Port length for 40 Hz tuning
At 40 Hz, a 2.5 ft³ net box needs a 4" round port 5 9/16" long, or a 2 9/16" × 13 1/2" slot port 18 11/16" long. Bigger boxes need shorter ports; bigger ports need longer ones.
At 40 Hz the box is louder and punchier around 40–60 Hz but rolls off sooner below; common for loud, bass-heavy builds. The port is shorter.
Round port length for 40 Hz (one port, flush on the baffle)
Round port length for 40 Hz (one port, flush on the baffle)| Net volume | 3" port | 4" port | 6" port |
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| 1 ft³ | 9 11/16" | 18 1/4" | 43 1/4" |
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| 1.5 ft³ | 5 3/4" | 11 3/16" | 27 3/8" |
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| 2 ft³ | 3 3/4" | 7 11/16" | 19 7/16" |
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| 2.5 ft³ | 2 9/16" | 5 9/16" | 14 11/16" |
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| 3 ft³ | 1 3/4" | 4 1/8" | 11 1/2" |
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| 4 ft³ | 13/16" | 2 3/8" | 7 1/2" |
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| 5 ft³ | 3/16" | 1 5/16" | 5 1/8" |
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Slot port for 40 Hz (13 1/2" tall, 14 sq in per ft³, 3/4" wall)
Slot port for 40 Hz (13 1/2" tall, 14 sq in per ft³, 3/4" wall)| Net volume | Slot W × H | Length | Takes up |
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| 1 ft³ | 1 1/16" × 13 1/2" | 20 1/2" | 0.29 ft³ |
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| 1.5 ft³ | 1 9/16" × 13 1/2" | 19 13/16" | 0.36 ft³ |
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| 2 ft³ | 2 1/16" × 13 1/2" | 19 3/16" | 0.42 ft³ |
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| 2.5 ft³ | 2 9/16" × 13 1/2" | 18 11/16" | 0.49 ft³ |
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| 3 ft³ | 3 1/8" × 13 1/2" | 18 1/4" | 0.55 ft³ |
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| 4 ft³ | 4 1/8" × 13 1/2" | 17 3/8" | 0.67 ft³ |
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| 5 ft³ | 5 3/16" × 13 1/2" | 16 11/16" | 0.77 ft³ |
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The formula
L = 1.463 × 10⁷ × N × R² ÷ (Fb² × Vb) − k × D, in inches, with Vb in cubic inches (1 ft³ = 1,728 in³), R the port radius and k × D = 1.463 × R for a port flush on the baffle. A slot uses the radius of a circle with the same area.
Real tuning lands within a few hertz of the math. Build the port a little long, test, and trim.
“Too small” means no length reaches that tuning: even with no tube the port tunes lower. Use a bigger port or more of them.
Port length for other tunings
Frequently asked questions
How long should the port be for 40 Hz?
At 40 Hz, a 2.5 ft³ net box needs a 4" round port 5 9/16" long, or a 2 9/16" × 13 1/2" slot port 18 11/16" long. Bigger boxes need shorter ports; bigger ports need longer ones.
Is 40 Hz a good tuning?
At 40 Hz the box is louder and punchier around 40–60 Hz but rolls off sooner below; common for loud, bass-heavy builds. The port is shorter.
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.