Acoustic Center

The exact spot a speaker's sound seems to come from, which is not always where the cone physically sits.

Each driver launches from its own acoustic centre at a different depth; engineers measure and time-align these so wavefronts arrive together at the listener.

What it is

The single point in space a speaker's sound appears to radiate from, often deeper or shallower than the cone itself.

Key facts

Speed of sound in air = 343 m/s at 20 C (rises about 0.6 m/s per +1 C)
Sound travels ~34.3 cm per 1 ms, so 1 cm of driver offset = ~0.029 ms (29 microseconds)
Acoustic centre = apparent launch point; tweeter vs woofer centres can differ 2 to 8 cm in depth
Time delay: t = d / c (t seconds, d distance in metres, c = 343 m/s)
Wavelength: lambda = c / f (lambda metres, c = 343 m/s, f frequency in Hz)
1 kHz wavelength = 34.3 cm; 100 Hz = 3.43 m; 10 kHz = 3.43 cm
Doubling distance from a point source = -6 dB (inverse-square law, free field)
Two identical signals time-aligned sum to +6 dB; 180 degrees out of phase = near total cancellation
Path error of half a wavelength (lambda/2) at crossover = a deep cancellation notch; phase = 360 x f x t
Precedence/Haas: arrivals within ~5 to 35 ms fuse to first; beyond ~40 ms = audible echo. Typical crossovers 1.5 to 3 kHz

How it works

Find each driver's acoustic centre with a measurement mic and dual-channel FFT (impulse response shows arrival time).
Read the impulse peak time for the woofer and tweeter separately; the gap is the offset.
Convert that time gap to distance: distance = gap x 343 m/s.
Delay the earlier (closer) driver so both arrive together at the crossover.
Check the phase trace through crossover; aligned drivers track in phase with no deep notch.
When stacking or flying boxes, measure from the acoustic centre, not the grille face, and time-align each box.

Real examples

A 2-way box: tweeter sits forward but its acoustic centre is ~3 cm behind the woofer's, so designers step the baffle back or add delay.
Subwoofer plus main top: sub acoustic centre is metres back, so you delay the tops to match and avoid a low-end notch.
Two flown line-array boxes splayed apart: each has its own acoustic centre, aimed so wavefronts knit together, not collide.
Studio monitor with a stepped or sloped front baffle: the step physically aligns tweeter and woofer acoustic centres.

How it helps in live sound

Use Smaart or REW dual-FFT to read driver arrival times before guessing delay values.
Delay your main tops to the sub's acoustic centre; start with a few ms and confirm low-end lifts, not dips.
Stack subs in a line or arc and measure from their shared acoustic centre when setting array delay.
Aim flown boxes using the acoustic centre as the pivot, not the grille, for accurate coverage angles.
Remember 1 cm error = ~0.029 ms; small physical shifts matter most up at 1 to 3 kHz crossover.
If a tonal hole appears at crossover, suspect acoustic-centre misalignment causing a lambda/2 cancellation.

Everyday analogy

It's like a starting line for sound: even if two runners' feet sit at different spots, what matters is where each actually launches from.

Watch out

Myth: the sound comes from the cone face. Truth: it radiates from the acoustic centre, which can sit centimetres behind the physical driver, so align by measured arrival time, not by the grille.

Fun fact

A tweeter's acoustic centre can shift with frequency, so a speaker's true launch point isn't even one fixed spot, it moves depending on the note being played.

Key takeaways

Acoustic centre = where sound seems to start, not where the cone sits.
Each driver has its own acoustic centre, offset by a few centimetres in depth.
1 cm offset = ~0.029 ms; convert offset to time with distance / 343 m/s.
Misalignment of lambda/2 at crossover causes a deep cancellation notch.
Aligned drivers sum to +6 dB; opposite phase cancels to near silence.
Always time-align and aim from the acoustic centre when stacking or flying.