Today's market is inundated with woofers that offer respectable performance and great cosmetics at affordable prices. The variety is terrific for consumers in terms of the buying options they have; but the sheer quantity makes it difficult to pick a speaker. Things are then further complicated by the issue of enclosure style. This is one of the more controversial topics in car audio. Everyone has an opinion as to which kind of enclosure is best, but some people like to think that there is only one truly superior box design. In fact, there are several styles to choose from and each one has its upside. In the end, the issue comes down to what you want to do or the kind of music you listen to. The focus of the following tutorial is to elucidate the popular enclosure designs, discussing it in laymen's terms, so that even passing enthusiasts will find it easy to read. I'll also explore the various pros and cons, and explain the trade-offs involved in a given enclosure design, in addition to providing a brief history of "making boxes for speakers" (a simple way to describe something that has become somewhat complex).
Everyone wants a clear-cut response to the following question: "What speaker enclosure produces the best sub-bass performance?" I'm asked that a lot; and to this day I have not arrived at any kind of absolute answer. While I have developed my own opinions on the various styles of enclosures, I would have to say that for the most part the good and bad aspects of the different styles tend to cancel each other out in the end.
Like I said, this is perhaps the most controversial area of car audio. Of course, that's because you have competing methods or technologies at work and no single enclosure design that provides users with all of the benefits the various boxes offer with none of the shortcomings. Many enthusiasts swear up and down that sealed enclosures produce the best performance, while others are equally adamant about the benefits of ported and bandpass designs. Most of these people are well-intentioned in recommending enclosures that they have had success with, but anyone who says that a given enclosure type is superior in every way is always wrong.
Infinite Baffle
Infinite baffles (IB), often termed "Free Air", are possibly the simplest enclosures to build, and a great way to conserve space. This design is defined as an enclosure that contains a greater volume of air than the Vas of the speaker. Applying an infinite baffle system in an automobile is relatively easy. The concept of an infinite baffle is to isolate the sound wave produced by the front of a speaker from the back wave. This normally involves mounting the speaker(s) on a board that seals off just behind the rear seat while using the trunk of the vehicle as the enclosure. When all is said and done, an IB enclosure is truly quite large.
Trade-offs come in many forms when working with IB enclosures. Sure, using IB enclosures conserves a lot of space, especially in the trunks of smaller cars. However, there are some losses to be noted. The most prominent is efficiency. Without a solid structure built around the woofer, it is difficult to reinforce the sound. Trunk interiors are filled with flimsy panels, thin sheet metal, and usually a measly spare tire -- none too pleasing for sound reproduction. This same issue holds value with the speaker's power handling. As much as 50% of the nominal power handling of a woofer can be lost when placed in an infinite baffle. Another anomaly that plagues IB enclosures is the tendency to lose control of cone movement when played below the Fs (resonance frequency of the driver), much like ported systems when played below the tuning frequency. For this reason, it may be desirable to utilize a subsonic filter below the Fs of the speaker.
When purchasing IB speakers, keep in mind the following parameters to assure that the speaker is right for the application. The first is the Qts (the total resistance of the driver at its resonance). Speakers with a Qts greater than 0.6 typically work well in an IB environment. The second is the Vas (the volume of air having the same compliance as the driver's suspension). The Vas should be smaller than the total volume of the environment the speaker(s) are being placed in.
Good: minimal space consumption; generally easy to build.Bad: poor efficiency; sometimes difficult to isolate front and back wave; limited cone control below speakers fs.
Sealed Enclosures
Sealed enclosures have stood the test of time and have grown in popularity throughout the years. This design was first patented by Harry Olson in 1949 and then exploited by Acoustic Research in the early 1950's. Today, sealed enclosures (aka: acoustic suspension) are the most commonly used by car audio enthusiasts around the world.
Industry aficionados often associate sealed enclosures with "sound quality". It is generally accepted that the transient response exhibited by these enclosures is superior to other box designs when they are built to the optimum size required by a particular speaker. Much of this can be contributed to the "acoustic-spring" that adds to the restoring force of the speaker. The acoustic-spring is a reaction from the air trapped within the enclosure that compresses or rarefies as the cone moves back and forth. This is greatly affected by the distance that the cone of the subwoofer moves. The longer the stroke of the woofer, the greater the force exerted by the acoustic-spring in the opposite direction. Power handling of a sealed system is a trait directly related to the acoustic-spring. It is the increasing load on the cone of the woofer that maintains control as the excursion increases, thus allowing for more power to be applied to the speaker. But power handling can be greatly affected by the size, or tuning if you will, of the enclosure. Larger enclosures have less of a limiting factor on the speaker cone, allowing the woofer to play lower with a more gradual role off while compromising power handling and some efficiency. On the other hand, decreasing the size of the enclosure, the acoustic-spring exerts more control over the cone motion with a trade-off in low frequency response. While this may increase power handling, the system will not play as low and often produce a rise, or bump, in the frequency response.
