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How To Build A Bandpass Subwoofer Box – Step by Step

Alright, folks, get ready because we're diving deep into the world of bass that'll rattle your bones – we're talking about making a car bandpass subwoofer box.

We will not only show you how to build it but also give you a step-by-step tour of all the necessary equipment and how to use them. As a bonus, we have also discussed how to calculate the Bandpass box Thiele/Small (T/S) parameters.

So, grab your tools, and let's get to work on making some noise!

Bandpass subwoofer boxes are the ninjas of the speaker world – they sneak in with precision and efficiency, focusing all their energy on a specific range of frequencies (usually those low, earth-shaking bass tones we all love) and cutting out the rest. They're like sniper rifles in a world of shotguns; while other enclosures scatter their sound, bandpass boxes hit the target with deadly accuracy. Now, we will discuss how to perform the building activities in 3 phases.

Phase 1: The Design Stage

We will first need to decide the build configurations to begin the building process. Here, we will see what sub to pick, how to decide the frequency range, and, most importantly, what enclosure type you should pick.

Choosing Your Subwoofer

We assume that you have already picked a suitable sub for your car audio setup. If not, then you need to choose one that best fits your choice and your car’s space.

This choice is crucial because the specs of your subwoofer will dictate the design of your bandpass box. You want a sub that can handle the type of power and frequency range you're aiming for. If you are not sure about what to buy, then you should definitely read our CAR SUBWOOFER BUYING GUIDE.

Deciding on Frequency Range

Next, decide on your desired frequency range. Most bandpass boxes shine in the bass region, around 30 Hz to 100 Hz. This range is where you'll get that punchy, guttural bass that makes your heart skip a beat.

Selecting the Bandpass Type

As we speak, there are lots of options when it comes to selecting the bandpass type. But which one should you go for? Let your sub’s manual decide that.

The manufacturer spends a lot of time and money to find out the optimum settings in which their sub will reproduce the best sounds. All this information is written in the user’s manual. So, you should have no difficulty in decoding that. 

But, if you want to explore your options, give this article a read Different Types of Bandpass Subwoofer Boxes

Calculating the Bandpass Build

Building a bandpass subwoofer involves a series of calculations to determine the optimal size and tuning of the enclosure for the desired frequency response. The process requires knowledge of the Thiele/Small (T/S) parameters of the subwoofer driver.

The Thiele/Small (T/S) parameters are a set of electromechanical specifications that describe the performance characteristics of a loudspeaker driver. While T/S parameters are not unique to bandpass subwoofers—since they apply to any loudspeaker driver—they are crucial for designing and optimizing enclosures, including bandpass subwoofers.

They are specifications provided by the manufacturer that describe the driver's performance characteristics. Key parameters include the speaker's resonant frequency (Fs), equivalent volume (Vas), and total electrical quality factor (Qts), among others.

Here's a rundown of the most relevant T/S parameters for designing a bandpass enclosure:

Parameter

Symbol

Description

How to Calculate

Resonance Frequency

Fs

The frequency at which the speaker's impedance is highest.

Provided by the manufacturer or measured with an impedance meter.

Total Q Factor

Qts

Measure of the total damping of the speaker, combining mechanical and electrical damping.

Calculated if Qes and Qms are known.

Equivalent Compliance Volume

Vas

Volume of air with the same compliance as the speaker's suspension.

Provided by the manufacturer or estimated through specific tests.

Electrical Q Factor

Qes

Electrical damping of the loudspeaker.

Provided by the manufacturer.

Mechanical Q Factor

Qms

Mechanical damping of the loudspeaker.

Provided by the manufacturer.

Voice Coil DC Resistance

Re

DC resistance of the loudspeaker's voice coil.

Measured with a multimeter.

Voice Coil Inductance

Le

Inductance of the voice coil, affecting impedance at high frequencies.

Measured with an LCR meter at 1 kHz.

Sound Pressure Level

SPL

Efficiency of the speaker, in dB at 1W/1m.

Provided by the manufacturer. Indicates loudness for given power.

Maximum Linear Excursion

Xmax

Maximum linear movement of the cone without distortion.

Provided by the manufacturer. Indicates low-frequency capability.

Motor Strength

BL

Magnetic strength of the motor structure.

Provided by the manufacturer. Related to driver control/efficiency.

