When two speakers play the same sound equally, you hear a voice floating in the middle even though no speaker sits there.
Identical sound from both speakers fuses into one ghost source dead centre, with no speaker there.
What it is
A centre image you hear floating between two speakers playing the same sound equally, with no speaker actually there.
Key facts
Phantom centre = identical signal sent equally to L and R; brain fuses it into one source at the midpoint.
Image sits dead centre ONLY with equal level + equal arrival time at both ears.
Speed of sound = 343 m/s at 20 degrees C (about 1 ms per 0.343 m, roughly 1 ft per ms).
Two correlated (identical) signals sum +6 dB at centre (20*log10(2)=6.02 dB); random/uncorrelated sum only +3 dB (10*log10(2)=3.01 dB).
Doubling power = +3 dB; halving power = -3 dB; doubling distance from a point source = -6 dB (inverse-square law).
Pan/stereo law pulls a centred sound DOWN ~3 dB (or 4.5/6 dB) so it is not louder than a hard-panned one.
ITD (time difference between ears) dominates below ~700 Hz; ILD (level difference) dominates above ~1.5 kHz; ears ~17-23 cm apart give max ITD ~0.6-0.7 ms.
Phantom image holds only inside the ~60 degree sweet spot: listener + 2 speakers as an equilateral triangle (~+/-30 deg).
Precedence/Haas: a 1-2 ms head start makes you localise to the NEARER speaker; window ~1-40 ms, fused even if the later sound is up to ~10 dB louder.
Flip one speaker's polarity (180 deg) and the phantom centre hollows out / cancels (out-of-phase mono); comb-filter first notch f = 343 / (2 x path-difference in metres).
How it works
Send the SAME signal to both speakers at equal level and equal delay.
Each speaker's wavefront reaches both your ears almost simultaneously.
Brain finds no time or level difference between ears, so it places the source straight ahead.
It invents a 'ghost' source in the gap between the speakers.
Pan a channel and you change relative level, sliding the phantom toward the louder side.
Step off the centre line and the nearer speaker arrives first/louder, so the image snaps to it.
Real examples
A lead vocal panned centre in any stereo song: no centre speaker, yet the singer is dead ahead.
Kick drum and bass guitar sat in the middle of a mix via equal L/R level.
A presenter's voice in stereo PA appearing centre-stage with only side hangs.
Stereo TV/hi-fi giving believable centre dialogue from two speakers.
Mono summed to both mains on a club system, voice locked centre for the sweet-spot crowd.
How it helps in live sound
Only the centre-triangle crowd gets a true phantom centre, so add a real centre cluster or run mono vocals if the audience is wide.
Run lead vocals MONO (centre) on a stereo rig so the voice does not smear or vanish off-axis.
Check polarity: one out-of-phase cable hollows the vocal and kills bass; sweep a mono track and listen for the centre disappearing.
Predict off-centre comb filtering with first-notch f = 343 / (2 x path-difference m) when mains are far apart.
Use the pan/stereo-link law (-3 dB centre) so a centred BV is not ~6 dB hotter than a hard-panned guitar.
Time-align and lean on precedence: a small delay to the far side keeps the image from collapsing for the near crowd.
Everyday analogy
Two mates standing left and right humming the exact same note in sync sound like one person humming right in front of your nose.
Watch out
Myth: the phantom centre is just '+3 dB louder' and works everywhere. Truth: identical signals sum +6 dB, and the image only holds inside the ~60 degree sweet spot, collapsing to the nearer speaker the moment you step off-axis.
Fun fact
Flip one speaker's polarity and the singer literally vanishes from the middle: the two identical waves cancel where your phantom centre used to be, the classic 'out-of-phase mono' test.
Key takeaways
Same sound, equal level, equal time = one ghost source in the middle.
No real speaker sits there, hence 'phantom'.
Identical signals sum +6 dB at centre; random signals only +3 dB.
Image only holds in the ~+/-30 degree stereo sweet spot.
Step off-centre and it collapses to the nearer speaker (precedence/Haas).
Reverse polarity and the phantom centre cancels and disappears.