This post is going to hit a nerve! People get very sensitive about this subject.
Plywood (left) and MDF (right) Subwoofer Enclosures
There are two main materials that we build boxes with. Plywood and MDF so here are the pros and cons of both.
Five Pro's of MDF:
MDF is heavy! You want heavy and dense for your sub box!
MDF is "inert" -- meaning it will not resonate and color the sound. It also does not expand and contract as humidity changes.
MDF is true to size! 3/4" MDF is 3/4" While plywood is often NOT
MDF is easy to Machine. If you are dong complex router work use MDF, it will not tear out like Plywood.
MDF is CHEAP! That is not reason to use MDF, it is a great material for the above reasons. So think of the lower price as a bonus.
Carpeted MDF Enclosure
Five Pro's of Plywood:
Plywood is light, up to 50% lighter than MDF.
Plywood looks awesome! If you know how to finish and stain wood then you can make some amazing works of art out of plywood.
Plywood is inert -- just like MDF the box will not color the sound. Especially if you use a high-quality plywood.
Plywood plays well with wood screws. It is less prone to splitting and the layers do a good job of holding on to screws.
Finally plywood is strong! How strong, check out this video!
Five Con's of MDF:
MDF is HEAVY! Yes, I listed that as a pro. But heavy boxes break backs! Plus they add weight to your car.
MDF does not like screws. It splits if you don't drill a piolet hole and you need coarse-grain screws or they will pullout.
MDF is hard to paint. This goes against conventional wisdom, but it is true. Especially the edges. You need to seal it before you paint it. But the best thing to do is cover it with carpet or veneer.
MDF generates nasty dust. Fine particles that get everywhere, like your lungs. You do need a dust mask when you do this kind of work.
MDF can't get wet! It will swell! Also, don't feed it after midnight.
Plywood Subwoofer Box
Five Con's of Plywood:
Plywood is expensive, at last the good stuff is.
You are not using good plywood. I see a lot of boxes online that use construction grade plywood and even if you opt for "birch" or other hardwood plywood from the local home center these are not hardwood. These have a decorative hardwood veneer, the plys are all pine.
The plys are visible, and they can be ugly.
Plywood is undersized -- it is typically 23/32" of an inch. So you need to take that variance into account when building your box.
It is hard to make it look good -- you need some experience with stain or the box will look nasty.
This is the legendary, and reliable Jasper Circle Jig. It is a must-have, and the best tool for DIY speaker cut outs. For those that don't like making Jeff Bezos even richer, consider purchasing one from Taylor Tool Works.
Go ahead and spend the money on a quality bit. You will use this, along with the circle jig and plunge router, to plunge into wood and cut holes for speakers, ports, and terminal cups. A straight bit will work, but these spiral bits plunge much better. Again, check check out Taylor Tool Works as an alternative to Amazon.
Even though I have a great table saw I still use my circular saw quite often. You can break down plywood and MDF to manageable sizes to use on the table saw, or just use your saw with a straight edge or edge guide. Go ahead and get a cordless one. The brand does not matter, get one that uses the same battery as your cordless drill.
This may be my favorite beginner/DIY tool. With a little practice you can get straight, accurate, and repeatable cuts. Plus this thing is CHEAP. Check out my video review:
Some people prefer "glue and screw." I prefer to use a brad nailer. I use Ryobi simply because they were the only cordless platform with an affordable cordless nailer. You can use a pneumatic one, but you will also need an air compressor, plus hosses and accessories.
Yes you DO need hearing protection. I have been using 33M Worktunes. These let you listen to music while you work, plus they are bluetooth so you can take phone calls.
All builds are budget builds, budgets are just different. Here are four budgets and some gear that will get you the bass you crave. Make sure you check out the end of the article for a few caveats*.
$300 Amazon Build
For this build we are going with a single 12" instead of an 8" or 10". Cone area is king.
That puts us right at $300 before taxes, most of it is on amazon, so shipping is free for prime members. I have personally reviewed the SKAR box and the LEGIS amp. If you go this route pay close attention to the subwoofer and the amp. Make sure you get the dual 2 ohm sub. Wire the two connections in parallel to get the most power out of the amp. When looking at the amp LEGIS also has a "1600 watt" version for the same price. Look at the fuses. The 500.1 D has 2 25amp fuses. The "1600 watt" version has 1 25 amp fuse. You cannot pull 500 watts through a 25 amp fuse, much less 1600. You will be hard pressed to do beat this set up for under $300. SKAR has the budget bass market cornered, and Recoil is putting out some great budget gear. The weakest link in this setup is the amp. You may want to spend some more and upgrade to the Recoil RED 600.1 for another $30. But that puts you over the budget. You may be tempted to go with a cheaper sub box. Please don't. Here is an image of the SKAR box nest to a cheap enclosure.
