Subwoofer Enclosure Types and Loading Methods
27th of January 2009
For many years, 12 volt guys have been misunderstood, mistaught or tend to make up their own beliefs on what type of subwoofer enclosures are better for what type results. The truth is that there are several types of enclosures, each with their own signature trait. To say that one type of enclosure is better for Rap, Rock, Jazz or Country music is just plain dumb. I’ve heard “sealed” boxes that are way too “boomy” (underdamped) that sound terrible, even though these boxes are thought to be the Audiophile’s choice. On the flip side, I’ve heard “vented” enclosures that sound phenomenal with great transient response. Or maybe you’re like me and like all music but hate that country garbage ;^) I like the overall response of a sealed enclosure because it mates very well with the transfer function of a vehicle for a nice even response down to the bottom octave. But then again, I have a serious fetish for body pulverizing bass. So what do I do? Build new enclosures every few months and upset the wife! Hey, it’s not like she didn’t know what she was getting into. Plus she listens to music louder than I do! She’s a Bass Freak too!!!
Let’s get one FACT straight right now… ENCLOSURE DESIGN IS EVERYTHING, PERIOD! You can take a Danish, high-end subwoofer that costs $1k and if you put in a poorly engineering enclosure or pre-made generic box, it’s going to sound like poop! Likewise, if you buy an average woofer from the swap meet and design a proper box, it will yield desirable results. I have spent over 2 decades studying acoustics for nothing. There is no God-given or Voodoo box that works for all types of music and for all types of woofers. A true professional installer/salesperson should ask YOU questions to derive the information he needs to properly design an enclosure that will fit YOUR needs, not his/hers. At that point, the proper subwoofers can be chosen. The Home Audio market utilizes all types of enclosures from entry level to high-end audiophile quality loudspeakers. Let’s take a brief overview of what types of enclosures have been used in car audio in the past couple of decades …
Infinite Baffle (aka Free-Air)
IB is an “enclosureless” way of loading subwoofers into a vehicle. An example would be to have subwoofer(s) mounted in the rear deck of a sedan, whereas the subwoofers use the trunk as the “enclosure”. This method was use more in the 80s when companies made Free-Air or Open-Air subwoofers specifically designed for this application, but rarely in the last 2 decades as transducer design has advanced and the need for massive enclosures has been eliminated for the most part.
Advantages: No enclosure.
Disadvantages: Lower power handling. Sacrificed sound quality.
Acoustic Suspension (aka Sealed)
This is the simplest type of enclosure to construct. Originally patented by Harry Olsen in 1949, this is possibly the most popular and accepted type of enclosure. The air inside acts like a “spring or coil” which controls the woofer. Smaller enclosures control the excursion of the woofer better, increase power handling, and can tend to yield a peak in response, depending on how small the enclosure is. Larger enclosures sacrifice some power handling, but allow flatter and lower response and will play louder due to less restriction from the “acoustic spring” (air volume).
• Advantages: Easy to build. Good power handling. Smooth frequency response. Good for most music material.
• Disadvantages: Not as efficient as Bass-Reflex or Bandpass alignments
Bass Reflex (aka Ported, Vented)
Seen more and more as time goes by, these enclosures have become very popular (especially in Sound Pressure Level (SPL) competition vehicles). While they are a more advanced design, they were actually patented in 1932 by A.C. Thuras, before Olsen had his patent for the “simpler” Acoustic Suspension enclosure. These enclosures take advantage of the back wave energy of the driver and couple it to the listening environment via a vent. The enclosure/vent dimensional relationship is critical and tougher to design. One cannot just take a random size box and install a random size vent. The vent must be of a certain size to properly tune the enclosure to the woofer. These enclosures yield higher output and lower frequency extension. While they have great power handling above the tuning frequency of the vent, the power handling below the tuning frequency is radically reduced. Below this point, the wave exiting the vent and wave coming off the cone are out of phase and work against each other, causing the woofer to become uncontrolled and have violent, damaging movement. At the tuning frequency, the cone excursion is almost nonexistent. This yields very low cone radiated distortion in this range. The efficiency is typically 3dB higher than an Acoustic Suspension enclosure, assuming that a “musical” enclosure is being used. (In an SPL competition vehicle, it is not uncommon to have 6-10dB of gain due to the higher tuning and much larger enclosure.)
Advantages: Efficiency, extended frequency response. Lower distortion around tuning.
Disadvantages: Difficult to design and construct. Low power handling below tuning.
