The car is equipped with a nice Harmon Kardon 7.1 speaker system comprising a front center channel, a component pair in the front doors, a two-way 6.5″ speaker in each of the rear doors and a single subwoofer mounted in the parcel shelf of the car.

Notice the dual wirings...

After some further research on the topic, I stumbled on some pictures of the installed factory subwoofer. If you zoom in on the picture, the print on the back of the voice coil seems to say the following.

So that gives us the data we need to look for an upgraded subwoofer for my car. It also tells us that the factory setup is not a sealed ‘free air’ install or infinite baffle install but rather a large vented setup using the entire 12.4 cubic feet of trunk space as a subwoofer box venting through about 6 2″ wide vents. Interesting.

Vented trunk space is clear in this picture

Now looking at the install pictures, I further wondered: Would it be easier to install a higher quality shallow mount subwoofer in the factory mounting location ? Or do I stand to gain some good bass by mounting a regular subwoofer to the same location on the opposite side of the parcel shelf protruding into the trunk ? To perform this comparison, I didn’t have any data about the factory subwoofer, however I thorough search on Amazon and came up with two possible candidates. Each of these candidates is an 8″ subwoofer. Both are electrically equivalent with an equivalent resistance of 2 ohms. Both are from the same manufacturer, which should make this a fair comparison of the design appropriation of each of these subwoofers for our application.

Product	Kicker 08CVT82	Kicker 07CVR84
Style	Shallow Mount	Typical Subwoofer
Size	8 inch subwoofer	8 inch subwoofer
Voice coil	Single 2 ohms	Dual 4 ohms
Power Handling	200W RMS / 400W Peak	200W RMS / 400W Peak
Frequency Response	25 to 350 Hz	30 to 500 Hz
Sensitivity	84.8 dB	83.1 dB
Displacement	30.4 Cubic Inches	41.1 Cubic Inches
Max Excursion (ExMax)	0.226″	0.408″
Qms	10.641	9.36
Qes	0.597	0.912
Qts	0.565	0.831
Vas	0.364 cu. ft.	0.495 cu. ft.
Mounting Depth	3.38″	4.31″
Resonant Frequency (Fs)	49.2 Hz	44.3 Hz

Somethings jump at me right off the bat here. The regular subwoofer has about 1″ more depth to it (or 25% deeper) but it has almost double the max excursion. Also if you look at the resonance figures (Qms, Qes, Qts) we see that overall the shallow mount is less resonant and more damped. This makes sense since this subwoofer has limited space to work with so it can’t be allowed to have a great excursion , and we expect to see it installed in tiny boxes which require more damping. The last thing to note here is a slightly higher resonant frequency (Fs) for the shallow mount which means I’d expect slightly deeper bass from the regular basket woofer.

Now this is all expectation and I wanted to go one step further and analyze my expectations against simulation results… To perform the simulation, I needed two figures from my enclosure to enter into the simulator:

My enclosure volume:
Is my trunk volume which is a massive 12.4 cubic feet as I looked it up online.

My enclosure tuned frequency:
I estimated this around 40 hz. If you listen to the sound system on my car, it plays songs like 50 Cent – Hustler’s ambition with authority. But the system struggles to play deeper bass notes stereotypical of some of the more experimental techno music I listen to. It also struggles to play some of the higher bass frequencies into the mid-bass region. So my guesstimate of 40hz is just that a guess, but it is probably true and serves well for illustrative purposes here.

Plugging all the numbers above into the simulator you can see here the response of the shallow mount on the bottom curve, with the response of the deep basket subwoofer on top.

There is no comparison here as the regular subwoofer holds at least a +3dB gain over the shallow mount almost for the entire frequency range. Amazing.

