Many people don't realize that, unless you have virtually unlimited resources, there are going to be trade offs when designing a system. If you want a system to produce a lot of SPL (but have limited power or number of speakers), you'll probably have to concentrate a lot of the acoustic energy within a small band of frequencies. This would allow you to generate the desired SPL and properly impress your friends. It would, however, leave you with a system that would have less than perfect frequency response and marginal performance at the low frequency end of the spectrum. You could design a system that would produce the desired SPL at all frequencies but it would require significantly more power and speakers than the aforementioned system with the narrow peak.
Installing car stereo equipment is no picnic. Cramming electronics and speakers into places they simply weren't meant to be presents a challenge to even the most savvy of installers. And the more exotic the install, the tougher the task. While each aspect of system design brings a unique set of challenges to the installation party, building custom speaker enclosures is perhaps the most creative aspect of the installation.
The box-building option is regularly exercised by enthusiasts in pursuit of smooth, deep bass – a tough nut to crack in the automobile environment. Three enclosure types that are practical for most automobile applications are the "sealed box", "vented box" and the "bandpass box". Although different in design and operation, these types utilize the same basic construction techniques. But before you grab a saw and carpenter's square, you have to decide which box will best suit your needs.
Manufacturers Recommendations:
Many manufacturers recommend an enclosure that's going to give you a peak like I mentioned earlier. This will give you good results for rap or other bass heavy music. If you listen to those types of music, this system may perform satisfactorily. If you listen to all types of music, you'd likely want an enclosure that would give a flatter response. To get a flatter response, you could increase the size of the enclosure but the overall response wouldn't be perfectly flat. To build a system that had a perfectly flat frequency response (generally desired by audiophiles), you'll have to take the vehicle's into account. When a manufacturer recommends a particular enclosure, you should ask yourself (or, better yet, the manufacturer) what the enclosure is optimized for.
Enclosure Types:
When laying out a system, you need to decide what type of enclosure to use. No enclosure is magic.
Sealed Enclosures
Straightforward in design, the sealed box is nothing more than an airtight enclosure whose purpose is to enhance speaker performance. When a woofer is installed in one, the sound waves that emanate from the front of the speaker cone are separated from the rear-firing waves. This improves bass response, since opposing waves can cancel each other out when they aren't isolated. Superb damping, good power-handling capability, and simplicity of construction make the sealed-box enclosure an ideal candidate for a variety of installations. A sealed enclosure will be the smallest (for a given response shape) and will have good low frequency extension but may not have the best low frequency extension. If space is limited, this may be your best choice.
Vented Enclosures
Vented enclosures – also known as ported or bass-reflex systems – are more complex than sealed boxes, but the extra construction work required to install a vent has its rewards. Vented boxes are more efficient than their sealed counterparts, since they channel sound waves from the front and rear of the cone into the listening environment. Other virtues of the vented design include better reproduction of low bass, a reasonably flat response curve, and low distortion – provided the box is properly constructed. A vented enclosure will generally have a better low frequency extension for a given response shape (alignment) but would require a larger enclosure. If you made the enclosure as small as the sealed enclosure but ported it to gain the low frequency response, the output would deviate from the desired flat response.
Bandpass Enclosures
A bandpass enclosure can sound good and give you a flat response but most of the generic bandpass enclosures are not designed for a flat response. They are designed to impress you in the stores. This means that they are built to produce a large peak at some frequency near 60hz. These enclosures will work well with something like rap music but generally won't sound good with other types of music. If you use a bandpass enclosure, it should be designed specifically for your speakers.
If you're building your first system, I'd recommend a sealed enclosure. It is the simplest enclosure and will be the easiest to get right. A sealed enclosure only needs to be the right size and well sealed. Alright... Enough of my opinions.
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System Design
The first step when designing an enclosure is determining how much space is available in your vehicle. If you own a hatchback, for example, calculate the cubic footage of the hatch area, keeping in mind that you'll still need access to your spare tire. The space available will dictate the size and shape of the enclosure, which in turn will narrow your choice of drivers. For example, one 10-inch subwoofer typically requires an enclosure with 1 to 1.5 cubic feet of internal volume for optimum performance, while a 12-inch subwoofer requires 2 to 3 cubic feet. It's important to note that any recommendation won't take into account the volume that will be displaced by the drivers, crossovers, internal braces, and, in the case of a vented system, the port. To compensate for these components, you must build your enclosure slightly larger than your measurements indicate. As a rule of thumb, it's generally wise to increase the volume by 20 percent.