Bandpass
Don’t even get me started on what orders are what, as this is a never ending battle that will never be agreed upon… The word bandpass accurately describes this type of enclosure as it only “passes” a controlled “band” of frequencies. These alignments increase efficiency substantially and all through a relatively small vent. The first recorded patent was way back in 1934 by Andre D’Alton, again before the Olsen patent (I still find this odd). There are single reflex, dual reflex parallel tuned, dual reflex series tuned, and a small assortment of other alignments that we will not discuss at this time. Bandpass enclosures allow a small driver to have fairly significant output. Some companies, i.e. Bose, frequently use sophisticated bandpass enclosures in their home audio systems with respectable results. A single reflex (Fig.4) houses the driver in a sealed box and couples to a vented box. The vented box serves as a “filter” or pseudo-crossover. A dual reflex (Fig.5), series or parallel tuned, houses the woofer in a bass reflex enclosure couple to another one. The elimination of the upper frequencies naturally diverts the energy in the passband response, increasing the overall output of the enclosure, if you choose to. There’s a choice? Imagine you have a balloon, but instead of being full of air, it’s full of bass! You have a fixed amount of energy generated by the woofer. You can squish the sides and get more output in a narrower passband, or you can squish the top and bottom and lengthen the passband and increase low frequency extension. They are difficult to build and very difficult to design so I highly recommend you get a proper design from the woofer manufacturer. A cool benefit of the bandpass enclosure is the ability to directly couple the output into the cabin. Huh? Ok, if you have a sedan or truck and don’t mind making a few modifications, you can fit the vent through the rear deck or cab wall to get 100% of the acoustic energy into the listening area, instead of wasting a ton of energy on vibrating panels. I included a “dual-reflex series-tuned tri-chamber, isobaric push-pull loaded bandpass just to give you an idea of how weird enclosures can get.
A couple words about premade, generic bandpass enclosures… THEY SUCK!!! With thousands of woofers on the market due to the different minds of different engineers, there is NOTHING GENERIC about acoustics, period! Sure, some drivers may work “ok” in one, but I highly recommend you either build your own to proper manufacturer specs, or have a professional build it for you. Bandpass enclosures hide audible distortion. It is not uncommon for a brand new woofer to blow up rapidly due to a generic or poor design, and you never would have heard it coming…
Advantages: Very efficient – up to 8-10dB or more! Easy to couple 100% of the energy into the cabin.
Disadvantages: Quite difficult to build and design. Distortion inaudible. Slightly less sound quality than other alignments (in my opinion). Dual reflex designed tend to have inferior transient responses as compared to the single reflex.
Regarding transient response… Transient response is the ability of the speaker to remain in control, and be able to make rapid changes with the dynamics of the music material being played, which is affected by the enclosure. Narrower bandwidth enclosures will better transient response as compared to wider bandwidth enclosures. Most trained ears feel that the acoustic suspension enclosures yield the best transient response.
Cone radiated distortion. At average power levels, the bass reflex and bandpass enclosures will yield less distortion than the acoustic suspension enclosures, but the difference is marginal and the average listener will probably not hear the difference. At high power levels, bass reflex and single reflex enclosures yield even less distortion than the acoustic suspension with the dual reflex having the least amount of distortion of all designs.
BASS CDs – While the synthesized bass CDs are fun and really show off a system’s capability, it is very important to understand that Acoustic Suspension and Single Reflex enclosure will reduce damage to speakers in the bottom octave. Playing these CDs in bass reflex and dual reflex bandpass enclosures can and will damage woofers.
Now, let’s address the whole “Isobaric” and “Push-Pull” terms which frequently confuse people. Both of these terms are methods of mounting woofers, NOT enclosure types. Isobaric has a definition of “(thermodynamics) Of equal or constant pressure, with respect to either space or time.” The first recorded patent was in 1975 by Linn Products (Scottish home audio company). When mounting 2 drivers to each other, moving mass (Mms) is doubled and Vas is halved, allowing the isobaric pair to be mounted in half the space as a single driver, and with identical frequency response. For example, if a 12” woofer needs 1 cubic foot of air space for a sealed enclosure, then an isobaric pair only requires only ½ cubic foot. There are two fashions to mount isobaric pairs of woofers – cone-to-cone (aka clamshell or push-pull, see Fig.6) or cone-to-magnet. The cone-to-cone alignment is much more common for the reason that it takes up less volume. With this mounting method, the reversed woofer must be wired out of phase (backwards). Since Isobaric loading is a method and not an enclosure type, it can be used in virtually every style of enclosure identified above. Keep in mind that in building an isobaric bass reflex enclosure, while the air volume will get smaller, the vent length in bass reflex enclosures will get longer which may inhibit your design. The downfall to isobaric loading is that the pair of woofers is 3db less efficient than a single driver. However this can be regained by simply doubling the power for an effective 3dB gain.
Push-pull aligned woofers can be isobaric, or they can also be in any type of alignment. For example a bass reflex enclosure with one driver mounted traditionally and the second driver mounted reversed. This is NOT isobaric and can be treated like a regular enclosure, just with different aesthetics. Another benefit to push-pull mounting, whether it be isobaric or not, is the ability to cancel out iniquities in woofer excursion. If a woofer is poorly designed and moves farther one way than another, the coupling of these woofers eliminates this resulting in less distortion and cleaner sound.
There are a lot of crazy things being done these days. Now you know the basics and should able to decide on which enclosure is better for you.
