Impedance

Acoustic impedance is the ratio of sound pressure to particle velocity at a point in a medium, expressed in Pa·s/m (rayl). It determines how sound energy transfers between media: when impedance is matched, energy transmits efficiently; when mismatched, energy reflects. The characteristic impedance of air is approximately 413 rayl (at 20°C), while dense materials like concrete have impedances millions of times higher. This mismatch is why solid walls reflect most airborne sound. Impedance concepts underpin the design of porous absorbers (gradual impedance matching), membrane absorbers (tuned impedance), and sound insulation systems (impedance discontinuities). In duct acoustics, impedance mismatches at openings and bends cause reflections that affect insertion loss calculations.