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lincolntc690
04-21-2009, 09:29 PM
Sealed - 2nd Order Acoustic Suspension

Basic Theory

Widely popularized by Acoustic Research back in the 50’s. The driver is mounted in a completely sealed, airtight enclosure, generally with the front of the driver facing outward but is not restricted to this method only. The system is designed to achieve a desired Q factor which controls the response characteristics of the driver/enclosure combination and usually ranging between 0.5 and 1.5. The total system Q (also known as Qtc) is dependent upon: volume of enclosure, T/S parameters of driver and internal treatment compounds including insulation, stuffing or wall-deadening materials. Best suited for drivers with an EBP of 50.0 or lower and drivers with Qts values above 0.40 but is not restricted to these exact alignments. Cutoff rate is about 12 dB/octave below f3, however higher system Q's result is a somewhat sharper roll-off while lower system Q's result in a slightly more shallow roll-off varying by about +/- 2 dB. Better damping and better transients is achieved by shooting for a lower system Q which can be accomplished by either making the enclosure larger or by adding stuffing/damping material such as polyfill, Dacron, fiberglass, acoustic foam, or any other suitable type of fill material. Box stuffing will also affect f3 by either raising it or lowering it depending upon the type and amount of stuffing used.

Advantages
Sealed enclosures are the simplest of all designs. They are easy to model with speaker software and it's easy to achieve predicted results. Box size and shape are generally the least complex. Great for both beginning and advanced DIY’ers. The exact response characteristics which are desired can be achieved by simply designing for a particular Qtc. You can alter its performance by varying the size of the enclosure and the amount of stuffing material used. Qtc ranges from 0.5 to 1.5 while 0.7 is generally considered ideal however some drivers may perform better (i.e., sound better) with Qtc values less than 0.7, especially where the driver dictates a smaller-than-optimal box at Qtc=0.7.. Slowest of all cutoff rates below fB of 12 db/octave. This results in much quicker group delay responses ranging from 1ms – 10 ms. Fast, quick, natural, smooth, tight, accurate, controlled and warm are some common terms that audiophiles use to describe sealed enclosures. Transient response is the best of all enclosure types. The excursion of the driver increases as the frequency applied decreases until fB is reached after which the driver excursion begins to decrease once again. There's no need for subsonic filtering due to the enclosure’s natural tendency to inhibit extremely low frequencies. This results in less bottoming out of drivers at subsonic frequencies. However as the enclosure size gets larger, more Xmax is required for any given input power. More extended low frequency response than vented enclosures given the same f3 for both. Phase shift is minimal within its normal operating frequency range.

Disadvantages
Very low frequency output is minimal. The f3 also know as (3dB down poin) is usually fairly high, above 30 Hz in most applications. Less power efficiency by about –3 dB as compared with vented enclosures. Lower over SPL capabilities. There's a strong need for drivers with a very large Xmax in order to ensure safe operation at least down to fB, especially if the box is designed for Qtc values < 0.7 Any enclosure volume that is modeled with the system Q larger than 0.707 results in higher f3. Lowest f3 is achievable only under an ideal Q = .707 alignment which may require unusually large and sometimes unacceptable enclosure volumes.

Best Applications
Best suited where a completely uncompromised sound quality is desired. Best for classical music and most rock and pop type music. Most widely used in car stereo systems where cabin gain can make up for its lack of low end <30Hz bass. Where size is an issue. Sealed boxes can be half the size of vented boxes yet can be made even smaller. May also be use for small to moderately sized Home Theaters. Usually is the easiest box to pass SAF (spouse acceptance factor). You should also go with sealed when the driver's T/S parameters dictate that the driver should be housed in a sealed enclosure due to a high Qts (above 0.4) or an EPB of 50 or lower.

