Speaker Enclosure Categories

By: cplkittle

Enclosures are categorized into orders and variants of orders. This overview will explain each design and give links to calculators and references. While researching this I did find some conflicting information. The information posted has the least common ideas/theories omitted. I highly recommend further research to anyone considering building their own enclosure. On recommended volumes from the owner’s manual, you will normally get a rather large area between the minimum and maximum volumes. Sometimes close to 1 cubic foot difference. These recommendations, in my opinion, are for the general first time buyer/builder. For more accurate volumes, use a calculator or design program that request Theile-Small parameters (T/S parameters) such as Q, F3, Qes, Qtc, Fs, and or Xmax. These parameters, when used in enclosure design, will give you a much more accurate volume or port length designed specifically for your driver.

The First Order, also referred to as infinite baffle, is basically a free air installation. Very few speakers are designed for this application because most require some resistance to prevent over excursion. You will notice that some sealed enclosures are referred to as infinite baffle as well, this is due to the fact that most free air speakers are mounted facing the cabin of a vehicle with the back facing the trunk. In this case, the trunk is used as the sealed box. The airspace has little or no resistance or effect on the speaker. Infinite baffle setups have little or no tuning or adjustment options.

The Second Order, or sealed box, is the simplest of all designs. The size and shape of sealed boxes are the least complex to build, and it is easy to achieve predicted results. There are two types of sealed enclosures. The infinite baffle has an air space larger than the compliance of the woofer’s own mechanical suspension. In other words, you are using the woofer’s own mechanical suspension to control excursion, as mentioned in the first order. The second type is the acoustic suspension. An acoustic suspension has air space smaller than the compliance of the woofer’s suspension. In this type of enclosure, the air is used as a cushion, preventing the speaker from bottoming out. The following information is based on acoustic suspension enclosures. Sealed boxes give a quick, tight, accurate, and controlled response. You can alter the performance of a driver by varying the size of the enclosure or amount of fill material you use. The disadvantages of sealed enclosures are that low frequency output is minimal, they are less power efficient, lower SPL capabilities, and you need a driver with a large Xmax to ensure safe operation of frequencies down to fB (enclosure resonance).

Third Order, or ported boxes take advantage of, and manipulate the rearward wave, giving either more low-end extension towards the bottom octave or more SPL in the lower midbass region by incorporating a tuned port. There is generally a 3dB increase in output compared to a sealed enclosure. Ported boxes give deep, powerful, full sound when designed properly. Xmax is reached easily below the tuned frequency, subsonic filtering is recommended.

Fourth Order boxes also known as dual chamber (ported/sealed) are a combination of second and third order. Fourth order or bandpass boxes have a driver mounted in a sealed enclosure with a ported enclosure on the front. The driver may be mounted either way as long as one side is sealed and the other vented. Very low F3 is possible with fourth order bandpass boxes, as well as less overall driver excursion. There is more control over cone movement, so over excursion is less likely. The disadvantages weigh heavy on bandpass boxes however. The combined volume for both chambers results in a large enclosure, which are difficult to design properly. Drivers can be blown due to lowered cone movement causing thermal limits to be exceeded. In some larger vehicles (SUV’s, vans) the cabin gain can accommodate minor miscalculations by giving a bandpass box a flatter and wider bandwidth while maintaining an increased SPL.

Sixth Order bandpass enclosures are ported on both sides of the driver. These enclosures are more efficient within their bandpass, and produce less audible distortion whereas the chambers act as filters removing unwanted noise. They tend to be larger than fourth order bandpass boxes, and are much more difficult to design properly. There are no publicly released exact formulas for calculating sixth order bandpass boxes, even the calculators and design programs are still operating in the experimental era. As with any bandpass enclosure, the size of the cabin affects the overall result of the sound, so experimentation is necessary to find the right sound for your vehicle.

Eighth Order enclosures are much more complex and near impossible to design. They consist of a sixth order bandpass with the ports from both compartments feeding into a third enclosed area which is also ported. A quasi eighth order design is the basic sixth order with a port between the two separate chambers instead of having a third enclosed area.

All other orders are variants of the bandpass theory. There are a few that don’t fall into an order category. These are the transmission line, folded horn, extended bass shelf (EBS) and Passive radiator; there is also a configuration called isobaric which I will explain.

The Transmission Line enclosure design is a driver mounted in a type of acoustical labyrinth which functions as a low pass filter with a 90 degree phase shift, absorbing all the rear wave energy of the driver except for frequencies below around 75Hz. The transmission line is almost always filled with some type or various types of filler or stuffing material to reduce the speed of sound through the line allowing shorter line lengths. The filler also dampens standing waves and back-wave reflections. The length of the transmission line is normally between 4-8 feet. The length corresponds to the ¼ wavelength of the resonant frequency of the driver. For example: A 50Hz frequency has a 23’ wavelength, giving you an estimated line length of 5.75’. Transmission line enclosures are for the serious audiophile with a ton of time and even more patience, who are looking for superior performance.

Folded Horn is similar to a transmission line with a fourth order bandpass basis. A driver enclosed in a sealed box with a transmission line instead of a port. The calculations are the same in that the volumes for each side must be in sync with each other.

 Extended bass shelf or EBS for short is only one of the variations of vented alignments. The idea is to design an enclosure to be 125%-150% larger than the optimal calculated volume and tuned lower as well. The result is a 2dB output gain at Fs (driver’s free air resonance) giving you a frequency response in the teens! An increase in efficiency below 25 Hz and decrease above 30Hz. The cutoff rate can be as high as 36dB/octave below fB. Transient response is degraded as a result. The bass sound from an EBS is very soft and very deep. Recommended more for large home theaters rather than car audio applications.

Passive radiators are used in conjunction with an actual driver and its purpose is to replace the port or vent of a third order enclosure. The resonant frequency of the PR is intentionally altered by the designer by adding or removing calculated amounts of mass to or from the cone of the radiator. The passive radiator is in essence a speaker without a voice coil, using either a magnet for resistance or basically just a cone resisted by the surround. Using the air movement in a sealed box from the driver, a passive radiator will move 180 degrees out of phase if there is no resistance or channeling within the enclosure. Adding weight to the radiator changes the resistance, therefore changing the frequency of the box.

Isobaric or dual drivers can be used with any enclosure. There are multiple ways to set up an isobaric configuration. One is to enclose them within the same airspace and set one of the drivers 180 degrees out of phase. Another is to mount them together in an enclosure with an air cavity between them, in this setup the first driver is pushing into a box which the second is facing out of. The third and most common isobaric configuration is to mount them face to face with minimal airspace, but enough so that they are not touching, and wire them 180 degrees out of phase. The idea behind this is that one driver pushes while the other pulls, lightening the load on each speaker. Advantages to this are improved bass response. Bass is said to be tighter, faster, and more accurate. Vas of the driver is cut in half, as well as the volume of the enclosure.

Isobaric enclosures are used where size is an issue, or where more accurate bass is more important than a lot of bass. Disadvantages include wasted amplifier power, and efficiency is down 3dB over that of a single driver.