Box Design Tool

The box design tool lets you see the frequency response of your driver in the enclosure of your choice.  You can compare different drivers or different enclosure designs by displaying multiple curves on the same graph.

Start by entering the T/S parameters (fs, Qts and Vas) of your driver.  Now you can experiment with various enclosure types and box volumes.  The results can be viewed in the frequency response window.

Hint: white data boxes are for user input, and blue data boxes show calculated results.

Subtopics:

Data Set

New Data Set

Delete Data Set

Data Description

Fs

Qts

Vas

Cu. Feet

Liters

Enclosure Type

Isobarik

Box Volume

Car Response

Update Plot

Fb (port frequency)

Box Volume for QB3

Recommended Fb (port frequency)

Ported Fraction of Box Volume (%)

Normalize plot to 0 dB

Sensitivity (dB)

Metric


Data Set

A set of woofer and enclosure parameters corresponding to a single curve on the frequency response plot.  Use the up and down buttons to select the data set you want to edit.

Note: to create a new data set, use the New Data Set button.


New Data Set

Creates a new data set.  (For convenience, woofer and enclosure parameters are copied from the active data set.)

A new data set is needed for each curve on the graph.  Multiple curves make it easy to compare different enclosure designs or compare different woofers.


Delete Data Set

Deletes the active data set (and its corresponding curve on the graph).

Note: the remaining data sets are renumbered to eliminate gaps.


Data Description

User supplied description for each data set.  These are used to label the curves on the plot.

For convenience, data descriptions are preassigned "New Data 1", "New Data 2" and so on.  These may be edited to any description up to 30 characters long.


Fs

Fs is the resonant frequency of the woofer.  It is one of three Thiele/Small parameters required to design a box.  This parameter can be found on the spec sheet from the manufacturer.

see also: Qts, Vas


Qts

Qts is the total Q at the resonant frequency of the woofer.  It is one of three Thiele/Small parameters required to design a box.  This parameter can be found on the spec sheet from the manufacturer.

Avoid drivers with unusually high Qts values - anything above 0.8 is too high.  High values are caused by undersized magnet structures and usually result in frequency responses with a large resonant peak.

see also: fs, Vas


Vas

Vas is the volume of air having the same compliance as the woofer.  It is one of three Thiele/Small parameters required to design a box.  This parameter can be found on the spec sheet from the manufacturer.

Note: this parameter may be specified by the manufacturer in cubic feet or liters.

see also: fs, Qts


Cu. Feet

Select this unit of measure if Vas is specified in cubic feet.


Liters

Select this unit of measure if Vas is specified in liters.


Enclosure Type

Choose from Sealed, Ported, Bandpass or Free-air.


Isobarik

Isobarik configurations replace each driver with two face-to-face mounted

drivers.

Isobarik (which means constant pressure) designs were first described by Harry Olson in the early 1950's.  The primary benefit of any isobarik configuration is that it allows you to cut the size of your enclosure in half with respect to that of a single driver.  This can be valuable if you are trying to squeeze a subwoofer box into a tight space.  It comes at the price of having to buy a second driver and power it.  The acoustic sensitivity of an isobarik pair is also 3 dB lower than that of a single driver (due to the doubling of the effective cone mass).

Of all the isobaric configurations, the front-to-front is the most useful for subwoofer applications.  (The back of a driver is as good as the front for deep bass.)  In addition to cutting the enclosure size in half, it also provides even-order distortion cancellation.

Isobarik loading can be used with sealed, ported or bandpass designs, although in practice, bandpass isobarik designs can be particularly difficult to get right.

When mounting the drivers, a spacer must normally be used between the two drivers to prevent the two surrounds from touching each other. You can use a ring of medium density fiberboard (MDF) with appropriately spaced holes to pass the mounting bolts/screws through.  Lay the bottom driver in the box after wiring it up (this driver should have its leads wired normally). Lay the MDF ring on top of this driver.  Invert the second driver over the first, line up the mounting holes, and screw the whole assembly to the enclosure.  The outer driver should have its leads wired reversed.  This will assure that both drivers are moving in the same direction when a voltage is applied. If you hook everything up and get no bass from your subwoofer, chances are that you've got a driver's polarity reversed.


Box Volume

The volume of air inside the enclosure, measured in cubic feet or liters.  Select your choice of units with the Metric checkbox at the bottom of the window.

For bandpass boxes, this number is the total of both sealed and ported chamber volumes.

Note: when calculating the volume of an enclosure, be sure to use the internal (not external) dimensions.  Subtract the volume of items which reduce the internal volume, such as bracing and the back of the woofer itself.  For ported boxes the volume of the port should be deducted too.  These volume adjustments normally amount to about a 5% reduction of the internal box volume.


Update Plot

Updates all curves on the frequency response plot and brings the Frequency Response Window to the front.


Fb (port frequency)

Fb is the frequency that a ported or bandpass box is tuned to, via the port.

When using the Port Design tool, enter the value from the Box Design tool.

