Crover

Free calculator

Vented box calculator: port length for the tuning frequency

Enter the net volume, the port diameter and the tuning frequency fB you want: the calculator gives the port length – with the same end correction as WinISD – and, with Sd and Xmax, whether the port is large enough.

Inputs and result

l
Hz
mm

The woofer's Sd and Xmax (for the minimum port size)

cm²
mm

Port length (each): 147 mm

  • Acoustic length incl. end correction: 183 mm
  • Total port area: 19.6 cm²
  • First pipe resonance (hint): 937 Hz
  • Minimum diameter for ≤ 17 m/s at Xmax: 35 mm

Formulas

fB = c / (2π) · √( N · S / (Vb · L_eff) )
L_eff = L + 0.732 · d
N · S_min ≈ Sd · Xmax · 2π · fB / 17 m/s
pipe resonance ≈ c / (2 · L_eff)
c = 343.2 m/s (20 °C)
Simulate in Crover with real measurements

Opens a new project with exactly this circuit. Works as a guest without an account; then load your drivers' FRD/ZMA.

Crover's enclosure planner is an Alpha: it calculates with estimates from T/S and geometry. Measure before you buy parts.

How a vented box is tuned

The air in the port moves as a mass on the spring of the air in the box – a Helmholtz resonator. Its frequency fB is the tuning: around fB mostly the port radiates, the woofer barely moves.

Air in front of and behind the port ends moves with it. So the port is acoustically longer than measured – the end correction, here 0.732 × diameter (one flanged and one free end, as in WinISD).

Limits

  • The volume is net: subtract driver, port, bracing and account for damping material.
  • Rectangular slots are computed as a round port of the same area; slots against a wall tune somewhat lower.
  • The minimum size is a rule of thumb for the air velocity at fB. For more power below fB or very small boxes only a simulation helps.
  • Which tuning suits your driver (volume and fB from its T/S parameters) is not in this calculator – that is enclosure simulation.

Sources: R. H. Small, “Vented-Box Loudspeaker Systems”, JAES 21 (1973); end correction after Levine & Schwinger (1948) and WinISD (0.732 · d); the minimum port area as a rule of thumb for 5 % of the speed of sound.