Go to…. Sealed ELFTM Aperiodic Ported/EBS Passive radiator Bandpass Enclosure aesthetics Practical considerations Array Isobaric Transmission line Horn Terminated line/inverted horn Tapered quarter wave tube Dipole

Sealed box

This is the simplest type of box. The back wave can't interfere with the front wave because it's trapped in the box. Typically, sealed box systems are the least efficient designs since half of the radiation from the driver (i.e. the back wave) is wasted. Despite this, sealed designs are quite popular because: 1) they're easier to build than other enclosure types, and 2) are better behaved below their tuning frequency. The greatest disadvantage of sealed enclosures has little to do with efficiency, but is that their lower -3 dB frequency (F3) is always higher than the driver's free air resonance frequency (Fs). Variants of the sealed box include the infinite baffle and the acoustic suspension. An infinite baffle is where the volume of the box is so large that the driver acts (approximately) as if it were suspended in free air, An acoustic suspension design uses a very small box so the air inside can be used as a pneumatic spring to support the driver. Typically, sealed boxes are stuffed with acoustic material to increase their apparent size by changing the enclosure's characteristics from adiabatic to isothermal. All sealed box designs ultimately roll off at 12 dB/octave.

ELFTM sealed box

An interesting variation on the sealed box is the Extended Low Frequency (ELF) design. (Note that "ELF" is a trademark of Bag End Loudspeakers, one of several companies supplying commercial subwoofers based on this design principle.) An ELF system is simply a sealed box which is operated only below the low frequency -3 dB point (F3). This has several advantages as well as several obvious disadvantages. The advantage include: Very small boxes are not only possible, but actually desirable since F3 becomes the upper limit rather than the lower limit of the passband. The impedance is quite constant and stable below F3. thus presenting an easier load for an amplifier. Nothing's free, however, and ELF designs suffer corresponding disadvantages: Since the amplifier must equalized to put out an extra 12 dB/octave below F3, efficiency is extremely low and the required amplifier power is exceptionally high. For example, Bob Carver's Sunfire "True Subwoofer" is an ELF design which includes a built-in, dedicated 2,700 watt amplifier! The driver must be capable of handling all this additional power. Generally speaking, the more power a driver must dissipate, the more distortion it can generate due to the effects of voice coil heating. The driver must be designed with a high enough excursion to be able to achieve its equalized low frequency potential. For more information on ELF designs, see the Bag End (for theory) and Elliot Sound Products (for information on the required electronic crossovers) web sites.

Aperiodic box

Essentially a poorly sealed box, an aperiodic box vents the inside of the box to the outside air via an "acoustic resistor". All other comments about sealed boxes generally apply. The purpose of an aperiodic design is to allow a smaller enclosure than would normally be possible. If you put a driver into a sealed box that's too small, it will exhibit high Q, causing a peak in its lower response and a high impedance peak. By letting the box leak air, both the response and impedance peaks are tamed. Aperiodic boxes should not be confused with ported boxes (discussed below). The essential difference is that a that the port in a ported box acts as a Helmholz resonator tuned to a specific frequency in order to enhance and extend the low frequency response. Aperiodic vent designs exhibit no such characteristic resonance and functions solely as an acoustic resistance. As with any other low frequency system, the smaller size is achieved at the cost of a somewhat higher F3 and a steeper 18 dB/octave roll off than a sealed system. Aperiodic vents are sold primarily by Dynaudio (which calls them "Variovents") and Scan-Speak (which merely calls them a "flow resistor").

Ported/EBS box

Also known as a vented box, a ported box is a non-sealed box with a port (a tube between the box and the outside air) which is tuned to extend LF response. The port usually uses a constant cross-sectional area (excluding flares on the ends). Both the port length and diameter may be varied independently to achieve the desired tuning. Ported boxes are typically 3 dB more efficient than a sealed box below the sealed box's F3 point, since the back wave is used to augment the driver's output. The characteristics of this augmentation are determined by the port's tuning. Response rolls off at a 24 dB/octave slope. Ported enclosures are typically lined with acoustic treatment to break up standing waves, and may contain a low density fill, but should never be densely stuffed. In either case, the filler should be held securely in place to prevent it from being blown out the port. A popular variation of the ported box is the Extended Bass Shelf (EBS) design. This is simply a ported system in a larger than optimal box, tuned to a lower than optimal resonance frequency. The frequency response is distinguished by a slightly depressed shelf at the bottom end. Although F3 is actually higher, usable in-room bass response is extended, while providing better phase and delay performance. For more information on EBS alignments, see The Subwoofer DIY Pages.

