Xmax and Excursion
Xmax and excursion, defined: excursion is how far the cone moves from rest; Xmax is the one-way excursion a driver can reach while staying within its linear range. Below the box's tuning or horn cut-off, Xmax sets the ceiling on clean bass output, and running past it is how drivers get damaged mechanically.
How Xmax is defined
There is more than one definition, so read datasheets with care:
- Geometric: the coil overhang, (voice coil winding height − gap height) ÷ 2. This is the travel before turns start leaving the gap and motor force begins to fall.
- AES2-1984: the peak displacement at which the "linearity" of the motor deviates by 10%.
- Distortion-based (Klippel's proposed method): the peak displacement at which total harmonic distortion or second or third order intermodulation distortion reaches 10%.
Two drivers with the same quoted Xmax may not be comparable if the makers use different methods.
Xmax is not the end of travel
Beyond Xmax there is still mechanical travel before the coil former hits the back plate or the suspension runs out. That limit is usually called Xmech or Xlim. Between Xmax and Xmech distortion climbs steeply; past Xmech parts collide, stretch or tear.
Excursion quadruples every octave down
For a constant sound pressure from a driver in its mass-controlled range, cone displacement is proportional to 1/f². Halve the frequency and the cone must move four times as far: 12 dB per octave. A driver comfortable at 50 Hz can be at its limit at 25 Hz with the same fader. That is why subs need a high-pass filter. In a reflex box it is essential: below the port tuning the box stops loading the cone and excursion rises fast for little output.
Displacement: Xmax × Sd
Excursion only matters in proportion to cone area. The volume of air a driver can move linearly is Vd = Sd × Xmax, and excursion-limited SPL scales with it: doubling Vd is +6 dB.
- WM1850/8: Sd 1244 cm² × Xmax 12 mm = 1493 cm³, on a 125 mm coil rated 1800 W AES, £275.
For how Vd separates 15s and 18s, see 15 vs 18 inch subs.
Xmax in scoops and horns
Horn loading raises the acoustic load on the cone across the horn's working band, so the cone moves less for a given output than in a reflex box. Below the horn's cut-off that loading disappears, and the high-pass filter does the job again. In reflex and sealed boxes, Xmax is one of the first numbers to check. For scoops and folded horns, our WM18X451 pairs a 115 mm (4.5") copper coil with a double spider, rated 1800 W RMS.
Read next: cabinet tuning and speaker distortion explained.
Sources: Klippel GmbH, Application Note 4: Assessment of Voice Coil Peak Displacement Xmax (AES2-1984 wording and distortion-based definition), checked October 2026; Wavemark WM1850/8 spec sheet and WM18X451 product data. Displacement relationships: L. L. Beranek, Acoustics.
Questions
What is Xmax on a speaker?
The one-way cone excursion a driver can reach while staying within its linear range. It is commonly calculated as the coil overhang, (coil winding height minus gap height) divided by 2, though some makers use a 10% distortion threshold instead.
What is the difference between Xmax and Xmech?
Xmax is the limit of linear travel. Xmech, or Xlim, is the mechanical limit before parts collide or the suspension is overstretched. Between the two, distortion climbs steeply.
Why does excursion increase at low frequencies?
For constant sound pressure, cone displacement is proportional to 1 over frequency squared, so each octave down needs four times the excursion. A high-pass filter stops the lowest frequencies driving the cone past Xmax.
What is Vd?
Displacement volume, Sd times Xmax: the air a driver can move linearly. The WM1850/8 has Vd of 1493 cm³ from 1244 cm² Sd and 12 mm Xmax. Doubling Vd raises excursion-limited output by 6 dB.