Team-BHP - Questions about subwoofers

This information below is taken from JLAUDIO site....I could have provided the link only but people tend to get confused thats why here is the complete writeup.....JL Audio

Sub-30Hz Behavior
Sealed box designs and single-reflex bandpasses are much better at controlling excursion at extremely low-frequencies (below 30Hz.) For this reason, they can usually handle more power in these frequency ranges than ported designs and dual-reflex bandpass designs which makes them less prone to low-frequency induced speaker damage. At frequencies below the tuning frequency of the port, a woofer in a ported box (or a dual-reflex bandpass) starts to de-couple. This means that the controlling function of the enclosure begins to disappear. The collapse is gradual rather than immediate, but at some point below the tuning of the port, the speaker behaves as if it were operating without an enclosure and suffers from potentially damaging over-excursion. (This is why it is a good practice to use an infrasonic filter when running a ported enclosure or a dual-reflex bandpass. Related to the loss of enclosure damping, ported and dual-reflex bandpass designs also exhibit higher distortion levels at very low frequencies than sealed or single-reflex bandpass designs. The importance of this is questionable, however, since little program material extends to below 30Hz. Sealed enclosures and single-reflex bandpass designs have a rather shallow low-frequency roll-off rate of around 12dB/octave, whereas ported enclosures and dual-reflex bandpasses typically exhibit 18- 24dB/octave roll-off. For this reason, sealed enclosures and single-reflex bandpass boxes can have much higher -3dB points (the frequency at which the output dips 3dB below the reference efficiency of the speaker) than ported designs while still producing very good ultra-low frequency output.

30-80Hz Behavior
This is the frequency range that is most important in that it encompasses the vast majority of low-frequency information present in music. Serious audiophiles assign much more importance to good performance in this range than in the extreme low-frequency range.At moderate power levels all of these enclosure types exhibit pretty decent manners. The ported box and the bandpass designs produce less distortion than the sealed box, but the difference is marginal.At higher power levels things change considerably. The dual-reflex bandpass, due to the fact that its ports control cone motion over a wider range of frequencies, produces the least distortion and exhibits the best power-handling characteristics. The ported enclosure and the single-reflex bandpass also do a very good job producing high-levels of undistorted bass output, again due to reduced cone motion in this frequency range. Bringing up the rear in this category is the sealed enclosure, which produces higher levels of distortion at high power levels. There is a common misconception that ported designs produce more distortion than sealed boxes. As you can see this is not entirely accurate; it depends on the frequency and the power level.

Transient Response
Transient response refers to the ability of the subwoofer system to reproduce quick changes (transients) in the program material accurately. This is often interpreted as "tightness" or "looseness" which is maybe a dangerous terminology since many people are more influenced by tonal characteristics when asked to qualify the "tightness" of the bass. Transient response is actually a function of accuracy in relation to time rather than frequency. In music, sounds like drum strikes and quick bass guitar pulses are good tests of a subwoofer system's transient performance. A system with good transient response will reproduce these sounds with clear, "tight" definition. A system with poor transient response tends to blur these sounds over time, due to the speaker's inability to stop and start quickly enough to react to the signal accurately.It is generally accepted that an optimized sealed enclosure exhibits the best transient response characteristics. The control provided by the air-spring in a good sealed system contribute to generally outstanding transient behavior (at very high power levels, the increased distortion can overshadow this advantage, however.) A ported enclosure can also achieve good transient behavior but never as good as an optimized sealed enclosure. It is possible, however, for a well-designed ported enclosure to have better transient response characteristics than sealed enclosures with higher Qtc's (above 1.0.) The specific alignment of the sealed and ported enclosures plays a huge role in determining the transient characteristics of each individual subwoofer system. Single-Reflex bandpass designs can also have good transient characteristics if their bandwidth is fairly narrow, but again, not as good as an optimized sealed enclosure. As the bandwidth becomes wider, their transient response can degrade considerably.Dual-Reflex designs generally exhibit inferior transient response characteristics when compared to the other designs. As with single-reflex designs, narrower bandwidths produce better transient performance than wider ones.

