How long a round or aero port must be to tune your box to 32 Hz (or any tuning), or what a port you have tunes to.
Find
Port
length
Tuning
(Hz)
Port length
Tuning (Hz)
Port length
Tuning (Hz)
Port type
Round
tube
Aero
(flared)
Round tube
Aero (flared)
Round tube
Aero (flared)
Number of ports
1
2
3
4
1
2
3
4
Inside diameter (in)
Net box volume (ft³)
Net: after sub, port and bracing displacement.
Tuning (Hz)
30–36 Hz is usual for a daily. Lower = deeper.
Port length (each)
10 5/16" (262 mm)
Port area
12.6 sq in, 5 sq in per ft³ (typical 12–16)
Port takes up
0.07 ft³ (2.1 L)
Box volume to build (net + port)
2.57 ft³ (72.9 L)
Formula
L = 1.463 × 10⁷ × N × R² ÷ (Fb² × Vb) − k × D
With
Fb = 32 Hz, Vb = 4,320 in³, R = 2", N = 1, k × D = 2.93", L = 10 5/16"
Starting points, not guarantees. Your sub’s manual wins: check its recommended box and tuning.
How it works
Enter the port’s inside diameter, how many ports, the net box volume and the tuning you want. The calculator gives the length of each port, its area per cubic foot and how much volume it takes up, so you can add it to the box. Or switch to “Tuning” to see what a port you already have tunes to.
Example: one 4 in round port in a 2.5 ft³ (4,320 in³) box tuned to 32 Hz: 1.463 × 10⁷ × 2² ÷ (32² × 4,320) = 13.23 in, minus the end correction 1.463 × 2 = 2.93 in, gives 10.30 in. The same port as an aero port: 9.83 in.
A 4 in port has 12.6 sq in of area, only 5 sq in per ft³ in that box, well under the usual 12–16: it may chuff at high volume. Two ports double the area, and each one must then be 23.53 in long.
Frequently asked questions
How long should the port be for 32 Hz?
It depends on the port size and the box volume. A 4 in round port in a 2.5 ft³ box needs about 10.3 in; bigger ports and smaller boxes need longer ports.
What is end correction?
The air just outside each end of the port moves with it, so the port acts longer than it is. The formula subtracts that extra length: 1.463 × R for a plain tube on the baffle, more for a flared port.
How do I check what my box is tuned to?
Switch Find to Tuning and enter the port length you built. The calculator turns the same formula around.