Ported - 4th Order Vented
Basic Theory

Also known as bass-reflex or ported. The driver is mounted into an enclosure which houses a large opening, port, vent or slot that extends into the cabinet a specified length. The length and size of this vent are extremely critical to the proper function of this enclosure. The port and driver contribute together to provide the desired response characteristic. The driver is generally mounted with the front facing outwards, but is not restricted to this method only. The vent which extends into the cabinet tunes the enclosure to a specific frequency (known as fB) thereby acting as a high pass filter on the driver. Driver excursion is at its minimum at fB where the vent then takes over and does most of the work. Cut-off rate below fB is 24 dB/octave but can be varied up or down 5-6 dB depending upon the exact tuning frequency and volume of the enclosure. There are various types of alignments that all fit into the ported 4th order category. Some common types are QB3, EBS, SBB4 and SC4. By varying the enclosure size and the tuning frequency, it is possible to achieve a variety of distinct low frequency performances from a single driver. The vent acts by damping the load produced by the driver above fB causing it to behave somewhat as if it were in a sealed enclosure. Best suited for drivers with an EBP near 100.0 or higher and Qts < 4.0 but is not restricted to these numbers only.
Advantages Extended low frequency response. 3 dB down points (f3) are capable of being near or even below 20 Hz. Increased power handling above fB due to reduced driver excursion at and while nearing fB. More efficient system. Generally 3dB increased output over sealed enclosures due to the combined output of driver and port. More overall SPL capabilities. Deep, powerful, full, loud, inspiring, incredible, and earth shattering are common terms associated with vented enclosures.
Disadvantages

Larger enclosure size. More difficult to accurately achieve predicted results. Misaligned enclosures can result in very poor bass quality. Very accurate T/S parameters of actual driver is required. Although sometimes you can get away with using manufacture’s specifications. Driver unloading or bottoming out below fB is very common. Xmax is reached easily below fB and may cause sever damage the the driver's suspension, voice coil or cone. This usually requires the need to install additional high pass filtering below fB. But is not a always a necessity as long as power levels and frequency content are kept within reason. Transient response is degraded, yielding typical group delay curves as high as 50 ms. Muggy, boomy, sluggish, one-note, slow, and inaccurate are common terms associated with vented enclosures. Port diameter must be large to avoid unwanted port noise, which in turn requires the port to be long for any given Fb, which then drives up the volume of the enclosure, sometimes to undesirably large proportions.
Best Applications Where the deepest and loudest bass is necessary. Where size is not a huge issue but may still be a definite factor. For Home Theater and music. May be best suited for sound reinforcement, theater, live performances, DJ and other situations where lots of loud deep bass is needed and transient response is less critical.

Bandpass - Dual Chamber Vented/Sealed 4th Order
Basic Theory

The front and the rear of the driver are housed in their own separate and distinct enclosures. The front of the driver is in a ported enclosure while the rear of the driver is in a sealed enclosure. The driver may be mounted the other way around however as long as one chamber is sealed while the other is vented. The enclosure is designed as a sealed enclosure but with the addition of an acoustic filter (the port) in series with the front of the driver that acts to limit the driver's bandwidth and therefore increase its SPL capabilities within its bandwidth.
Advantages Very low f3 is possible at the expense of lower efficiency and increased ripple. Extremely high SPL is also possible at the expense of a higher f3 and narrower bandwidth. Less overall driver excursion. More control over cone movement. Bandwidth and efficiency are inversely proportional.
Disadvantages Combined volume of both chambers results in large overall enclosures. Difficult to design properly. Results may vary substantially due to misalignment of both front and rear chambers as well as tuning frequency. Tend to have "one-note" bass, especially if designed or built poorly. In order to achieve a wide useable bandwidth, there must be some amount of mid-band ripple as well as decreased efficiency. Drivers can be easily blown due to high compression factors because of lowered cone motion and thereby exceeding the thermal limits of the driver before exceeding its mechanical limits. Bandwidth and efficiency are inversely proportional.
Best Applications Where the large size of enclosure is of little concern. In cars where the design calls for high SPL where the limited bandwidth which results can be increased due to cabin gain. The cabin gain will help achieve a flatter and wider bandwidth across the desired range while maintaining the increased SPL of the enclosure. Very popular in car applications for this reason.

lincolntc690
04-21-2009, 09:29 PM
Bandpass - Dual Chamber Vented 6th Order
Basic Theory


