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4. Subwoofers

This section describes some elements necessary for understanding subwoofers - how they operate, how to build proper enclosures, how to pick the right driver for you, and how to have a computer do some of the work for you.

4.1 What are "Thiele/Small parameters?" [CD, RDP]  
4.2 How does speaker sensitivity affect real world SPL? Will a higher sensitivity give me a larger SPL? [MS]  
4.3 What are the enclosure types available? [JLD, JG]  
4.4 Which enclosure type is right for me? [IDB, DK]  
4.5 How do I build an enclosure? [AO]  
4.6 MDF for Dummies [IDB]  
4.7 What driver should I use?  
4.8 Is there any computer software available to help me choose an enclosure and a driver? [JSC, MH, DK]  
4.9 What is an "aperiodic membrane?" [CD, DK]  
4.10 Can I use my subs in the winter? [MS]  
4.11 How can I carpet my enclosure? [AO]  
4.12 Are large magnets always better than small magnets? [ST]  
4.13 I know the box volume required for my subwoofer, but what are the best dimensions for my enclosure? [IDB]  


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4.1 What are "Thiele/Small parameters?" [CD, RDP]

These are a group of parameters outlined by A. N. Thiele, and later R. H. Small, which can completely describe the electrical and mechanical characteristics of a mid and low frequency driver operating in its pistonic region. These parameters are crucial for designing a quality subwoofer enclosure, be it for reference quality reproduction or for booming.

`Fs'
Driver free air resonance, in Hz. This is the point at which driver impedance is maximum.

`Fc'
System resonance (usually for sealed box systems), in Hz

`Fb'
Enclosure resonance (usually for reflex systems), in Hz

`F3'
-3 dB cutoff frequency, in Hz

`Vas'
"Equivalent volume of compliance", this is a volume of air whose compliance is the same as a driver's acoustical compliance Cms (q.v.), in cubic meters

`D'
Effective diameter of driver, in meters

`Sd'
Effective piston radiating area of driver in square meters

`Xmax'
Maximum peak linear excursion of driver, in meters

`Vd'
Maximum linear volume of displacement of the driver (product of Sd times Xmax), in cubic meters.

`Re'
Driver DC resistance (voice coil, mainly), in ohms

`Rg'
Amplifier source resistance (includes leads, crossover, etc.), in ohms

`Qms'
The driver's Q at resonance (Fs), due to mechanical losses; dimensionless

`Qes'
The driver's Q at resonance (Fs), due to electrical losses; dimensionless

`Qts'
The driver's Q at resonance (Fs), due to all losses; dimensionless

`Qmc'
The system's Q at resonance (Fc), due to mechanical losses; dimensionless

`Qec'
The system's Q at resonance (Fc), due to electrical losses; dimensionless

`Qtc'
The system's Q at resonance (Fc), due to all losses; dimensionless

`Ql'
The system's Q at Fb, due to leakage losses; dimensionless

`Qa'
The system's Q at Fb, due to absorption losses; dimensionless

`Qp'
The system's Q at Fb, due to port losses (turbulence, viscosity, etc.); dimensionless

`n0'
The reference efficiency of the system (eta sub 0) dimensionless, usually expressed as a percentage

`Cms'
The driver's mechanical compliance (reciprocal of stiffness), in m/N

`Mms'
The driver's effective mechanical mass (including air load), in kg

`Rms'
The driver's mechanical losses, in kg/s

`Cas'
Acoustical equivalent of Cms

`Mas'
Acoustical equivalent of Mms

`Ras'
Acoustical equivalent of Rms

`Cmes'
The electrical capacitive equivalent of Mms, in farads

`Lces'
The electrical inductive equivalent of Cms, in henries

`Res'
The electrical resistive equivalent of Rms, in ohms

`B'
Magnetic flux density in gap, in Tesla

`l'
Length of wire immersed in magnetic field, in meters

`Bl'
Electro-magnetic force factor, can be expressed in Tesla-meters or, preferably, in meters/Newton

`Pa'
Acoustical power

`Pe'
Electrical power

`c'
Propagation velocity of sound at STP, approx. 342 m/s

`p'
Density of air at STP 1.18 kg/m^3 (rho)


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4.2 How does speaker sensitivity affect real world SPL? Will a higher sensitivity give me a larger SPL? [MS]

When it comes to mids and highs, efficiency (sensitivity) is a fairly good indicator of output differences at the same power level. When it comes to subwoofer performance, the driver's sensitivity is irrelevant unless you are also specifying a box volume.