For parameters that are typically provided by the manufacturer, like Fs, Vas, SPL, Xmax, and BL, these are determined through standardized testing procedures that are difficult to replicate accurately without specialized equipment. Qts, Qes, and Qms are also usually provided, but they can be calculated if the other values are known and the formulas are applied.

Re and Le can be measured directly using electrical instruments like a multimeter (for Re) and an LCR meter (for Le), making them some of the more straightforward parameters to obtain for an unknown speaker.

In practical speaker design, obtaining these parameters from the manufacturer is preferred for accuracy, but DIY methods and software tools exist for enthusiasts who want to measure or estimate these parameters themselves.

If you are one of those enthusiasts, we recommend you this virtual calculator. You may also read this one to get a more detailed insight into the bandpass calculation system. 

Now, let’s move into phase two, or groundwork, and see how to assemble the box.

Phase 2: The Build

It is time to get your hands dirty as the assembling starts.

Gathering Materials

You need a lot of tools while doing this DIY project. Here is a brief list of all the necessary tools.

Material/Equipment

Use

Medium Density Fiberboard (MDF) or Plywood

For constructing the enclosure's panels.

Speaker Terminal Cup

To connect the internal wiring to the external amplifier.

Acoustic Foam or Damping Material

To reduce internal resonances and reflections.

Sealant (Wood Glue, Silicone Caulk)

To assemble panels securely and ensure an airtight seal.

Screws

To reinforce the joints and secure the box assembly.

Wire (Speaker Wire)

To connect the subwoofer to the terminal and amplifier.

Port Tubes

For tuning the ported section of the enclosure.

Circular Saw, Jigsaw, or Table Saw

To cut the panels to required dimensions.

Drill

For making holes for screws, terminal cups, and ports.

Router

To cut precise holes and round off edges for a finished look.

Sander or Sandpaper

For smoothing edges and surfaces before finishing.

Screwdrivers or Power Screwdriver

To drive screws during assembly.

Measuring Tape and Square

For accurate measurement and alignment of panels.

Clamps

To hold panels in place while the glue dries.

Paint, Veneer, or Carpet

For finishing the exterior of the box.

Sealant or Wood Filler

To smooth gaps or screw holes before applying finish.

Cutting the Panels

Use the measuring tape and square to measure and mark the cut lines on your MDF or plywood accurately.

Using the circular saw, jigsaw, or table saw, carefully cut the MDF or plywood to create the panels for your box based on the design specifications.

Mark the positions for the speaker and terminal cup on the appropriate panels. Use a drill to start the holes and a jigsaw or router to cut them to the correct size.

Assembling the Box

Arrange all panels to ensure they fit together properly.

Apply wood glue along the edges of the panels and fit them together. Use clamps to hold the panels in place as the glue dries. Once the glue has set, reinforce the joints with screws.

Apply silicone caulk or sealant around the inside joints to ensure the box is airtight.

Installing the Subwoofer and Terminal Cup

Install the terminal cup in its cutout, securing it with screws.

Connect the speaker wire to the subwoofer terminals and run the other end to the terminal cup. Ensure the polarity (+/-) is consistent.

Secure the subwoofer in its cutout using screws. If using a gasket, place it between the subwoofer and the box to improve the seal.

If your design includes a ported section, cut the PVC pipe or port tube to the calculated length. Install it in the pre-cut port hole, securing it with glue or sealant if necessary.

Phase 3: Finishing Touches

Now, for the final part. In this phase, we will ensure the longevity of the setup and see if everything works well.

Sealing and Damping

Attach acoustic foam or damping material to the inside walls of the box to minimize resonances.
Sand the exterior of the box to prepare it for finishing.

Apply paint, veneer, or carpet to the box's exterior according to your preference. Use adhesive for veneer or carpet, and apply paint with a brush or spray can for an even coat.

Use wood filler to fill any screw holes or gaps, then sand smooth before applying the final finish.

Testing and Tweaking

Connect your subwoofer box to your amplifier and test it with music or tones to ensure it performs as expected. Listen for any air leaks or unwanted resonances and adjust if necessary.

The Endnote

We know the entire process can be challenging to follow. But with our easy guide, you have all the information in one place.

As this article focuses on how to build a bandpass subwoofer box, it will answer all of your relevant queries. Most importantly, while deducing the calculations, remember to tweak a little to get the best sound that complements your environment.

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