When using the Box Design tool for ported boxes, a recommended value is provided in the Recommended Fb box.  (Double-clicking the Recommended Fb value will copy it into the Fb box.)  You may also choose to experiment with other values.

When using the Box Design tool for bandpass boxes,  Fb is automatically optimized as you adjust the Ported Fraction of Box Volume.


Box Volume for QB3

QB3 stands for "Quasi Third-Order Butterworth" alignment.

Alignments are a classification of box size and tuning that provide a reasonably flat frequency response.  QB3 is the most commonly used ported alignment because it yields a smaller box and lower cutoff frequency for a given driver Qts.

Note: QB3 alignments exist only for drivers with Qts less than or equal to 0.50.


Recommended Fb (port frequency)

When using the Box Design tool for ported boxes, a recommended value for Fb is provided in the Recommended Fb box.  You may also choose to experiment with other values.

Double-clicking the Recommended Fb value will copy it into the Fb box.


Ported Fraction of Box Volume (%)

When using the Box Design tool for bandpass boxes, this is the percentage of the total box volume used by the ported chamber.

Typical values are between 25 and 50 percent.  High values produce high sensitivity, but narrow bandwidth and increased center frequency.  Bandwidths narrower than an octave (2:1 frequency ratio) produce an objectionable "one-note" type of response.


Normalize plot to 0 dB

When using the Box Design tool for bandpass boxes, this checkbox selects whether or not to normalize the frequency response curve to 0 dB at the bandpass center frequency.

With normalization off, 0 dB represents the sensitivity of the driver as specified by the manufacturer.  This setting is useful for seeing how the system sensitivity is affected by the box design.

With normalization on, the frequency response curve is shifted  to 0 dB at the center frequency.  (This is equivalent to readjusting the subwoofer amp gain for proper level matching.)  This setting is useful for seeing how  the system bandwidth is affected by the box design.

see also: Sensitivity (dB)


Sensitivity (dB)

When using the Box Design tool for bandpass boxes, Sensitivity (dB) shows the relative gain of the system with respect to the manufacturer's specified driver sensitivity.

This feature is of most importance when the Normalize plot to 0 dB is on.  (With Normalization off, the response curves show how system sensitivity is affected by box design.)


Metric

This checkbox selects either Metric or English units of measure.  The selection affects Box Design, Port Design and Woofer Selection tools.  The setting is stored as a user preference to eliminate the need to set it each time the program is started.

Note: units for Vas are not affected by this checkbox.


Port Design Tool

The port design tool lets you design single or multiple ports for ported or bandpass boxes.  This tool should be used after completing your design with the Box Design tool.

Ports are assumed to be cylindrical and flush-mounted in the cabinet wall.

Hint: white data boxes are for user input, and blue data boxes show calculated results.

Subtopics:

Ported Enclosure Volume

Number of Ports

Port Inner Diameter

Calculated Port Length

Metric


Ported Enclosure Volume

For ported designs, enter the box volume from the Box Design tool.

For bandpass designs, enter the volume of the ported chamber.  This is equal to the box volume (from the box design tool) multiplied by the ported fraction of box volume (from the Box Design tool).  For example, a box volume of 1.0 cubic feet and a ported fraction of box volume of 30% would yield a ported enclosure volume of 0.3 cubic feet.


Number of Ports

Normally, single ports are used.  Multiple ports can be used when the look of several smaller diameter ports is preferred to that of a single large port.

Unlike some other programs, the Subwoofer Design Toolbox uses the more accurate divided chamber method for performing multiple port calculations.

Note: when using multiple ports, all ports are assumed to be the same diameter and length.


Port Inner Diameter

The inner diameter of the port tube.  Ports are assumed to be cylindrical and flush-mounted in the cabinet wall.

To reduce port nonlinearity, use a port diameter at least one third the woofer diameter.


Calculated Port Length

The length of the port tube.  Ports are assumed to be cylindrical and flush-mounted in the cabinet wall.

The minimum practical port length is equal to the thickness of the cabinet wall.  Avoid long ports, as they can create audible wind noises and resonances.

Note: an elbow may be used to create a bend in a port where the port length exceeds the box depth.


Woofer Selection Tool

The woofer selection tool lets you quickly evaluate and compare different drivers without having to design an enclosure for them.

Unless you've already chosen a woofer, use this tool as the first step in your subwoofer design.  Start by entering the T/S parameters (fs, Qts and Vas) of a driver you are considering.  The woofer selection tool will provide you with useful performance and application information for that driver.  Compare this information against that from other drivers you're considering.

Hint: white data boxes are for user input, and blue data boxes show calculated results.

Subtopics:

Sealed & Ported Bass Response Figure of Merit

Estimated Free-Air Bass Response

This Driver is Best Suited for

Use Port Only if Box Volume Exceeds

Estimated Box Requirements

Metric

see also: Power Handling, Driver Sensitivity, Bass Response, Cone Material, Surround Material


Sealed & Ported Bass Response Figure of Merit

This number rates the bass response of the driver for sealed and ported applications.  The bass response figure of merit is based on how low the cutoff frequency would be in a reference enclosure.  This number is of lesser importance for bandpass applications.