An efficient sub requires a larger box to achieve equivalent extension to a less efficient sub. In a small box, the less efficient sub will actually be LOUDER at low frequencies at the SAME POWER as the more efficient sub.

Linear excursion is a very good indicator of ultimate output capability (given sufficient power to drive the speaker to that point.) To make sound you must move air; therefore, the more air you move, the more sound you make. When comparing two speakers of equal surface area, the one with greater excursion capability will play louder given sufficient power.


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4.3 What are the enclosure types available? [JLD, JG]

Only the order of the enclosure itself is shown here. The addition of a crossover network increases the order of the system by the order of the crossover. Example: If a First-Order, 6dB/Oct. crossover (single inductor in series with the speaker) is used with a Fourth Order enclosure, the total system is a fifth order. Note: Air volumes and ratios shown here may not be to scale. This is designed to provide order information only.

 
                       First Order                
                       Infinite-Baffle or Free-Air
                         
                               |                  
                               |                  
                              /                   
                             /                    
                           ||                     
                           ||                     
                             \                    
                              \                   
                               |                  
                               |                  
 
 
 Second Order                        Second Order
 Acoustic- or Air-Suspension         Isobaric* Acoustic-Suspension
 or Sealed                           (Compound Loaded)
  _______________________             _______________________
 |                       |           |                  _____|
 |                      /            |                 /    /
 |                     /             |                /    /
 |                   ||              |              ||   ||
 |                   ||              |              ||   ||
 |                     \             |                \    \
 |                      \            |                 \____\
 |_______________________|           |_______________________|
 
 
 Fourth Order            Fourth Order            Fourth Order
 Bass-Reflex or          Passive Radiator        Isobaric*
 Vented or Ported        Bass-Reflex             Bass-Reflex
  _______________         _______________         _______________
 |               |       |               |       |          ____ |
 |              /        |              /        |         /    /
 |             /         |             /         |        /    /
 |           ||          |           ||          |      ||   ||
 |           ||          |           ||          |      ||   ||
 |             \         |             \         |        \    \
 |              \        |              \        |         \____\
 |               |       |               |       |               |
 |               |       |              /        |               |
 |               |       |             /         |               |
 |           ____|       |            |          |           ____|
 |                       |            |          |
 |           ____        |             \         |           ____
 |               |       |              \        |               |
 |_______________|       |_______________|       |_______________|
 
 
 Fourth Order                    Fourth Order
 Single-Reflex Bandpass          Isobaric* Single-Reflex Bandpass
  _________________    ____      _______________________    ____
 |         |       |  |    |    |               |       |  |    |
 |        /        |  |    |    |              / \      |  |    |
 |       /                 |    |             /   \             |
 |     ||                  |    |           ||     ||           |
 |     ||                  |    |           ||     ||           |
 |       \                 |    |             \   /             |
 |        \                |    |              \ /              |
 |_________|_______________|    |_______________|_______________|
 
 
 Fourth Order                      Fourth Order
 Three Chamber                     Three Chamber Isobaric*
 Single-Reflex Bandpass            Single-Reflex Bandpass
  ____________   ____________      ______________   ______________
 |      |     | |     |      |    |       |      | |      |       |
 |     /      | |      \     |    |      / \     | |     / \      |
 |    /                 \    |    |     /   \           /   \     |
 |  ||                   ||  |    |   ||     ||       ||     ||   |
 |  ||                   ||  |    |   ||     ||       ||     ||   |
 |    \                 /    |    |     \   /           \   /     |
 |     \               /     |    |      \ /             \ /      |
 |______|_____________|______|    |_______|_______________|_______|
 
 
 Fifth Order = Fourth Order Enclosure + First Order Crossover
             = Third Order Enclosure + Second Order Crossover, etc.
 
