Bandpass Enclosures - The MAGIC BOX?
Bandpass enclosures have gained popularity over the last couple of years. Some people seem to think that bandpass enclosures are the "magic" enclosure to solve all problems. Bandpass enclosures like all the other enclosure designs have both benefits and disadvantages. Understanding the performance characteristics of the bandpass enclosure will help you choose if a bandpass enclosure is for you.
Bandpass enclosures differ from the rest of the enclosures in that all of the acoustical output is radiated from a vent or port. There are three different types of bandpass enclosures: sealed, vented and series vented. In general bandpass enclosures have improved mechanical power handling over other types of enclosures.
In general, bandpass enclosures are more difficult to construct than conventional enclosures. Minor deviations in the enclosure volumes and tuning frequency have a huge effect on the performance of the enclosure. When constructing bandpass enclosures, special care is needed to make sure your calculations are correct.
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topWhat bandpass enclosures can do
Bandpass enclosures can be optimized in two different fashions. One way is to maximize the output is a narrow frequency range. In this situation, the woofer will play louder I this bandwidth that the earlier enclosures described but will not have a wide of bandwidth. Another design of a bandpass can maximize the bandwidth of a woofer. The size of the enclosure will affect the output efficiency of the enclosure. There are woofers that are not well suited to some or all types of bandpass enclosures and these woofers will offer superior performance with conventional enclosures regardless of enclosure volume and tuning frequency.
Sealed Bandpass
Sealed bandpass enclosures are sealed enclosures with the output filtered though a secondary tuned enclosure. Figure 7 From a mechanical standpoint, the sealed bandpass enclosure performs similar to a conventional sealed enclosure. The primary differences between the two different types of enclosures are is the reduced excursion of the woofer at the tuned frequency of the vented chamber and the increased efficiency if the enclosure volume is large enough.
Contrary to popular belief, the low frequency output is determined by the combination of the sealed and vented enclosure volumes. Simply making the enclosure larger will not necessarily improve the performance. The port tuning primarily affects the upper bandwidth efficiency of the low frequency system. It also affects the bandwidth depending on the interaction between the enclosure, woofer and the tuned vent.
Of all bandpass enclosures, the sealed bandpass enclosures are the easiest to construct. To maximize the performance of a bandpass design, we strongly recommend a computer aided design program. Additionally, experimentation is needed on these enclosures because port and woofer placement in the interior of the enclosures has a great effect on the performance of the enclosure design that cannot be modeled by commercially available computer software.
Designing Sealed Bandpass Enclosures
If you are looking for general enclosure recommendations, the sealed bandpass is useful for output in the 40 Hz to 90 Hz region with a reasonable amount of gain. Again, not all woofers are suited for this application. Remember it is not just the woofer or just the enclosure, but the combination of all factors that determines performance. For woofers that are suited for this application, a good starting point is to build the sealed section of the enclosure to a Qtc between 1.1 to .707. Generally "Q" values outside this region yield bandwidth frequency response irregularities that can degrade sound quality. The vented side of the enclosure can vary between an equal volume of the sealed chamber up to twice the volume. The reason for the variance in the vented side of the enclosure will depend on overall output efficiency desired, and tuning ability. The tuning of the vent can vary between 60 Hz to 75 Hz depending on all the other factors concerned. The larger the vent the better. To small of a vent will severely limit the output capability as well as cause audible vent noise. The port area should not exceed the surface diameter of the woofer/s.
Vented Bandpass Enclosures
Vented bandpass enclosures tune both chambers for a potential increase in output capability. Figure 8 The word potential is used because this is very woofer dependent. If the woofer is not suited for this application. The trade-off for this enclosure design is a decrease in bandwidth when compared to other enclosure types.
Enclosure construction is more difficult than other enclosure due to the nature of the enclosure. Minor changes in the dimensions or vent tuning can make or break the performance. While longhand formulas exist for vented bandpass enclosure design, we strongly recommend the use of a computer aided design program. Placement of both the woofer and the vents are critical to get the performance predicted by computer aided design programs or longhand calculations.
Series Vented Bandpass Enclosures
Series vented bandpass enclosures are a hybrid design. Figure 9 Like a sealed bandpass all the output if filtered through a vent. Like a vented bandpass, both chambers are tuned for maximizing the bandwidth limits of the enclosure design.
Things to consider
Series bandpass enclosure differ from the two previous bandpass enclosures in that the low frequency limit of the woofer can be excursion limited by the tuned port that is vented to the outside of the enclosure. This is very dependent on the tuning frequency the enclosure volume and the vent area relative to the radiating area of the woofer or woofers used in the enclosure. As before, we strongly recommend the use of a computer aided design program. Placement of both the woofer and the vents are critical to get the performance predicted by computer aided design programs or longhand calculations.
Maximizing Performance
Maximizing the performance of a bandpass enclosure is not as easy as the other enclosures. The interaction of the woofer placed inside of the enclosure and the tuned vent cannot be easily determined before enclosure construction The simpler the bandpass design (use of multiple woofers and vents in a shared volume is not simple) the more consistent the results. Listed below are some suggestions to maximize the performance.
Seal the enclosure: Any loss in the enclosure is a reduction in output. This is both true for both the woofer mounting and the hole for the speaker wire.
Mount the woofer to a solid baffle: The less the enclosure flexes, the greater transfer of mechanical energy.
Mount woofers on same plane: When using multiple woofers, mount all woofers on the same plane (side) in the enclosure. Failure to due this can cause a reduction in output efficiency
Prevent port from interfering with woofer: When possible, mount the so that the port does not interfere with the output of the woofer. Although alternative placement can work, this will ensure trouble-free performance
Keep the bandpass enclosure simple: Separate chambers for woofers is more consistent that single chambers for the overall enclosure. Combinations with single woofers and multiple ports or single ports with multiple woofers will usually not produce problems. Multiple ports and multiple woofers sharing airspace can be difficult at best, destructive at worst
Do not block the port air flow: Leave a minimum distance of the diameter of the port away from any obstructions that can block the airflow
Do not build a perfect cube enclosure: This will alter the frequency response of the woofer due to standing waves created inside the enclosure. For rectangular shaped enclosures, the Golden Ratio between the dimensions is 0.8 to 1.0 to 1.2
Angle the enclosure: When ever possible, put an angle in the enclosure. This will reduce the possibility of destructive interference in the enclosure
Brace the enclosure: Bracing the enclosure increases the mechanical transfer of energy and reduces the flexing of the enclosure with can reduce the efficiency of the system.
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