Efficiency
The term "efficiency" refers to the ability of a speaker system to convert electrical energy (power from the amplifier) into acoustic output. Consequently, it also serves as an indication of which system will produce the loudest possible output given the same size amplifier (assuming they can all handle the power.) For the purposes of this comparison, we are looking at efficiency in the 40-80Hz octave.Generally speaking, the most efficient enclosures are the two narrow-bandwidth bandpass designs with the dual-reflex version having a slight edge. Next in line, the wide-bandwidth dual-reflex and the ported enclosure exhibit very good efficiency as well. The sealed enclosure and the wide-bandwidth single-reflex bandpass are the least efficient designs.

Midbass transition
For sub-bass to sound natural, the system must have good midbass capability as well. These two are interrelated because harmonic components of the sounds produced by instruments that play in the sub-bass range must be accurately reproduced in the mid-bass range for a system to sound accurate.In car audio, we normally don't have the luxury of using very large drivers to reproduce midbass. For this reason, the ability of a subwoofer system to smoothly transition to the mid-bass region becomes very important to achieving top-notch fidelity.The sealed and ported enclosures, because the speakers play directly into the listening environment usually produce the smoothest midbass transition. Wide bandwidth bandpass designs are a little more ragged, but still deliver good midbass reinforcement. The narrow bandwidth bandpass designs can create serious problems because their high-frequency roll-off can begin as low as 75-80 Hz and the amplitude of their response peaks is very high, which necessitates the use of larger, very capable midbass speakers in order to blend smoothly with the sub-bass.

There Is No Free Lunch
As you can see by the comparison, no enclosure design is superior in all respects. They all have advantages and disadvantages. Analyzing the characteristics of each enclosure type will help you decide which enclosure type is right for your application. An informed decision involves an analysis of the following factors: the space that you want to make available in your car for the enclosure, your performance expectations (loudness, tonal qualities, etc,) the amount of amplifier power you will be using, and of course, your budget. Top-notch car audio specialists will weigh all the factors and consider all enclosure types before recommending a subwoofer system. Many will even show you specific data to support their suggestions.Remember that the information presented here assumes that each enclosure type has been properly designed and executed. This means that the speaker and the enclosure are carefully matched as a system. The skills of a competent designer, installer and cabinet builder are every bit as important to the end result as the design of the box or the type of woofers that you buy.Despite the very general scope of this piece, we hope it leaves you with a better understanding of subwoofer systems. At the very least, we hope that the next time you hear someone say "whatever you do, don't port the box" or "sealed boxes don't sound good," you will ask them to thoroughly explain their position. It could be amusing.

The sealed (aka: air-suspension, acoustic-suspension) enclosure is a classic box design. Patented in 1949 by Harry Olson and popularized in the 1950's by Acoustic Research, this design has stood the test of time and has been adopted by many home and car audio companies.

Primary Advantages:Small, good low frequency extension and control, good transient response, simple to build

In a sealed enclosure, the woofer is tightly controlled by a trapped volume of air in the enclosure which acts as a spring (hence the name "air-suspension.") The woofer must literally pull the air with it as it moves outward thus decreasing the air pressure inside the box and compress the air inside the box when it moves inward, which increases the air pressure inside the box. Since the air pressure inside the box seeks to equal the barometric pressure of the atmosphere, it acts as a controlling force over the motion of the speaker. The more the speaker moves inward or outward, the greater the pressure exerted by the air-spring of the sealed enclosure in the opposite direction. The relationship between the parameters of the speaker being used and the volume of air inside the enclosure dictates the performance of the sealed subwoofer system. By making the box larger, the air spring limits cone motion less and allows the system to play lower and with flatter overall response (lower Qtc) at the expense of power handling. If you go too large, however, you begin to lose efficiency in order to gain the additional low frequency extension. By making the box smaller, the air spring exerts more control and limits cone motion at low frequencies which increases power handling but does not let the system play as low and produces a more peaked response (higher Qtc.) For any speaker competently designed for sealed box applications there is a range of enclosure volumes that will produce good high-fidelity sound. Changing the enclosure volume within that range can fine-tune the response to suit the tastes of the listener and/or the acoustic properties of the vehicle.