A rating of zero (or a negative rating) indicates poor bass response.  A rating of 100 (or higher) indicates exceptional bass response.  Most drivers have a rating between 40 and 70.

Note: smaller drivers generally have a higher bass response figure of merit than larger drivers since they provide deeper bass in a small or normal-sized box.  Larger drivers generally offer increased power handling and the potential for deeper bass in very large enclosures.  If you're willing to use a very large box, use the Box Design Tool to compare woofers of different sizes.

Important: bass response, together with driver power handling and sensitivity should be used as the basis for choosing a woofer.  Consult the spec sheet from the manufacturer for these numbers.


Estimated Free-Air Bass Response

This is the estimated cutoff frequency of the driver in a free-air application.


This Driver is Best Suited for

This is the recommended enclosure type for the driver.  The possible results are:

Sealed

Sealed or Ported

Ported

Suitability for bandpass designs is best determined by using the Box Design tool.


Use Port Only if Box Volume Exceeds

Small boxes should always be sealed, and large boxes should generally be ported.  This is because porting helps large boxes, but hurts small ones.  A ported box should be considered only if the box volume exceeds the volume shown.

Sealed boxes offer other advantages over ported boxes, so you may decide to use a sealed box for a large enclosure anyway.  See Sealed enclosures for details.


Point Tracking

Point tracking allows you to determine the exact frequency and magnitude for any point on the frequency response plot.  This feature converts the cursor arrow to crosshairs when on the frequency response plot.  This feature is useful for determining the precise cutoff frequency of a response curve.

Note: the point tracking on/off setting is stored as a user preference to eliminate the need to set it each time the program is started.


Switch to design panel

This bar along the bottom of the frequency response window brings the design panel window to the front.

This is a particularly handy feature if the frequency response window is maximized or sized to cover most of the screen.


Print with Symbols

Print with symbols improves the readability of frequency response printouts with black and white printers.  For color printers, this option can be turned off.

This feature can be activated through the frequency response window File menu or through the print selection dialog box that appears with the main window print command.

Note: this setting is stored as a user preference to eliminate the need to set it each time the program is started.


Print

Print allows you to select the printer and printer properties as well as print range and number of copies.  Print is found in the frequency response and main window File menus.

Printing from the main window will activate the Print Selection dialog box, allowing you to select which tool results to print.  Printing from the Frequency Response Tool automatically prints only the Frequency Response Plot.

Data from the Box Design Tool, Port Design Tool, Enclosure Design Tool and Woofer Selection Tool are printed in tabular form.  The Frequency Response Plot and Cutting Template are printed graphically.

Note: the frequency response printout is sized to make full horizontal use of the paper (excluding the user specified margins in Page Setup).  The relative text size and the aspect ratio (width to height) of the plot can be manipulated by resizing the frequency response window on the computer screen to the desired appearance.


Free-air

Free-air subwoofer mounting uses the trunk as a large speaker enclosure.  The driver may be mounted on the rear deck or behind the back seat.

Assuming you have a trunk and a suitable mounting location, free-air mounting eliminates the need to build or buy a box.  The problem is that most trunks do not provide good sonic isolation.  This means that sound from the back of the speaker can be heard in the passenger compartment, canceling out deep bass.  To see this for yourself, have a friend with a deep voice talk into your open trunk, from the outside.  You should be listening inside the passenger compartment, with the windows rolled up.  If you can barely hear him, then free-air subwoofer mounting will work in your car.  Otherwise, choose one of the other enclosure types.


Sealed

Sealed boxes offer a number of advantages over the other enclosure types.  They are the easiest box type to design and build.  Sealed systems are the least sensitive to using a box which is a little too big or small, or to drivers which are different than their published specs.  This is a big benefit if you are planning to use component woofers in a prefab box.  The spring effect of air in a sealed box provides woofer protection against turn-on thumps, subsonic bass energy and going over bumps.

Sealed boxes provide the deepest bass if you must use a small enclosure.  The big drawback of sealed boxes is that they cannot provide bass as deep as ported or bandpass boxes for larger enclosures.  However, the low frequency rolloff characteristic of sealed systems is relatively shallow compared to ported and bandpass systems, resulting in useful deep bass below the "official" -3 dB cutoff frequency.


Ported

Ported enclosures make constructive use of the sound from the back of the speaker.  By doing so, it becomes possible to achieve deeper bass than with a sealed box.  In a ported system, the sound from the back of the speaker produces a resonant output at the port, much like producing a tone by blowing air across the mouth of a soda bottle.  The port is tuned to provide constructive reinforcement of bass at the proper frequency.

