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How to Port a Sealed Sub Box

Ported enclosures by nature can deliver more output than a sealed box at any given amplifier wattage. For this reason, converting a sealed enclosure into ported makes perfect sense.

Most car audio enthusiasts, bassheads and audio fanatics are tinkerers, and the ability to squeeze every bit of performance out of a sealed subwoofer box is too tempting a project to not undertake.

Porting a sealed subwoofer box is more complicated than simply drilling a hole in an enclosure. It requires the knowledge, tools and skills necessary to manage this intricate task.

To make the whole process as straightforward as possible, we’ve pulled together this step by step guide that will answer all your questions on how to port a sealed subwoofer box.

Can You Convert a Sealed Enclosure into Ported?

Technically, the answer is yes. You can convert a sealed subwoofer box into ported, but it might not sound like a box that’s built to be ported from the get go (more on this down below). This is especially true if the sealed prefab box doesn’t meet the internal cubic feet of a ported subwoofer box.

When it comes to porting a subwoofer box, there’s a lot to know. First of all, it’s important to realize the differentiation between building a ported subwoofer box from scratch and converting a prefab sealed one into ported. You port a sealed subwoofer box differently than you port a vented enclosure in the making. Regardless, the idea is to get more output out of your subwoofer box and increase volume and bass response.

The reason for porting a sealed box is to increase bass output at specific lower frequencies. The byproduct is that the port will redirect the sound from the rear of the cone and adds it to the sound coming from the front, making the bass louder. This can be an advantage when extreme output levels are desired or if you are wanting to maximize the output of a subwoofer with a smaller power amplifier.

It must be noted that the enclosure’s air volume, vent area and length are critical for proper tuning and power handling. Failure to follow the recommended guidelines and specifications can result in poor bass response and/or damage to the woofer.

Why It’s Not Recommended to Port a Sealed Box

Although it’s possible to port a sealed subwoofer box and turn it into a ported one, it’s something we don’t recommend you to do at all unless you are just experimenting.

The reason why we don’t recommend this is because it’s tricky, and you can easily mess things up. This is especially true if you’re a relative greenhorn when it comes to car audio or if you’ve never built a subwoofer box before.

On the other hand, sealed enclosures don’t have enough cubic feet of space. In other words, the ideal volume between a sealed and ported box for the same subwoofer usually differs by up to 50%. What this means is that you can’t just slap a port into a sealed box and have it sound decent.

Although stuffing the box with some Pollyfill — to fool the woofer into thinking that it’s in a larger box (one with more air, or internal volume, in it) – can help somewhat, but only to a certain extent.

I mean, no matter how much you stuff your subwoofer box, you won’t make up for the difference. That’s not to say that Pollyfill doesn’t work. Quite honestly, Pollyfill does work for sealed boxes, and there’s no denying that it can make a sealed box louder. However, it’s not ideal for vented enclosures since air gets pumped in and out. This will make it fall apart over time and get blown into cabin space/trunk. This is the reason why you hardly see vented boxes with any stuffings.

Not only that, the vent on the box is tuned to a specific frequency, which is in direct correlation with the volume of the box. Speaking of which, tuning of the port is critical to maximizing the output capability of the subwoofer.

Finally, the other reason why we don’t recommend porting a sealed box is because it will take time and it might cost you more than buying a ported box, at which point it’s just not worth it, and you’re better off buying a brand new ported box.

Nevertheless, if you want to give it a shot, and you don’t mind spending some time and money, read on…

How to Port a Sealed Subwoofer Box

As mentioned above, porting a sealed box isn’t that easy. There are several factors that must be taken into account, the most crucial of which are:

  • Enclosure’s net cubic air space (net = space left after the subwoofer is in).
  • Port tuning frequency – Known as “Fb” which is the frequency that the port is tuned to for a ported enclosure. It’s determined by a combination of port diameter, port length, and net volume of the subwoofer box.
    Vent tuning changes the frequency at which the frequency response peaks (is loudest) which consequently changes the way a subwoofer box sounds in your setup. Accordingly, your tuning frequency choice will be determined by your goal.
    If you want more SPL than SQ (getting as loud as possible), you’ll need to tune fairly high (around 45Hz or possibly higher). However, if you’re looking for more SQ (Sound Quality), you’ll need to tune fairly low (around 30Hz – 25Hz). This will allow your setup to hit the ultra-low notes.
    Generally speaking, a range between 30Hz and 35 Hz should hit that sweet spot for most setups and should result in a perfect mix between SPL and SQ.

Now that we’ve made these two crucial things clear, it’s time to get your hands dirty.

What You’ll Need to Get This Job Done

  • Safety goggles
  • Circle cutter
  • PVC pipe
  • Silicone glue
  • Handsaw
  • Sandpaper
  • Heat gun
  • Kitchen metal bowl
  • Polyfill
  • Subwoofer enclosure calculator

Before you begin, you need to make sure the internal cubic volume of your sealed box is sufficient for being ported.

To do that, use this subwoofer enclosure calculator to work out the internal cubic volume. All you need to do is fill in your box’s measurements into the calculator and it’ll work it out for you. This calculator will also help you work out port tuning frequency, factoring in the diameter and port length.

Whichever calculator you use, you’ll find that your sealed box is too small to be ported. This is true to some extent. However, there are a few ways to increase the net volume of sealed subwoofer box so that you can port it.

  • Inverting the subwoofer: an inverted subwoofer is a fancy term used to describe a subwoofer that is mounted “backwards” or “upside-down”, so the magnet structure is exposed outside of the enclosure. This allows to free up a large amount of internal cubic feet as the magnet takes a huge amount of space up inside the box.
  • Using Polyfill: Also known as polyester fiberfill, it’s a white fluffy stuffing that is placed inside a subwoofer box. It can be found almost anywhere and is extremely affordable.The main purpose for using Polyfill is to slow down sound waves inside the box, making the subwoofer perform as if the box were bigger.
    Generally speaking, you can get anywhere around 25% to 35% of “space gain” by using 1 to 1-1/3 pounds of stuffing per cubic foot of box volume.
P.S: You can either buy a port tube for your box, or build one using a PVC pipe.


Once you’ve figured out what tuning you want, you may want to either increase or decrease the port length. So, use a handsaw to cut the PVC pipe you’re going to be using as a port tube, and file down the edge using sandpaper.

To prevent port noise at loud volumes, you’ll need to flare the opening of the PVC port.

Flaring a PVC pipe is easy and you can do it without the need for special tools. All you need is a heat gun and a metal bowl.

So, place the end of the PVC pipe onto the side of the metal bowl, keep twisting it while applying heat directly to it until the end becomes flared.

Next, place the PVC aero port at the front of the subwoofer box you want to add it to, and draw around it using a marker pen. Then, use a circle cutter to cut into the MDF. After that, sand down the edges for the perfect finish and slide the port in.

Finally, consider using some silicone inside the box to seal around any air leaks around the vent. This is highly recommended to prevent port noise.

Alex Brown

Hey There, my name is Alex Brown, I'm an LA-based sound engineer with over 10 years experience installing, troubleshooting, and repairing commercial, automotive, and household sound equipment. I've installed highly competitive car audio systems, and everything from navigation systems to full car stereo systems, remote starters, alarms and beyond. I enjoy creating solutions and simplifying everyday needs. I also love helping people get great sounding gear, thereby, saving the world from bad sound one customer at a time.

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