The size of the enclosure also determines the shape of the response. This is stated as the "Qtc." To be specific, Qtc is the value of damping provided of a speaker system. To give an example, a sealed system of optimum performance has a Qtc of .707. What this means is that the frequency response of the subwoofer will roll off at a -10.5dB/octave slope in the lower frequencies. Figures 1 and 2 give examples of different Qtc values and the shape of their response.
Altering the Qtc is not only dependent on the size of the speaker enclosure. Another way to change this is by adding "enclosure fill." Using fill can be an invaluable tool to alter box response in order to achieve certain parameters and Qtc value that may be difficult to achieve due to size constraints within the vehicle. Dacron, polyfill, fiberglass and acoustic foam are among the most commonly used enclosure fills. Adding these materials within an enclosure can effectively reduce the Qtc, reduce standing waves, and lower the resonance of the system. As well, proper selection of the amount, type of material, and the location of the material within the enclosure can also increase efficiency by up to 15%.
Although not every woofer works well in a sealed box, there are many competent speakers designed for this type of application. Most will operate in a variety of enclosure volumes that can produce good high-fidelity sound, and most consist of a simple design and easy construction. Perfect for the do-it-yourselfer.
Good: easy to build; small enclosures; high power handling; excellent damping and transient response.Bad: sub-par efficiency.
Ported Enclosures
The reputation of ported enclosures has not been one of quality sound. Rather, they are often perceived as being sloppy and generally not very accurate. This common misconception is likely a product of a poorly designed or an improperly ported enclosure. It is not uncommon for ported (vented) enclosures to have better transient response than underdamped sealed enclosures with a high Qtc.
As odd as it may sound, ported enclosures have been around longer than the simpler sealed box. The patent on this design was first issued to A.C. Thuras of Bell Labs back in 1932. His work contributed to basic theory of the design, describing in detail the interaction between the driver and vent. It was not until 1961 that A.N. Thiele published a landmark paper that was later (1973) refined by R.H. Small who defined the behavior of a speaker, thus allowing for ported enclosures to be perfected. Their contribution is well known as Theile-Small parameters and most commonly related to ported speaker systems.
Ported enclosures are often referred to by many names such as bass reflex, tuned, or more commonly, vented. What distinguishes a ported enclosure from that of a sealed is the port or duct that allows the back wave to interact with the front wave. The coupling of a port to the air inside the enclosure reinforces the low-frequency response the subwoofer system. By changing either the length or surface area of the port, the resistance to motion of the column of air within the port changes its resonant characteristics, thus causing the tuning frequency of the enclosure to change.
In a ported enclosure, there are three main factors that have to be considered: the parameters of the speaker being used, the volume of air in the enclosure, and the size (diameter and length) of the port. Proper integration of these three factors will have a positive effect on the output of a ported system. The primary goal of integrating these three factors is to delay the rear output wave of the speaker so that as it comes out of the port it is relatively in phase with the wave produced by the front of the speaker. The result is a gain in efficiency of about 3dB compared to a sealed system using the same woofer. It is this low-frequency reinforcement that is one of the big advantages of a well-designed ported system. Another big advantage is that near the tuning frequency, almost all output is from the vent so cone movement is controlled to the point of almost no movement. That results in lower distortion at higher volume levels in the frequency range controlled by the port. But not everything is perfect with ported enclosures. They suffer from one main drawback: the response of the speaker below the tuning frequency. The speaker no longer has the restoring force of the air in the enclosure, and therefore the cone acts as though it were not enclosed at all. This can lead to serious damage to the speaker itself and leave you stranded without sound. The laws managing the performance and proper design of ported speaker systems are considerably more complex than those for sealed enclosures. It is very easy to screw up a ported system, so make it a good practice to follow the manufacturer' advice, or the advice of an experienced system designer. Guessing at the port dimensions will likely create a bad sounding enclosure and may also damage your expensive speaker.