Ported systems are more difficult to design and build than sealed systems.  They are more sensitive than sealed systems to using a box which is a little too big or small and to drivers which are different than their published specs.  Ported boxes provide no woofer protection against turn-on thumps, subsonic bass energy and going over bumps.

The big plus of ported boxes is that they can provide bass at least an octave deeper than sealed boxes.  That's the difference between 80 Hz bass and 40 Hz bass.  To do this, however, requires a much bigger box - typically five times bigger to get that octave improvement.

A primary factor in whether to use a ported box or a sealed box is how big a box you can tolerate.  Small boxes should always be sealed, and large boxes should generally be ported.


Bandpass

Bandpass enclosures are based on ported enclosures, except that the direct sound from the speaker is blocked off using a secondary sealed chamber.  This means all you hear is the tuned output from the port.  Unlike the other enclosure types, bandpass boxes only pass sound over a narrow frequency band.  This reduces the requirement for a steep low-pass slope in a subwoofer crossover.  Like the tone from a soda bottle, however, bandpass designs run the risk of sounding like a "one note" system.

Bandpass systems are more difficult to design and build than either sealed or ported systems.  They are very sensitive to speaker parameters, box volumes of the sealed and ported sub-enclosures and port tuning.  Because of this, using component woofers in a "one size fits all" prefab bandpass box is insanity.

Bandpass systems produce deep bass which is comparable to equally sized ported systems.  Like ported systems, bandpass systems require big enclosures.  A unique property of bandpass systems is that they allow you to trade efficiency for bandwidth by changing the sealed fraction of the box.  Very efficient bandpass designs have inherently narrow bandwidths, resulting in a "one note thump monster."


Power Handling

How much power a subwoofer driver can handle is limited by two things - the diameter and maximum excursion of the cone and the power rating of the voice coil.  Either factor can be the weak link in power handling.

At low frequencies, a subwoofer has to move lots of air to create loud bass.  How much air it can move is proportional to the area of the cone multiplied by the maximum excursion of the cone.  Since area is proportional to cone diameter squared, large drivers have the potential to move a lot more air than small ones.  Cone excursion means how far out the cone can realistically move before distortion or damage to the speaker occurs.  This is shown as Xmax on subwoofer spec sheets.

Note: be careful when reading manufacturers' literature - some list Xmax as total excursion, others as one-way excursion, still others as one-way +15%!

Voice coil power handling depends on the diameter of the voice coil, the gauge of wire used and other factors affecting how much heat the voice coil assembly can handle and effectively dissipate.


Driver Sensitivity

Driver sensitivity tells you how loud a speaker will play at a specified power level.  This number is given in dB and is normally measured at one meter using 1 watt.  90 dB is a typical value, and higher numbers are better.

For example, a subwoofer with 90 dB driver sensitivity is 3 dB better than one with 87 dB.  This means that it is 3 dB louder - equivalently, it would require 3 dB less amplifier power to play at the same volume level.  The table below illustrates how driver sensitivity affects how much power you would need to produce the same volume level.

driver	required

sensitivity	amp power

87 dB	200 W

88 dB	158 W

89 dB	126 W

90 dB	100 W

91 dB	 79 W

92 dB	 63 W

93 dB	 50 W

The table shows that the 87 dB driver would require 200 watts of power to play as loud as the 90 dB driver using 100 watts.  This seems dramatic, but you may not achieve the big benefit you expect, since high sensitivity drivers often produce higher cutoff frequencies.  This means that the gains you pick up in sensitivity may be lost in having to boost the deep bass frequencies.

The bottom line is not to overemphasize the importance of driver sensitivity.  Consider it along with bass response and power handling to paint a more complete picture.


Bass Response

Bass response is just another way of saying "how low will it go."  The bass response of a subwoofer system depends on two things - the driver itself and its enclosure.  In general, the larger the enclosure you are willing to use, the lower the bass response will be for a properly designed sealed, ported or bandpass system.  For free-air applications, bass response depends only on the driver.

The most accurate way to compare the bass response potential of various drivers is to simulate the frequency response of each of them in your intended application.  This would entail a fair amount of work, not to mention the fact that you may not have even decided what size your intended enclosure should be.  An easier alternative is to use the Woofer Selection Tool.


Cone Material

Cone material is especially important in the harsh automotive environment, but it's hard to wade through the hype.  Polypropylene cones are extremely resistant to environmental deterioration, but are a temperature sensitive plastic, and become soft in the heat, and hard when cold.  Paper cones deteriorate with sunlight and moisture, so you must keep them out of the sun.  Coated paper is a good compromise.  Other materials such as graphite, Tri-Laminate, resin laminate, carbon blended poly, Kapok, poly-graphite, graphite-quartz, Foam-Infused  IMPP, Titanium Composite, fiberglass and Kevlar offer the promise of superior performance.  Don't expect a big improvement over coated paper.

see also: Surround Material


Surround Material

The surround is the soft ring around the outside of the cone with the bulge in it.  Foam deteriorates with sunlight - choose rubber if your subwoofer will be exposed to the sun.

see also: Cone Material


Dual Voice Coil Subs

A dual voice coil speaker is simply one in which two separate voice coil windings and sets of terminals are provided.  It's more expensive to wind and terminate dual voice coils but you typically pay only a small premium compared to a similar single voice coil speaker.

