Choosing an enclosure alignment depends on what your client is hoping to gain from an audio system. Here’s how each enclosure measured up.

WORDS BY CAMERON “CHIMPO” POWELL

It’s often difficult to describe the audible differences between alignments of subwoofer enclosures. In the beginning of my career in mobile electronics, I used the program WinISD to design random enclosures to see how the responses would change from alignment to alignment. Back then, I also had the opportunity to build some of those designs myself. Additionally, I sent other designs to people around the country to see how they would perform in real life applications rather than just viewing computer-generated graphs.

At LIS Audio, we traditionally build or install four different types of subwoofer enclosures. Our go-to professional window tinter, Steve Newendyke of Cutting Edge Window Tinting, wanted to add a bit of bass to his factory system. We installed the amplifier and used a prefabricated enclosure with plans for a custom enclosure later down the line. We figured this would be a great opportunity to test the different options to see and experience the results. I will be covering these designs used in both daily driving and “Show Car” applications. Here are some key details on each of the four:

Sealed (Acoustic Suspension): The interior of this enclosure has an airtight seal from the exterior environment. A sealed enclosure alignment can be tuned by adding or subtracting to the internal air space, which will change the frequency response of the driver installed in the enclosure. This design is commonly used in Sound Quality (SQ) competition as it provides a very accurate and tight response, though it does lack a bit in output, comparatively. Sealed alignments are one of the most commonly found enclosures in car audio due to their ease of design and petite size.

Vented (Ported or Bass Reflex): This enclosure design is commonly found in the car audio industry, and applies to both prefab and custom designs. It offers

the ability to tune the enclosure close to, or around, a specific frequency. To tune this alignment, one must change the diameter and length of the port against the amount of the internal air space. Vented enclosures are more ideal for output as opposed to sound quality applications, which is why they are commonly found in Sound Pressure Level (SPL) competition audio systems.

Dual Reflex Band-Pass (6th Order): This design is like that of a single reflex bandpass enclosure, except both chambers are vented. The two chambers must be tuned 180 degrees out of polarity from

each other and must be tuned at least one octave apart to avoid cancellation issues. Dual reflex band-pass low frequency sys- tems generally exhibit a roll-off of about 18 to 24 dB per octave on the high pass, and 12 dB per octave on the low pass. The biggest benefit to a 6th order enclosure is amazing bass from smaller drivers, but they can be very difficult to design and build correctly.

Most prefabricated enclosures are designed to work within the specific parameters of many different subwoofers. Each subwoofer has its own set of speci- fications and parameters that allow us to know what enclosure alignments would be ideal for them individually. Manufac- turers may offer information on whether the subwoofer performs best in sealed, vented, 4th order, or 6th order enclosure alignments. They may also list recom- mended airspace and efficient tuning ranges. Each manufacturer has differ- ent specifications, so it’s important for technicians to know these details before promising an end result.

What’s the Difference?

I collected data from the four enclosure types to offer a visual rep- resentation of the differences. For the tests, I kept all variables except for one independent variable—the enclosure. We used a 2013 Nissan Altima that still has a factory radio, factory electrical with an AudioControl LC800.1 amplifier and an Audio Dynamics 12-inch 2100 sub- woofer. The amplifier tuning was kept the same throughout the experiment. The stereo volume remained at 20 with pink noise played to show the lower end curve.

We set the amplifier to a safe level with the SMD DD-1 upon installation. I chose the following tools to measure and depict the output of the different enclosures: the SMD DD-1 CD, WinISD, and the AudioControl DM-RTA paired with a CM-10 Mic. The CM-10 will only accurately record up to 136 dB, so I decided to keep the mic sensitivity set so it wasn’t necessary to max out the volume of the vehicle’s stereo, which helped ensure accurate results. The rear seat was left up to create a daily driver environment for testing.

Let’s take a look at some details and data from each enclosure I built for this experiment.

Sealed Enclosure

The sealed enclosure was built using ¾-inch MDF to specifications, like a prefabricated sealed enclosure for the 12-inch subwoofer. It has .81 cu. ft. of inner air volume. The sealed enclosure comes to about 1.19 cu. ft. of external volume with the measurements being 14” by 14” by 10.5”. This can help show how a sealed enclosure performs against the rest of the enclosures and how much room it may take up in the cargo area. Many of the sealed enclosures we build at LIS Audio are fabricated into the rear hips of vehicles, dependent on space allowed, but generally have about the same amount of airspace as a generic sealed prefab.

