How a Port Works
When a sub pushes forward, its back pushes into the box just as hard. In a sealed box that back push stays trapped. A port gives it a way out. Near one frequency, called the box’s tuning, the air in the port swings in and out in step with the cone and adds to what the cone is playing.
Around tuning the port does most of the work and the cone hardly moves. That is why a ported box gets so much louder than a sealed one near its tuning, for the same power and with less cone travel.
A Spring With a Weight on It
Blow across the top of a bottle and it plays a note. A ported box works the same way. The air inside the box acts like a spring, and the plug of air in the port acts like a weight sitting on it. Push the weight and it bounces at its own speed. That speed is the tuning.
- More box air makes a softer spring, so the box tunes lower.
- A longer port holds more air, a heavier weight, so it tunes lower.
- A bigger port opening lets the air move more easily, so it tunes higher. To keep the same tuning with a bigger port, it has to get longer too.
Try It
Move the sliders and watch the tuning and the loudness curve move with them.
Tunes to 29.7 Hz
An example 12-inch sub, outside a vehicle. 0 dB is its level at 120 Hz.
Look at the shaded part of the chart, below tuning. Down there the port stops helping and the loudness falls off fast. Worse, the box air no longer holds the cone back, so the cone can flap far past its limits on very little power. A subsonic filter set a few Hz under tuning keeps that energy off the sub.
The port’s air also sticks out a little past each end, so a port acts slightly longer than it measures. The tuning maths allows for this. A port running along a box wall acts a little longer again, which is one reason a finished box can land a few Hz off the plan.
Port Size and Port Noise
At tuning, all the air the cone moves goes through the port. A small opening means the air has to rush through it. Past a point it chuffs and whistles, and the turbulence costs output, so the box stops getting louder as you add power. That is called port compression. It also pushes the box’s working tuning up a little at full power, so a quiet sweep can read slightly lower than the box behaves on the meter.
More port area slows the air down. A common rule is at least a ninth of the cone area, better a quarter or more, and close to the full cone area for very high power. The catch is that a bigger port needs to be longer for the same tuning, and it takes up space in the box, which in a car is space you rarely have. Rounded or flared port ends also help the air in and out more quietly. The Box Designer sizes the port area for your sub and power so the air stays at a sensible speed.
Finding Where Your Box Really Tunes
Plans are an estimate, so measure the finished box. A sub’s impedance (its resistance to the amp, in ohms) changes with frequency. In a ported box it rises into two humps, and the low point between them is the box’s tuning. An impedance meter or sweep tool will draw this for you. The impedance sweep page walks through it step by step.
The same example sub in a 3 cubic foot box tuned to 32 Hz.
The humps also tell you how well the box is built. Tall, sharp humps mean a tight box. Short, soft humps and a raised middle mean air is leaking out somewhere it shouldn’t, like a seam, a terminal cup or a loose screw. A leak costs output right where the port should be strongest, so seal it before you meter.
Ports in a Vehicle
A car is a small room, and it has a note of its own. Below a certain frequency the cabin pressurises and bass builds up, and most vehicles have one Hz where they ring loudest. What the meter reads is the box and the vehicle together, so the loudest score usually comes where the box’s tuning and the vehicle’s own peak work together.
That is why two people with the same box can hit very different numbers, and why moving the tuning a few Hz is one of the biggest changes you can make. Use the port tuning calculator to work out a port, measure the box when it’s built, then try a few tunings in the car on the meter.