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Some thoughts on sub-box designs (and other stuff)

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 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.

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%.

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.

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 X 12 inches X 12 inches. 12 X 12 X 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, 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.

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.

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).

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.

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.

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. Rodin (formerly Phoenix Gold) http://www.rodinaudio.com and Rockford Fosgate's ACI division (602-967- 3565) http:///www.rockford.com sell fabric for this purpose. Phoenix Gold's 4 Season carpet is available in eleven colors and costs about $12 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 $10 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.

Damping

Pollyfil is a sound absorption, dampening fiber that will deepen bass, tone down any unwanted reverberation and improve the dynamic extension of any speaker system which will give you tighter and more accurate bass. It also helps to give a flatter frequency response to the overall sound, which is good. Besides this, it also helps reduce the vibration of the box, which results in less distortion and "tricks" a sub into thinking it is in a bigger box. There is a big difference to the bass when you have an insulated box.

Damping also increases subwoofer efficiency by dissipating some energy that affects the sub, particularly the voice coil. Pillow polyfill and fiberglass insulation are common, though polyfill is a lot easier on your skin. Most sealed and vented enclosures require 1/2 lb. of dampening material per cubic foot of internal enclosure space per chamber. For best results it is recommended to loosely fill the material throughout the enclosure on the bottom and sides using staples to sucure it. The reason it's not put at the top is that it could fall down into the subwoofer and cause problems.

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.

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.

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.

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).

You should not exceed the RMS watts of a driver by more than 10% at the rated power.

If for example your amp is rated stable at 4Ω don't subject it to 2Ω or less, it will go into protect mode and stay that way until you resolve the load issue, or it will fry.
MOST two-channel amps when bridged will only handle 4 ohms.

Here is the formula for subs in parallel, where Z is the total impedance and sub# is each sub (or each coil in a multi coiled sub):

Z = 1 / (1/sub1 + 1/sub2 + 1/sub3 + ....)

For series, just add them up (sub1 + sub2 + ...).

You can mix 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.

To calculate total watts for two or more subs, just add the RMS watts of each sub together. For instance, two 300 Watt RMS connected in series OR parallel will be 600 Watts RMS total. Just remember that the ohms will be different between series and parallel.

See the section on Subwoofer Wiring Methods for more detail.

You will also need what's called a "Line Level Converter" or LOC "Line-Out Converter" if your stock head unit doesn't have RCA outputs or your amp doesn't have high-level inputs. You can find these at Wal-Mart.

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.

L = (1.463 X 10^7 X R^2 / (fb^2) X Vb) - 1.463 X R

Current Draw, Power Requirements and Power Wire Guage Charts

ATTENTION: The following link is THE most important tool you could possibly have in designing and setting up your system!

This is a guide that covers pre-installation as well as post.

This link will take you to a download site for the Audio-Calc.XLS worksheet.

The Excel worksheet will help with gain settings, power wire sizes and the current (amperage) requirements of the vehicle.

http://www.datafilehost.com/download.php?file=6d26c621

New version has amplifier efficiency as an input http://www.box.net/shared/uchv4dbk88

You will need Microsoft Excel, a multi-meter (AC volt meter) and a way to burn an audio CD from an MP3.

If you don't have Microsoft Excel, use this link http://www.box.net/shared/nkkw1dhk4g

or e-mail me Sparky3489@yahoo.com

Subwoofer Wiring Methods

You want to know, so here we go...hang on!

Sub wiring depends greatly on the method used to match impedances. You have series or parallel, or any combination of the two.

Series adds the ohms together, sub1 + sub2 + sub3 + ....

Parallel is a little tricky. There is a formula and its this, where Z is total resistance(ohms) and sub# is each sub (or each coil in multi-coiled subs):

Z = 1 / (1/sub1 + 1/sub2 + 1/sub3 + ...) as many as you have.

You can mix and match these equations to get the right impedance required by your amp.

An easy method to figure parallel is (as long as BOTH ohm values are the same on each sub and are single voice coiled subs) the ohms will get cut in half for two subs. Example, two 4 ohms subs wired in parallel will result in a 2 ohm load.

These ohm measurements must match an amp as well as the total watts RMS.