The main advantage of dual voice coil speakers is wiring flexibility. A single dual voice coil driver offers the user three hookup choices: parallel, series and independent. In a parallel hook-up the drivers impedance will be half that of each individual coil (a dual 4 ohm speaker would be a 2 ohm speaker in parallel).  A series hook-up results in twice the impedance of each single coil (a dual 4 ohm speaker results in 8 ohms if its coils are wired in series). Finally, you can wire each voice coil to a separate channel of your amplifier, which can be useful if your amplifier is not mono-bridgeable or if you are bridging a four channel amplifier down to two channels to run your sub.

If a dual voice coil subwoofer is wired to two independent channels, the inputs to both channels should ideally be the same (summed mono).  If there is any difference between the signals driving each coil, the voice coils effectively fight each other.  This results in wasted amplifier power, reduced headroom and increased distortion.

A common misconception with regard to dual voice coil speakers is the assumption that nothing changes if you use only one of the voice coils.  With only one coil hooked up, a dual voice coil speaker will suffer a loss in reference efficiency of about 3 dB as well as a significant shift in its Thiele/Small parameters (Qts will go way up).  This renders any enclosure calculations inaccurate unless you measure the speaker parameters with only one coil hooked up.  Failure to account for the different parameters of a dual voice coil speaker with only one coil powered can result in very poor performance.


How low do you need to go?

Deciding how low in frequency you want your subwoofer to go is usually a compromise between performance and practical box size.  It's easy to weigh the acceptability of a particular box size, but how do you weigh the importance of bass response?

FM radio broadcasts are limited to between 30 and 50 Hz.  With CD players, you can expect response down to 20 Hz or even lower.  This doesn't mean your music goes that low, it just means that your CD player can pass it if it does.

In spite of the fact that humans can hear (or feel) bass below 20 Hz, 30 Hz is the realistic lower limit of recorded music.  The truth is, very little recorded music (including CD's) contains much below about 40 Hz.  Between 40 and 60 Hz, kick drums, tympani, and the low incisive "chunk" of bass guitars kick in.  Between 60 and 100 Hz is where you find the power of drums and the deep, tight, strong bass that makes rock solid.


Multiple Drivers in One Box

Using two (or more) identical woofers in a single subwoofer cabinet can provide a number of advantages over single subwoofer boxes.  This approach can be used with sealed, ported or bandpass designs.

The standard configuration can be thought of as two identical subwoofer systems joined together, with the separating wall removed.  It thus doubles the size of your enclosure and the power handling as well.  Because the cone area is doubled, the acoustic sensitivity is 3 dB higher than that of a single driver.

For two drivers in one box, follow the same box design procedure as you would for a single driver, but assume that you are using a driver having a value of Vas which is twice that of an actual single driver.  This method will end up giving you a box which is twice the original size, and any port calculations will come out right.

see also: isobarik


Box Materials

A good speaker box should be both rigid and non-resonant.  Choosing the right box material is an important step in ensuring this.

Subwoofer boxes should be constructed of 3/4" thick particle board or MDF (medium density fiberboard).  Both of these materials are rigid, non-resonant, uniform, easy to work with and best of all, cheap.  MDF is approximately 35% stronger than particle board, but is more expensive and harder to find.  Other materials such as hardwood, plywood or OSB (oriented strand board) may be used, but none of them are as good as particle board or MDF.

If you want a cylindrical enclosure, use large cardboard tubing called Sonotube® (manufactured by Sunoco for forming concrete pillars).  It's available at construction supply outlets.  The cylindrical shape provides superior rigidity even though its wall thickness is only 1/4" to 1/2".  You can also use large diameter PVC pipe, available in diameters as large as 12" from pipe suppliers. Cut circles of particle board or MDF to fit inside the ends and fasten with Liquid Nails or similar construction adhesive.

see also: Box Shape


Box Shape

Rectangular boxes are the easiest to build, but to take advantage of an odd-sized space, you may want to use a wedge or other shape.  For subwoofers, the shape and relative dimensions of the box do not affect performance.  You can build a perfect cube, a tall and thin box or a pyramid - it doesn't matter, so long as the volume is the same.  (This is not true for full-range loudspeakers, where the shape influences midbass and higher frequency response.)

Note: when calculating the volume of an enclosure, be sure to use the internal (not external) dimensions.  Subtract the volume of items which reduce the internal volume, such as bracing and the back of the woofer itself.  For ported boxes the volume of the port should be deducted too.  These volume adjustments normally amount to about a 5% reduction of the internal box

volume.

see also: Box Materials


Box Assembly

The most popular method of box construction today is to build a six-sided box without removable access panels.  This provides maximum strength and makes the box more airtight.  Access to the inside of the box is provided through the speaker cutout.  The speaker must be mounted from the outside with this method.

