It is the smallest change in a sound's position you can actually notice, basically how fuzzy your directional hearing is.
Directional resolution is razor-sharp dead ahead (~1 degree) and roughly 10x blurrier out to the sides, driven by tiny time/level gaps between your two ears.
What it is
The smallest change in a sound's direction your ears can reliably detect, and it gets worse off to the sides and behind.
Key facts
Front (0 degrees azimuth): localization blur is about 1 degree, the sharpest your hearing gets.
Sides (90 degrees, directly left/right): blur balloons to roughly 10 degrees, about 10x worse than front.
Rear (180 degrees, behind): blur about 5-6 degrees, still fuzzier than front.
Up/down (vertical/elevation) blur is roughly 9-22 degrees even straight ahead, far worse than horizontal.
Two main horizontal cues: ITD = Interaural Time Difference (which ear the sound hits first), and ILD = Interaural Level Difference (which ear is louder).
Max ITD = Interaural Time Difference is about 0.6-0.7 ms (milliseconds) for a sound at 90 degrees to the side.
Ears can resolve an ITD = Interaural Time Difference as tiny as ~10 microseconds (0.00001 s) for front sounds.
Speed of sound in air = 343 m/s (metres per second) at 20 degrees C; ~331 m/s at 0 degrees C.
Head width ~0.18 m (18 cm) gives that ~0.5-0.7 ms max time gap between the two ears.
How it works
A sound arrives, hitting one ear slightly before the other (ITD = Interaural Time Difference).
The head shadows the far ear, making it quieter (ILD = Interaural Level Difference).
Brain compares these two cues to estimate the angle.
Cues are strong and clean when the source is in front, so the estimate is sharp (~1 degree).
At the sides the cues change very little per degree, so the estimate gets fuzzy (~10 degrees).
The fuzzy spread of likely positions IS the localization blur.
Real examples
Centre vocal panned dead centre: a 1-degree shift is audible, so precise panning pays off.
A guitar hard-panned to the side: nudging it a few degrees is basically inaudible, blur hides it.
Front-of-house cluster slightly off-axis: small aim errors front-and-centre are noticed first.
Surround/atmos speaker behind the audience: exact placement matters less, ears can't pinpoint it.
Stereo image collapse: if both ears get identical timing/level, the source snaps to dead centre.
How it helps in live sound
Spend your panning precision on centre sources (lead vocal, kick, snare); be loose with hard-panned edges.
Keep main L/R clusters symmetrical and aimed at centre: front errors are the most audible.
For surrounds/effects speakers, prioritise even coverage over pinpoint placement, blur forgives position.
Watch arrival times: a >1-2 ms delay between sources can yank the image off via the precedence effect.
Mind interaural cues: phantom-centre vocals need matched L/R level and time or the image smears.
Everyday analogy
It is like the pixel size of your sense of direction: tiny sharp pixels dead ahead, big blocky pixels out to the sides and behind.
Watch out
Myth: panning is equally precise everywhere. Truth: directional resolution is ~1 degree at front but ~10 degrees at the sides, so side panning is far coarser.
Fun fact
Your localization blur sideways is so wide that a sound at 80 vs 90 degrees is often indistinguishable, yet you can hear a 1-degree wobble straight ahead, finer than the width of your own nose at arm's length.
Key takeaways
Localization blur = the smallest position change you can actually detect.
Sharpest in front (~1 degree), fuzziest at the sides (~10 degrees).
Driven by ITD (timing) and ILD (loudness) differences between your two ears.
Vertical (up/down) localization is much blurrier than horizontal.
Pan precisely for centre sources; relax for hard-panned and surround sources.