The SKAR is bigger, a bigger box will give you more bass. It is called Hofmann's Iron Law:
$500 NVX Build
Have you heard of Sonic Electronix? If not you should check them out. Their house brand,NVX, provides excellent bang for the buck. For the $500 build we are going to kick things up to a pair of 12's and 700 Watts total system power. If you purchase directly from NVX use the COUPON CODE DIYAUDIO10 for a discount!
That brings us to $410. If you use the discount code it brings you down to $370. Plenty of room to throw in some NVX Sound Deadener to keep your trunk from rattling or a quality bass enhancer to integrate with your factory radio.. The subwoofers are dual 4 ohm, when you wire everything up in parallel you will get a 1 ohm load. Double check when ordering to make sure you have the dual 4-ohm subs in your cart. The amp is a "Brazilian style" full-bridge. Some people do not like Brazilian style amps. One alternative is this N-Series amp. Another alternative is the Recoil RED 600.1:
$1000 Build
At this level we kick things up to a pair of 12's and bigger amp. This is the threshold for what you can do with a stock electrical system. You may need to consider upgrading your battery or alternator (or both) before ordering this gear, but it is not 100% necessary. All of this is available from Down4Sound.
This is a package deal. I personally run the single 12 version of this with the same amp and I am very happy with the performance. The official ratting on this amp is 700 watts, but this amp will give you over 1200 watts IF you have a good enough electrical system. Expect 900 watts at the most with stock electrical. I tested it on my bench and was able to get 900 watts before my power supply was unable to keep up. But I still got over 800 just running off of the battery. Plus the JP amps have the best bass knobs in the business.
Before we look at the next budget, we need to draw a line right here. At this point we move from "daily driver" systems that can work OK on stock electrical to a system that requires upgraded electrical. Most people will install a second battery, but that is the wrong thing to do. Most people do that because it is cheaper than doing it the right way. What is the right way? Upgrade the alternator. You might end up spending $300 to $600 on this, you will probably need a new serpentine belt. You will also need to look very close at your available space. The boxes get huge, so you will need to make sure your subs will fit.
That puts us at $1910, so that gives us some extra wiggle room. There are lot of other good options out there at this price point. I went with products from Down 4 Sound because I have personal experience with both the pro-fab enclosure and the JP amplifiers. The Down 4 Sound organization focuses on the "basshead" market so if you want to make big bass that is where you want to look. This option includes sound deadener because it is no longer just an option or a good idea. It is a must have for a system this big, I would recommend spending the rest of your $90 budget on some more. If you are looking for an alternative to the Sundown Subwoofers, then I suggest that you look at the Kicker Q class 15's.
Over $2000
You would not know it by looking at social media, but builds this big are pretty rare. Most car audio consumers are running 10s and 12s on less than 1000 watts. Remember that cranking up the power means you need to make other upgrades. If you want to spend over $2k on just the sub stage then you will need to do a lot more research, or visit a local shop.
Here are the caveats I mentioned above.
In
this post we are just looking at the subwoofer part of the system. You
will need a head unit or a good line output converter (LOC) to get
signal to your subwoofer. Most amplifiers will accept speaker level
inputs. Not all of them are good.
These may or may not fit in your car. Measure the size of your trunk opening (not the trunk, the trunk opening)
If you are on a budget you probably don't have tools, so we are looking at pre-fab boxes only.
Good installation is important. A lot of budget builds are garbage due to improper installation, tuning, and cheap wire.
If you have some extra money, buy sound deadener. It makes a huge difference.
Pay
attention to the OHMs and the voice coils. If you are going to be
using a 1 OHM amp you need to make sure you can wire your speakers to 1
OHM
Prices change over time, so you will need to follow the link to get updated prices.
I
am including an amp wire kit in my budget. Don't cheap out on the
wire. Go ahead and get OFC wire, avoid CCA. If you insist on getting
the cheaper wire then you will need to go up in size. Keep in mind that
smaller AWG numbers are lager wires.
I tried to group these all together so that you can order everything you need from one place. The links in this video are affiliate links, and I may earn a small commission if you make a purchase.
What exactly is a passive radiator, when and why would you use one?