When figuring out the shape of your box, keep in mind these "ideal" dimension guidelines: Depth should be 0.7 x Width and Height 1.4 x Width. Also, avoid constructing an enclosure with walls that have equal dimensions; this could result in a resonant peak in the midbass region.
If the enclosure is going to be irregularly shaped – as most are – it's easiest to think of its interior as a collection of conventional shapes, calculate the volume for each, and add these volumes together. For example, the volume of a box with a slanted front is most easily determined by adding the volume of the square or rectangular portion at the bottom to the volume of the triangular portion at the top. Also, it's easier to measure in cubic inches and then convert to cubic feet than to work with cubic-foot figures alone.
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Calculating Speaker Enclosure Volume
As you already know, for speakers (especially woofers) to work properly, they must be in the proper size enclosure. The manufacturer can give you the required enclosure volume but can't give the exact dimensions of an enclosure that will work in all vehicles. So that you can build an enclosure of the proper size for your vehicle, this page will explain how you calculate the total volume of the enclosure.
Magic number:
OK... It's not magic but the number is 1728. If you forget it, just remember that the number is 12 inches * 12 inches * 12 inches. 12*12*12=1728.
Square or Rectangular Enclosures:
These boxes are the easiest to calculate the internal volume. You simply measure the height, width and depth (in inches), multiply them together and then divide that number by 1728. If the box has internal measurements of 6" high x 18" wide x 12" deep then the volume of the box is 1296/1728=.75 ft³.
Wedge Enclosures:
Since wedges are comprised of two shapes, a rectangle and a triangle, they are fairly easy to caculate as well. Simply seperate the shapes, and calculate for each. Use the same method above for the internal measurements of a rectangle, then use (H X W X D) / 2 for the triangles internal volume. Add these together to get your total internal volume.
Area (rectangle) = Height x Width
Area (triangle) = 1/2 Height x Width
Area (circle) = 3.141592654 x Radius²
Area (circle) = .7853981634 x Diameter²
Volume (rectangular box) = Area (of one side) x Depth
Volume (triangular enclosure) = Area (triangle) x Depth
Volume (cylinder) = Area (circle) x Depth
1728 Cubic Inches = 1 Cubic Foot
28 Liters = 1 Cubic Foot
If you want to compensate for the volume taken up by the woofer, you can use the following approximations. Keep in mind that these are for 'normal' woofers. If you're using a competition woofer with a huge frame and magnet structure, refer to the manufacturer for the actual displacement volume of the woofer. Even if you're not using competition woofers, most high quality manufacturers provide this spec in the woofer's datasheet.
8" = .03 ft³
10" = .05 ft³
12" = .07 ft³
15" = .10 ft³
For multiple-driver enclosures, you'll get the best performance by isolating each driver in a separate chamber. This prevents variations in speaker tolerances and input power from restricting system output. If building separate chambers isn't feasible, you can make sure that each driver receives the same amount of power by mounting two identical drivers in the same cavity and wiring them in parallel keeping in mind that ths will half the impedance.
Given the extraordinary number of choices, selecting drivers can be a bewildering task. It requires an understanding of specs as well as a sharp set of ears. For some guidance, see the "Wire Service" column in the September/October CSR and talk to an experienced installer.
Once you've selected an enclosure type and the appropriate drivers, map out the design on paper. This will help you visualize the shape of the enclosure and may prevent unnecessary cutting. Before you take out your trusty saw, be absolutely sure that the enclosure will fit in your vehicle.
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Cautions
If you pick a tuning frequency above the lowest frequency that you intend to drive into the speakers, the speaker may well be damaged due to the lack of damping. Below the tuning frequency, all of the control will depend on the speaker's suspension. If you drive the amplifier into clipping, the problem will be even worse. If you're not sure about the proper tuning frequency, use a sealed box or at the very least keep an eye on your woofers for the first few times that you drive them hard with very low notes .
You can not have a port length shorter than the thickness of the wood that you're using.
I don't recommend using a port diameter of less than 3 inches for subwoofers.
The port length must be short enough to allow a clearance of the port's diameter between the end of the port and the wall of the box. If you're using a 3 inch port, allow at least 3 inches between the back end of the port and any wall of the enclosure.