Passive radiator (PR)

This is quite similar to a ported enclosure, except the mass of air in the port is replaced by one or more additional cones. These additional cones are called, variously, passive radiators, auxiliary bass radiators, or "drone cones". They are, quite simply, speakers without voice coils or magnets. Sometimes, the rear spiders are also omitted, but PR's with spiders are highly preferred. As with ported enclosures, PR enclosures are lined but not stuffed. Although not always suitable as a general-purpose port replacement, PR's offer some unique advantages of their own, especially in the frequency range around and below the tuning frequency. PR systems are also desirable in cases where the tuning would dictate a vent that's too big for the box. PR systems are tuned by varying the size of the enclosure and the mass of the cones ("cone" is somewhat of a misnomer since the best PR's use massive flat diaphragms made of wood). For more detailed information on PR's, see the PR FAQ and the DIY PR design tutorial published by Lambda Acoustics, the premier OEM supplier of PR's. Lambda PR's for hobbyists. Since PR enclosures work similarly to ported enclosures, it's also easy to realize EBS designs (see discussion of ported/EBS designs above) using PR's.

Bandpass

A bandpass design is a special type of ported box. It contains two or more chambers, any of which may be sealed, ported to the outside, or mutually ported with another chamber. The driver is mounted on a baffle between two of the internal chambers, so the sound can only get to the outside through a port. As with ported enclosures, bandpass enclosures are lined but not stuffed. As their name implies, bandpass designs exhibit both low- and high-frequency roll offs of 12 dB/octave in the case of 4th order systems. In all but the best implementations, this leads to boomy, one-note bass - impressive for casual listening, but not particularly accurate. As a result, bandpass enclosures are mostly used to provide the perception of lots of bass from a small box. Good bandpass designs are possible, but they remain subject to Hoffman's Iron Law. All Bose® systems are bandpass designs. Bose® also claims patent rights over commercial implementations of some sorts of bandpass designs. Since bandpass designs have been around for a while, these patents might not survive a vigorous challenge in court from someone with as much money to spend on lawyers as Bose®. Few such companies, however, consider the advantages worth the expense.

Array

The amount of low bass a system can deliver is directly proportional to the amount of air it can move, i.e. it's proportional to the driver's Vd spec. Although the obvious solution to low bass is to simply use a larger driver, this approach has some drawbacks: The large box is constrained due to the dimensions of the driver, i.e. there has to be at least one face of the box large enough to fit the driver. As a driver's diameter increases, its useful off-axis response decreases. For example, the average 12" driver has a conservatively rated upper limit of ~550 Hz while the average 5" driver has a conservatively rated upper limit of ~1300 Hz. Note that this limitation is imposed by the physics, and is always there regardless of the driver's on-axis frequency response. The next obvious solution is to use multiple smaller drivers. Typically, these are use in vertical arrays. Doing so provides the same wide horizontal dispersion as a single small driver (albeit at the expense of vertical dispersion (which is often a good thing since it eliminated floor and ceiling bounce). On the other hand, arrays of subwoofers typically are put on adjacent sides of a box rather than aligned vertically on a tall box. Array systems may be sealed or ported. The only caveat for designing arrays is that too many people base their estimates of how much bass they can achieve by simply multiplying the number of drivers by the individual drivers' area (Sd). This is deceptive since bass is dependent on Vd, not Sd. An array of drivers works as if it is a single driver with an Sd value equal to the sum of all the drivers in the array, but a Vd value calculated from the Xmax value of a single driver. One final note: Sooner or later, many people's research into arrays leads them to Bessel arrays. A Bessel array is a specially wired vertical array whose apparent vertical size varies with frequency, thereby avoiding the ragged vertical polar response of a normal vertical array. This appears to be an attractive design. However, most references to Bessel arrays fail to mention that they're only effective at listening distances equal to, or greater than, approximately ten times the array length. This makes them unsuitable for most home application, although they're used to good effect in many stage and professional systems. For more information, a good source is the Meyer Sound web site.

Isobaric

In order to get more bass out of a smaller box, isobaric driver mounting was invented. An isobaric system is constructed of two (usually) identical drivers mounted in a small sealed subenclosure. Wired in phase, the drivers are mounted front-to-back with the same orientation (as shown). Wired out of phase, the drivers may be mounted with either with a front-to-front ("clamshell") or back-to-back orientation. Such opposed mounting is actually preferable since it discourages standing waves between the two drivers' cones. As opposed to arrays, an isobaric system may be considered a multiple driver system with its drivers acoustically in series rather than acoustically in parallel as in an array. Isobaric designs are inefficient, both acoustically and financially, yet they are a legitimate means of reducing the size of a system. Unlike arrays, isobaric systems enjoy no Vd advantage over single driver systems.. Isobaric systems may be either sealed or vented.

