How to Build a Car Subwoofer Enclosure?


To have strong bass in your car, you need to have a subwoofer. If you need a customized size or design, you can build a subwoofer box for your vehicle from scratch, and this is the cheapest way to make a box exactly like you need.

But how build a perfect subwoofer enclosure for your car? Let’s find out.

To build a car subwoofer box, you need to consider several factors that will affect its design:

  • Define where in the car you will install the box.
  • Define the shape of the box, whether it will be standard or customized to fit in the vehicle’s body.
  • Define the number of speakers and their size that will determine the volume of the box.
  • Choose the type of the box: sealed, vented, or bandpass.

When you have the above information, you need to measure how much space you have in the car, determining any further adjustments to the size.

Make sure you consider the suitable material, which in most cases will be either fiberboard or plywood. Besides, in some subwoofers, you will need a damping material like fiberglass, and if will decide to make a vented enclosure, you will need a plastic port.

As an Amazon Associate, ImproveCarAudio will receive a small commission from qualifying purchases made through the links in this article.

Tip: To finish a box and make it look nice, you will need a thin subwoofer carpet. But this is not all. You will also need some tools, like an electric drillcircular saw, and jigsaw cutter. If you need some great tools to finish your project, the above links will get you to the Amazon, where you can check out the latest prices.

In the article below, I will go through all of the materials and technology to make sure you will make your perfect box with the least hassle possible.

Should I Buy or Build Subwoofer Box?

Buying an enclosure is very simple and quick. You go to the store, pay, and return home with a box in the trunk.

So now you have a box, but something is missing. There is no job added, no personality. It is just a purchase, and in some worst cases, there is a chance that a pre-built enclosure will not fit your car as you would like.

Making a subwoofer box all by yourself is a great way to add personality to your car audio system.

how to build a car subwoofer enclosure

Just as satisfying for me is installing new speakers or amplifiers and putting a car back together, making a subwoofer box, and installing it gives me the same pleasure.

Building a bass enclosure is not complicated, and it should not take more than several hours to finish (this does not count waiting time for drying), well, unless you want to go ahead with the extreme modification and change the whole shape of the trunk.

Regardless of how you will go and which design of the box you will choose, the most important thing you have to remember is accurately calculating the enclosure’s shape and applying the correct dimensions to the material used.

If you are not happy with the bass you hear after tests in the car, you will most likely return to the planning board because this is where the most mistakes happen. It is better to spend an hour longer on the design board than to start all over again.

How to Calculate Subwoofer Box?

Making correct calculations is the most critical part of the whole project. You have to put your idea on paper, and that is the area where you can make all of the changes without any cost.

But before you start with the subwoofer box design, you need to know the space in the car in all three dimensions. Do not ignore this step; the last thing you need is to make a box too high for your trunk by half of an inch.

The first thing you have to do is to check the volume recommendation for your speaker. 

The volume defined by the speaker type is the most important value that will determine the size of your box. While you can be flexible with the shape of the box and adjust lengths, angles or roundness, the volume for your subwoofer box is fixed, with tight tolerance.

You can check the recommended volume on the speaker’s paperwork or look at the manufacturer’s manual. For example, here is the specification for JL speakers 10W1-4 and 10W1-8 subwoofers, and let’s use this example for our next steps.

JL subwoofer box specification

The specification gives you basic dimensions for both types of boxes and gives you details about material thickness. In our case, it is either 0.75″, or 0.625,” and this is the minimum.

You should not use thinner wood for the walls than 0.625″ because the enclosure will not be solid enough and may start rattling with time.

The best effect you will achieve with direct factory recommendations.

Dimensions on the specification are not fixed. Although the producer recommends 18″ x 11″ x 10.25″, and you can adjust the sizes to make the enclosure better fitting into your car, however, the volume of the sealed subwoofer box must remain close to the 0.75 cu ft.

For example, if you want to reduce the box’s depth, you can increase either width or height to compensate for volume loss. There are, of course, limits due to the speaker’s size, but some adjustments to the box size can be made.

Below are examples of the external box sizes combinations you can make for 0.75ft3, with material 0.75″

Width18″17.5″12.5″
Height11″14″12.5″
Depth10.25″8″12.25″

What Is the Volume of a Cubic Foot?

The volume of a cubic foot, when converted to inches, it gives precisely 1728. This is a constant figure and never changes. In other words, 12″ x12″ x12″ = 1728 cubic inches in every cubic foot.

In our example, the given box volume is 0.75 cubic feet, and that is 1296 cubic inches.