 
 Sixth Order                        Sixth Order
 Dual-Reflex Bandpass               Isobaric* Dual-Reflex Bandpass
  ____    _____________    ____      ____    ____________    ____
 |    |  |       |     |  |    |    |    |  |      |     |  |    |
 |    |  |      /      |  |    |    |    |  |     / \    |  |    |
 |    |  |     /               |    |    |  |    /   \           |
 |           ||                |    |          ||     ||         |
 |           ||                |    |          ||     ||         |
 |             \               |    |            \   /           |
 |              \              |    |             \ /            |
 |_______________|_____________|    |______________|_____________|
 
 Sixth Order
 Three Chamber                        Quasi-Sixth Order
 Dual-Reflex Bandpass                 Series-Tuned Bandpass
  _    _________   _________    _      _________________    ____
 | |  |   |     | |     |   |  | |    |           |     |  |    |
 | |  |  /      | |      \  |  | |    |          /      |  |    |
 |      /                 \      |    |         /               |
 |    ||                   ||    |    |       ||                |
 |    ||                   ||    |    |       ||                |
 |      \                 /      |    |         \               |
 |       \               /       |    |          \              |
 |________|_____________|________|    |       ____|             |
                                      |                         |
                                      |       ____              |
                                      |           |             |
                                      |___________|_____________|
 
 
 Seventh Order = Sixth Order Enclosure + First Order Crossover, etc.
 
 Quasi-Eighth Order               
 Series-Tuned Dual-Reflex          Eighth Order
 Bandpass                          Triple-Reflex Bandpass
  _    _______________    _         ____________    _____________
 | |  |        |      |  | |       |            |  |             | 
 | |  |       /       |  | |       |            |  |             |
 |           /             |       |                             |
 |         ||              |       |                             |
 |         ||              |       |                             |
 |           \             |       |____    _____________    ____|
 |            \            |       |    |  |      |      |  |    |
 |         ____|           |       |    |  |     /       |  |    |
 |                         |       |            /                |
 |         ____            |       |          ||                 |
 |_____________|___________|       |          ||                 |
                                   |            \                |
                                   |             \               |
                                   |______________|______________|
 
 
 * Isobaric or Coupled Pair (Iso-group) Variations:
 
 A variety of configurations may be used in the isobaric loading
 of any order enclosure. Physical and acoustic restrictions may
 make one loading configuration preferable to another in a
 particular enclosure.
 
 Composite or Push-Pull                  Compound or Piggy-Back
 or Face-to-Face Loading                 or Tunnel Loading
  _________________                 ___________________________
 |                 |               |                       ____|
 |                / \              |                      /   /
 |               /   \             |                     /   /
 |         >>> ||     || >>>       |               >>> ||  || >>>
 |         >>> ||     || >>>       |               >>> ||  || >>>
 |               \   /             |                     \   \
 |                \ /              |                      \___\
 |_________________|               |___________________________|
 
 Back-to-Back Loading                    Planar Loading
  _________________________         ___________________________
 |                _________|       |                        |  |
 |                \       /        |                       /   |
 |                 \     /         |                      /    |
 |              >>> || || >>>      |                    || >>> |
 |              >>> || || >>>      |                    || >>> |
 |                 /     \         |                      \    |
 |                /_______\        |                       \   |
 |_________________________|       |________________________|  |
                                                            |  |
                                                           /   |
                                                          /    |
                                                        || <<< |
                                                        || <<< |
                                                          \    |
 >>> indicates direction of                                \   |
 >>> simultaneous cone movement.                            |__|


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4.4 Which enclosure type is right for me? [IDB, DK]

This answer is not designed to tell you exactly what kind of enclosure to build, but rather to give an idea of the advantages and disadvantages to the simple configurations (Infinite baffle [1st order], Sealed [2nd order], Ported [4th order] and basic bandpass). Building and designing more complicated systems (order > 4) is not for the light at heart.

4.4.1 Infinite Baffle ("free-air")  
4.4.2 Sealed Box  
4.4.3 Ported Box  
4.4.4 Bandpass Box  


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4.4.1 Infinite Baffle ("free-air")


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4.4.2 Sealed Box


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4.4.3 Ported Box


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4.4.4 Bandpass Box


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4.5 How do I build an enclosure? [AO]

These instructions are for building a first order (sealed) subwoofer enclosure. Building ported or bandpass boxes is more difficult, and those designs are less forgiving of mistakes. These instructions apply for all box designs, but be sure of the measurements before you make your cut. Building your own enclosure can save you a lot of money, but only if you don't need to buy all of your materials twice because of mistakes!