What this means is that I stand to DOUBLE my bass loudness by simply swapping the top mounted shallow mount for a regular subwoofer trunk mounted, using the same amplifer, and the same ‘enclosure’! That is awesome for a low buck upgrade to my factory sound system without the complication of a full subwoofer box, amps, wiring …etc

Then I thought, what do I stand to gain if I sealed the bass ports and turned this into a free air install with a sealed trunk acting as my box…

As you can see from the picture above, sealing the bass ports on the factory trunk would give me a reasonable gain in the subsonic frequencies which would be fun for some genres of trance music, but I will loose all that nice authoritive bass when listening to hip hop and other genres of music (a 15dB drop will kill the bass). So unless I was going to upgrade to a higher powered amplifier, going to a sealed solution was probably the wrong way to go in this application.

Then I thought, I wonder how differently the two subwoofers in question here would perform in their own optimized enclosures… The optimum enclosure for the shallow mount is a tiny 1.1 cubic foot box tuned for a frequency of 35 Hz, while the optimum enclosure for our regular subwoofer is a much larger 5.4 cubic foot box tuned for a much lower frequency of 21 hz.

Looking at the plot above, you can see that shallow mount, when used properly can give a very good frequency response playing as deep as 30hz (where the gain rolls off to -3dB). However, the regular subwoofer, with it’s lower resonant frequency, slightly less damped cone, and greater excursion is able to add another 10hz of subsonic playing as low as 20hz (where the gain rolls off to -3dB)…. So let’s recap what we just found out:

1- In a hap hazard enclosure, the regular subwoofer will play louder and deeper than the shallow mount.

2- As we already knew, sealed enclosures play deeper into the subsonic but there may be a loss compared to a tuned vented box depending on port tuning. For factory systems with limited power from the factory amplifier, vented enclosures are probable the best bang for the watt.

3- Shallow mounts can perform really well if used in the right sized enclosure and are a great option when space limitations are an issue.

Very interesting results and I’m really considering now purchasing myself a single Kicker 07CVR84 and installing it in my trunk using an 8″ spacer ring (at least 0.4″ thick to allow it to reach max excursion without touching the bottom of the parcel shelf).

Note: Click here to purchase Kicker 08CVT82 Shallow Mount Subwoofers and make the most out of your limited space enclosure! Or click here to purchase the more powerful Kicker 07CVR84 subwoofer for your car.

Technorati Tags: 2 ohm, 4 ohm, DVC, Shallow Mount Subwoofers, SVC

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1. Choosing a stable amplifier for your car sub woofer

The thing is, a lot of first time diy’ers build their first DIY Subwoofer box using a single undivided enclosure with dual subwoofers and a 2 channel amplifier. Compared to having no bass at all, then a single enclosure with dual subs will sound like the greatest addition to the car’s sound system since sliced bread. I know I for sure loved mine, just as much as my friend Julian loved his dual 10s in his CRX hatch, and as much as my friend Dan love his dual 12s in the trunk of his Oldsmobile.

If you think about a typical single sealed box subwoofer install, then the amount of air trapped inside the enclosed and sealed box is fixed. The volume of the box is not fixed though because one of the box walls has a variable excursion part to it which we call a subwoofer. As the subwoofer moves in and out with reference to the box wall when the music moves the speaker cone, the volume of the box increases (when the sub moves out) and decreases (when the sub moves in).

As we know from physics density = mass / volume. So a fixed mass of air is manipulated inside a variable volume box and what this does is create a variable density area behind the speaker cone inside the box. As the sub moves outwards, the desnity and pressure of air inside the box drops. This low pressure behind the speaker (with regular pressure outside of the box) creates a suction on the speaker which tries to dampen it and return it back to its resting position. The opposite is true when the speaker dips into the boxes volume, as the pressure increases inside the box trying to push it back out.

This ‘overdamped’ nature of sealed boxes is what helps them create TIGHT bass with minimal distortion even when the speaker is slightly overpowered as the physics of the box dampen the possible distortions of the speaker motion and accelerate its return to its resting position to prepare it for the next bass hit, even when faced with a series of fast paced rolling bass lines.

