If you are a car audio addict, then you have heard about bandpass enclosures, specifically the 4th order and 6th order versions. But what does bandpass mean? How do they work?

Well, in this post, I will break down what a bandpass enclosure is, and how they work. For the sake of simplicity, I will focus on the two types of bandpass enclosures that make up the majority of enclosures you will see in the car audio world.

How about a little background before we start, shall we? Though extremely popular with car audio today, did you know that the bandpass enclosure is a really old loudspeaker enclosure design? The first patent for the bandpass enclosure type was filed by Andre d’Alton way back in 1934. This actually makes this enclosure type 88 years old. 

We all know there are heaps of car audio enthusiasts that install some variation of the bandpass enclosure within their vehicle, but why? What are the advantages of the bandpass enclosure? The simple answer is that bandpass enclosures allow for lots of output and low-frequency bass response with less power than a typical sealed, or even the traditional ported subwoofer enclosure. This is the reason why most home audio systems utilize a bandpass enclosure for the subwoofer.

Now that we have a little background on the bandpass enclosure, let’s take a look at the types of bandpass enclosures. These are the two major bandpass categories that the different variations will fall under, depending on the overall design. The two basic types of bandpass enclosures are single-reflex and dual-reflex enclosures. To better understand what the nomenclature means, let’s look at both types.

Single-Reflex Bandpass Enclosures

The single-reflex bandpass enclosure is the most popular type of bandpass enclosure in car audio. IF you take a look at most, if not all, of the “pre-fab” bandpass enclosure on the market today, you will see that they are, indeed, a single reflex bandpass.

The major characteristic of the single-reflex bandpass enclosure is that the rear chamber of the

the enclosure is a sealed chamber. In the single-reflex bandpass, the rear chamber’s main purpose is to act as a high-pass filter. The airspace, or volume, of that rear sealed chamber, controls the lower -3dB point. The single-reflex bandpass exhibits a shallow frequency roll-off (typically 12dB octave). This refers to the rate at which the frequency is attenuated once the frequency passes the cutoff point. For example, if your subwoofer’s low pass filter is set at 80Hz, and with a slope of 12dB/octave, for each octave below 80Hz the level of the frequencies will diminish by an additional 12dB. The single-reflex also has a better transient response than other enclosure designs.

Changing one of the chambers of this type of bandpass enclosure will result in changes needed to be made to the corresponding chamber. For instance, if the volume of the sealed chamber is changed, the ported chamber will need to be changed to re-tune the chamber. If this is not done properly, there will be a mistuning situation and the output of the subwoofer will be decreased, or just plain sound bad. 

Dual-Reflex Bandpass Enclosures

In a dual-reflex bandpass enclosure, both of the chambers of the enclosure are ported into the listening area or vehicle cabin. To add another twist to this, there is also another type of dual-reflex bandpass enclosure known as a “series tuned” bandpass. In this type of bandpass

enclosure, the rear chamber of the enclosure is ported into the front chamber, which is then ported into the listening area. 

A dual-reflex bandpass enclosure is more efficient than a typical single-reflex enclosure. It also controls the subwoofer’s cone more effectively over a wider range of frequencies because both chambers can be tuned independently of each other, thus covering a wider spread in the frequency range, as compared to the single-reflex enclosure. A dual-reflex enclosure, because of its design, will also carry a sharper low-frequency roll-off (typically around 18 – 24dB/octave). Because of this characteristic, the dual-reflex enclosure is typically larger in volume to produce the same low-frequency extension as the single-reflex design. 

Should You Build A Bandpass Enclosure?

Compared to other enclosure types, the bandpass enclosure is one of the most difficult and complex to design and build. These types of enclosures are very unforgiving when they are not designed properly. The slight error in volume or port calculations can leave your killer-looking design sound, simply put, like trash.

If you intend on using a bandpass enclosure in your car audio build, I would highly recommend that you have a highly-experienced, reputable enclosure designer, or builder, complete this daunting task for you. This way you can be sure that the enclosure and sound you are looking for will definitely be the enclosure and sound you get. 

Another thing to take into consideration when deciding whether or not to use a bandpass enclosure is space. Does your vehicle have enough space to house such an enclosure? As I stated previously, by design, the bandpass enclosures tend to run much larger than your typical ported and sealed enclosures, so make sure your vehicle can fit a bandpass enclosure.

Off The Shelf Bandpass Enclosures

While the idea of the “off the shelf” bandpass enclosure can be appealing, it is best to leave these enclosures alone. A bandpass enclosure is most definitely NOT a one size fits all subwoofer enclosure. Even some of the “loaded” bandpass enclosures are built incorrectly for the subwoofer that they house. In all honesty, if you want a bandpass enclosure, please don’t waste your money on a “pre-fab” bandpass, have a custom enclosure designed to match the parameters of your specific subwoofers, as I stated previously.


For many years, the bandpass enclosure, in its various iterations, has been used within the car audio community with great success. A properly designed bandpass enclosure will allow your subwoofers to outperform the same subwoofers installed into traditional ported and sealed enclosures, which can make them an awesome option when designing your next car audio build.

I hope this post has given you a little more information on the enclosure type that is the bandpass. Though not all information about the bandpass enclosure was written here, it was my goal to give the car audio community, especially the newcomers, some basic information to help them on their way to becoming a true car audio addict. 

Bandpass Subwoofer: You Are Doing it Wrong. Bandpass MYTHS BUSTED!

About The Author Brandon L

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