You will need:

Wood
I only use MDF See section 4.6 MDF for Dummies [IDB], but others have reported success using other hardwoods like birch and oak. Do not use plywood - it's far to flexible and porous. Use a minimum of 3/4" wood -- flexing sub enclosures lose precious energy!

Screws
For one inch wood use #8 2 inch wood screws. For 3/4 inch wood use #8 1 3/4 inch screws. Double grip Drywall screws also work well.

Adhesive
I use "Liquid Nails" which comes in a caulk tube or a bucket, but any paste type of adhesive will work. Spray adhesives will not work.

Silicone sealant
White, brown, clear, caulk tube, or squeeze bottle, it doesn't matter. Make sure you don't get silicone lubricant (which comes in a spray can)!

Terminals/Terminal Cup
To allow easy connections from your amp.

Besides these materials you will need several tools:

Table saw or radial arm saw
I use a radial arm saw, just because it's a little easier and accurate, but a table saw will work also. You can use a circular saw, but be very careful to make your cuts straight.

Jig saw
For cutting your speaker hole.

Drill
You will also need a 1/8" drill bit, a screwdriver bit, and a countersinking bit.

Pencil
To mark your cuts, make notes, etc.

Measuring tape

Safety Goggles

Face Mask
Breathing MDF dust has not been proven to cause health-related problems, but hang around with with a few installers at your local shop for an afternoon and you'll see why you need a face mask. :-)

Start by marking the cuts you need to make on your wood. Double check your math, and your measurements.

Use the table or radial arm saw to cut your wood. When you're done you should have six pieces of wood which fit together tightly to form a box.

At this point you will need to trace the cut out for your subwoofer onto the front of the box. Remember that if you have a 10 inch subwoofer you do NOT want a 10 inch cutout. The 10 inch measurement is from the outside of the mounting ring. The actual cutout diameter should be with your instructions. Transfer the proper sized circle onto the sub box and cut it out with the jig saw. If you have trouble starting cuts with a jig saw, drill a 1/2" hole in the wood inside the circle. You can drop your blade into the hole and then cut out to the edge of the circle and around.

After you have cut out your mounting hole you will need to cut out a square on one of the sides for your terminal cup. Transfer the proper size rectangle onto the wood and cut it out with the jig saw.

Now you are ready to start assembling the enclosure. Choose one of the ends, and one of the sides. Apply a bead of adhesive along the edge of the end piece. Affix it to the edge of the bottom piece. Flip it over (have a friend hold the other end and hold the end in place,) and screw the edge to the end. Use one screw at each corner and then one more screw about every 8 inches. Drill a pilot hole with your 1/8" drill bit, then drill a countersink with your countersinking bit. Finally, drive the screw in. Make sure that you don't strip the hole.

Repeat the above procedure with the other end. You should now have the two ends connected to one side. Affix the other three sides the same way.

Finally, you'll want to seal the insides of the box with silicone. Apply a bead of silicone across all the inside edges and around the terminal strip.

Allow the box to dry over night and then place your speaker into the hole. Screw it down and you're done!


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4.6 MDF for Dummies [IDB]

Since MDF (Medium Density Fiberboard) is used so frequently in building subwoofer enclosures and other projects, this section provides some more detailed information.

4.6.1 What is MDF? [PS]  
4.6.2 Where can I get MDF? [PS]  
4.6.3 What type of saw blade works best when cutting MDF? [PS]  
4.6.4 What type of router bits work well with MDF? [PS]  


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4.6.1 What is MDF? [PS]

MDF (Medium Density Fiberboard) is a hardboard product comprised of hot compressed wood fibers glued together. The color of this material can vary from light to dark brown depending on the brand of manufacturer. Particle Board or Flakeboard consist mostly of recycled wood chips. The size of the wood chips used varies which means that the coarser the wood chips reduces the structural density of the material.


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4.6.2 Where can I get MDF? [PS]

Wholesale distributors of lumber products. Home Improvement Centers, such as Home Depot or Home Base, and Lumber Yards may or may not stock MDF products, but they will be able to order it for you. Contact woodworking or cabinet making shops in your area, ask them where to get it or if it may be possible to buy the material from their business.