SO remember, whenever you want to make a subwoofer box, keep in mind the number 1728, which is the starting point for all further box calculations.

For example, when you want to place a 10″ subwoofer in the square box as close to the frame diameter as possible, so let’s make it 10″ x 10″.

Will the subwoofer fit into this box?

Well, from the specification, you see mounting hole diameter is 9.125″, so you have to add to this value 2 x wall thickness, and that makes:

9.125” + 2 x 0.75” = 10.625”

From above, you see that the speaker will not fit into a 10″ x 10″ enclosure, and if this is your limit, you cannot use this speaker in your car. The smallest box has to be 10.625″ x 10.625″, so to make calculations easier, let’s make it 11″.

What is the depth of the 11″ x 11″ box?

To calculate the inner of the depth in your box, you have to use the following formula:

1296 / ((11” – 0.75”) x (11” – 0.75”)) = 12.33” this is the inner depth of your enclosure.

To figure out the external box depth, you have to add 2 x wall thickness to the result:

12.33″ + 2 x 0.75″ = 13.83″

You can round up depth to 14″. It will not affect sound quality, and again, it will be easier for further calculations.

Also, to the calculated depth, you have to add 2″ for the baffle to which you will mount a subwoofer, so in or example total box depth will be 16″.

A baffle or walls with double thickness should be used only on the top side of the box, and that is critical for the powerful subwoofers that are mounted in the boxes made of thin materials.

One point to remember -you should avoid rounding down any values for the box calculations because the speaker placed in a too-small box can create too much air resistance. In effect, a subwoofer may not work in its full amplitude or will require much more power from the amplifier to achieve the same sound effect.

Now, when you have the dimensions of your subwoofer box:

11” (width) x 11” (height) x 16” (depth)

It is time to calculate the actual sizes for all six elements.

Front and back walls will be 11″ x 11″

For side walls, you have to deduct material thickness, and with adjustments, you will have:

2 pcs 11″ x (16″ – 0.75″ – 2.75″) = 11″ x 12.5″

2 pcs (11” – 2 x 0.75”) x (16” – 0.75” – 2.75”) = 9.5” x 12.5”

The above example is the simplest way to calculate a sealed box to understand basic volume calculation rules.

If you want to make a box with customized shapes or vented with one or more ports, feel free to create one example project on the paper, you will understand better size relations.

Or, you may need to use design software. I use very good and user-friendly software for fast designing different subwoofer boxes, WinISD (link open in the next window).

You will have to fill in all the needed information regarding the box type, choose a subwoofer driver, and receive a box design with all dimensions.

Now, when you have all sizes for your subwoofer walls, the next step is to transfer them onto the material you will use.

One more point, JL specification says about the size of a mounting hole for the driver, which should be 9.125″. It is good to increase the hole size by 0.125″ because the hole’s inner edge will cover with carpet.

What Is the Best Material to Build a Subwoofer Box?

When you build a subwoofer box, you have to remember that it has to be as rigid as possible without flexing.

While it should not be a problem with thinner material for the small sealed boxes, you may face more challenges when making a vented or bandpass box.

The most recommended thickness of the material, also which I always use, is ¾”, or 0.75″. For the smaller subwoofers 6.5″, or 8″, you can use ½” or 5/6″ thick wood, but I have found myself most comfortable with ¾.”

You are probably wondering, what is the best material for a subwoofer box? Well, there are several options available, so let’s go through the most popular:

MDF (Medium Density Fiberboard)

MDF (Medium Density Fiberboard) is the most popular material for the standard enclosures’ types. This manufactured wood is highly durable and dense that helps the sound quality.

I have found it my favorite, very easy for cutting or assembly. MDF, because it does not warp or expand, is perfect, especially for any places with high humidity or intense winters. And, because of its thickness, it does not cause any sound distortion.

MDF has one significant disadvantage, and this is its weight. While it does not matter for the smaller boxes (well, maybe unless you carry it every day back and forth), it is crucial for complicated multi-driver constructions.

In many cases, subwoofers with 2 or 3 large drivers will weigh over 200lb. In this article, you will find a weight overview of several types of boxes and speakers if you would like to know more about the subwoofers’ weight (opens in a new window).

Plywood

Plywood is another popular material used for subwoofer boxes. It is less dense than MDF, so it is lighter but strong enough not to rattle. When you use plywood for your project, make sure there are no surface defects that could cause vibrations or rattling of the walls.

Fiberglass

Fiberglass is an excellent choice for light constructions, and it is often used for complicated and fully customized shapes. So, if you want to accommodate your whole trunk for several subwoofers, you should use fiberglass for this, and the result will be outstanding.