Ported enclosures (aka: Bass-Reflex, Vented) have actually been around longer than sealed designs. The ported enclosure was patented in 1932 by A.C. Thuras. Further development since then has defined the behavior of ported systems much more precisely. A.N. Thiele and Richard Small are generally credited with having done the most definitive work in this area, which is why enclosure/speaker parameters are commonly referred to as Thiele-Small parameters.

Primary advantages:Efficient, low distortion around port tuning

The coupling of a port or duct to the air inside the enclosure allows the subwoofer system to take advantage of the work being done by the rear of the woofer cone to reinforce the low-frequency response. The resonant characteristics of the column of air in a port, when installed in a given box, are adjusted by altering its resistance to motion, which is accomplished by changing the dimensions of the port. In some designs, instead of a port, a speaker cone with no motor assembly or a flat diaphragm is used to achieve the same effect. This is known as a passive radiator. The resonance of a passive radiator system can be adjusted by altering the radiator's surface area, mass and compliance (stiffness of suspension.)

In a ported enclosure, there is a delicate relationship between the volume of air in the box, the resonant effect of the port, and the parameters of the speaker being used. When these three factors are correctly integrated, the rear output wave of the speaker is delayed just enough so that when it comes out of the port, it is in relative phase with the wave being produced by the front of the speaker. The result is constructive output from the port limited to a desired low- frequency range. This low-frequency reinforcement is one of the big advantages of a well-designed ported system. Using the work of the rear of the cone in a constructive manner means that a gain in efficiency of about 3dB over a broad band in the sub-bass range can be achieved as compared to a sealed enclosure using the same woofer. The other big advantage is that the interaction of the port, the enclosure and the speaker's resonant characteristics also reduces cone motion and, therefore, distortion at higher volume levels in the frequency range controlled by the port. The down side is that at frequencies below the tuning of the port, the speaker gradually begins to act as if it were not enclosed at all (more on this later.)

The increased output combined with reduced distortion in the "meat" of the bass range (35-60Hz) is a big reason why many home speakers and high-power sound-reinforcement systems use ported enclosures for low-frequency reproduction. Many high-output studio monitors also use ported enclosuresfor the same reasons.

The rules governing the behavior and proper design of ported speaker systems are considerably more complex than those for sealed enclosures. For this reason, it is a good practice to follow the advice of the speaker manufacturer or an experienced enclosure designer when it comes to designing a ported system. It is very easy to screw up a ported box if you just guess at the size and length of the port or the tuning frequency for the box. Not only will a poorly designed box sound bad, but it can easily damage the speaker if it is played hard.

Enclosure Types: Bandpass

These enclosures have, at times, been popular in the car audio world. It would probably surprise many people to know that this is a very old design. The first patent for a bandpass enclosure was filed in 1934 by Andre d'Alton. Many home sub/satellite speaker systems currently use bandpass designs for low-frequency reproduction.

Advantages:Can pass lots of output through a small opening.

In a bandpass box design, the woofer no longer plays directly into the listening area. Instead, the entire output of the subwoofer system is produced through the port or ports. In a conventional sealed or ported subwoofer system the low-frequency extension is controlled by the interaction of the speaker and the enclosure design, but the high frequency response is a result of the speaker's natural frequency response capability (unless limited by a crossover.) In a bandpass enclosure, the front of the speaker fires into a chamber which is tuned by a port. This ported front chamber acts as a low-pass filter which acoustically limits the high- frequency response of the subwoofer system. The name "bandpass" is really pretty descriptive in that it refers to the fact that the enclosure will only allow a certain frequency "band" (range) to "pass" into the listening environment.