Now although sealed boxes are great for clean tight bass, they don’t always hit as hard or as low as we’d like them to, especially when compared to a tuned vented box or a band pass system. One solution to helping a sealed box reach max excursion and create more bass is to neutralize the high and low pressure waves existing inside the sealed box. This gets even worse on a typical ‘newbie’ DIY box with TWO subwoofers in a single enclosure. As both speakers move inwards and outwards in phase, the pressure waves inside the box are exaggerated and neither speaker is allowed to go to maximum excursion.

Isobaric literally means of equal (iso) pressure (baric). And the way we achieve this ‘isobaric’ region is to operate two identically matched subwoofers within a tight enclosure. The two cones of the speakers are arranged so that they can move OPPOSITE each other in a way that the total volume of the box is fixed. Since the amount of air trapped in the box is fixed, and the total volume of the box stays fixed, then the pressure inside the box stays fixed (iso-baric) and the pressure inside the box is always equal to the pressure outside of the box. This way both subwoofers are surrounded on both faces by equal pressure and are no longer over damped by the box physics but rather damped by their own physical damping (electrically by the voice coil and mechanically by the design of the basket and surround material).

There are many different subwoofer arrangements that will allow you to reach an isobaric condition in your sub enclosures, but the simplest are by altering:

1- The direction of subwoofer mounting (flush into the box, or protruding with the basket ouside of the box)
2- The polarity of the wiring of one of the two subwoofers so that it moves in opposite direction to the other subwoofer

So what is the advantage of using an isobaric (or push pull as some may call it) subwoofer network ?

Well one advantage is that as stated before the subwoofer is no longer overly damped, which means more excursion and louder bass.
The second advantage is that now that the subwoofer is neutrally damped, it can actually increase the sound quality of the bass in your system so long as the subwoofer is not overdriven or overpowered.
The third and most interesting advantage of an isboaric arrangement is that you are now driving more power into the same box which combines the effects of both subwoofers resulting in a ’super sub’. This means that if you have a tight box space, that you can build a smaller box to give you the same operating characteristic as you need for a single box.

For example you purchase a 15″ subwoofer that requires a minimum 3.5 cubic feet of enclosure space. However, in the spare tire well in your car you measure out an available 2.0 cubic feet of space which is not enough for this sub. Setting up an isobaric setup with two identical 15″ woofers lower your space requirements by a factor of 2 down to only 1.75 cubic feet. What this also means is that any extra space you get over that 1.75 cubic feet will show up as bass extension as your isobaric subwoofer network will be able to hit harder and reach a LOWER corner frequency, as low as half an octave lower if you use an isobaric network in a 3.5 cubic foot box as recommended.

Here is an example of isobaric loading using two 15 inch subwoofers in a tuned vented box in the trunk of a Golf GTI. This box has been tested and proven to hit as low as 16hz!

So how do we use this information ?

1- If you (like i did) have a regular box equipped with two undivided subwoofers sharing the same space, then reversing the polarity of one of those subs will result in a stronger bass hit, and a lower overall frequency response. Have a box loaded with two 15 inch subwoofers ? Simply reverse the polarity on one of them and gain more power around the 16hz region simulating a big 18″. Have two 8 inch subs but want more of a 12″ sound ? Again do the same.

2- Sometimes you are limited for space, such as on a work truck. Work trucks need pretty much every last inch of space to haul tools and carry people around. So some people resort to installing midbass drivers (such as 6.5 inch “subs”) in their door panels. Mid bass drivers are great for 250 to 600hz mid-bass but they don’t really reach that far down into the bass frequencies on the frequency spectrum, especially if they are NOT installed in a ported enclosure with the right resonant frequency. One smart way to increase the frequency dynamic range of your truck would be to use a 6.5″ spacer ring to isobarically load your door mounted midbass drivers with two identical drivers mounted in plain sight. This will not only double your usage of your tiny in-door enclosure (as explained earlier about cutting your space requirements in half), but it will also allow you to gain more true bass from your midbass drivers and increase the overall feel that you get from such a small and minimally invasive install. (To pull this off correctly the speakers have to be wired out of phase (opposite polarity) so that they move in the same direction with the distance between them constant to maintain the isobaric region between them).