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4.6.3 What type of saw blade works best when cutting MDF? [PS]

If you're using a circular saw, a good 40 tooth carbide tipped blade works best. If you're using a table saw, a 50/60 tooth carbide tipped combination or ripping blade will provide smooth cuts.


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4.6.4 What type of router bits work well with MDF? [PS]

As far as router bits go, use only two flute carbide bits and make several passes as opposed to one single pass. If possible, trace the outline of what you intend to router and remove the majority of the material with a jigsaw which will reduce the amount of material being removed and will put less strain on the router as well, not to mention, this will all reduce the amount of MDF dust.


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4.7 What driver should I use?


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4.8 Is there any computer software available to help me choose an enclosure and a driver? [JSC, MH, DK]

Various enclosure design software is available via ftp from

ftp://ftp.uu.net/usenet/rec.audio.high-end/Software/.

The most popular program there is Perfect Box, which is in the file `perf.uu' (or `perf.zip').

Note that NO program can tell you what enclosure is best for YOUR car! The program does not take into consideration your space limitations, the type of car you drive, the type and number of mid-bass drivers you use, your musical preferences and the goals you have for your system. Many people follow (blindly) what a computer program says is "optimal," and end up unhappy with the results. Therefore, it is always a good idea to discuss a design you think looks good with a qualified installer or (even better) with the manufacturer.

For an overview of many programs and devices available for enclosure design, obtain the file `sahfsd01.doc' at the ftp.uu.net archive. The filename stands for "Software and Hardware for Speaker Design", and was added to the archive in June 1994 by an anonymous contributor.


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4.9 What is an "aperiodic membrane?" [CD, DK]

An aperiodic membrane is one part of a type of subwoofer enclosure. It is an air-permeable sheet which has frequency-dependent acoustical resistance properties. The original design goes back to Naim, for use in home systems, but has been applied by several individuals and companies in car audio.

The completed system will be aperiodic, which means it will prove to be over-damped with a Q well below 0.7. In contrast, the most commonly used sealed enclosures have Qtc's in the range of 0.8 to 1.1 which are considered, by definition, to be underdamped. When improperly used, a high-Q system may have poor transient response, nasty peaks in frequency response, and high rates of roll-off. Aperiodic systems will feature excellent Aperiodic systems are characterized by better transient response, flatter frequency response and somewhat extended low frequency response.

Another benefit of the system is that you can pretty much choose whichever driver you'd like to use, as long as they are big. The Thiele/Small parameters (which would normally determine what kind of box would be used) are taken into consideration by the membrane designers so that the response is extended and overdamped, regardless of the characteristics of the driver.

Physically, the aperiodic membrane isn't for every car. It requires sealing the trunk from the passenger compartment in an air-tight manner, as well as sealing the trunk from the outside for best results. The drivers are then mounted into the baffle between the passenger compartment and the trunk, as would be standard in an infinite-baffle/free-air set-up. The aperiodic membrane is then placed either in front of the driver or behind the driver, depending on the type. When mounting behind the driver, the membrane is used as the rear-wall of a very small box which the driver sits in (as in Richard Clark's infamous Buick Grand National). So, in short, it's not suitable for trucks, jeeps, R/V's, or hatchbacks.

You should probably only get an aperiodic membrane if you've got money to burn, lots of amplifier power, some big subs, a sedan, a desire for trunk space, and no wish to boom. If your tastes lean towards bass-heavy booming, as opposed to well-recorded acoustic instruments, you're not going to be pleased with the result.


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4.10 Can I use my subs in the winter? [MS]

The following applies to all speakers in extremely cold conditions, but the question most often occurs in reference to subwoofers.

The suspension of the speakers will stiffen considerably at very cold temperatures (lower than 30 degrees F). So will certain cone materials which may become more brittle.

If a very cold speaker is played very hard there is a small potential for damage because more stress is placed on the cone's neck. The likelihood of damage is minimal for well-constructed and well-designed automotive speakers, however.

Thermally, the danger is minimal because the ambient temperature and the coil temperature are so low that it is highly unlikely that a coil will overheat and burn, despite limited movement and ventilation.

At temperatures between +20 degrees F and 0 degrees F, it is a good idea to play the system at a moderate level until the car's heater has warmed the vehicle interior. As the speakers warm up, they will play louder and lower signifying that their suspensions are warming up and returning to nominal compliance.

