Acoustic Impedance

Same wave, two boundaries: matched air-to-air transmits (R=0); air-to-wall mismatch reflects (R≈1).

What it is

Acoustic impedance is how hard a medium pushes back against a sound wave trying to move through it.

Key facts

• Symbol Z, unit the rayl; SI rayl = 1 Pa·s/m (pascal-second per metre).
• Specific acoustic impedance Z = pressure p divided by particle velocity u (Z = p/u).
• Characteristic impedance Z = rho × c, where rho = density of the medium and c = speed of sound in it.
• Air at 20°C: rho ≈ 1.2 kg/m³, c ≈ 343 m/s, so Z(air) ≈ 413 rayls.
• Water Z ≈ 1.48 million rayls; that is ~3,600× higher than air — sound barely crosses the air/water boundary.
• Concrete/brick wall Z is ~15,000× air, so almost all airborne sound reflects rather than passes through.
• Reflection at a boundary: R = (Z2 − Z1) / (Z2 + Z1); equal impedances (Z1 = Z2) give R = 0 = perfect transmission.
• Big impedance mismatch → most energy reflects (loud echo, poor transmission); matched impedance → energy passes cleanly.
• Speed of sound in air ≈ 343 m/s at 20°C (rises ~0.6 m/s per °C).
• Doubling sound power = +3 dB; halving power = −3 dB; doubling sound PRESSURE (or perceived distance halving) = +6 dB.

How it works

1. A sound wave hits a boundary between two media (air to wall, throat to air).
2. Each side has its own impedance Z = density × speed of sound.
3. If the two Z values match, the wave hands its energy across and keeps going.
4. If they are mismatched, a chunk bounces back as a reflection.
5. Bigger the mismatch, more energy reflects and less transmits.
6. Horns and transformers gradually change Z so energy passes efficiently instead of bouncing.

Real examples

• Brick wall (huge Z mismatch with air) reflects bass back into the room instead of letting it escape — neighbours still hear it via vibration, not air.
• A horn-loaded speaker is an impedance transformer: it steps the driver's high Z down to air's low Z so it's far louder and more efficient.
• Open window passes sound straight out — air-to-air, no impedance change, no reflection.
• Ultrasound gel matches the probe's Z to skin Z so the beam enters the body instead of reflecting off an air gap.
• Heavy stage curtains/drapes sit between hard wall and air to soak reflections and tame slapback.

How it helps in live sound

• Horn-loaded tops (e.g. JBL/RCF) are impedance matchers — that's why they throw louder and further than a bare cone for the same watts.
• Reflective hard walls (brick, glass) = huge Z mismatch = slapback and comb filtering; hang drapes or move PA off-axis to them.
• For low-end isolation, mass + air gaps (decoupling) beats thin panels — bass needs big impedance change to stop, not a curtain.
• Don't leave a horn driver's throat blocked or gasketed wrong — it kills the impedance match and you lose top-end output.
• Subs against a wall/corner gain ~+3 to +6 dB from boundary loading — use it on purpose, not by accident.
• Outdoor gigs lose the room's reflective help: no impedance boundaries bouncing energy back, so you need more boxes/SPL.