Zaph|Audio - SB Acoustics 2-Way : Box Design

Box Design Considerations

Box design is generally the first consideration. The reason is that the baffle dimensions must be known before any layout or other work can commence. At minimum, the rough volume is needed even if a box kit is to be selected.

There are several considerations used in selecting the box, including aesthetics. The latter often has more influence than many people may be willing to admit. In this case, maximal system response quality headed the list with selection of a widely available kit box a close second.

The factors to take into account are listed below, not necessarily in order of importance. We'll examine the them and how they influence the design process.

• Box Type (Closed, Vented, Passive Radiator)
• Bass Extension
• Box Volume and Dimensions
• Stand Mount vs. Floor Standing
• Baffle Dimensions
• Complexity of Construction
• Cost
• Aesthetics

Everyone wants bass extension. The problem is that there is a limit to it for any given driver beyond its free-air frequency response. This limit is largely dictated by the Thiele/Small parameters that are inherently a part of the driver with some control retained through selection of some of the other factors listed above. In addition, adding extension also has drawbacks due to the limits of driver displacement. We'll examine all of this and how it may be used to decide on the box and tuning.

The best way to select the box requirements is to use a software design program made for this purpose. Some excellent free software is available for this and can be easier to use than some of what comes with proprietary software. We'll be using the freely available Excel spreadsheet called Unibox. Another possibility is a newer spreadsheet by Jeff Bagby called the Woofer Box and Circuit Designer. Both require Microsoft Excel.

Where to Begin - Frequency Response

The key to designing the box for a 2-way starts with the midwoofer. In this case, the midwoofer choice has already been made. The T/S parameters mentioned previously are all entered into the software, then analyses of various box configurations can be made. For this SB design, the T/S parameters supplied by the vendor are to be used. Given the multitude of possibilities, the range of box tunings makes it impractical to show much other than the final settings. The versions tested and presented later for the closed box and passive radiator boxes are shown below as well as for a ported variant not tested. We'll see why a 22L box was chosen for the volume as a result of this exercise. The graphs and comparisons here were run initially using 20L when generating these graphs, but the change is small moving to 22L. In addition, the box is to be stuffed about 50%, so apparent volume will be different again.

The results can be compared as overlays within Unibox and are shown below, beginning with the frequency response. These results indicate that a ported box would have the most extension, this for a 2" diameter port approximately 8" long. While that can be realized, it does make for a rather long port. In addition, after construction, the tuning cannot easily be changed, one advantage of a passive radiator.

A port has other issues. It has a self-resonance based on its dimensions. You'll see this in the frequency response curves around 765Hz. A port also is more prone to noise (reflections/resonances) exiting from inside the box through the port. It makes the location on the box more critical. The location of the inside of the port opening can also have an influence, though that magnitude of that is not easy to determine. One last concern is that a port reduces the internal volume of the box by the volume of the port, since it is usually mounted inside of the box.

The passive radiator also has issues. It usually has a bit of loss, so the extension is not quite as good as a port. It also has a self-resonance associated with the surround. This causes a rolloff to initially appear to be 6th order, even though the asymptote of the system response is 4th order, the same as it is for a port. As an aside, a passive radiator is the full implementation of a bass-reflex system while a port is a special case, having no suspension (spider and surround).

Advantages of a passive radiator are that it can be re-tuned at any time (within limits of added weight) and it tends to block spurious sounds (reflections/resonances) from exiting. PR systems have often been characterized as sounding different than ported ones, certainly a purely subjective appraisal. That may have as much to do with ported systems that have often been tuned too high, either by intent or by limitations of port dimension requirements.

Excursion

When we look at the excursion curves, we'll see another set of issues. Notice that this midwoofer is displacement limited below 100Hz for everything but a bandpass that we won't examine. Above 100Hz it is power limited. That is, if the rated power is applied, below 100Hz it will be driven beyond its rated displacement while above 100Hz it will remain below that displacement. In essence, its safe operating range as a midwoofer in a 2-way is only valid down to 100Hz at higher powers. This is not unusual, as woofers and midwoofers tend to be displacement limited, its a matter of determining that and designing around it, if possible.

The interesting point to notice is that all three standard arrangements, closed, ported and passiver radiator boxes, all show the same transition point. That is, all of them show displacement limitation below 100Hz. For a 2-way, the maximum output as a system is often limited by the midwoofer displacement. Tradeoffs must be made and accepted.

The limits at the lower end make a port appear to be best, but notice as well that above about 55Hz, all three boxes are nearly identical in displacement. They diverge below that, but all three are already well above the rated displacement for rated power, anyway. However, if when the power is applied to that which would be within the limits at the point where the closed box continued to increase, the port and passive radiator systems are both held within the rated displacement down to essentially the same point, about 29Hz. The closed box has continued to get worse.

There's one last item. The output of the port and passive radiator diverge at about 27Hz, with the displacement of the port increasing much faster than for the passive radiator. The latter will provide more protection at sub-30Hz levels in essence, though neither one is as good as the closed box in this regard. The ported and passive radiator systems are both going to allow for a higher system power output for all music signals above 29Hz. This is a real bonus for music, not so for movie soundtracks, but a 2-way isn't really much good for that, anyway.

Conclusion on Box and Tunings

The box size selected allows usage either as a closed box or passive radiator, a somewhat unusual situation made possible by the specifics of the T/S parameters of the SB17 midwoofer. The port length required is usable, but has not been implemented nor tested. In addition, if the passive radiator is used, it may easily be reverted to a closed box by covering the radiator or removing it and covering the opening. Fully stuffing the box would then be required, but there would be no port to interfere.

The Other Factors

This system is intended to be stand-mounted, away from the back wall. There will be some bass reinforcement if placed close to a wall and the fact that it will be either a closed box or passive radiator makes that a bit more flexible since the PR radiation is from a large surface and should not be as sensitive to proximity to a back wall. Ports need a bit more "breathing room", so-to-speak.

Being stand mounted also allows for more flexibility in box dimensions. The only two requirements other than volume are to have sufficient front baffle area for the drivers and one surface sufficient for the passive radiator. Ports can be more limiting due to lengths that may be required.

Three options for the boxes exist, build the box from scratch, purchase the front baffle from Parts Express (details later) or purchase a full box kit from Parts Express. The cost difference is relatively significant, but for those without box skills, the full box kit purchase is a viable option. That said, you'll still have to cut out the openings for the passive radiator and/or drivers.

David L. Ralph © 2009