I remember my first encounter with a passive radiator. I was in high school and I was browsing a catalog (way back in the stone age before we had the internet). I was smitten with some very sexy tower speakers that had multiple 6.5" drivers. Reading through the stats I noticed that 2 of the 3 drivers were passive radiators. That same catalog had pre-loaded car audio subwoofer enclosures designed to fit under the front seat of a car, these designs used passive radiators, also called drone cones. Several manufactures have made these over the years. The modern version is the JBL Fuse, pictured below. My initial gut reaction was that this was a rip-off. A speaker with no magnet or voice coil that gave the illusion of multiple drivers. I could not have been more wrong.
JBL Fuse Subwoofer (Click on Image to View on Amazon)
It turns out that these drone cones have some interesting uses and are a great way to get some extra bass out of small enclosures. Back in the 1970's when Kicker started building subwoofer cabinets for cars they used passive radiators. Why? Because passive radiators shine when you are trying to build small enclosures designed to fit into tight spaces. Kicker has even brought back a line of enclosures that utilize passive radiators. Heck they will even sell you a passive radiator that you can use in your own custom enclosure.
Kicker 8" Passive Radiator (Click on the Image to View on Amazon)
What is it and when do you use one?
For all practical purposes a passive radiator is nothing more than a very complex port. That sounds strange. I have a blog post on how a port works. Here is the tl;dr version. When a subwoofer compresses and rarefies the air in a ported enclosure it will cause the air in the port to resonate. The port air has mass, so it takes some effort to move it, so it only resonates at a very specific band of frequencies. When you do move it it will cause sound waves in the exact same way that a speaker cone causes sound waves. You can adjust the port tuning by changing the opening (the cross section) and the length of the port.
Passive radiators have mass, you tune them by increasing they size (or number) and by adding additional mass to them. They are not as effective as ports, and they can be a lot more expensive. But they have one specific situation where they really shine. Small enclosures.
As I discuss in my port tuning series on YouTube as the enclosure gets smaller the port has to get longer in order to maintain the same tuning frequency.
So if you want to fit a ported enclosures behind the seat of a regular cab truck, under the seat of a crew cab truck, under the front seat of a car, or ANY other tight spot you will need to make the enclosure very small. So then you need an absurdly long port. You can't get the port to fit in the required space, so you use a passive radiator. There really is no other good reason to use one. I will say that again just so we are clear. A port will out-perform a passive radiator at a lower cost in all but one case, when you are trying to make a very small enclosure.
JBL Bluetooth Speaker (click on image to view on Amazon)
Passives also work very well in boomboxes and "Pill" shaped Bluetooth speakers. Parts express offers several compact Bluetooth boomboxes that make use of a passive radiator.
I own a quad-cab dodge truck and I desperately need some upgraded bass. So I have been experimenting with pervasive radiators. My eventual plan is to create a custom fiberglass enclosure and make use of passive radiators to kick my performance up to the next level. I have already experimented with some test boxes and have learned a lot about them. Stay tuned so you can see what happens when I figure out how to work with fiberglass!
*Links on this site may be affiliate links, if you click on an affiliate link and make a purchase then I might get paid a small commission. So thank you for clicking the links!
Josef Anton Hofmann was a physicist, audio expert and entrepreneur.
In addition to earning Ph.D. in physics from Harvard and working on the Manhattan Project he was a co-founder or partner in no less then three groundbreaking audio companies; KLH (the H stood for Hofmann), Advent, and Acoustic Research. These companies, which today are all shells of their former selves, were the giants in hi-fidelity home audio back in their heyday. Acoustic Research, for example, was the first company to produce an
acoustic suspension (i.e. sealed) loudspeaker. Interestingly enough a
simple sealed speaker enclosure was not invented until the 1950's by
Edgar Villchur, the founder and president of Acoustic Research, while
the vented enclosure was patented by a scientist Bell Labs in the 1930's.
We would not have the field of acoustics, much less the market for hi-end audio equipment, as we know it today with out Dr. Hofmann. His work would would become the backbone that Neville Theil and Richard Small would use to create what is now known as the Theil-Small parameters.
When explaining the mathematics behind enclosure design people often ask me why this stuff seems like rocket science. Simple answer: It is. I cannot find any direct evidence that Dr. Hofmann was a literal rocket scientist. However, he was clearly an intellectual giant and could have easily pursued this field. Instead he focused on things that were equally as complex like semiconductors, solid state devices, and of course, the pursuit of audiophile quality sound.
Hofmann's Iron law is elegant in it's simplicity.