Can You Use Speaker Wire for a Subwoofer?

In general, you should use a wire gauge between 12 and 16 for a subwoofer, depending on the power. If you are going extremely strong, use a smaller gauge.

Where you are unsure, it is always better to round up and use a smaller gauge (thicker wire) than round gauge up and to limit the subwoofer’s performance by using too thin wire.

Remember, a gauge of the wire inside of the box should never be higher than the wire from the amplifier to the enclosure.

Subwoofers require much more power than speakers, so their wires should be able to transfer this. Not only the connection between the amplifier and the terminal in the enclosure should be strong, but also all connections inside the box.

I saw many subwoofers with wires so thin, which seemed more like head unit harnesses rather than the wiring of the subwoofer. In this article, Crutchfield explains how to choose the right wire gauge, depending on the power of the subwoofer.

How to Make Subwoofer Box?

When you have the design ready, all needed materials, and tools, it is time to start making the actual box.

To put a subwoofer box together, you will need strong glue, screws, and caulk. I am using Elmer’s Professional, and with MDF, I have had great results so far. It is strong and fills small gaps with an excellent sealant.

But glue is not enough. To make the box solid, you have to use screws, but do not go too long. 2″ will be enough. I am using Hillman fasteners, and I am pleased with them with all wall thicknesses. 

For subwoofer boxes, remember not to use nails. Over time, they will become loose from vibrations.

One more thing to remember, you should place braces between the walls. They provide a box with additional stability and are especially important for large sizes and more than one driver.

You can use 2″ x 2″ wood strips and glue them between the walls. Although it is recommended to use them for the large boxes, I am using them for all.

Let’s start with making the box.

Draw All Subwoofer Walls on the MDF

Apply all external measurements of the box panels on the MDF. You should have a total of 7 pieces, six walls, and one extra panel for the front to which you will mount a driver. From our earlier calculation, you will have:

  • 3 pcs – 11″ x 11″ – tops and back
  • 2 pcs – 11” x 12.5” – side walls
  • 2 pcs – 9.5” x 12.5” – side walls

Cut All Pieces Using a Table Saw.

Cut all panels using a table saw, for example, from DEWALT. If you do not have a table saw, it can be any that will make straight cuts. I do not have a table saw, and I use this circular saw from Makita with a carbide-triple blade, and it works excellent with MDF.

Make sure however, to keep all cuts square. When cutting MDF, you will have a lot of dust in the air, so it is worth wearing a mask unless you are doing it outside. Remember also to use glasses and protect your eyes.

Glue Together Two 11″ X 11″ Front Panels

Connecting two 11″ x 11″ panels and double the thickness will make the front wall of your subwoofer box.

Use strong wood glue and cover with it the whole panel’s side. I have found that Franklin International 5005 Titebond II Premium Wood Glue is excellent and makes a solid connection for larger areas. 

Do not leave any air gaps or empty edges that can later cause any rattling. Hold both pieces together with a few sheet metal screws until dry.

I am leaving it for 2-3 hours in a well-ventilated place or outside before cutting the mounting hole, and it is enough.

Mark the center point and cut the mounting hole

When dried, find a center point on the glued panel and mark a 9.5″ circle from it. You should have a hole template received with your subwoofer so you can use it. If you do not have a template, you can use a compass for marking.

Drill a hole inside of the circle, close to the edge. The hole should be big enough to fit in the standard jigsaw blade. The blades are either 1/4″ or 3/8,” so I use a 1/2″ drill.

Cut the mounting hole in the doubled panel. The best for this is a jigsaw, and make sure you follow the circle line.

Make a small hole in the back panel for a subwoofer terminal.

Make a hole in the back panel. In this hole, you will later place a subwoofer terminal. The hole size depends on the terminal model you have, and it will either be round or square.

To save time, I do this while waiting for the panels to dry, following the same steps as for the driver mounting hole. When you have a terminal hole ready, do not mount the terminal yet, you will do it after covering the finished box with the carpet.

Mount all panels together

Now is the time to mount all panels together. I am starting with large walls and the back of the box, then small walls between the large ones. In the end, I am covering all walls with a doubled front panel.

Remember that before connecting the walls with screws, you have to pre-drill holes close to the panels’ edges. Pre-drilling is essential, especially with MDF that tends to split when you screw it too close to the sides.

I am drilling the holes using 5/64″ drills, with spaces from 5″ to 6″ from each other, so in this box, every edge should have three holes.