So what? Couldn't the same thing be accomplished by placing a low pass crossover on the subwoofer system? Yes, it could, but a bandpass enclosure can produce significant performance benefits in terms of efficiency and/or deep bass extension that would not be possible in conventional designs of equal size.

By adjusting the volumes of the front and rear chambers and the tuning of the port or ports, significant performance trade-offs can be created. When box parameters are adjusted for a narrower bandwidth, the efficiency of the subwoofer system within that bandwidth increases and can reach gains of up to 8dB (sometimes even higher.) As box parameters are adjusted for wider bandwidths, very impressive low-frequency extension can be produced from extremely compact enclosures at the expense of efficiency and good transient response. Intermediate bandwidths can also be designed which create a compromise between all these characteristics. As if that is not confusing enough, within each bandwidth range, the designer can also manipulate box parameters to shift the range of operation up or down the sub-bass range which also has an effect on efficiency.

As you can see, bandpass enclosures can have very different sound characteristics based on the designer's choice of box parameters. As such, it is not always possible to make blanket statements as to the performance benefits and drawbacks of bandpass enclosures in general.

One characteristic of bandpass enclosures which is universal is that they exert greater control over cone motion over a wider frequency band than conventional designs. Due to controlled, rapidly changing air pressure on either side of the woofer, the woofer is capable of producing high levels of acoustic output without physically moving very much. This means that the woofer is less likely to encounter excursion limits in the main part of the sub-bass range. However, just because the cone isn't moving as much doesn't mean that the speaker's motor assembly isn't still trying to drive the cone hard; it just means that the speaker cone is encountering resistance to motion. This resistance can be very hard on speakers, especially when crazy car audiophiles are at the controls. The conflict between the force generated by the motor assembly and the air pressure in the enclosure can impose extreme stress on the glue joints and suspensions of the woofers. You can literally tear a speaker apart in a bandpass enclosure if you apply too much power. Because the speaker is not moving as much and because noises are masked by the front chamber, it is also very difficult to hear when a woofer is in serious trouble. Many people have been known to crank bandpass enclosures up and blow the speaker to bits within a few minutes because they did not realize that the speaker was having a heart attack. Choosing the right amount of power and carefully setting amplifier gains is very important in order to ensure long- term reliability.

Bandpass enclosures can be divided into two basic types: single- reflex and dual-reflex. In a single-reflex design, the rear chamber is sealed and the front chamber is ported. In a dual-reflex design, both front and rear chambers are ported into the listening area. A variation of the dual-reflex and single-reflex, known as "series-tuned," has a port which connects the rear and front chambers.

The differences between single-reflex and dual-reflex bandpasses are similar to the differences between sealed and ported enclosures. A single-reflex typically exhibits a shallower low-frequency roll- off rate (approximately12dB/octave) and better transient response. A dual-reflex is more efficient and controls cone-motion over a wider range but typically has a sharper (18-24dB/octave) low- frequency roll-off. Because of the difference in low-frequency roll- off rates, a dual-reflex usually has to be larger in size to produce the same low-frequency extension as a single-reflex design.

As compared to more conventional enclosure designs, bandpass enclosures are very complex to design and build. The rules governing the performance of bandpass enclosures leave no room for error. Slight volume miscalculations or sloppy construction can turn a good design into a poor-performing box. Integrating the proper size port or ports can be extremely challenging and often renders designs that looked great on paper completely impractical. The design of these boxes should definitely be left to people with extensive enclosure- building experience.For many years now, bandpass enclosures have been quite popular both for their aesthetics and performance. A properly designed, constructed and implemented bandpass enclosure can and often will out-perform the same driver or drivers in a more conventional sealed or ported design in terms of sheer output and/or low-frequency extension. A bandpass alignment also allows the installer to funnel a potentially large amount of low-frequency energy into the vehicle's cabin through a relatively small opening. This can be particularly useful in some vehicles such as European sedans whose tank-like construction does not facilitate a satisfactory transfer of low-frequency energy from the trunk into the cabin.

