Sealed Subwoofer Excursion & Safe Power Calculator
Enter the driver small-signal data, sealed-box volume, and amplifier rating. The calculator estimates peak cone excursion at one frequency and solves the amplifier power that would reach one-way Xmax.
Maximum safe amplifier power233.2
- One-way peak cone excursion
- 17.57
- Xmax utilization
- 146.43%
- Excursion headroom
- -3.31
- Closed-box resonance (Fc)
- 46.75
- Closed-box total Q (Qtc)
- 0.64
- Excursion status
- Over Xmax
Electrical and motion checks
| Quantity | Value |
|---|---|
| Amplifier output voltage | 44.72 V RMS |
| Safe output voltage | 30.54 V RMS |
| Input impedance magnitude at test frequency | 4.49 Ω |
| RMS displacement sensitivity | 0.2778 mm/V |
Assumes one driver in an ideal sealed enclosure with no leakage, stuffing correction, filters, equalization, or clipping.
Safe power is excursion-limited at the selected frequency only; it is not a thermal voice-coil rating.
How to use this calculator
- Enter Fs, Vas, Qes, Qms, Re, Le, Sd, and Xmax from the driver specifications.
- Enter the finished sealed-box net air volume after subtracting displacement.
- Choose the frequency where you want to check excursion, often a low bass frequency.
- Enter the amplifier rated power and the load impedance used for that rating.
- Read the predicted excursion, Xmax utilization, and excursion-limited safe power.
How the excursion result is calculated
This calculator uses a linear electromechanical Thiele-Small model for one driver in an ideal sealed box. Vas and box volume are converted from liters to cubic meters, Sd from cm² to m², Le from mH to H, and Xmax from mm to m. Air density is set to 1.204 kg/m³ and sound speed to 343 m/s.
The driver suspension compliance is Cms = Vas / (ρ c² Sd²). From that, the moving mass is Mms = 1 / ((2πFs)² Cms), mechanical resistance is Rms = 2πFs Mms / Qms, and force factor is Bl = sqrt(2πFs Mms Re / Qes). The sealed box adds an air spring with Cmb = Vb / (ρ c² Sd²), so the combined compliance is Cmt = 1 / (1 / Cms + 1 / Cmb).
At the selected frequency, the mechanical impedance is Zm = Rms + j(ωMms − 1 / (ωCmt)). The electrical input impedance is Ze = Re + jωLe + Bl² / Zm. With a 1 V RMS source, current is I1 = 1 / Ze, cone velocity is u1 = Bl I1 / Zm, and displacement sensitivity is Hx = u1 / jω meters RMS per volt.
The amplifier rating is converted to RMS voltage with V = sqrt(Pamp × Zrated). Predicted one-way peak excursion is xPeak = sqrt(2) × |Hx| × V. Xmax utilization is 100 × xPeak / Xmax, and headroom is 20 log10(Xmax / xPeak). The safe-power result reverses the same sensitivity: Vsafe = Xmax / (sqrt(2) × |Hx|) and Psafe = Vsafe² / Zrated.
What moves the result most
Frequency, box volume, and Xmax usually dominate the answer. Excursion rises quickly below the sealed-box resonance because the cone is doing more motion for less acoustic output. A smaller sealed box raises Fc and Qtc, which may reduce very-low-frequency travel but can create a peaky alignment. Re, Qes, Qms, and Bl-related behavior determine how much electrical damping the driver has at the check frequency.
Closed-box resonance and Q
The calculator also reports the common small-signal sealed-box values Qts = 1 / (1 / Qes + 1 / Qms), Fc = Fs × sqrt(1 + Vas / Vb), and Qtc = Qts × sqrt(1 + Vas / Vb). These values describe the ideal low-level alignment, not the large-signal limits of the driver.
What this tool leaves out
The model does not include suspension, motor, or inductance nonlinearities near full stroke. It also excludes acoustic stuffing, leakage, panel losses, room gain, cabin gain, equalization, high-pass filters, amplifier clipping, and thermal power handling. Treat the safe-power number as an excursion limit at one frequency, not as a general wattage rating.
Worked example
For a driver with Fs = 30 Hz, Vas = 50 L, Qes = 0.45, Qms = 5, Re = 3.4 Ω, Le = 1.5 mH, Sd = 480 cm², and Xmax = 12 mm in a 35 L sealed box, checked at 20 Hz with a 500 W amplifier rated into 4 Ω, the amplifier voltage is sqrt(500 × 4) = 44.72 V RMS.
The model gives an RMS displacement sensitivity of about 0.0002778 m/V. Peak excursion is therefore sqrt(2) × 0.0002778 × 44.72 = 17.57 mm. That is 146.43% of the entered Xmax, so the status is over Xmax. Reversing the same sensitivity gives 30.54 V RMS at the excursion limit, or 233.20 W into the rated 4 Ω load.
Common questions
How much amplifier power can my subwoofer handle at 20 Hz?
Use the frequency field for 20 Hz and enter the driver, box, and amplifier-rating data. The safe-power result is the amplifier rating whose equivalent RMS voltage reaches the entered one-way Xmax at 20 Hz. It is an excursion limit only, not a thermal rating for music or long-term heat.
Why can a subwoofer exceed Xmax below its box resonance?
Below resonance, acoustic output falls while cone motion can remain high, so the driver may need a lot of stroke for little sound pressure. A sealed box controls excursion more gently than many vented alignments, but it does not prevent over-excursion. A high-pass filter or less low-frequency boost may be needed when the calculated utilization is high.
Should I use Re or nominal impedance?
Use Re, the voice-coil DC resistance from the driver data sheet or a measurement. Nominal impedance is a broad compatibility label such as 4 Ω or 8 Ω, while the model needs the coil resistance used in the small-signal parameters. The separate rated-load field is only for converting amplifier watts to RMS volts.
What is one-way Xmax compared with peak-to-peak excursion?
One-way Xmax is the peak linear travel in one direction from rest. Peak-to-peak excursion is twice that value because it spans both directions of motion. This calculator expects one-way peak Xmax, so do not enter a peak-to-peak number unless you divide it by two.
How does sealed-box volume affect excursion, Fc, and Qtc?
A smaller sealed box adds a stiffer air spring, which raises Fc and Qtc. That can reduce excursion at very low frequencies but may create more response peaking near resonance. A larger box lowers Fc and Qtc and lets the driver behave more like its free-air suspension.
Does a high-pass filter reduce subwoofer excursion?
Yes. A high-pass filter reduces voltage below its cutoff, so cone excursion below that point usually falls sharply. This calculator does not include filter transfer functions; enter the amplifier power or voltage equivalent that actually reaches the driver at the frequency you are checking.