Transmission line (TL)

Think of a TL as a pipe containing a driver in one end and with the other end open. Traditionally, a TL had no tuning frequency (Fb) in the conventional sense as with a sealed or ported box. "Tuning" a TL was a "simple " matter of making the line length 1/4 wavelength of where the driver begins to roll off, so that the rear wave can reinforce the front wave. The line was tuned to either the driver's F3 (-3 dB) or F10 (-10 dB), depending on whose design methodology you used. Similarly, unlike a port, which acts as a Helmholtz resonator, the parameters of a TL were independent, the cross-sectional area being determined by the driver's Sd, and the length being determined by the driver's F3 or F10, as previously noted. Some TL's (notably the Focal Daline series) were hybrids, utilizing a small enclosure which vents to the outside via a more conventional TL. Even in a Daline, though, the cross sectional area of the TL where it joins the box was much larger than a typical port, usually 1-1.5 times the SD of the driver. TL's are usually stuffed, often with stuffing materials of varying density. Sound complicated? It was, which was why many people avoided TL's as involving as much luck and voodoo as science. Well, that was then, this is now… Beginning with some ground breaking theoretical work by G. Augsburger (AES preprint #5011), TL's have finally yielded to rigorous theoretical analysis, which has led to the development of effective computer models. Some of the most significant work has been done by Martin J. King, whose free MathCAD model is listed as the second entry in Appendix C. Using King's computer model, it is now possible for almost anyone to design and build a working TL with excellent and predictable performance. Interestingly, his model can also be used to analyze some existing designs, at times revealing a design's shortcomings.

Horn

Horns for bass systems are almost always folded to conserve space. Both the geometry and subsequent design can be tricky and will require the greatest woodworking skills of any enclosure type. Horns are characterized by their flare rate - i.e. the rate at which the cross-sectional area increases between the driver and the mouth. Lacking finite element analysis tools, the best way to design a horn is using a tractrix (a specific geometric shape) profile. There are several software packages (none free that I know of, though) which can be used to design tractrix horns (some also do hyperbolic and/or exponential profiles as well). The mouth area and length of the horn determine the LF cutoff of the horn. Raise the cutoff frequency of the horn, and the enclosure size drops dramatically. Much of the traditional literature describes horns which are front-loaded, i.e. the driver is in a box (which can be sealed or ported, BTW - there are some good articles in recent issues of Speaker Builder magazine discussing horns with drivers in ported sub-chambers) which, in turn, drives the horn. Much contemporary interest in horns involves the use of a direct radiating, high-efficiency, full- or wide-range driver, with horn loading of the driver's back wave. A horn acts as an acoustic transformer, greatly increasing the efficiency of the system. Horns exhibit a LF cutoff, based on the circumference of the mouth. Below this cutoff, output drops precipitously - typically 24 dB/octave or greater.

Terminated line/inverted horn

One interesting design variant which superficially combines aspects of both horn and TL design is the terminated line, also known as an inverted horn. More closely related to aperiodic enclosures, unlike horns or TL's, terminated lines don't have to be particularly large to achieve their design goals. A terminated line approximates an ideal infinite baffle enclosure by sending the rear wave down a tapered tube containing damping material. If properly designed, the rear wave will be completely dissipated in the line. The line may be sealed or open. If sealed, the enclosure will be pressurized by the driver and so the low frequency cutoff and Q can be adjusted just as in a conventional sealed design. If open and stuffed, the design acts at the lowest frequencies just as an aperiodically loaded system. If open and lightly stuffed or unstuffed, it will act as a cross between a TL and a complex ported system where the port is merged with the box. Currently, the only companies building tapered lines are B&W and nOrh, both high-end companies.

Tapered Quarter Wave Tube (TQWT), also known as a Voigt pipe

The TQWT is a hybrid technology, related to both TL's and horns. Although more obscure than TL's, TQWT's have traditionally been backed by more solid theory. This is due to the fact that they were the work of one highly respected audio researcher, Paul Voigt, in the early 30's. Both straight and folded versions are known. Also like TL's, TQWT's benefit greatly from a good stuffing strategy. Although arguably as arcane as TL's, the TQWT's principle use has been as a low cost, easy to build alternative to horns. TQWT's are often used with full-range drivers, such as those listed in Section 3. Having said all that, if you want something different, yet lack the woodworking skills to build a good folded horn or TL, a TQWT might be just the project you're looking for. Important note… The same pioneering computer models developed by Martin J. King, and noted above in the discussion of TL's, also describes the traditional TQWT. His models is based on an analysis of pipe enclosures as a general class, with the choice of open or closed ends, and driver selection and positioning all being variables.