Cover all mounting surfaces on the panels with a thin layer of glue

When you have all holes pre-drilled, cover all mounting surfaces with a thick layer of glue. Make sure you do not leave empty gaps that later can make strange noises when not sealed properly. For the subwoofer box, it is better when you use too much glue than too little. Any excess you can wipe off after fastening the whole enclosure.

Tighten all walls together using screws

Start screwing the whole box together in pre-drilled places. For side walls and back of the box, I use #6 x 2″, or #7 x 2″ screws, but for the front of the box, due to its 1.5″ thickness, I use 3″. Do not use 2″, this part of your subwoofer box has to be the strongest, so especially for MDF, the longer screws, the better.

This was the last part of building a box, and as long as you are happy with the final shape, you did a great job. After this part, I am leaving the box for a few hours to let the glue dry. The last few more steps are to finish the box and to give it a nice look.

Seal all gaps between the wooden panels

When the glue has dried, I cover all internal seals with a silicone caulk, which will seal any overlooked gaps in the glue.

After covering all inner corners, I leave a box for the full dry until the next day. Although some silicones need up to 48 hours, the next step will be to carpet a subwoofer box, and this will be done only from the outside.

How to Carpet Subwoofer Box

To cover a subwoofer box with a carpet, I do not use any complicated tools, only a razor blade, spray glue, and a metal ruler. If you would like to finish your subwoofer box using different materials, you will find other options like Vinyl in this article from Crutchfield.

Measure the carpet and cut it to the correct size

If you want to go ahead with the carpet, the first step is to precut it to match the box’s size.

In our case, I cut from the whole carpet a 43″ x 70″ piece, and this will be enough to cover the entire box.

You can use the whole carpet, but I do not like it too large on my table, so I use the following formula to determine how much of the carpet I really need:

Carpet width – front width plus the length of the sidewalls

11” + 2 x 16” = 43”

Carpet length – the perimeter of the box from the front side, plus one front for overlap

11” + 2 x 16” + 11” + 16” = 70”

Cover the enclosure with the carpet

Now when you have carpet prepared, let’s carpet the box.

Because it is better to show carpeting on the actual example, the video below will drive you to step by step the whole process.

Install the wire terminal in the mounting hole

Now, when you have a box nearly finished, the next step is to install a subwoofer terminal in the hole. For this, I use small 1/2″ screws and silicone all from inside to ensure there are no unwanted air leaks.

Connect a wire from the subwoofer to the wire terminal

The next step is connecting a subwoofer wire to the terminal inside of the enclosure. Ensure the terminal is correctly sealed in the back wall. Otherwise, it may become the weakest point in the sealed speaker and can ruin the whole project. Another important thing you have to remember is to cut the wire as short as possible. You do not want a wire to start rattle inside of the box from the vibrations.

Fill the interior of the enclosure with polyfill.

With wire inside of the box, I am starting to fill a box with a polyfill. I am not filling the box entirely, just covering side and back walls using spray wood glue, so the driver has room to breathe and will not overheat.

Filling a bass box also guarantees that loose wire inside will not rattle against covered walls. The best effect that I noted, however, is in the sound. Because polyfill absorbs the sound, the bass is deeper.

Mount a subwoofer in the enclosure and test it.

When the box is finished, sealed, and connected to the speaker, the last step is to mount the speaker to the enclosure and test the subwoofer. 

Before screwing the speaker into the box, it is worth placing a rubber gasket on your speaker’s bottom sideThis will allow avoiding any air leaking from the enclosure.

Place a subwoofer into the mounting hole and make sure there is no gasket sticking out. To screw a subwoofer, I use 1″ screws. Do not use longer ones because the front panel’s thickness is 1.5″, and it is better when screws do not go entirely through.

Install a bass box in the car

Take the box to the car, plug a speaker wire into the terminal, and turn the radio on. Do not give too much volume at the beginning.

You may still need to adjust filters in the amplifier and crossover to get the best result, but I am sure if you have done everything with attention to detail, you will be pleased with the result.

Related Questions

What Is The Best Frequency for a Car Subwoofer?

Although subwoofers, in general, create audible sounds in frequencies between 20 and 200Hz, it is recommended to limit the top frequency between 80 and 100Hz by crossover. Car speakers are capable of running at 100Hz and above, so the subwoofer should run only the lowest tones. Many subwoofers can run frequencies below 20Hz, but these are felt tones, not heard.

Martin

Welcome to ImproveCarAudio! I am Martin, and I love to write about everything related to car sound systems. I strive to provide the most accurate and helpful information about car audio through extensive research, as well as my experience with car audio installations.

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