12 Volt Encyclopedia · Volume V · Article 9
Subwoofer Excursion and Motor Strength: Xmax, 70% BL, Xmech and Power Compression
Xmax is useful only when its definition is known. One company may publish a geometric calculation while another reports excursion at a specified loss of motor force or rise in distortion. Large numbers cannot be compared fairly without the method.
Geometric Xmax
For an overhung coil, one common one-way estimate is (winding height − top-plate thickness) ÷ 2. It describes how far the winding can move before leaving the nominal gap geometry. It does not measure suspension nonlinearity or the actual BL curve.
BL versus position
BL(x) plots force factor as the coil moves. At center, BL is typically highest. As the coil travels, less conductor remains effectively coupled to the gap and force declines. Symmetry matters because unequal decline creates distortion and offset tendencies.
The 70% BL convention
A 70% BL limit identifies the excursion where force factor falls to 70% of its centered value. This corresponds to a meaningful nonlinear threshold but is not the only accepted definition. Some reports use a percentage deviation, distortion limit or the most restrictive of motor and suspension criteria.
Suspension-limited excursion
Cms(x) describes compliance through travel. A motor may remain strong while the spider or surround becomes sharply stiffer, shifts asymmetrically or buckles. Useful Xmax should consider both motor and suspension behavior.
Xmech and physical clearance
Xmech is a mechanical travel limit based on collision, bottoming, lead stress, surround or spider limit, or loss of safe alignment. It should exceed intended linear excursion. Reaching it is not normal operation and may cause immediate damage.
Thermal power compression
As the voice coil heats, resistance rises. At fixed amplifier voltage, current and motor force fall, so output gains less than expected. Adhesives, former and winding temperatures can reach limits even when excursion looks safe.
Displacement and real output
Sd × linear Xmax estimates one-way displacement capability. Large Sd can reduce excursion required for a given output. Enclosure loading, frequency and compression determine how much of that theoretical displacement becomes sound.
Compare honestly
Ask for definition, measurement direction, symmetry, test conditions and whether the published number is one-way. A smaller rigorously measured Xmax can be more useful than a larger unexplained figure.
Installation and testing checklist
| Checkpoint | What to verify |
|---|---|
| Compatibility | Source output type, receiving input type and allowable voltage range match. |
| Routing | Signal wiring is protected, secured and separated from likely interference sources. |
| Termination | Connectors fit firmly without excessive force; no cable weight hangs from a jack. |
| Signal | Every channel is identified, clean and unclipped at the intended source level. |
| Final test | Polarity, channel assignment, noise floor and operation are verified with the vehicle running. |
Frequently asked questions
Is Xmax one-way or peak-to-peak?
Usually one-way, but always verify the manufacturer’s definition.
What does 70% BL mean?
Motor force factor has fallen to 70% of its centered value at that position.
Is Xmech usable excursion?
No. It is a survival or collision boundary, not a clean-operation target.
Why does output stop increasing with power?
Thermal compression, excursion nonlinearity, enclosure limits and amplifier limits all contribute.
Does more Xmax always mean more bass?
Only if cone area, motor, enclosure, cooling and power support its use.