Here are a few wiring methods for SVC (single voice coil) subs:

This is how a series is wired for a single voice coil, mono or bridged amp:

This is how parallel is wired for a single voice coil, mono or bridged amp:

In any case the watts RMS gets added together when wiring multiple subs together. For instance, two 300 watt RMS subs equal 600 watts total when wired together. You must also match the total watts RMS and ohms to an amps total watts RMS and impedance for best performance.

For instance, if you have two 200 watt RMS @ 4 ohms subs, then you could either go with a 200 watt RMS X 2 @ 4 ohms amp. This would be to have each sub wired to a channel on the amp.

-or-

Go with a 400 watt RMS X 1 @ 2 ohms amp. This would be to wire the subs in parallel.

Either case is acceptable. If the amp in the first example is bridgeable, then it could become 400 watts RMS X 1 @ 2 ohms like the second example.

Here is a way to wire four subs to a mono or bridged amp. Each sub is 4 ohms and the resulting resistance is 4 ohms. The amp must be four times as powerful as one sub (if each sub is equal in power).

Here are two wiring methods for DVC (dual voice coiled) subs:

Parallel-series. If the impedance (ohms) of each coil is 4 ohm, then the resulting resistance will be 4 ohms. This can also be used for a mono or bridged amp.

Tri-way parallel-series for three subs. With 4 ohms each coil in a DVC sub, the resulting impedance will be 2.67 ohms which would be fine for a 2 ohm stable amp. Again, mono but maybe not a bridged amp. Most bridged amps cannot handle loads below 4 ohms.

Here is one wiring method for two QVC (quad voice coiled) subs:

Each coil is 4 ohms so the resulting impedance is 4 ohms per channel. When you get in to quad voice coils, it can get confusing real fast, like so.

These are but a handful of options to sub wiring.

Stiffening Capacitors

A capacitor is only good so far as the audio system isn't trying to pull too much from the electrical system. A cap doesn't provide more power, it's designed to 'stiffen' the voltage to the amp, nothing esle. If the current isn't there, a cap won't help. The alternator must have at least 20% more amperage power than the entire vehicle and sound system combined for a cap to be of benefit which is ironically the same requirements for an amp to be efficient.

It's recommended for every 500 watts MAX you use a 0.5 Farad cap
Example: 1000 watts MAX - 1 Farad cap

A capacitor must be charged just before you supply power to it once installed.

It's best to install it as close to the amp as possible. You may need to get additional power and ground wire the same size you have now. Otherwise, cut the ground and power wire leaving enough room to mount it. The overall length of the ground wire should as short as possible (18" or less if possible, not critical).

Remove the fuse at the battery. If you don't have one, you need one that is rated the same as the total fuses that are on the amp. It must be no more than 18" from the battery. This fuse is really just to protect the battery (and the car from catching fire) from shorts in the power wire, nothing else.

Connect the ground from the chassis to the cap then to the amp.

Connect the the power wire (from fuse by the battery) to the cap then to the amp.

To charge the cap, get a 12v light bulb with leads. Temporarily connect the power wire from the battery to one side of the bulb (at the fuse) and connect the other side of the bulb to the wire going to the cap. The bulb will light and over a period of time (10-20 seconds) it will get dimmer. When the bulb goes out, the cap is charged. Basically, your going to put the light bulb where the fuse was.

Now you can re-install the fuse.

_______________________

IMPORTANT: Make/print a sign that states:

"WARNING: This vehicle employs a large electrolytic capacitor for the electrical/audio system. The capacitor holds a charge EVEN when disconnected. When changing the battery, remove this fuse. Take caution and charge the capacitor prior to replacing this fuse. SEVERE INJURY OR DAMAGE MAY RESULT IN FAILURE TO COMPLY!"

Have a big red arrow pointing to the fuse and one of those yellow triangles with the exclamation point in the center. Laminate this sign and place it near the fuse by the battery.

Something like this:

Amplifier Compliance

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!

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.

December 22 11:33 PM | Permalink | Blog it

How to create a VU meter

As with any project, you need to get your ducks in a row.

Tools and Equipment required:

You’ll need some kind of circuit designing CAD program to make a schematic and layout for your board, I use EagleCAD by CadSoft

http://www.cadsoft.de/

Copper Clad Board - 1 oz copper or less recommended http://www.digikey.com

single sided is fine for this project
Don't buy “pre-sensitized” boards; they are for transferring the image photographically.