The front and rear of the driver are mounted in separate enclosures and tuned to specific calculated figures. Resultant output is suppose to be better than any of the other designs mentioned previously. Bose owns the rights to the exact details behind this design. They explain the theory like this, "The low-frequency speaker drivers are located between separate acoustic compression chambers inside a patented Bose Acoustimass module. As each speaker cone moves, it excites air in the chambers. Trapped in the chamber, this air acts as an acoustic spring, which interacts with the air in the port to produce more low-frequency sound with less power. The system is more efficient and requires less cone motion, which in turn produces less distortion. In the event that any otherwise audible distortion is produced, the patented design traps it inside the acoustic chambers -- so it never enters the room. The result is an Acoustimass module with no audible distortion that can be located anywhere in the listening area." www.bose.com
Advantages


More efficient system within its bandpass. More control over cone movement. Less audible distortion. This doesn't mean that there is a true reduction in distortion, but that any distortion that is present form the driver can't be heard as well due to the chambers acting as filters on any unwanted noise.
Disadvantages


Combined volume of both chambers may result in large overall enclosures. Very difficult to design properly. You may have to experiment for months before getting this design to sound acceptable. Results may vary substantially due to misalignment of both front and rear chambers as well as tuning frequency of each chamber. Drivers can be easily blown due to high compression factors because of lowered cone motion and thereby exceeding the thermal limits of the driver before exceeding its mechanical limits. The driver may in fact tear itself to pieces. There are no exact parameters or calculations for designing 6th order bandpass enclosures due to the patent owned by Bose. So if you build one, you're basically on your own.
Best Applications


For more information on 6th order bandpass enclosures please visit www.decware.com

EBS - 4th Order Large Vented Enclosure with Low Tuning
Basic Theory


EBS- Extended Bass Shelf. This is only one of the various different types of vented alignments which are possible and follows many of the same characteristics of vented enclosures. The idea is to intentionally design the enclosure to be 125-175&#37; larger than the optimal calculated volume and then tune the enclosure much lower than optimal as well. The result is a significant amount of extended low frequency response. When the response curve is simulated, a visible "shelf" can be seen in the curve just above the tuning frequency before it sharply rolls off. The LEAP manual explains EBS theory like this: "The name [EBS] was derived simply from the visible appearance of the response curve. The bass response is extended to a lower frequency than would be possible from the QB3 alignment, but at a lower level or shelf relative to the mid band level. Although the EBS alignment is not a nice neat flat alignment such as the QB3, it is very often a much better choice than the QB3. The EBS alignment has some interesting features. Consider a loudspeaker with a Qts of 0.30, the QB3 alignment would have about 2dB more output at a frequency of twice the Fs, while the EBS alignment would have over 2db more output at Fs. In most cases the EBS alignments will have far more subjective [low] bass than the QB3 alignments. Also, if you were to equalize the responses flat to Fs, 10db more boost would be required for the QB3 versus the EBS. This can dramatically consume large amounts of headroom in the power amplifier, and may also far exceed the linear excursion limits of the speaker. The EBS alignment will maintain much lower cone excursion at frequencies near Fb than is possible with the QB3 alignment. This can be very important for high power systems."
Advantages


Extended low frequency response down into the teens. Subsonic, earth-shattering bass response. Increased efficiency at the lower frequencies (below 25 Hz) but decreased efficiency at higher frequencies (above 30 Hz). This is a rough figure since many different combinations can be designed to yield specific results. In general, low frequency is extended and efficiency increased at the expense of reduced efficiency at higher frequencies.
Disadvantages


Cut-off rate can be as high 36 dB/octave below fB. Transient response is degraded as a result of this. However it may be argued that because the tuning frequency is so low, that is it far enough out of normal operating range that it may be considered a negligible downside. The enclosure size is huge. Anywhere from 5-15 cubic feet depending on the size of the driver being used. Power handling capability of driver is reduced anywhere from 25-50%. Driver may reach Xmax sooner above fB even if it never reaches Xmax right at or below fB. Lack of real presence. Lack of kick or punch. The overall impact of the bass is much softer. Signals between 40 and 60 Hz are significantly reduced. Harder to "sell" because most people are more receiving to a pronounced upper bass response rather than an incredibly low and deep bass response . It takes 8 times as much power (as well as moving air) to make 20 Hz sound as loud as 40 Hz.
Best Applications


Where the truly deepest of all heavenly deep bass is desired. For drivers with a large Xmax and the ability to consume large amounts of power. For drivers whose T/S parameters dictate an optimal enclosure size that's smaller than what the designer wants to build. Large Home Theaters and varying kinds of music with heavy bass tracks would take the best advantage of this enclosure alignment.

