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Free Tool · Cone Excursion & Safe Power

Subwoofer Power Handling Calculator

Xmax does not change with box size. What changes is how far the cone travels per watt. Pick a driver, two sealed volumes, and an amplifier power, and see exactly where the cone runs out of travel and how much power the box can safely take at every frequency.

CalculatorHow It WorksRead the GraphExamplesFAQ
Quick answer: A subwoofer's real-world power handling is set by whichever limit it hits first: Xmax (the cone runs out of linear travel) or Pe (the voice coil overheats). Box size changes how much the cone moves per watt, so a bigger sealed box lowers safe power down low, and a subsonic filter raises it back by keeping the cone out of trouble below tuning. This tool plots both boxes against the driver's Xmax so you can see the trade in seconds.
Expert-built Built & reviewed by Scott Welch — IASCA & MECA Sound Quality Champion with 40+ years in car audio.

Cone Excursion & Power Handling Calculator

Sealed enclosures. Pick a sub we carry to auto-fill its manufacturer T/S, or enter any driver by hand.

Excursion vs frequency · Box A vs Box B

Amp power W
fcHz
Box A Box B Xmax Subsonic fc
Hover the chart to read cone excursion at a frequency.
Box A
Box B
FrequencyBox A safeBox B safeLimited by (A / B)

Safe power = the RMS wattage at which the cone reaches Xmax at that frequency, capped at the driver's thermal Pe rating. Independent of the amp-power slider, which drives the excursion graph above.

How Subwoofer Power Handling Actually Works

Every subwoofer has two separate limits, and the lower one wins at any given frequency:

Xmax (mechanical)The furthest the cone can travel in one direction and still stay linear. Push past it and the coil leaves the magnetic gap, distortion climbs fast, and the suspension takes the beating. Xmax is a fixed property of the driver.
Pe (thermal)The continuous RMS power the voice coil can dissipate as heat without cooking. Also fixed by the driver.

Here is the part people miss: cone travel per watt is not constant across frequency. In a sealed box it peaks near and below the system resonance (Fc) and drops off higher up. So a sub is almost always excursion-limited down low and thermally-limited up top. The frequency where those two limits cross is the knee you see on the graph.

Safe power (f) = min( Pe , (Xmax / excursion-per-watt at f)² )

Box size moves that knee. A smaller sealed box raises Fc and Qtc and reins in low-frequency excursion, so the driver takes more power down low. A bigger box lowers Fc for deeper extension but lets the cone move farther per watt, so safe power drops below tuning. A subsonic (high-pass) filter cuts the signal below its corner frequency, pulling the cone back under Xmax where it would otherwise run away, which is why it buys back real power handling.

How to Read the Graph

The curvesCone excursion in mm at your set amp power, one per box. Box A (blue) and Box B (amber).
The red lineThe driver's Xmax. Anywhere a curve climbs into the shaded zone above it, that box is over-excursing at that power.
The dashed lineYour subsonic filter's corner frequency, when the filter is on. Watch the curves collapse below it.
The tableThe flip side: the maximum safe RMS power at each frequency, and whether Xmax or Pe is the limit there.

Model scope: small-signal, sealed only, half-space. It does not add port or cabin gain, power compression, or large-signal nonlinearity, so read it as the mechanical ceiling, not a license to run a coil hot for hours. Xmax is entered one-way (linear, one direction). For a driver rated 39 mm peak-to-peak, enter 19.5.

Worked Examples: Subs We Carry

Manufacturer T/S, run through the model in a typical sealed box at 500 W. "Over Xmax below" is the frequency under which the cone exceeds its linear limit at that power.

SubwooferXmaxPeBoxOver Xmax below (500 W)
Image Dynamics IDMAX10 V419 mm500 W0.6 ft³Stays under Xmax
Image Dynamics ID10 V414 mm250 W0.6 ft³Thermal (Pe) limits first
JL Audio 12TW3-D415.2 mm400 W1.75 ft³~28 Hz
Proline X Driv 85014 mm800 W0.5 ft³Stays under Xmax

Run yours above, then browse our subwoofers. All preset T/S is manufacturer-published.

This Is One Step — The Rest of the Suite

Power handling tells you how hard you can push a driver in a given box. Pick the box style and size it with the rest of the tools:

EBP CalculatorSealed or ported? Decide from Fs and Qes Airspace CalculatorRecommended internal volume Box Calculator SuiteSealed & ported box designers

Rather We Just Build It Right?

We CNC-cut Proline X enclosures to your driver's exact sealed target, spec-matched and built in Tennessee, so the box protects the cone instead of fighting it.

Shop Proline X Enclosures Ask Scott a Fitment Question

Power Handling & Excursion FAQ

Does a bigger box increase or decrease power handling?
In a sealed box, bigger usually lowers low-frequency power handling. A larger box lets the cone move farther per watt below the system resonance, so it reaches Xmax at a lower wattage. You gain deeper extension but give up excursion headroom down low. A smaller sealed box does the opposite: tighter control, more low-end power handling, less deep extension.
Why does my subwoofer handle less power at low frequencies?
Because cone travel per watt is highest down low. Near and below the box's resonance the cone moves the most for a given input, so it hits Xmax first there. Higher up it barely moves, so the voice coil's thermal rating (Pe) becomes the limit instead. That is why the safe-power curve rises from left to right and then flattens.
What is Xmax and what happens if I exceed it?
Xmax is the one-way linear excursion limit: how far the cone travels in one direction while the voice coil stays fully in the magnetic gap. Past Xmax the motor loses control, distortion rises sharply, and the suspension and coil former take mechanical stress. Brief excursions past Xmax are survivable; sustained over-excursion is how cones and coils get destroyed.
How does a subsonic filter protect my subwoofer?
A subsonic (high-pass) filter cuts signal below its corner frequency. Since the lowest content is what drives the cone past Xmax, filtering it out keeps the cone in its linear range and lets you run more power safely. On the graph, turn the filter on and watch the excursion curves drop below the corner frequency.
Should I use a 12 or 24 dB/octave subsonic slope?
A 24 dB/octave slope cuts far more aggressively below the corner, so it protects the cone harder against deep sub-tuning content, which is why it is the common choice for ported builds and high-excursion drivers. A 12 dB/octave slope is gentler and preserves a little more low-end feel. For sealed boxes many builders skip the subsonic entirely or set a gentle one well below Fc.
Is this calculator only for sealed boxes?
Yes. It models sealed enclosures, where excursion behavior is predictable and the math is honest. Ported boxes minimize excursion at port tuning but let it rise sharply below tuning, which needs port and room-gain modeling this tool does not do. For ported sizing, use the box suite and always run a subsonic filter.
Where do I find my subwoofer's Thiele-Small parameters?
On the manufacturer's spec sheet or product page: Fs, Qes, Qms, Vas, Sd, Re, Xmax, and Pe. Pick your sub from the dropdown above and we fill in manufacturer-published values for the drivers we carry. Bought from us and don't see it listed? Contact us and we'll pull the numbers.
My sub isn't in the list. Can I still use it?
Yes. Choose Custom in the dropdown and type in the eight parameters from your spec sheet. Enter Xmax as the one-way figure. If your sheet only lists a peak-to-peak excursion, halve it. If it only lists Qts and Qms, derive Qes with Qes = (Qts x Qms) / (Qms - Qts).
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