If the temperature is extremely cold (less than 0 degrees F), you should avoid playing the system at all until the vehicle interior is warm. This is to avoid stress fractures in the surround material (especially with rubber surrounds).


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4.11 How can I carpet my enclosure? [AO]

What you will need:

Before you start, find a large, clean, flat surface on which to set the box as you carpet it. Start by unrolling the carpet onto the surface, smoothing it out so that its flat, and setting the box on top of it edgewise. Also, make sure that you remove the speaker, any ports and terminal cups from the box.

The instructions on how to carpet the box are as follows:

  1. Place the box such that it is centered on the carpet lengthwise, and one edge of the box is about one inch from the edge of the carpet.

  2. Roll the box back so that the side of the box that was previously done faces forward, and the carpet beneath it is exposed. Coat both the box and carpet with adhesive, but do NOT apply the carpet to the box - the adhesive needs a few minutes to set up (follow the instructions from your adhesive to find out how long you should wait).

  3. After the adhesive has set up, roll the box back into position. Wait a few more minutes for the adhesive to bond.

  4. Now coat the side of the box adjacent to the remaining carpet (the side facing backwards) and the carpet next to it with adhesive, let the adhesive set up, and roll the side you just coated onto the coated carpet. Repeat this until three sides of the box are carpeted.

  5. Before carpeting the next side, the 1 inch of carpet sticking over the edge must be removed. To do this, rotate the box so that the first side that was carpeted is up. Pull the carpet sticking over the edge down towards the uncarpeted edge and cut it off with the knife, flush with the uncarpeted side of the box. You will have to run the knife nearly parallel to the uncarpeted side to get a perfect cut.

  6. This done, spray the remaining side and carpet, and roll the box onto it. Shear off the remaining carpet sticking out from all edges with the scissors leaving a 1 inch border everywhere.

  7. Clean up the ends of the box so that the carpet is flush with the sides of the box as in step 4.

  8. Next cut off the remaining 1 inch flap of carpet (located at the point where you began carpeting). This is the tricky part, as you don't want to be able to see this seam. Again, pull the flap down over the edge of the box, but this time cut it at roughly a 45 degree angle. If you are successful you shouldn't be able to see the wood under the seam, but will probably see the white of the adhesive and the back of the carpet.

  9. Soak some of the solvent onto a rag and use this to scrub the edge you just cut off. It should dissolve the adhesive and the carpet backing somewhat, causing the carpet on the edge to become fuzzy. Keep scrubbing the edge until you can no longer see the seam.

  10. Now carpet the ends of the box. Cut two pieces of carpet slightly larger than the ends of the box and lay one of them flat on the surface. Spray the carpet and one end of the box with adhesive, and set the end of the box on the carpet, so the box stands on end.

  11. After the adhesive has dried sufficiently cut off the remaining border of the carpet as in 7 and 8.

  12. Repeat step 9 and 10 for the other end of the box.

Congratulations! You've just carpeted your box!


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4.12 Are large magnets always better than small magnets? [ST]

Magnet size is meaningless!

Every speaker will have an optimal BL (See section 4.1 What are "Thiele/Small parameters?" [CD, RDP],) product, the field strength in the air gap multiplied by the length of the voice coil wire in the field.

If the BL product is too low, the speaker is electrically not very well damped (which will result in a woofer with a high Qts). A bump in frequency response and a level drop in midband efficiency may be the result. If the BL product is too high, the speaker is electrically overdamped (Low Qts woofer). A very high midband efficiency, but the driver starts to roll of early.

An high BL product can be achieved in a number of ways: increase field strength; or increase wire length in magnetic gap.

The increase in field strength is limited; so some manufacturers use very thin wire for the voice coil, as such they can achieve a high BL product with a low field strength (cheap magnet). Or they use an 8 layer voice coil... needless to say that electrical power handling will decrease enormously.

Long stroke woofers, having only a part of the voice coil in the air gap, need a very high field strength to achieve a high BL product. Often this means a big magnet as well...

Use magnet size as an indication, but as nothing more than that.


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4.13 I know the box volume required for my subwoofer, but what are the best dimensions for my enclosure? [IDB]

The specific dimensions of a subwoofer enclosure aren't really important. Once you know the appropriate volume of the box, and you know where in your car you want to install it, you will have some idea of the restrictions in the dimensions. For example, if the distance between the floor of your trunk and the bottom of the rear deck is 16", then you probably shouldn't make your box any taller than 16". Likewise, if width of the trunk (between the wheel wells is 38", then you've got that much space to work with.

