A sound off to one side is a little louder in the near ear because your head blocks some of it from reaching the far ear.
Near ear gets the loud copy; the head shadows the far ear, and the dB gap tells your brain which way to point.
What it is
A sound off to one side hits the near ear louder than the far ear, because your head blocks part of it, and your brain reads that loudness gap to find the direction.
Key facts
ILD = Interaural Level Difference: the dB difference in sound level between your two ears.
Formula: ILD (dB) = SPL_near_ear minus SPL_far_ear. SPL = Sound Pressure Level in decibels.
Works best ABOVE ~1.5-2 kHz, where wavelength is shorter than head width so the head casts an acoustic shadow.
Below ~500 Hz ILD is almost zero: long bass waves (0.69 m at 500 Hz) just bend around the head (diffraction).
Max ILD reaches roughly 15-20 dB at high frequencies (>5 kHz) for a sound at 90 degrees (full side).
Sound dead ahead (0 deg) or dead behind (180 deg) = 0 dB ILD: both ears equal.
Average human head width (ear to ear) is about 17.5 cm (0.175 m); ITD partner cue maxes ~0.6-0.7 ms.
Speed of sound in air = 343 m/s at 20 deg C (rises ~0.6 m/s per +1 deg C).
Wavelength formula: lambda = c / f. c = speed of sound (343 m/s), f = frequency (Hz). 2 kHz = 0.17 m, same as head width.
+6 dB = double the sound pressure; +10 dB = perceived 'twice as loud'; -3 dB = half the power (half-power point).
How it works
A sound arrives from one side, off-centre to your head.
The near ear gets the full wavefront at higher SPL.
Your head physically blocks high frequencies, casting a quieter 'sound shadow' on the far ear.
Each ear's loudness is sent to the brainstem (lateral superior olive) for comparison.
Brain subtracts far-ear level from near-ear level to get the dB gap.
Bigger gap = more to the side; zero gap = dead centre.
Real examples
Whisper into someone's left ear: their left ear hears it loud, right ear barely catches it (big high-freq ILD).
Pan a hi-hat hard left in a mix: only the left speaker/ear is loud, brain places it left.
A reversing-beeper truck on your right side sounds clearly louder in your right ear.
Bass guitar panned hard left still feels fairly central in low end because bass gives almost no ILD.
Cupping a hand behind one ear boosts that ear's level, faking a stronger ILD toward that side.
How it helps in live sound
Pan-pot level panning IS ILD: it changes L/R loudness, the cue your high-freq localisation relies on.
Pan cymbals, hats and effects wide for placement; keep kick, bass and lead vocal centred (mono) since low end gives weak ILD anyway.
Headphone mixes lean HARD on ILD (no room) so over-panning sounds extreme and fatiguing: ease pans in 'cans.
For mono PA clusters, don't pan for a crowd: only listeners on the centre line get matched ears.
Keep bass and sub feeds mono/centred: panned sub wastes the ILD-poor low frequencies and can phase-cancel.
Check mono-fold compatibility: hard ILD pans can vanish or shift when a venue sums to mono.
Everyday analogy
Hold your hand up to shade a lamp: the far side of your hand sits in shadow and looks dimmer, and your head shades sound the exact same way so the far ear hears a quieter copy.
Watch out
Myth: panning works equally at all frequencies. Truth: ILD is a high-frequency cue (above ~1.5-2 kHz); below ~500 Hz the head can't shadow long bass waves, so low-frequency panning barely moves perceived position.
Fun fact
Your brain runs TWO direction systems at once and hands off between them: ILD (loudness gap) for highs above ~1.5 kHz, and ITD (tiny arrival-time gap, up to ~0.7 ms) for lows below ~1.5 kHz. The messy middle band is the 'duplex theory' crossover.
Key takeaways
ILD = how much LOUDER the near ear is than the far ear, in dB.
It exists because your head blocks (shadows) sound on its way to the far ear.
Strong cue for HIGH frequencies (>~1.5-2 kHz), weak-to-zero for bass.
Max gap ~15-20 dB at full side (90 deg); 0 dB straight ahead.
Level panning in a mix is literally ILD: that's why hard pans feel 'placed'.