Photo Paper

This will be hit or miss depending on the type used. High-Gloss is best

PCB Etchant Solution

Radioshack usually has this in stock
Active ingredient: Ferric Chloride

Nail Polish Remover

Nylon Abrasive Pad

The thin green Scotch-Brite scouring pads cut into 1.5 inch squares
Do NOT use steel wool

Laser Printer

Clothes Iron

Hot Water

Latex/Rubber Gloves

precision finger drill

I've seen them at Sears in the tools section

Power or Cordless Drill

Soldering equipment (soldering iron, solder, moist sponge, flux and the knowledge to use it all)

The Procedure:

Preparation

You first need to design and layout your board. Create the schematic and then the layout taking care that each component you plan to use is placed correctly and will fit with the other components in your drawings.

Order parts.

Be sure parts fit your design. I will print the schematic on my regular printer and "fit" the components to the paper and make any adjustment to the design if needed before I print to photo paper.

Print the Design

Print the design from a laser printer (NOT ink jet) using the darkest settings and highest resolution (e.g. Quality=Best, Intensity=Darkest, etc…). The goal is to get the greatest amount of toner onto the photo paper. Also, be sure to select “Photo Paper” or “Glossy Paper” as the paper type.

Now cut out the design around the boarder leaving about 1/8” of extra paper on three sides and 1” of extra paper on the fourth side (to grab it by).

Never touch the board part of the photo paper before or after it has been printed.

Transfer Design to Copper Board

Cleaning

First, you must prepare the copper board. Scrub the board with the Scotch-Brite pad in two orthogonal directions (up-and-down then side-to-side). Use a lighter pass at the end so the board is not too rough. Next, clean the board with a paper towel soaked with acetone until no more discoloration is seen on the paper towel. Press firmly and continue to use fresh parts of the paper towel.

Protect your skin: Use rubber gloves to when working with the acetone!

Ironing

Place the copper board on a flat, heat-resistant surface such as the back of wood cutting board and align the printed design face down on the copper board. Now use a clothes iron (maximum heat, no steam) to apply firm pressure to the back of the photo paper. Hold firmly like this for about 30 seconds. Now that the paper is stuck and there is no risk of slipping, go over the whole board with the tip of the iron, keeping the iron flat but torquing it forward. This should help get the toner to really stick to the copper. The whole ironing process should last about 5 minutes.

Remove Photo Paper

Within about a minute after ironing the design on the board, you need to place the board in HOT water. You can heat up some water on the stove if you your faucet water does not get hot enough. Let the paper soak for at least 20 minutes – times will vary. You can peel off a little at a time, but if you encounter resistance stop and let the board soak longer.

If some paper residue remains on the board, use your thumb and the hot water to rub it off.

If things go wrong: If the toner transferred well except for a couple spots you can fill in the gaps with a resist ink pen (Sharpie). If there are more than just a few bad spots, you have the option of redoing the whole process. Just use the acetone and a Scotch-Brite pad to rub off the toner and start over!

Etch the Board

Pour enough etchant (ferric chloride) in a flat, shallow dish placing the copper board face up in the dish so that it is just covered by a layer of the etchant. Closely monitor the progress over the next 20 minutes or so as the copper is etched away. Here are some ways to speed up the process:

• Heat the solution in the oven or microwave to 110°F
• Jiggle the dish to provide some gentle agitation
• Gently wipe the surface of the board with a balled up paper towel as it etches

Keep if off the clothes: Be careful when working with the etchant solution. It will stain almost everything it touches! Flush it down the toilet if you have a steel sink.

Be careful not to over etch the board or your traces will begin to eat away from the side. If the whole board is etched (translucent looking) except for some stubborn spots, pull the board out of the solution and attack those spots individually. A Q-tip works well for this.

Remove Toner

Use the acetone and the Scotch-Brite pad to rub off the toner from all the pads and traces. Rinse the board thoroughly with hot water when you are done to get rid of the corrosive acetone residue. Now is also a good time to test for shorts and open circuits using the continuity function of your multimeter. Be prepared to do some surgery with a magnifying glass, hobby knife (for shorts), and some solder (for open circuits).