At JL Audio, we only recommend single-reflex bandpass designs (as opposed to dual-reflex or series-tuned bandpass designs). Cubic foot for cubic foot, a single reflex-bandpass characterized by a sealed "rear" chamber(s) and ported "front" chamber(s), typically offer better transient response and low frequency extension than its more elaborate cousin. In addition, while still a fairly difficult system to construct properly, single-reflex designs are a bit more forgiving of minor errors in calculation and assembly when compared to more sophisticated bandpass types. All things considered, we have found that single-reflex designs offer the best combination of reasonable enclosure size, good transient response and predictable behavior.
For the duration of this tutorial, the term "bandpass" or "bandpass enclosure" will refer only to single-reflex bandpass enclosures unless noted otherwise.

So how do they work?
A bandpass enclosure is, by definition, simply a sealed enclosure with an acoustical filter in front of it that serves to limit the upper-end of the driver's frequency response. This natural limiting of the high-frequency response of the system makes the selection of mid-bass drivers critical. If your vehicle cannot fit larger midbass drivers (such as a 6 1/2" or larger), a bandpass enclosure is probably not the best choice for you. Using a bandpass enclosure with insufficient mid-bass reinforcement will lead to sluggish, sloppy, muddy, low frequency response. In short--it will sound like a soggy pancake hitting a cardboard box.

Once adequate mid-bass reinforcement has been selected to complement the sub-system it will be necessary to add additional electronic filtering to further limit the upper frequency output of the enclosure. Contrary to popular belief, a bandpass enclosure (of any type--single reflex, dual reflex, series-tuned, etc.) does require the use of an electronic crossover to achieve optimum performance since the acoustical low-pass filter is not a very effective filter. What proponents of "crossover-less" bandpass enclosures neglect is that there is a considerable amount of high frequency output (called "out-of-band noise") that can get to be quite annoying. It is for this reason that JL Audio recommends that all bandpass enclsoures be supplemented with an electronic crossover. If you would like to find out more about electronic crossovers, see your local authorized JL Audio dealer.

In order to understand how a bandpass enclosure works, it helps to break the enclosure itself into three parts: the sealed (rear) chamber, the ported (front) chamber, and the port itself; but before we get started, we need to define some of the basic terminology used in this tutorial so we can make sure everyone is on the same page.

The Sealed Chamber
The sealed chamber's primary purpose is to serve as a high-pass acoustical (as opposed to electrical) filter and it's volume controls the lower -3dB point or FL. By changing the size of the sealed chamber , we can see a corresponding shift in FL that follows these simple guidelines:

The bigger the sealed chamber is, the lower the FL will be. The smaller the sealed chamber is, the higher the FL will be.

Any changes made in the rear box volume require a corresponding change in the tuning of the front chamber(s) of the enclosure. Failure to retune the front chamber(s) will result in a mis-tuning and the box will more than likely sound really, really bad.

As is Mother Nature's style, we can't get something for nothing so as we adjust the volume of the rear chamber(s) it is important to keep the following in mind:

The bigger the sealed chamber is, the lower the driver's mechanical (also called "displacement-limited") power handling will be. If the rear chamber(s) is/are too big, the amount of power the system can handle is reduced.

The smaller the rear chamber(s) is/are, the higher the driver's displacement-limited power handling will be but of course, the low-frequency extension will suffer as a result.

The Ported Chamber
The ported chamber controls the bandwidth and efficiency of the system and behaves as follows:

As front chamber volume(s) increase, the system becomes more efficient. As front chamber volume(s) decrease, system effiency decreases.