Note: Click here to start building your own isobaric network using undivided isobaric sub enclosures for two 12″ drivers and save $20 off retail.

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Technorati Tags: Car Subs, sub enclosures, Subwoofer design

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The following article talks about the theory and application of installing the right subwoofer capacitor to improve the performance of your bass system. It discusses some of the concepts and calculations of subwoofer capacitors and gives my recommendations on how to choose the right size capacitor for your audio system.

Article:

Here’s a little secret about audio and power ratings that most people don’t know….
The maximum RMS power you can deliver to a resistive load is equal to V*V / R

Where V = your power supply regulated voltage (of 12 to 14 volts in a car audio system)
and R is the resistive load of your subwoofer which ranges from 1 ohm to 8 ohms for different car subs and configurations.

And thus there are 2 ways to get more peak power out of your subwoofer setup…

1- Get an amplifier that has a built in power supply with a step up DC to DC converter, for example boosting the internal supply voltage of the amplifier from 12-14volts to 24-28volts using a capacitor ladder or voltage doubler circuit.

2- Speakers are not entirely resistive load they are more of an inductive load (combined of a resistive part and an inductor coil). At the same time the amplifier is not a pure power source and has its own internal resistance and a capacitive nature as it stores and delivers power to the subwoofers. By studying and exploiting these characteristics of subwoofers and amplifiers it becomes possible to momentarily deliver a voltage spike between the amplifier and subwoofer giving it a higher momentary peak power. This is where the term PMPO (peak momentary power output) comes from and in the more educational circles is referred to as an ILS rating “If Lightning Strikes the amplifier, it may deliver 3000 watts to a 2 ohm load for a very short duration of time effectively consuming all of the energy inside the amplifier to overdrive the subwoofer for that short duration of time”

Anyhow, both the methods mentioned above of delivering more power to an unchanged load rely on the concept of power storage and momentary power delivery…. If we can charge an ignition coil using 14 volts for 1 minute, and then use that same amount of charge to deliver 24,000 volts to the spark plug for 10 milli seconds to ignite the mixture inside the engine, then similarly, we can charge our amplifier circuitry using 14 volt power and then deliver a momentary 200 to 300 volts (inductive load spikes on 14 volt fuel injectors with tiny solenoid coils can be as high as 60 volts and so on a larger coil such as a subwoofer voice coil higher voltage spikes are not out of the ordinary), then we can over extend our subwoofer’s performance and power delivery.

This is where a good audio capacitor comes into the question… An audio capacitor keeps a good amount of power stored near the amplifier, this doesn’t really affect RMS audio performance that much so long as the alternator charging system is capable of delivering a steady 14 volts to the amplifier at the back of the car. However, when it comes the time to reach deep into the amplifier to deliver our peak momentary power output of 3000 watts into a single channel (or however much your amplifier is rated for per channel) then having a capacitor bank charged and ready to delivery such power is critical. What’s even more important, and what makes a capacitor shine over having a second battery installed in the trunk of the car, is that a high quality capacitor, has a really low series resistance built into it. This extremely low E.S.R (equivalent series resistance) that is characteristic of a good audio capacitor allows the capacitor to deliver it’s stored power to the amplifier (and thus to the subwoofers) almost instantaneously. This is even more important when you think about a 125hz audio signal that need power to be charged and delivered in the amplifier 125 times per second.

Here’s a great example of a power audio subwoofer capacitor. The boss audio Cap10 capacitor here has an 10 Farad rating, a digital control circuit, and a voltage indicator, with an ultra low E.S.R. of less than 0.002 ohms.