PR - Passive Radiator
Basic Theory

A passive radiator is used in conjunction with an active driver and its purpose is to replace the port or vent of a typical 4th order enclosure. A passive radiator is sometimes referred to as a drone cone. A passive radiator (PR) enclosure is most similar to a vented enclosure in that they acoustically behave very much the same. Response characteristics include: there is a notch at the Fp of the passive radiator (resonant frequency of PR) and the typical cut-off rate below fB is 36 dB/octave. The resonant frequency of the PR is intentionally altered by the designer in order to achieve the proper fB of the enclosure. In other words, it is used to tune the box to the desired frequency and for optimum performance. This is done my adding or removing calculated amounts of mass from the cone of the PR. More mass = lower tuning. Less mass = higher tuning. The increases mass also helps to lower the Fp of the PR which in turn moves the undesired notch out of the passband resulting in improved transient response. In theory, the Vd (volume displaced by PR) should be at least 2 times the Vd of the driver it is being used with. Yet in practice, a good rule of thumb to go by is to have anywhere from 3-4 times as much displacement in the PR. This is to ensure the longevity of the PR's by preventing excessive continual over-excursion.
Advantages

Simplicity of tuning. By merely adding and removing small amounts of mass (usually measured in grams) the tuning frequency of the enclosure may be changed up or down by as much as 15 Hz or as little as 0.1 Hz. Precision tuning is very possible. Ability to tune small enclosures to very low frequencies without the loss of volume due to internal ports taking up enormous amounts of precious space. No port noise or any kind of air- turbulence mach levels to worry about. Pipe resonances and port standing waves are non-existent because there are no ports or vents in this system. Better driver stability below fB due to increased damping on the driver below fB. This is because of specific compliance characteristics of the PR which help to keep the driver under better control at subsonic frequencies.
Disadvantages Steepest of all cut-off rates with a roll-off 36 dB/octave below fB. Given the same size volume and tuning frequency of its vented counterpart, the PR alignment would result in a slightly higher f3. The expense of of PR's can be quite high when compared to a simple piece of PVC pipe. Especially when 2 or more PR's is needed, which is usually the case when using high-power high-excursion drivers.
Best Applications Best suited just about anywhere a regular vented enclosure would be used. May be used in applications where a smaller box is desired while wanting to maintain a 4th order alignment. The PR alignment can be made half the volume of the same vented alignment and still be designed to have the same tuning frequency. Though this theory has been argued.

lincolntc690
04-21-2009, 09:30 PM
TL - Transmission Line
Basic Theory


The driver is mounted in a type of acoustical labyrinth or long pipe commonly referred to as a transmission line. The length of this transmission line is usually somewhere between 4-8 feet and is dependent upon the Fs of the driver and the fill material used in the labyrinth. This t-line may have a tapered effect or maintain the same cross-sectional area throughout its length and may also contain various folds which help reduce the overall size of the enclosure. The length of the transmission line corresponds to the 1/4 wavelength of the resonant frequency of the driver. The t-line is almost always filled with various types of stuffing material which help reduce the speed of sound through the t-line allowing shorter line lengths while still achieving the proper tuning. For example, a transmission line for a given driver with an Fs of 25 Hz without any fill would need to be nearly three times longer than the same driver in a transmission line that was damped with 8kg/m^3 of wool.
Advantages Transient response is considered equal to (and sometimes better than) a 2nd order sealed enclosure and is considered far superior to that of a vented enclosure. The cut-off rate is somewhat shallower than a typical sealed enclosure and may be as low as 10 dB/octave or lower. This results in improved deep bass performance. Less upper bass coloration due to reduced impedance peaks. A more pure, cleaner, and deeper bass.
Disadvantages


Difficult to design as well as build. A transmission line is every carpenter's nightmare. Extensive knowledge of wood working is required. Not all drivers will work well for a TL enclosures, yet there is no specific guideline for which drivers may work well. There are no concrete methods or formulas for designing TL's. It's mostly trial and error. Not recommended for the novice DIY'er. Enclosure size may be very large.
Best Applications


Where you've got a ton of room, a ton of time, and a ton of patience. TL's will work well basically wherever any of the other alignments would work. It's unique performance characteristics make it suitable for even the most serious audiophile.