As always, Mother Nature has her dirty little hands in our enclosure design so we have to consider what she is doing:

As the size of the ported chamber increases, the bandwidth decreases. So, the more efficient we make the system, the smaller the passband will be. If the system is made too efficient (outrageously large front chamber), we'll wind up with "one-note" bass so typical of a myriad of mis-tuned bandpass enclosures on the market today. On the other hand, as the front chamber volume increases, the better the transient response of the system will be, and this is good.

As the size of the ported chamber decreases, the bandwidth increases. This may sound desirable (especially to those with smaller mid-bass drivers), but of course Mother has dictated that as the ported chamber shrinks the group delay becomes undesirably large and the system starts to sound very sluggish and muddy thus ruining our hopes of using a bandpass system with our factory-installed 4" speakers!

The Port
The port is probably the single most critical variable in the bandpass equation. The port MUST tune the front chamber to the exact center of the passband or the box will sound like total garbage. The center of the passband corresponds directly to the sealed-box resonant frequency or fc of the rear chamber. If for some reason the port is of the incorrect dimensions and tunes the front chamber too high, the frequency response will be skewed creating a really nasty response peak in the lower mid-bass range, whereas if it is tuned too low the response will be very peaky in the lower frequencies and the bass will sound unnatural and boomy.

"Universal" Bandpass Designs
The term "universal bandpass" itself is really an oxymoron (like "fresh-frozen", "jumbo-shrimp", "one size fits all", etc.) in that there truly is no one design that will work with all drivers! To imply that such a design is feasible is to totally ignore the very nature of bandpass enclosures: they are extremely driver sensitive and enclosure sensitive! If you have read all of this tutorial up until this point, it shouldn't take much persuasion to lead you to believe that "yes, bandpass designs are picky and easy to mess up". Not all 10" drivers are alike...not all 10" drivers from one manufacturer are alike, so why should they use the exact same box?

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Enclosure Types: Isobaric

About isobaric enclosures...

Again this is not a new concept, having been originally introduced by Harry Olson in the early 1950's. Technically, "isobaric" is not really an enclosure type; it is a loading method. This loading method involves the coupling of two woofers to work together as one unit. This is typically accomplished either by placing two woofers face to face or by coupling two woofers with a small chamber. The result of coupling the two speakers is that the coupled pair (iso-group) can now produce the same frequency response in half the box volume as a single speaker of the same type would require. For example, if a speaker is optimized for performance in a 1 cu.ft. sealed enclosure, one iso-group of the same speakers can achieve the same low frequency extension and overall response characteristics in a 0.5 cu.ft. sealed enclosure.

There is, of course, a penalty involved. Whenever you use isobaric loading, you are sacrificing 3dB of efficiency compared to a single driver in twice the air space. In practical terms, this is not usually a big deal since the powerhandling is doubled (two speakers instead of one) so the end result is about the same output as the single driver in the bigger box assuming you double the amplifier power.

Isobaric loading can be used within any enclosure type, including bandpass designs. The ported and bandpass isobaric designs can be difficult to design and build due to very small enclosures with large port requirements. Isobaric bandpass designs, in particular, can be literally impossible to build with certain speakers. There are some things to look out for with each type of isobaric design, such as mechanical noise and uneven heat dissipation which can present potential sound quality and reliability problems. All the methods which involve opposite cone motion require that the speakers be wired in reverse polarity relative to each other. These designs also provide a performance advantage in that their opposed cone motion averages out suspension non-linearities (differences in inward and outward suspension control,) which reduces distortion.

If you are strapped for space and can afford the extra speakers and more complex enclosure, the ability to have a compact subwoofer system with no real sacrifice in performance is well worth the extra effort and expense. On the other hand, if you have a lot of space and are looking to get the maximum amount of output without sacrificing sound quality, using multiple iso-groups can give you the best cone area/box volume ratio while still retaining good fidelity. Isobaric loading is not as popular today as it was in the early days of car audio subwoofer design. Why? Today's car audio woofers are designed for much smaller enclosures than their forebears, making the need for isobaric loading rare.