A 10 Farad capacitor running at 14 volts can store up to 140 Coulombs of charge or 980 Joules.
When we look at the fastest possible ‘bass’ frequency of 250hz then we have a wavelength with a duration of 4ms.
If we deliver half of our charge energy of 980 Joules to our subwoofer in 4ms, and keep the capacitor running at 50% (i.e. it is charging and discharging at the equal rate of 4ms which is the most we can take without draining it) then the maximum power we can deliver with such a capacitor without draining it (for the next coming bass hit) will be:

Peak power (Watts = Joules / Second) = (980/2) / (0.004) = 122,000 Watts !

Hrm, so why do we need such a large capacitor for a 1000 to 4000 watt system ?

The reason we need such a large capacitor, is that although capacitors can discharge power very rapidly, they take a long time to charge up. For example our 10 Farad Boss Audio Cap 10 , connected to the battery power through a low resistance 2 ohm power cable, will draw 7 amps of power for 100 seconds just to reach full charge… if we use higher resistance lower quality power wires, this charge time will grow even more.

So we have to use a larger capacitor bank because we have to find a balance between the capacitor charge and discharge characteristic as follows.

Say we have a 3000 Watt sound system wired up to the Cap 10. That 3000 Watt system for a slow 250hz bass hit will consume 12 Joules of energy in those 4 milli seconds.
E=0.5CVV and so V= sqrt ( 2E/C)
We started with 980 Joules of eneregy at a fully charged 14 volts.
We delivered 12 of those 980 Joules to our 3000 watt system in 4 milli seconds.
Our new Energy level is 968 Joules and our new voltage is 13.9 volts.

Let’s redo this calculation with a 1 Farad capacitor and a 3000 watt system
1 Farad gives us an energy of 98 Joules at 14 volts.
To reach that level of charge it takes the capacitor 10 seconds of total charge time (which makes sense since it’s 1/10th the capacitance, it takes 1/10th the charge time).
But look at what happens when we deliver 12 joules to our 3000 watt system in 4 milli seconds?
Our new energy level is 86 Joules and our new voltage is 13.1 volts

Now you see from this comparison that the 10 Farad cap is still fully charged after the first 3000 watt bass hit, on the other hand our 1 Farad cap has lost 1 volt which it has to replenish.
How long does it take the 1 Farad to re-charge again ? The answer is about 5 seconds. So, a couple of consecutive 3000 watt bass hits and your 1 Farad cap becomes a drain on your system rather than a power source, whereas the larger 10 Farad still has a stable 13.x volt power supply to your amplifier.

Let’s go one step further and try to scientifically answer the question: How big a cap do I need for my system ?

The following is calculated as a minimum subwoofer capacitor required to maintain a voltage of 13.8 to 14.0 volts for the power level indicated….

Now this is the first time I crunch these numbers but it’s very interesting to look at things this way.

Another great feature of the Boss Cap10 is that it has built in digital circuitry to do 2 things:

1- Monitor your running voltage level and warn you if your voltage dips below a safe level.
2- To monitor the charging and discharging of the capacitor. A charged directional electronlytic capacitor can be dangerous without proper monitoring circuitry.

Reverse the polarity of the wiring on a bare electrolytic capacitor, charge it for 3 seconds and watch it explode (I’ve seen this mistake first hand in my EE250 laboratory back when I was in college). Reverse the polarity on a 10F capacitor and you’re asking for trouble.

Then again, fully charge a 10F capacitor and then accidentally short it, and watch the flames. As we stated before these caps have an ultra low ESR (equivalent series resistance) and so if you short a 14 volt capacitor that has an internal resistance of 0.002 ohms then the peak current it can deliver is 7000 amps. You really don’t want to be the fire starter.

So to be able to deliver to you a powerful, effective, and SAFE product Boss Audio has included an charging and discharging circuit monitor to shutdown against reverse polarity wiring and protect against short circuits and thermal overload protection. Totally awesome.

Note: Read more about the Boss Cap 10 Subwoofer Capacitor, deliver more peak power to your system, and save 150 dollars off of MSRP.

Technorati Tags: 10 F

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