Dipole

A dipole system is simply one where both sides of the driver contribute to the total sound output. The obvious problem is that the front and rear waves of the driver are 180 degrees out of phase and so begin cancelling each other as their wavelengths approach the dimensions of the mounting baffle. For this reasons, most dipole systems use extended baffles, often with "wings" (as shown), in order to provide a longer cancellation path. Because of cancellation effects, dipole systems exhibit a corner frequency (or range of frequencies in better designs) based on baffle geometry and therefore independent of the driver's actual Fs. Below this frequency, the in-room response falls at 6 dB/octave. For this reason, dipole systems always require electronic equalization to achieve really low bass. For more technical information on the design and implementation of dipole systems (as well as many other audio and speaker building topics), there's no better resource than Linkwitz Lab. Another excellent resource, also developed with the assistance of Dr. Linkwitz, is on The Subwoofer DIY Pages.

Enclosure aesthetics

Although the technical design of loudspeaker enclosures is of greatest importance since it directly affects the sonic quality of the system, visual aesthetic considerations also play a part in any design. Let's face it, an attractive loudspeaker is more pleasant than an ugly box - whatever you may consider "attractive" and "ugly" to be. The appearance of a speaker is also of paramount importance for the majority of us who share our living space with a spouse or significant other. In Section 16 listing various kit designs, I've made an attempt to quantify the so called "SAF" (Spouse Acceptance Factor). If pleasing a cohabiting partner is a consideration for you, I recommend you read the discussion there.

Practical considerations

Aside from technical issues, aesthetics, SAF, and cost, there are additional practical considerations to be considered: Size  Even aside from SAF issues, you must consider whether your listening room can accommodate the design you choose. If you want or need an extremely large enclosure, will it fit? If not, can built it into an unused closet, the floor/basement, or ceiling/attic? If you're building a line array or using a large planar magnetic driver, will it clear the ceiling and doorways by enough to allow movement and installation? Weight  It's not uncommon for someone to design a killer enclosure, then be unable to move it. Even worse, if installed on a second floor or above a basement, it can cause structural problems as well. In such cases, the weight can also greatly increase coupling between the enclosure and structure, thereby causing subfloor structures and areas to resonate. Environmental  Only sealed, ELF, aperiodic and PR enclosures offer any protection for speaker internals. (The same is true of sealed arrays and sealed inverted horns.) In some climates, a common problem is insects or other vermin which like to crawl inside and make their home (hey, I wouldn't like to live inside an operating loudspeaker, but I'm not a bug!) People with pets and large vented enclosures can often have the same problem. (It's not at all uncommon to hear of people whose cats have crawled up through their large diameter ports and into their enclosures. In such cases, when suddenly subjected to 90+ dB inside the box when the system is turned on, the cats often try to claw their way out. Their chosen exit path may be back out the port, or it may be through the woofer cone -  not good! Either way, wires which get in the way are history.) HINT: Downward firing drivers or vents are less accessible to pets and vermin. Grilles?  Closely related to the environmental issues noted above is the issue of speaker grilles. Grilles on speakers almost invariably degrade the sound, but are tolerated (at least for times when the speakers aren't on) because of the protection offered the driver cones. Some of the same approaches used to mitigate environmental hazards can also be applied in conjunction with grilles. If you're going to use a grille, consider how to design it to cover vent openings as well as just the drivers. Are you using down-firing drivers or vents? Consider ways of covering the outside of the resulting circumferential exit slot with a grille. Are you using a rear-mounted vent? So much the better, cover it with a small piece of grille cloth to keep out the things you don't want to get in - since it's in the back, aesthetics aren't a major issue. "Clamshell" isobaric mounting is not only an aesthetic and SAF nightmare, but also is an open invitation to damage. Driver materials  No matter how hard you try to address environmental issues, or protect your precious drivers with grilles, there will always be some pet, vermin, or child determined to undermine your best efforts. Children just love to poke pencils through paper cones! If you live with potentially destructive small animals of any sort, it may be the better part of valor to simply choose drivers with less vulnerable cone/diaphragm and/or surround materials. Location also plays a part here. A small silk dome tweeter hidden at ear level behind a grille presents a much less attractive target for small children than a large woofer exposed at floor level.

©1998-2007 by Bob Stout, all rights reserved

Tested with: .

Support: