Bass Reflex Port Length Calculator

What this tool does

A ported box is a Helmholtz resonator: the air in the port bounces on the springy air in the box, and its tuning depends on the box volume and the port's area and length. The air just beyond each end of the tube also moves, so the acoustic length is longer than the tube — about 0.73 diameters for one flanged end and one free end — and that is subtracted to get the length to cut. Narrow ports make the air rush and 'chuff' at high volume, which is why wider ports, needing longer tubes, are preferred; the box volume should be net of the driver, bracing and the port itself.

Formula

f_b = (c ÷ 2π) × √(A ÷ (V × L_eff)) ⇒ L_eff = c² A ÷ (4π² f_b² V) · cut length = L_eff − 0.732 × D

Variables

Worked example

Limitations

• The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
• Coverage and consumption figures vary between products. Take the values from the manufacturer's datasheet for the product you are actually using.