It is very easy to see on the WinISD graph that the initial curve of the sealed enclosure, which is the blue line, has a very smooth roll-off. This shows there might be a fairly smooth transitions from note-to-note as it gradually gets weaker the lower the notes go. From the DM-RTA readings, the most pronounced frequencies in the vehicle are between 45Hz and 55Hz. I set the pink noise to play the same 60 seconds of a specific section of the track for every trial to maintain consistency. This alignment produced an average maximum of 105.0dB. This setup performed most music genres well, overall.

Prefab Vented

Next, we have the prefabricated vented enclosure, single bass-reflex. The enclosures external measurements are 13.25” by 16” by 18” and the port appears to be tuned to 38Hz. This would put the enclosure at 2.20 cu. ft. of external volume—which is how much room it will take up in the intended cargo area. The interior volume is estimated to be 1.5 cu. ft. before sub displacement. As mentioned earlier, this alignment is often used in SPL competitions, and is commonly built by shops due to the ease of design.

The prefab enclosure design depicted on the graph (red line) shows that we can expect an increase in output, which

may be most prominent around 57Hz. The roll-off looks to steepen compared to the previous sealed design. Observe the RTA results and it is clear that the sub-stage output has increased simply due to swapping out the enclosure. The data shows this alignment commonly peaks through the 47Hz to 57Hz range and has an overall maximum output of 111.7dB. While listening to music it is apparent this setup plays higher notes more aggressively, yet tapers off when low notes begin to come in. It performs better with music genres like classic rock and metal, than it does with hip-hop or dubstep.

Custom Vented

The custom vented enclosure is designed around the subwoofer’s

specifications and to the client’s preferences of music. To build this enclosure, I factor the inner air volume against the port tune while also factoring storage space constraints. The exterior dimensions are 28” by 14” by 14” which is just shy of 2.5 cu. ft. overall. The inner air volume comes to 1.75 cu. ft. after port displacement and prior to subwoofer displacement. I built this enclosure to be larger than optimal and tuned it to the client’s preference. He wanted more output than the sealed enclosure provides, but wanted to hit lower notes more aggressively.

On the RTA graph of the custom LIS Audio vented enclosure, we were to expect a decrease in total output that peaks near 55Hz. Due to the lowered tune of the port, compared to the prefab, the roll-off seems to be smoother and not quite as steep. When we see the RTA results, we notice a tremendous increase in output, which was noticeable immediately once playing music. Not only did it look and feel louder, the lower-end frequencies had much more presence. The data shows the most prominent range of this setup in-car was between 38Hz and 60Hz, with 115.3dB was the average maximum output.

6th Order

The last enclosure alignment is what Pete at Audio Dynamics refers to as “6th Order Series Tuned Bass Reflex.” This enclosure has two chambers, one ported into the next. The enclosure is measured at 36” by 14” by 18”, externally

taking up 5.25” cu. ft. The 6th order utilizing this design tends to allow for a bit more driver power handling than the traditional, or more popular internally loaded 6th order enclosures. The largest disadvantage of building this alignment is having a custom enclosure that is nearly twice the size of a vented enclosure. For many of our clients the amount of audible output and frequency response gained is worth giving up the additional space.

As seen in the graph drafted in WinISD, the 6th order band (turquoise line) was predicted to cover more frequency range and offer more output than the other alignments. The top of the band starts to taper off at about 73Hz and begins to get steeper after 47Hz. The in-car measurements of the frequency response on the RTA show the most prominent frequencies are between 40 Hz and 63Hz. While collecting data it was noticeable that this entire range would seem to spike near the same decibel level with frequencies as low as 34Hz short behind. The average maximum output was 116.4 on pink noise. This alignment plays most music genres just as well as the previous alignments, and with noticeably more output.

Which Enclosure Fits Your Client’s Needs?

Changing only the alignment of the subwoofer enclosure can make a difference in output and frequency response—enough so it’s noticeable visually, audibly and physically. Keep in mind that some subwoofers will not perform in all alignments well, and may only perform efficiently in a single design. Though, many quality subwoofers on the market are surprisingly versatile. Also, keep in mind that the shape of the vehicle and any obstructions between the enclosure and vehicle cabin can make a big difference in how each of the alignments perform.

I hope this helps shed some light on how enclosure alignments and designs alone can impact the entire sub-stage. I wouldn’t personally say that one alignment is better than another, but I do think it’s important to know which enclosure would best suit the client’s preferences to meet their expectations. Overall, our client was very impressed with the quality and output of the 6th order enclosure—so, needless to say, he decided to keep that design in the vehicle. Now it sounds like there’s a gorilla in his trunk!

Cameron “Chimpo” Powell is a fabricator and co-owner of LIS Audio in Spring Hill, Kan.