If you want to use square ports, use a port who's height multiplied by its width equals the 'port area'. The port length will be the same as the round port.
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Materials
MDF
Medium Density Fiberboard is a compressed type of 'wood product' It is similar to particle board but is much 'friendlier' to work with than is particle board. The wood cuts with less dust and leaves a good clean cut. It also resists chipping when screwing close to the edge. You should still pre drill the screw holes when assembling a box with drywall screws because the wood will split if a screw is driven into the end of the board. Many people use a pneumatic stapler and a good quality wood glue to assemble boxes. Some people use liquid nails type adhesives to seal the joints but the solvents in construction adhesives can soften the adhesives used on some speakers (which could lead to premature speaker failure if the speakers are installed before the adhesive has dried completely). You should also realize that the fumes are flammable (and may be explosive when contained). If you have a loose speaker connection on the speaker terminals, you may have a fire/explosion hazard if the speakers are played before the solvent has fully evaporated. Silicone adhesive has acetic acid which is released as the adhesive cures. This acid will corrode speaker baskets if the speakers are reinstalled before the silicone has completely cured. The best way to make sure the enclosure is sealed is to make good quality cuts. It will take less time to make good cuts than it will for the sealant to dry/cure (24 hours).
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Construction
Draw out each piece of the enclosure before cutting so you don't waste material. Once individual sections have been cut, temporarily assemble the box to make sure the pieces fit snugly together. One option to consider at this juncture is the use of internal braces, which, depending on the overall size of the box, may result in a sturdier enclosure. Usually, 1.5 inch x 1.5 inch strips of material will suffice. Generally, you'll need to add braces only for boxes that have internal volumes of more than 5 cubic feet.
If all of the pieces fit and the edges match up, the box is ready for permanent assembly. First make sure all joints are free of debris. Then slowly run a bead of carpenter's glue along the line where you'll be making the first joint. Carefully assemble the two pieces and secure them using screws or nails. Use one screw every 6 inches or one nail every 3 inches. If you're using screws, drill pilot holes to prevent the wood from splitting. And for a more polished look, countersink the screws. (Note that the type of joint that you select is not critical as long as the box is strong and airtight.)
Continue assembling the box but don't attach the baffle board – the front panel that holds the speakers – until you've made sure the enclosure is sealed. To do this, fill a caulking gun with silicone sealant and run a bead along the inside of each joint. When you're finished caulking the box, secure the baffle board in place.
With the box assembled, you are now ready to cut the speaker openings and the vent, if you've chosen a vented design. I've always found that it's easier to work on the wood once the box is assembled. The vent tube can be made of plastic, cardboard, or – for a square or rectangular vent – wood. First, calculate the center of each hole and draw a circle of the appropriate diameter using your compass. Make 3/8-inch starter holes with a drill and then cut out the holes using a jigsaw.
Once the speaker and port openings are cut, put some silicone sealant on the tip of your index finger and reach into the enclosure to seal the inside joints of the baffle board. It's very important for the enclosure to be completely airtight; air leakage reduces the system's power-handling capacity and causes distortion. Once the joints are sealed, I recommend covering three adjacent inside walls – back, top, and bottom, for example – with 1 to 2 inches of polyfill (available at fabric stores for about $1.50 a bag). The use of filler will help minimize midbass resonances. Let the silicone completely cure for at least 24 hours before you install your subs. The fumes can actually cuase your subs to come apart.
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Sealing around speaker
If the speaker has no gasket and the speaker box isn't covered in carpet or vinyl, You can use an open cell foam weather stripping around the cutout in the baffle. The weather stripping should be about 3/8 to 1/2 inch wide and 1/2 inch thick. You need to make sure that the area around the cutout is clean and dry so that the weather stripping will stick. I recommend wiping it down with solvent and allowing it to dry before applying the weather stripping. If you REALLY want it to stick, apply a single coat of contact cement to the area around the hole and allow it to dry for 10 minutes or until it no longer sticks to your fingers when you touch it. When applying the weather stripping to the coated area, you get only ONE chance to lay it down in the right place. As soon as the weather stripping touches the contact cement, it's not coming back up.
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Finishing
The physical appearance of your enclosure is strictly a matter of personal taste. Although you may not think it's important, most people want the cabinet to complement the vehicle's interior. Phoenix Gold (800- 445-9229) and Rockford Fosgate's ACI division (602-967- 3565) sell fabric for this purpose. Phoenix Gold's 4 Season carpet is available in eleven colors and costs about $10 per square yard. Perfect Interface's Overdrive is a cross between carpet and felt that's available in twelve different colors; it'll run you about $12 per square yard. Your local installer might be able to help out here, too.
