(ρc), where ρc is the characteristic impedance of air.