[Paul Spencer]

There is a good summary of different subwoofer types at [1] which discusses them all in more detail. This is intended as a guide for beginners, therefore discussion is limited to the basic factors that matter most to the decision of where to start.

Type	Pros	Cons
1. sealed	small & simple	limited output
2. vented	more output & depth than sealed	difficult to avoid vent noise with some drivers
3. EBS	bass extension below fs	less midbass; 50% bigger than vented
4. passive radiator	no vent noise	extra cost
5. Transmission line	natural & articulate bass	large
6. bandpass	very high output	difficult to design; less accurate
7. acoustic lever	very high output	large; special software required
8. bass horn	very high output; very accurate	very large; difficult to design & build
9. Infinite baffle	can use many drivers hidden	cost of drivers
10. open baffle dipole	minimal room interaction	very low efficiency

Conventional subs

1. sealed

The driver is placed in a sealed box which acts as an air spring. The fs of the driver will be raised from say 20 Hz to 40 Hz and below this there will be a 2nd order roll off. This is fine for music and rooms which have room gain below this point. This is the easiest sub to get right, and if you don't have TS parameters for your driver, as is often the case with cheap or car audio drivers, then this is the best choice. You can add eq to get down as low as you desire, although output will be limited by the excursion capability of the driver. One option is to use a Linkwitz Transform.

2. vented

A vented box achieves more output and lower bass extension. A driver with an Fs of 20 Hz can achieve an F3 at this point. You can expect typically 6db more output from tuning to an octave above tuning. This tuning point reduces cone excursion and hence lowers distortion. Below tuning, the roll off is steeper - 4th order. Also cone excursion increases more rapidly than a vented box below the F3. As a result, a rumble filter is essential.

3. EBS

This is a special class of vented box. The box is made 50% larger and the tuning point below fs. Making the box larger increases the vent efficiency. This is required to get extension below fs. Some midbass output is sacrificed by this approach, but in conjunction with room gain, you can achieve very low in-room response, perhaps even as low as 10 Hz! 15 Hz extension is readily obtainable. This may be the subwoofer of choice if you don't mind a large box, have a high excursion driver, and you are impressed by subsonic LFE.

4. passive radiator

This is similar in performance to a vented box, however passive radiators are used instead of vents. A passive radiator (PR) is a driver without a motor. Weight is added to the cone (often flat) to achieve tuning. Since such a design has no performance advantage over a vented box, it is wise to first attempt to design a vented box. This will save you money which is better spent on a quality driver. However, in some cases, you will find that it is not possible to meet your performance goals for your driver with a vented box. Suppose you have an 85L box and a 12" high excursion driver. To achieve 20 Hz tuning with a 100mm vent requires a 618mm long vent, however with some very high excursion drivers, you will have turbulence due to high velocity air in the vent. Moving up to a 150mm vent may reduce vent velocity enough to remove the problem, but now you have a 1.4m vent! This problem is common with drivers with an xmax greater than ~15mm. A passive radiator removes this problem and gives you the freedom to use an ultra high excursion driver in a box that isn't huge.

5. Transmission line

In the past, design of transmission line (TL) speakers was considered "black art" and proceeded more by experimentation as there were no simulation tools. This is no longer the case, since the work of Martin King who has graciously provided a means to accurately simulate TLs. A typical TL will have a tube with a lenth that is a quarter of the wavelength of the desired tuning frequency. The bass is natural and uncoloured. TLs are normally used to get 25 Hz bass out of small midbass drivers with say a 1.8m folded tapered tube tuned at 40 Hz. With a sub driver, very low tuning is not necessarily a requirement.

High output subs

6. bandpass

There are a number of types of bandpass types. The simplest is 4th order bandpass, which has a sealed chamber and a vented chamber. The response is similar to a vented box but the vent tuning determines the upper end of its bandwidth. A 6th order bandpass has a vented chamber on either side of the driver, each with a different tuning to determine the upper and lower extents of the bandwidth. The 8th order bandpass has another vented chamber in which the output of both of the other chambers are combined.

While bandpass subs are technically interesting, situations in which they are a good idea for hifi are rare. While they achieve higher efficiency, power handling and output than vented subs, this is typically at the cost of accuracy. They are difficult to get right. More suitable for pro sound.

7. acoustic lever

This is a relatively new type of subwoofer, invented by Dr Earl Geddes. It is similar to a bandpass, but it is claimed that it eliminates some of the disadvantages. It uses a number of passive radiators. As this is relatively new and not well known, special software designed by Dr Geddes is required. There is also a legal limit on the number of them that you can build. There is more information on acoustic lever subs in the Loudspeaker Cookbook.

Ultimate Subs

These are for the extreme enthusiast! The enthusiast who has had some other subs, and now wants to build "the subwoofer to end all subwoofers!" These subs are the most expensive but also the most rewarding. Even when cost is no object, there isn't a clear winner, each have a mix of pros and cons. It's a matter of preferences, priorities and perception!

8. bass horn

Bass horns are unique in that they can offer two things that rarely go together - maximum output and maximum fidelity! A bass horn can do just about anything any other sub can do to achieve fidelity except improve room interaction like an open baffle dipole. A bass horn improves efficiency, adding typically 10db to efficiency, while also increasing power handling. To achieve their goals, bass horns must be VERY large. A vented subwoofer can achieve 20 Hz extension with an 85L box and 12" high excursion driver. By contrast, you could likely achieve a similar response with an 8" horn-loaded driver. The output can be as high as 130 db but the horn may be as big as the room itself! A solution for extreme enthusiasts is to build a basement horn, or some other architectural solution, or settle for a "compromised horn." Some use a bass horn down to 40 Hz then supplement with a vented sub below.

9. Infinite baffle

This is a great option for those who own their own home. A number of high excursion drivers can be mounted in the floor/walls/ceiling so that they are invisible and don't occupy space in the room. As a result you can fit in many drivers, typically 4 or more 15" drivers which would normally require large boxes. Low distortion is achieved by having so much displacement (excursion x driver piston area). It is often claimed that not having an air spring also achieves very accurate bass.

10. Open baffle (OB) dipole

Many who have built or heard open baffle dipole bass comment that it's "the best bass I've ever heard!" Perpundicular to the baffle, the sound from the front and rear of the driver cancels, creating a "dipole null." All other subwoofers are monopoles, which means that they are omnidirectional. Stand behind a conventional speaker and the bass is unattenuated while the treble is missing and the midrange is reduced. Partly as a result of the dipole effect, OB subs interact with the room less, and respond to room modes differently. The result is the ultimate in accuracy, but the downside is output. Depending on the design, four drivers may not achieve the same output as a single driver in a sealed box! In fact, it's best for this reason to limit the lower cut off to 40 Hz for an OB dipole and use a monopole if 20 Hz bass is desired. This combination can work well as down this low you may not in fact have any room modes. Where there are no modes, the room is uniformly pressurised.

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