Typically, spray adhesives are used to bond fabric to the surface of an enclosure. (3M Spray Adhesive sells for about $8 a can.) You'll need your razor knife to trim away excess material and cut around the speaker openings. You also can use paint or Formica to finish your enclosure. Although Formica looks good, it's very difficult to work with. Paint, on the other hand, is easy to apply but can make your box look rough.
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Mounting the Drivers
Before mounting the speakers, you'll need to drill a few holes. First make a hole at the bottom of the enclosure's rear panel for the speaker-cable connector. Then place the drivers in their respective holes and mark the location of each screw hole with a pencil. Remove the speaker and drill pilot holes. Before proceeding, remove all wood shavings. Snake a generous length of speaker cable through the hole in the rear of the enclosure. Then mount the connector of your choice (Radio Shack sells many different types) and, using your finger, seal the opening with silicone. Next, hook the speaker wire to the connector. If you don't use connectors (in the case of a permanent installation, perhaps), tie a knot in the speaker cable at the point where it exits the enclosure.
Now mount your crossover devices, and wire the cables to the speakers. (Don't forget about polarity.) With all wires securely in place (I recommend soldering), secure the drivers to the baffle board, using extreme caution – the last thing you need at this stage is a screwdriver-size hole in the speaker.
Covering the speakers with some type of grille may be a good idea depending on the location of the box in your vehicle.
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Using T-Nuts
If you have very heavy woofers or will need to remove your woofers frequently, you'll need to use T-nuts. T-nuts are threaded metal fasteners that are used to provide a solid mounting point in wood. They are inserted into the back side of the baffle board. A hole is drilled just large enough to accept the cylindrical part of the T-nut. You insert the T-nut into the hole and (if possible) give it a good solid whack with a hammer. If you can not get to it to hit it with a hammer, they can sometimes be pulled into place by simply tightening the screw. This works fine on softer woods but won't always work with MDF. Sometimes the threads strip or the screws break before the T-nut is fully seated. If you're having trouble getting them to pull all of the way down, use a C-clamp to seat them. If you don't get them to seat fully before mounting the speaker, the screws will continue to loosen as the T-nuts continue to pull down.
Only after they are fully seated will the screws stay tight.
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Securing the Enclosure
Once your box is complete, the next step is to secure it in the vehicle. A careful inspection of the area surrounding the enclosure will help you determine the best and safest means for doing so. L-brackets usually suffice, or you can remove the drivers and bolt the box directly to the vehicle. If you choose the latter approach, be extremely careful when drilling holes. It's amazing how much damage can be done to a vehicle if you don't look where you're drilling. (Don't overlook the gas tank.)
The final step: Connect your new creation to its power source and hit the play button on your head unit. Now you can give yourself a pat on the back for enhancing the performance of your auto sound system. I've built many enclosures over the years, and with each one I've picked up a new trick or two. Patience is key, but it is even more important to talk to manufacturers and experienced installers. Following their directions makes for smooth sailing on the road to good sound.
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Formulas and other tips
You should always match up RMS (Root Mean Square) watts of Subs and amp per channel as well as impedance's (resistances).
If for example your amp is rated at 4Ω don't subject it to 2Ω, it will fry.
Here is the formula for subs in parallel:
Z = 1 / (1/sub1 + 1/sub2 + 1/sub3 + ....)
For series, just add them up (sub1 + sub2 + ...).
You can mix and match these equations to get the right impedance required by your amp.
Example 1: Amp is 2 channel and rated at 4Ω bridged, 4 subs @ 4Ω each. If I connect two in parallel and the other two in parallel and connect the two pairs in series, I would have a total impedance(resistance) of 4Ω. So to the amp, it's just one sub in bridged mode.
Example 2: Amp is rated at 1Ω mono, 2 subs @ 4Ω each dual voice coil. If I connect two voice coils in parallel and the other two voice coils in series and connect the two subs in parallel, I would have a total impedance of 1.6Ω. Which would be perfect for a 1Ω stable mono amp.
You should use a capacitor as well. They help "stiiffen" the power your battery can't provide as well as lengthening the life of your equipment and electrical system..