The law focus on the interplay between enclosure size, efficiency, and low end extension.
Efficiency is a way of quantifying loudness relative to power level. A more efficient speaker can play louder with the same amount of amplifier power as a less efficient speaker. With today's modern class D designs amplifier power has become cheap. But imagine trying to fill a room with sound using a tube amp from the 1950's or transistor radio from the 1970's.
Low end extension is just what it sounds like, the ability to play bass frequencies. With the massive subwoofers available today this is trivial. But again, think the post WW2 era. Long before Cerwin-Vega started producing strokers for movie theaters. Long before Kicker started putting enclosures behind the seats of regular-cab pick-up trucks. Back then not even the best hi-fidelity sound systems could play 20 HZ with authority.
Imagine that you are Dr. Hofmann and your boss, the inventor of the sealed loudspeaker, needs you to build a hi-fidelity speaker that could reproduce quality music, covering wide band of frequencies, in a way that had never before been done.
That is where enclosure size and Hofmann's iron law comes into play. Dr. Hofmann determined that if you wanted and enclosure that was efficient and had good low end extension the key to getting low and loud was a large enclosure. Especially at at time when adding more power was not easy or even feasible. Hofmann's Iron Law has been summarized like this: with a given amount of power you can be low, you can be loud, or you can have a small enclosure. Pick two. This means that a small enclosure can either be efficient or it can have low end extension, but not both. An efficient enclosure can either be small or it can have low end extension but not both.
I have been unable to find any specific mathematical formulation of Hofmann's Iron Law written by Dr. Hofmann. A mathematical formulation does exist, but it makes use of T/S parameters. For those that are interested you can find some equations on the DIY Audio Forums. If any reader has information on these formulas or happens to have access to any of Hofmann's original research please let me know. I would love to make a post breaking these down into common-sense terms. In the mean time the best explanation of the math that I have found has been this facebook post.
Implications for today
Simple. It takes space to make bass. Especially ultra low frequencies. Consider this review of DIY home theater subwoofers:
In that video Nick from Toid's DIY Audio showed off a 15" subwoofer in a 5 cubic foot enclosure. I have the 10" version of that subwoofer. The manufacturer recommends 1.1 cubic feet. My plans call for a 1.5 cubic foot enclosure. Why so big? Because Joseph Anton Hofmann is smarter than I am, and he is probably smarter than you as well. So if you insist on installing a 12" subwoofer in a pitiful box like this one go ahead.
I am going to build a box that is about 50% bigger and my box will be at lest 3DB louder at every frequency, plus my port will be tuned lower so I can hit frequencies that your box cannot and I can do all this without the subwoofer unloading. Your only chance of keeping up with me is to double your amplifier power. When you do that you will need to go ahead and buy an upgraded aftermarket alternator. The only downside to a bigger box (besides the space taken up by a bigger box) is cone control. Under high power the driver will exceed X-max and distort. Fortunately, the larger box is more efficient, so Hofmann's Iron Law tells us that we don't need as much power.
If you find yourself in a situation where you are forced to use a small box, like an under-seat setup in an crew cab pickup truck then you are stuck. You can either choose to sacrifice low frequencies for efficiency or efficiency for low frequencies. The iron law has you. You can't do anything about it, other than throwing power at the problem.
Note: Most of the links in this article are affiliate links, I may earn a small commission if you click on a link and make a purchase.
One of my patrons over on Patreon wanted some help with a box design.
He is trying to fit a 12" subwoofer in the middle of the floorboard of
his crew cab pickup truck. For the driver we are using Dayton Audio Reference High Output 12" subwoofer.
The enclosure is 2.5 cubic feet and tuned to 32 HZ. For the port we
went with a 2" by 19.5" slot port that is 36 1/4" long. The f3 is 28
HZ. Here are the frequency response and cone excursion plots from WINisd.
Frequency Response Ported (Green) vs Sealed (Blue)
Cone Excursion Ported (Green) vs Sealed (Blue)
For more information on how these were modeled in WINisd give this video a look:
If you would to build the box for yourself the you should be able to piece it together with the dimensions in these in these pictures.
Circular Saw:
The port air has mass, so it takes some effort to move it, so it only resonates at a very specific band of frequencies. When you do move it it will cause sound waves in the exact same way that a speaker cone causes sound waves. You can adjust the port tuning by changing the opening (the cross section) and the length of the port. 
They are not as effective as ports, and they can be a lot more expensive. But they have one specific situation where they really shine. Small enclosures.