Because of the speaker cutout, the front panel is usually the weakest.  It should either be braced or constructed of a double thickness of particle board.  If you use the double thickness approach, you may want to precut the speaker cutout in each thickness before gluing them together, depending on the capabilities of your jig saw.

All box joints and bracing should be glued generously then nailed or preferably, screwed together.  Use a wood glue such as Elmer's® Carpenter's Wood glue.  2" drywall screws work well for this application.  If you don't have an electric drill and screwdriver bit for driving screws, you can use 6d (2") finishing nails.

see also: Box Bracing, Bandpass Box Construction


Box Bracing

Bracing refers to fastening strips of wood (usually 2" x 2" lumber) along internal seams, across panels or between panels.  Bracing strengthens the box and improves its performance by reducing the sound which the box itself radiates.  Corner bracing is important where the precision of cuts for the box pieces was limited by using a hand saw, jig saw or circular saw.  Bracing across panels is beneficial for large enclosures (greater than 1 ft3).

Because of the speaker cutout, the front panel is usually the weakest.  It should either be braced or constructed of a double thickness of particle board.  If you use the double thickness approach, you may want to precut the speaker cutout in each thickness before gluing them together, depending on the capabilities of your jig saw.

All box joints and bracing should be glued generously then nailed or preferably, screwed together.  Use a wood glue such as Elmer's® Carpenter's Wood glue.  2" drywall screws work well for this application.  If you don't have an electric drill and screwdriver bit for driving screws, you can use 6d (2") finishing nails.

see also: Box Assembly


Bandpass Box Construction

Bandpass designs present some unique challenges when it comes to box construction.

In other types of designs, the woofer is attached from the outside so the box can be constructed without any removable access panels.  This is not the case for bandpass boxes.  Furthermore, a bandpass system requires sealed and ported chambers.  The normal construction technique for a bandpass box is to build the outer cabinet, then add an inner partition to create the two chambers.  The removable access panel can be on the sealed side or the ported side.  If you want to see your woofer in action, you can use 3/8" or 1/2" thick plexiglass for the access panel.  It should be sealed with foam gasket material or non-hardening caulk then secured with a generous number of screws.

For the most part, woofer direction isn't critical.  The front of a woofer provides the best high frequency response.  For full range systems, good high frequency response is an asset, but for bandpass systems it's a liability.  For this reason, it's better to have the back of the woofer in the ported chamber.  (If you use a crossover with a steep slope, this factor shouldn't matter.)  A second benefit of this orientation is that it provides better cooling for the motor structure of the driver.  This points to putting the access panel on the sealed chamber.


Constructing Ports

Ports can be constructed of PVC pipe (the large diameter plastic pipe plumbers use).  You can also buy prefab speaker ports which you trim to the desired length.  There is also an adjustable type which has a sliding outer tube.  The advantage of prefab ports is their attractive appearance - they have a contoured mounting flange and are black.  The disadvantage is that the length you need in the diameter you want may not be readily available.


Installing Ports

Ports may be located on any side of the enclosure.  Ports should be placed a minimum of 4" to 6" from the woofer to prevent interaction between the cone and the vent.  The back of the port should be a minimum of 3" from the opposing wall to prevent a reduction of air flow.

To install the port, hold the small end of the port tube against the box where you want it to go, and trace around it with a pencil.  Use a jigsaw to cut just inside the pencil mark to provide a snug fit.  You will then need to file or sand the hole to make it exact.  Use Liquid Nails or a similar construction adhesive to glue the tube to the hole.

Tip: allow adhesives to cure before installing your woofer since the fumes released by some sealants during curing may have an appetite for foam surrounds.


Enclosure Damping

Enclosure damping refers to using fiberglass, polyester or dacron batting or similar materials to line or stuff a speaker enclosure.

Enclosure damping is traditionally used for two reasons.  First, it reduces the effects of internal reflections within a speaker enclosure.  One to two inches of fiberglass are normally used to line the internal walls of an enclosure for this.  These reflections can cause resonant peaks and dips in the frequency response at midbass and higher frequencies.  This is important for full range systems, but because subwoofers are used only for very low frequencies, lining them with damping material provides no benefit.

The second use of enclosure damping is to increase the effective size of a sealed box.  Fully stuffing a sealed box can increase its effective size by 15% to 25%.  This can be a useful technique where space is extremely tight, but otherwise isn't worth the trouble.  Care must be taken to keep damping material out of and away from the rear basket of the speaker itself.  If you plan on stuffing, use only materials which do not shed short fibers.  These can find their way into the voice coil gap and cause rubbing and distortion in the speaker.  Enclosure stuffing should not be used for ported or bandpass boxes.


Finishing Touches

Many products are available to give your hand-crafted subwoofer box a professional appearance and protect it from damage.  These include custom carpet, corner protectors, handles and grilles.

