Subwoofer Port Calculator
Inputs in cubic feet + inches. Outputs in inches. Air velocity shown in ft/s.
Configuration
Port Type
Formula (Helmholtz, inches/ft³)
L = (c² · Atotal) / ((2πf)² · Vin³) − (k · Deff)
c = 13,560 in/s, A in in², Vin³ = Vft³ × 1728, f in Hz
Results
PORT LENGTH
–
inches
PORT AREA
–
sq. in (total)
AREA RATIO
–
sq. in / ft³
PORT VELOCITY (EST.)
–
ft/s (rule-based)
✓ Port length looks reasonable
✓ Good port area ratio
✓ Port velocity acceptable
“Velocity (est.)” is a fast sanity check. Real port velocity depends on the driver, excursion, and alignment.
Best Practices (Quick, Real-World)
1 Sub vs Multiple Subs
  • One sub: tune a touch lower for deeper daily bass (common sweet spot: 32–36 Hz for trucks/cars).
  • Two+ subs: you move more air, so protect the port. Increase total port area and/or lower tuning slightly to keep it clean.
  • Rule of thumb: when you double cone area and power, don’t keep the same port. Add area or you will hear port noise first.
  • Space reality: bigger port area usually means a longer port. Plan the fold path before you cut wood.
Port Design Tips (So it stays quiet)
  • Port area matters more than port length. Length is what you end up with to hit tuning.
  • Keep slot ports tall enough. Very short slot heights can “hiss” even when the math says it works.
  • Round ports: flare the ends when you can. It buys you cleaner output at high excursion.
  • Bracing: brace long ports. A vibrating port wall makes the box sound “cheap.”
Cars + Trucks: Best Output Direction
  • Most common winner: sub + port firing toward the rear of the vehicle (uses cabin gain and longer path).
  • Behind-seat truck boxes: sub forward (into cabin) often feels “punchy,” port can be side or down if rear space is tight.
  • Avoid choking: keep at least one port diameter of clearance from walls/seat backs (more is better).
  • Test fast: if you can rotate the box, do it. The “best” direction is the one that loads the cabin best in that vehicle.
Boats: Port + Sub Direction
  • No cabin gain like a car. Boats need more cone area and/or power for the same “impact.”
  • Protect the port from spray: don’t aim the port at areas that get wet. Water + port velocity is a bad combo.
  • Keep ports away from loose storage: anything that can block the port will kill output and raise distortion.
  • Structure matters: boats rattle easily. Use real bracing and solid mounting or you lose bass to vibration.
Simple build sanity checklist: confirm net volume (after sub + port + bracing), confirm port clearance, brace large panels, and seal every joint.