Port length for 30 Hz tuning
At 30 Hz, a 2.5 ft³ net box needs a 4" round port 12 1/8" long, or a 2 9/16" × 13 1/2" slot port 37 1/16" long. Bigger boxes need shorter ports; bigger ports need longer ones.
At 30 Hz the box plays very deep and smooth, but it needs a big box and a long port and gives up some punch higher up. Good for deep-bass music and SQ builds.
Round port length for 30 Hz (one port, flush on the baffle)
Round port length for 30 Hz (one port, flush on the baffle)| Net volume | 3" port | 4" port | 6" port |
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| 1 ft³ | 19" | 34 11/16" | 80 1/4" |
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| 1.5 ft³ | 11 15/16" | 22 3/16" | 52 1/16" |
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| 2 ft³ | 8 3/8" | 15 7/8" | 37 15/16" |
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| 2.5 ft³ | 6 1/4" | 12 1/8" | 29 1/2" |
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| 3 ft³ | 4 7/8" | 9 5/8" | 23 13/16" |
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| 4 ft³ | 3 1/8" | 6 1/2" | 16 3/4" |
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| 5 ft³ | 2 1/16" | 4 5/8" | 12 9/16" |
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Slot port for 30 Hz (13 1/2" tall, 14 sq in per ft³, 3/4" wall)
Slot port for 30 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" | 38 13/16" | 0.54 ft³ |
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| 1.5 ft³ | 1 9/16" × 13 1/2" | 38 1/8" | 0.69 ft³ |
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| 2 ft³ | 2 1/16" × 13 1/2" | 37 9/16" | 0.83 ft³ |
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| 2.5 ft³ | 2 9/16" × 13 1/2" | 37 1/16" | 0.97 ft³ |
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| 3 ft³ | 3 1/8" × 13 1/2" | 36 9/16" | 1.1 ft³ |
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| 4 ft³ | 4 1/8" × 13 1/2" | 35 3/4" | 1.37 ft³ |
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| 5 ft³ | 5 3/16" × 13 1/2" | 35" | 1.62 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 30 Hz?
At 30 Hz, a 2.5 ft³ net box needs a 4" round port 12 1/8" long, or a 2 9/16" × 13 1/2" slot port 37 1/16" long. Bigger boxes need shorter ports; bigger ports need longer ones.
Is 30 Hz a good tuning?
At 30 Hz the box plays very deep and smooth, but it needs a big box and a long port and gives up some punch higher up. Good for deep-bass music and SQ builds.
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.