Speaker carpet is available in colors to match every automotive interior.  It provides a more durable finish than paint, and conceals minor imperfections in the cabinet.  Carpet is normally applied to all sides of a speaker box using spray adhesive, hot glue or a staple gun.

Plastic corner protectors reduce damage to the box and other objects if you bang a corner against something.  They also make corners look better - it's difficult to make a corner carpet seam look good otherwise.  Plastic or metal speaker grilles protect your expensive woofers from damage by objects in your trunk, dropped screwdrivers and pets.


Estimated Box Requirements

Estimated Box Requirements show you what box size to expect for a particular driver.  The table shows the enclosure volume and type (sealed or ported) for bass response down to 30, 40, 50 and 60 Hertz.

Note: the values in the table are intended for woofer evaluation purposes only.  Use the Box Design Tool to accurately design your optimum enclosure.


Frequency Response Window

The frequency response window lets you view the frequency response curves produced by the box design tool.

Subtopics:

Switch to design panel

Point Tracking

dB/div


Page Setup

Page Setup allows you to adjust paper size, source and orientation, as well as the top, bottom, left and right print margins.  Page Setup is found in the frequency response and main window File menus.

Note: the frequency response printout is sized to make full horizontal use of the paper excluding the user specified margins in Page Setup.  The relative text size and the aspect ratio (width to height) of the plot can be manipulated by resizing the frequency response window on the computer screen to the desired appearance.


Contents

Getting Started

   Box Design Tool

   Port Design Tool

   Enclosure Design Tool

   Woofer Selection Tool

Box Design Tool

   Data Set

   New Data Set

   Delete Data Set

   Data Description

   Fs

   Qts

   Vas

   Cu. Feet

   Liters

   Enclosure Type

   Isobarik

   Box Volume

   Car Response

   Update Plot

   Fb (port frequency)

   Box Volume for QB3

   Recommended Fb (port frequency)

   Ported Fraction of Box Volume (%)

   Normalize plot to 0 dB

   Sensitivity (dB)

   Metric

Port Design Tool

   Ported Enclosure Volume

   Fb (port frequency)

   Number of Ports

   Port Inner Diameter

   Calculated Port Length

   Metric

Enclosure Design Tool

   Box Style

   Wall Thickness

   Outside X Dimension

   Outside X1 Dimension

   Outside X2 Dimension

   Outside Y Dimension

   Outside Z Dimension

   Volume

   Show Template

   Metric

Woofer Selection Tool

   Sealed & Ported Bass Response Figure of Merit

   Estimated Free-Air Bass Response

   This Driver is Best Suited for

   Use Port Only if Box Volume Exceeds

   Estimated Box Requirements

   Metric

Frequency Response Window

   Switch to design panel

   Point Tracking

   dB/div

Printing

   Print Setup

   Page Setup

   Print

   Print with Symbols

Subwoofer Design Notes

   Types of Enclosures

   Free-air

   Sealed

   Ported

   Bandpass

   Choosing a Driver

   Power Handling

   Driver Sensitivity

   Bass Response

   Cone Material

   Surround Material

   Dual Voice Coil Subs

   How low do you need to go?

   Multiple Drivers in One Box

   Box Construction

   Box Materials

   Box Shape

   Box Assembly

   Box Bracing

   Bandpass Box Construction

   Constructing Ports

   Installing Ports

   Enclosure Damping

   Finishing Touches


Quick Help

Quick Help directly takes you to a topic of interest without having to navigate through Help Contents or perform a Help Search.

Use Quick Help by clicking on a data field or control with your mouse then hitting the F1 key on your keyboard.


Box Style

Choose the style of box you plan to use.  Choose from a rectangular box or a wedge-shaped box.

A rectangular box is easier to build.  A wedge can make better use of available space and lets you fit a box with a larger volume behind a truck seat.


Wall Thickness

Enter the thickness of the particle board or other material used to construct the enclosure.

 0.75" (19 mm) walls are recommended for most designs.  For very small or thin enclosures, a thinner wall can substantially increase the internal volume for a given set of external dimensions.

Note: the Enclosure Design tool assumes the same wall thickness for all walls.


Outside X Dimension

This is the external X dimension of the enclosure, as pictured in the diagram on the upper right of the Enclosure Design tool.

You can enter the X dimension you want, or you can calculate this dimension based on the other dimensions and box volume by clicking the adjoining Calc button.


Outside Y Dimension

This is the external Y dimension of the enclosure, as pictured in the diagram on the upper right of the Enclosure Design tool.

You can enter the Y dimension you want, or you can calculate this dimension based on the other dimensions and box volume by clicking the adjoining Calc button.


Outside Z Dimension

This is the external Z dimension of the enclosure, as pictured in the diagram on the upper right of the Enclosure Design tool.

You can enter the Z dimension you want, or you can calculate this dimension based on the other dimensions and box volume by clicking the adjoining Calc button.