Recommend for every 500 watts MAX you use a .5 Farad cap
Example: 1000 watts MAX - 1 Farad cap
You will also need what's called a "line level converter" if your stock head unit doesn't have RCA outputs or your amp doesn't have high-level inputs.
Port tube length calculations:
input = Radius of Port (R), tuning frequency in Hz (fb), Volume of the box in cubic inches (Vb), (L) will be the resulting length of tube.
(1.463 X 10^7 X R^2)
L = ————————————
(fb^2 X Vb) - 1.463 X R
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Current Draw and Power Wire Guage Charts
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Look for this when buying an amp...
CEA-2006 Compliant
On May 28, 2003, the Consumer Electronics Association published standard CEA-2006, "Testing & Measurement Methods for Mobile Audio Amplifiers." This "voluntary" standard advocates a uniform method for determining an amplifier's RMS power and signal-to-noise ratio. Using 14.4 volts, RMS watts are measured into a 4-ohm impedance load at 1 percent Total Harmonic Distortion (THD) plus noise, at a frequency range (for general purpose amplifiers) of 20 Hz to 20,000 Hz. Signal-to-Noise ratio is measured in weighted absolute decibels (dBA) at a reference of 1 watt into 4 ohms. This applies to both external amplifiers and the amplifiers within in-dash receivers.
If an amp doesn't have this rating, it may not produce the watts stated!
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Some unknown truths
Some people get really confused with certain aspects of the mechanics of how subwoofers and amplifiers work. There are even nasty myths that underpowering a sub will cause damage.
Too little power will only cause the maximum output level to be low. Abuse and the defective 'wing nut' (an idiot) connected to the volume control blow speakers with low powered amplifiers. If driving a speaker with low power would cause them to fail, speakers would fail every time you lower the volume on the head unit.
Clipping and Distortion
Clipping is one form of distortion that occurs when an amplifier is overdriven, which happens when it attempts to increase voltage or current beyond its limits.
Distortion refers to any kind of deformation of a waveform, compared to the input.
The picture above shows an oscilloscope screen of an amplifier "clipping." The amplifier should be outputting a clean sine wave, but instead the top and bottom of the waveform is cut off, or "clipped." This can be observed because the tops and bottoms of the wave, which should be rounded, are flat. The term "clipping" is used because the top and bottom of the waveform appear to visually have been "clipped" with a pair of scissors.
When an amplifier is asked to create a signal greater than its maximum capacity, it will amplify the signal only up to its maximum capacity, at which point the signal will be amplified no further. As the signal simply "cuts" or "clips" at the maximum capacity of the amplifier, the signal is said to be "clipped." The extra signal which is beyond the capability of the amplifier is simply cut off, resulting in a fixed signal. Note that this fixed signal suffers from other forms of distortion, such as total harmonic distortion.
Because the clipped waveform has more area underneath it than the smaller unclipped waveform, the amplifier produces more power in clipping. This extra power can damage any part of the loudspeaker, including the woofer or the tweeter.
The clipping introduces additional high frequency components, meaning the clipped signal will be weighted more towards treble than the unclipped signal. Some people believe this additional treble weighting is dangerous to tweeters. Others believe that it is not dangerous, noting that normal music recordings sometimes have significant treble energy and yet don't damage tweeters.
Here are some more unknown truths:
I can drive speakers with a 100% clipped square wave signal all day long with no problems as long as the thermal and mechanical limits of the speaker are not exceeded.
I can feed a speaker 100% distortion all day long with no damage as long as the thermal and mechanical limits of the speaker are not exceeded.
I can exceed the thermal and/or mechanical limits of a speaker and watch it fail in short order.
These are electrical and physical truths and anything else is a myth.
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My plans (below) are for a 4th order band passbass box (A.K.A. single reflex bandpass) that holds two 12's. There is also plans for a fan cooling system with an automatic thermostat, a blown fuse indicator and a 4th order Linkwitz-Riley 33 Hz highpass filter with 24 db/octave rolloff.
The picture quality isn't that good on here, but if you'd like I will provide them to you free of charge. Just e-mail me at sparky3489@yahoo.com
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My system
I chose Blaupunkt TSw1200 subs with a Sony Xplōd XM2200GTX.
Amp is 1200 watts (200 watts RMS X2), subs are 600 watts each (200 watts RMS), cap is 1.5 Farad w/digital read-out
SPL is about 112 db
Total cost(so far) = $410
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