Bandpass Enclosures
By now, most car audio enthusiasts are probably familiar with bandpass enclosures. Starting in the early '90's, they became the rage of the car audio world. But bandpass enclosures are far from being a new design concept. It would probably surprise most people to know that these designs have been around since the 1930's. The original patent for a single reflex bandpass was filed in 1934 by Andre d'Alton. Just over five decades later, Bose was granted a patent on dual reflex bandpass enclosures (1985). Three years beyond that, Jean Margerand republished the methodology of two French designers, Augris and Santens. It was their hand-calculator design system for single reflex bandpass speakers that is likely responsible for the eruption of popularity amongst manufacturers as well as consumers. In the last ten years, considerable progress has been made in defining the behavior of these speaker systems in both home and car audio.
So where did the term "bandpass" derive from? This is really pretty simple. The term refers to the fact that the enclosure will only allow a certain frequency range (band) to play (pass) into the listening environment without the aid of a crossover. In order to for this to happen, the frequency response of the speaker must be acoustically limited by the enclosure.
In a bandpass enclosure, the woofer resides between two separate chambers. Instead of playing directly into the listening area, the entire output of the speaker system is formed through port(s). These ports act as an acoustic filter and limit the frequency response of the speaker to a specific bandwidth. Adjusting the size of the enclosure and/or the dimensions of the ports, the frequency response (bandwidth) can be greatly altered.
But why would anyone want to use a bandpass enclosure over a sealed or ported? Well, besides the natural filter acting as a crossover, a bandpass enclosure can deliver considerable performance benefits in terms of efficiency and low frequency extension that would likely not be possible in conventional designs of similar size. This of course is a trade-off with regards to the parameters of the enclosure. When enclosure parameters are attuned for a narrower bandwidth, gains of up to 8-10dB in efficiency are not uncommon within the bandwidth of the subwoofer system. Adjusting the box parameters for wider bandwidths, low frequency extension can greatly be improved, but at the expense of efficiency and good transient response. Using an intermediate bandwidth can create a compromise between these characteristics.
Bandpass enclosures come in two flavors: single and dual reflex. The more popular of these is the single reflex bandpass, otherwise referred to as a sealed rear chamber bandpass. In a single reflex bandpass, the rear of the speaker resides in a sealed chamber while the front fires into a chamber that is tuned by a port. This front chamber acts as a low-pass filter, which acoustically limits the high frequency response of the speaker. The roll off of this type of bandpass is typically 12dB per octave, much like that of the common sealed enclosure with better overall transient response.
Dual reflex enclosures are very similar in design to single reflex. The difference comes with the placement of port(s) in the rear chamber of the enclosure. This addition makes use of both the front and back wave of the speaker to improve efficiency over that of a single reflex. The additional port(s) also allows for extended low frequency response. In a dual reflex, the cone motion of the speaker is generally very well controlled within the bandwidth. But, as with everything there is a downside. This is apparent in the low frequency roll off that can be sharp, often in the range of 18-24dB per octave. This steep roll-off in the low frequency implies that a dual-reflex usually has to be larger in size to produce the same low-frequency extension as a single-reflex design. The low frequencies are a direct effect of the roll-off of the enclosure design; and as the enclosure is made larger to compensate, the transient response can degrade considerably.
In comparison to more conservative box designs, bandpass enclosures are can be very complex to design and construct. Their complexity leaves little or no room for error without suffering consequences. The slightest miscalculation in volume size or port length can create drastic changes in the response of the system. It is highly recommended that the design and construction of these boxes should be left to shops or people with extensive enclosure experience.
When it comes time to build an enclosure, it is important to keep in mind that there is no such thing as a "magic box." Every enclosure has its benefits that make it that better than the next design. No one box will yield the highest SPL, best transient response, smoothest frequency response, as well as power handling. So when it comes time to build the enclosure, be sure that you use the recommendations provided by the manufacturer to assure that the speaker performs the best that it possibly can.
In part two we'll explore alternative enclosures. Stay tuned.
Single Reflex Bandpass
Good: can be very efficient within the bandwidth; controlled cone excursion at low frequencies; increased power handling within the band.
Bad: distortion hard to hear so it's very easy to damage woofers; relatively large enclosures; difficult to design and build; sensitive to design and construction errors; stressful on speaker joints and suspension.
Dual Reflex BandpassGood: can be even more efficient than a single reflex bandpass; excellent damping at resonance; broad bandwidth with extensions at low frequencies.
Bad: Extrememly difficult to design and construct; not very forgiving of construction errors; like a ported enclosure it looses woofer control below f3; steep low frequency roll- off (18 to 24dB); very large enclosures; stressful on speaker joints and suspension.