Outside X1 Dimension

This is the external X1 dimension of the enclosure, as pictured in the diagram on the upper right of the Enclosure Design tool.

You can enter the X1 dimension you want, or you can calculate this dimension based on the other dimensions and box volume by clicking the adjoining Calc button.


Outside X2 Dimension

This is the external X2 dimension of the enclosure, as pictured in the diagram on the upper right of the Enclosure Design tool.

You can enter the X2 dimension you want, or you can calculate this dimension based on the other dimensions and box volume by clicking the adjoining Calc button.


Volume

This is the internal volume of the enclosure.

You can enter the volume you want, or you can calculate the volume based on the enclosure dimensions by clicking the adjoining Calc button.


Enclosure Design Tool

The enclosure design tool helps you design rectangular and wedge-shaped boxes.  It also lets you calculate the volume of prefabricated boxes.  This tool can be used after completing your design with the Box Design tool (to design a box with a desired volume), or it can be used beforehand to determine the volume of a box that will fit in your application.

This tool will calculate the internal enclosure volume if you specify the external box dimensions.  It will also calculate any external dimension if you specify the other external dimensions and the box volume.  Just enter the parameters you want to specify, then click on the Calc button to the right of the desired parameter.

After you've specified the enclosure dimensions, click on the Show Template button to view a printable cutting template for the enclosure pieces you'll need.

Subtopics:

Box Style

Wall Thickness

Outside X Dimension

Outside X1 Dimension

Outside X2 Dimension

Outside Y Dimension

Outside Z Dimension

Volume

Show Template

Metric


Car Response

Operating a subwoofer in a small enclosed space (such as a car) provides a natural low frequency boost.  The Car Response feature lets you see the effect of operating your sub in a wide range of vehicle sizes.

For home subwoofers or very large vehicles (such as full size vans) choose None (flat).  For smaller vehicles, choose the size that most closely describes your vehicle.  The table below gives car volumes and examples of each car type to help you decide.

Car	Car Volume

Type	(cubic feet)	Examples

None (flat)	      -	convertibles

Subcompact	     55	Geo Metro, Kia Sephia

Compact   	     70	Ford Escort, Dodge Neon, Chevy S-10 (std cab)

Midsize   	   110	Pontiac Grand Am, Toyota Corolla

Fullsize  	   180	Buick LeSabre, Ford Taurus

Minivan   	   300	Dodge Caravan

Important: The None (flat) setting applies to any size convertible (even with the top up) and to any vehicle with the windows rolled down.

If you like to drive with the windows down, you'll want to design your subwoofer so it gives acceptable response with the None (flat) setting.  In any vehicle, use the bass control of your stereo to compensate for the response difference between windows up and windows down.  You've probably done this all along without even thinking about it - now you understand why.


dB/div

Sets the coarse divisions and display range of the magnitude scale of the Frequency Response plot.  Choose from 3 dB/div or 5 dB/div.  Selecting 3 dB/div gives a display range of -12 to +9 dB.  Selecting 5 dB/div gives a display range of -20 to +15 dB.

Note: the dB/div setting is stored as a user preference to eliminate the need to set it each time the program is started.


Show Template

After you've specified the enclosure dimensions, click on the Show Template button to view a printable Cutting Template for the enclosure pieces you'll need.


Cutting Template

After you've specified the enclosure dimensions in the Enclosure Design Tool, click on the Show Template button to view a printable cutting template for the enclosure pieces you'll need.  Print the cutting template using the main print menu.

For wedge style boxes, angled cuts are required on some of the edges, as shown on the template diagram.  The cutting angle is specified as it would be set on a standard table saw indicator dial - in other words, a cutting angle of zero degrees means a standard perpendicular cut.  Note: Dimensions T1, B1 and F1 are given as pre-angled-cut dimensions.  This gives you the option of first cutting the pieces in these dimensions with standard perpendicular cuts, then running them through the saw a second time to produce the angled edges.

Tip: cut side pieces of wedge style boxes starting with a single rectangle S2 tall by (S1 + S3 + a little extra) wide.  The "little extra" is to account for the kerf of the blade.  Draw a diagonal pencil line dividing the rectangle into the two side pieces, then cut along the line.  Don't use this technique if the material you're cutting has a "good side" and a "bad side" - you'll end up with the "bad side out" on one side.


Print Selection

The Print Selection dialog box is activated when printing from the main program window.  It allows you to select which tool results to print.  (Printing from the Frequency Response Tool automatically prints only the Frequency Response Plot.)

Data from the Box Design Tool, Port Design Tool, Enclosure Design Tool and Woofer Selection Tool are printed in tabular form.  The Frequency Response Plot and Cutting Template are printed graphically.

Note: the Frequency Response Plot can be printed with symbols to improve readability with black and white printers.  For color printers, this option can be turned off.


Print Setup

Print Setup allows you to select the printer and printer properties as well as adjust paper size, source and orientation.  Print Setup is found in the frequency response and main window File menus.