It is the natural built-in resistance to sound that a particular medium, like open air, always has.
Air's fixed resistance to sound: Z0 = density × speed of sound ≈ 413 rayl, and the driver↔air mismatch is what bleeds energy.
What it is
The fixed, built-in resistance a medium like air offers to a passing sound wave, set only by the medium itself.
Key facts
Symbol Z0, unit = rayl (Pa·s/m); 1 Pa·s/m = 1 kg/(m²·s)
Formula: Z0 = ρ × c, where ρ = density of the medium and c = speed of sound in it
Air at 20°C: ρ ≈ 1.204 kg/m³, c ≈ 343 m/s, so Z0 ≈ 413 rayl (often rounded ~415)
Water Z0 ≈ 1.48 million rayl — about 3,600× stiffer to sound than air
Z0 of air FALLS as temperature rises, and FALLS with altitude as air thins (lower ρ)
It is intensive: a property of the STUFF, not of room size, walls or objects
For a plane wave, acoustic impedance Z = pressure p ÷ particle velocity u = Z0 = ρc
Huge air↔driver mismatch is why a bare cone radiates poorly — horns/baffles bridge it
+6 dB SPL = pressure ×2; doubling distance in free field = -6 dB
-3 dB = half the acoustic POWER (the half-power point); +10 dB ≈ perceived 'twice as loud'
How it works
Take the medium's density ρ (air ≈ 1.204 kg/m³).
Take the speed of sound c in that medium (air ≈ 343 m/s).
Multiply: Z0 = ρ × c (air ≈ 413 rayl).
Compare source impedance (cone) to load (air) — big mismatch = poor energy transfer.
Add a horn or baffle to step the impedance down toward air and couple energy efficiently.
Real examples
Air ≈ 413 rayl — the baseline every PA speaker has to push against.
Water ≈ 1.48 million rayl — airborne sound transfers into it far harder, so a swimmer hears little of what is shouted from the poolside.
A horn-loaded compression driver works because the horn gently matches high cone impedance down to air's low Z0.
Hot outdoor gig: warmer air = slightly lower Z0, subtly changing how cones load.
A foam earplug works partly by being an impedance mismatch that reflects sound back.
How it helps in live sound
Use horn-loaded tops + horn/folded subs outdoors: horns impedance-match the driver to air for far higher efficiency than a bare cone.
Keep subs on the ground or in a corner — boundary loading raises the effective load impedance, gaining +3 to +6 dB of low end for free.
Expect outdoor low end to feel weaker: no walls means no boundary reinforcement, so bring more sub headroom.
Account for temperature: as air warms through a gig, c rises ~0.6 m/s per °C, shifting delay-line timing and array tuning.
Don't expect a tiny driver in free air to move real SPL — the air/cone mismatch is why you need horns, baffles or arrays.
Everyday analogy
Air has a fixed 'thickness' to sound the way honey is always thicker than water no matter how hard you stir, and a speaker fighting that mismatch is like trying to shove honey with a teaspoon.
Watch out
Myth: impedance is about your speaker's '8 ohm' rating. Correction: characteristic ACOUSTIC impedance (Z0 = ρc ≈ 413 rayl for air) is a property of the air itself, totally separate from the electrical ohms on the speaker terminals.
Fun fact
The air↔water acoustic impedance mismatch is so huge (~3,600:1) that about 99.9% of sound energy hitting a water surface bounces straight back — which is why you can shout over a pool and a swimmer underwater barely hears you.
Key takeaways
Z0 = ρ × c — density times speed of sound, nothing else.
Air ≈ 413 rayl at 20°C; it's a fixed property of the medium.
Big impedance mismatch = poor energy transfer; that's the core speaker problem.
Horns, baffles and boundaries exist to bridge the cone↔air mismatch.
It's acoustic rayls, NOT the electrical ohms on your amp.