Isobaric - Dual Drivers

Basic Theory

Two drivers are mounted together in an enclosure with a cavity of air between the two drivers. The drivers must operate in phase with each other. The cavity of air between the drivers should be made as small as possible without compromising the operation of either driver. The modeling for this type of enclosure is done just as you would any other speaker enclosure except you take the Vas of the driver and divide by 2. This will in effect make all your speaker enclosures half as big as they would normally be for any particular driver.
Advantages Improved sonic bass response. Bass is claimed as being tighter, faster, more accurate and more pure. Vas of the driver is cut in half. The volume of enclosure required to obtain a specific frequency response can be achieved in only half the volume. This is where isobaric enclosures have their biggest advantage.
Disadvantages Wasted amplifier power to driver the internal sub. Efficiency of the system is down 3 dB as compared to a single driver due to the added cone mass and the reduced Vas. When you compare isobarics to a system which houses two drivers each in their own enclosure, this system would actually be 6 dB less efficient.
Best Applications Where size is a big issue. When you want the box to be very small. Where more accurate bass is more important than lots of bass. If you have a hefty amplifier with plenty of juice to spare and a driver that can handle a good amount of power. Suited for music, home theater and car.

Compound ~ Push/Pull Dual Drivers
Basic Theory

Two drivers share an acoustic volume of air within a single enclosure. The best way to take advantage of this alignment is to mount one driver facing outwards with the other driver inverted and facing inwards. The drivers are then wired so that they are electrically out of phase while remaining mechanically still in phase with one another. Odd ordered harmonics are cancelled out by using this approach according to Vance Dickason. According to M&K who specialize is push/pull subwoofers claim that this approach cancels out even ordered harmonics. So take your pick. Either way, harmonic distortion is reduced in that any anomalies or variations in the two driver's spider, cone or suspension characteristics are canceled out by the other driver's inversely proportional anomalies and variations. The sound is as accurate and pure as it can possibly be with each driver "correcting" the other driver. Of course many times two drivers will share the same acoustic volume of air while maintaining the more traditional look of having both drivers fire forward into the listening environment. Though this does not have the same harmonic cancellation effect, all other characteristics between the two alignments is identical. Box volume must be twice that of a single driver. This can be easily modeled by taking the Vas of a single driver and multiplying it by two. The system has an increased efficiency of 3dB over a single driver. Power handling for the system is twice that of single driver. Frequency response is the same for a single driver in an enclosure excaly half the size.
Advantages Increased output and power handling. Very high SPL capability.
Disadvantages One single huge speaker enclosure that may be both unattractive and hard to move. Response it essentially identical to building two smaller enclosures of exactly half the size but without the versatility of placement of two separate subs. If one of the drivers blows due to too much power, the whole speaker will no longer perform because the blown driver will begin to act like a poorly-tuned passive radiator. There are no real disadvantages to building this kind of enclosure as the speakers will behave just as they would in enclosures by themselves. It's very common to make MMT style speakers and use the two drivers in the same enclosure.
Best Applications

Where one sub just isn't enough. High power high output applications. If you choose to do the push/pull configuration, the sonic advantage may make this sub more suitable for audiophile music and critical listening experiences.

lincolntc690
04-21-2009, 09:30 PM
long but well worth the read

ascitiesburn69
04-21-2009, 09:34 PM
Nice read, lots of good info there

brian84corvette
04-21-2009, 09:52 PM
super good T line info.

bose needs to let the 6th order out of the bag, seems impossible to calculate as of now.

lincolntc690
04-21-2009, 09:57 PM
they'd be out of alot of money if they did :D

lincolntc690
04-22-2009, 05:01 AM
up for the morning crew