You can also infer some other information about your box, from the speaker specifications (for a PPI PC10):

 
     Mounting Depth:         4.5625" 
     Speaker Displacement:   .032 ft^3 

Here is an ASCII drawing of a subwoofer enclosure to help illustrate a few things:

 
      |--- L ---|
      __________     ___
     /         /|     |
    /     +   / |     |         + = Center of speaker
   / x       /  |     D         x = center of port
  /_________/   |     |
  |         |   |    _|_        W = Width of box
  |         |  /     /          L = Length of box
  |         | /     W           D = Depth of box
  |_________|/    _/_


Because of the speaker's mounting depth, you know that the box MUST be at 5.5" deep (it's always good to leave at least 1" of space behind the speaker, but leave more if you can). If you use a straight port (3" diameter PVC) then the box will need to be at least 13" deep (leaving 2 inches between the end of the port and the back of other side of the enclosure), assuming that your port will reside completely within the enclosure. If you use Flex-Port or choose to have a portion of the port extending outside of the enclosure, you can make the box less deep.

We also know that with a 10" woofer, the length and width should both be a minimum of 12" (leave 1" on either side of the woofer). Obviously both dimensions can't be 12" otherwise we can't put the port on the same face as the speaker.

With volume, remember a couple of things. Recommended volume is the Net Internal volume. Both the speaker and the port will displace volume from the box -- this means that you MUST account for these (i.e., make the interior volume of the box larger than the recommended value). In this case, you know the following:

 
     Required Volume:       1.25 ft^3
     Speaker Displacement:  0.032 ft^3
     Port Displacement:     ??

3" (interior diameter) PVC will usually have an exterior diameter of 3.5" (0.25" wall thickness). Volume of a cylinder = pi*r^2*h, where r = 1.75" and h=12.25" (13" port length - 0.75" for the thickness of the MDF). Therefore the port displacement is: 117.9 in^3 or 0.068 ft^3 (1728 in^3 = 1 ft^3).

 
     Required Volume:       1.25 ft^3
     Speaker Displacement:  0.032 ft^3
     Port Displacement:     0.068 ft^3

     TOTAL INTERIOR VOLUME: 1.35 ft^3 or 2332.8 in^3

Now, to calculate the dimensions of the box: For a rectangular box, L x W x D = Volume.

If we know that the minimum interior depth is 15" (because the port is 13" and you should leave 2" between the port and the wall of the enclosure), we can say,

 
     L x W x 15"  = 2332.8 in^3   or
     L x W        = 155.52 in^2

Since we know that L must be at least 12" (since the speaker is 10" in diameter), we can continue:

 
     12" x W      = 155.52 in^2
     W            = 12.96"

So, we have determined that the interior dimensions of the box must be 12" x 12.96" x 15". Obviously this can't work, since there is nowhere to put the port! You will either have to have the port extend outside of the enclosure, or use flex-port. Keep in mind that if you have part of the port "sticking out" of the enclosure, the Port Displacement will also change!!.

For simplicity, (in this example) let's use flex-port. This will let you decrease the thickness of your box to about 6". So,

 
     L x W x 6"  = 2332.8 in^3
     L x W       = 388.8 in^2
     12" x W     = 388.8 in^2
     W           = 32.4"

This is a little more reasonable. This will be a flat, wide box. Again, keep in mind that these are internal dimensions. If you use 3/4" MDF (and you should) to build the enclosure, this will add 1.5" to each of these figures for the external dimensions. e.g.:

 
    13.5" x 33.9" x 7.5"

Remember, the 6" depth and 12" length are just the minimum values, you can alter them if, for example, you don't have enough space between your wheel wells for a 34" box. If you only have, say, 28", then:

 
     maximum external width = 28"
     maximum internal width = 26.5"

     L x W x D        = 2332.8 in^3
     L x 26.5" x 6"   = 2332.8 in^3
     L                = 14.7"  (internal)

Got it? It can be a pain, but that's part of the fun in building a speaker enclosure.


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This document was generated by Ian D. Bjorhovde on February, 7 2006 using texi2html