Bass on a Budget

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.   

1. Enclosure:  SKAR Audio SK1X12V     $94
   
2. Sub:  SKAR Audio SDR-12 D2             $90
3. Amp:  LEGIS Audio 500.1 D                 $66
4. Wire: Recoil OFC 8 Gauge Kit               $50
   

LEGIS Audio 500.1 D

 

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.  

SKAR Audio SK1X12V

 

 

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!

1. Enclosure: NVX XMDFP122  $120
2. Subs:  NVX NSW 124 v2     $90
3. Amp:NVX XAD 11   $110
4. Wire: NVX 4 AWG Kit:  $90

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.
 

1. Enclosure, Subs, and Amp:  Down 4 Sound Bass Package:  $900
2. Wire:  Down 4 Sound 4 Gauge OFC kit $70
   

JP8 AMP 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.

 

Down 4 Sound Pro-Fab Enclosure

$2,000 Build

Now we are really kicking things up!  We are moving to some big beefy 15" subwoofers in a HUGE enclosure.  

1. Enclsoure:  2 x 15 Profab Enclosure  $550
   
2. Subs:  Sundown Audio SA V2  $770
   
3. Amp:  JP23  $350
   
4. Wire:  Down 4 Sound 0 gauge wire kit  $140
   
5. Deadener:  Down 4 Sound Deadener:  $100

 

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.

1. 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.
   
2. These may or may not fit in your car.  Measure the size of your trunk opening (not the trunk, the trunk opening)
3. If you are on a budget you probably don't have tools, so we are looking at pre-fab boxes only.
   
4. Good installation is important.  A lot of budget builds are garbage due to improper installation, tuning, and cheap wire.  
5. If you have some extra money, buy sound deadener.  It makes a huge difference.
   
6. 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 
7. Prices change over time, so you will need to follow the link to get updated prices.
8. 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.
9. 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. 
   

Passive Radiators

An 8" Passive Radiator on the Back Side of a Small BoomBox

 

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!

Hofmann's Iron Law

Dayton Audio Max-X 10" Subwoofer in a 1.5 cuft Ported Enclosure

 

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.  

Kicker Comp R 12 in a 2.5 cuft Ported Enclosure

 

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.

Cheap, Undersized, Flimsy Subwoofer Enclosure.

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. 

Free Subwoofer Box Plans: Dayton Audio Reference High Output 12

 

 

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.

Front

Back

Left and Right Side

 

Top and Bottom

Port Detail

Window Brace

Here is a cut list for  your convenience.

Cut List

Build Plans: Home Theater Subwoofer

In this post I am going to share with you plans for a home theater subwoofer with some interesting design elements.

 

 

 

 

 

 

Parts Used*

1. Dayton Audio Classic Series 4 Ohm 12" Subwoofer
2. Young 300 W Plate Amplifier
3. B52 3" x 5 " port flares
4. Birch Veneered Plywood
   
5. Wood Glue
6. Birch Edge Banding
7. Clear Poly
8. Textured Coating
   
9. Pocket hole screws
10. Cap Head Hex Wood Screws
    
11. Brad Nails
    
12. 5 minute epoxy
13. Polly-Fill Quilt Batting
    
14. Sound Deadener 
    

2 Design Goals 

 

The hardest part of building a speaker is making it look good, especially if it is a budget build.  Wood veneer is a simple solution that that looks great, but it can really run up the cost of a project.  It is no more expensive to use cabinet grade as compared to veneering MDF.  The problem with plywood is the exposed layers on the edges. Some people like the look, I don't.  So, this design is going to make use of overhangs to cover as many edges as possible, and edge banding to cover the rest.

As far as the sound goes, the goal is a a relatively flat response that reaches as low as possible, without exceeding X-MAX.  The design will make use of 2 3" ports with flares on both ends.  The port area is not big enough to prevent port noise, so flares are used to fix this problem. 

Cut List

• Baffle (2):  15" X 21.25"
  
• Back (1):  15" X 21.25"
• Sides (2):  21.75" X 21.25
  
• Top/Bottom (2):  22.25" X 17.5"
  
• Window Brace:  15" X 19"
  
• Port Support: 15" X 7" 
• Ridge Brace:  15" x 7
  

 

Dimensions 

 

 

You are free to cut up your plywood anyway you like. But, take some time to plan ahead.  Unless you have a large table saw with some big in-feed and out-feed supports you will want to have the plywood cut down to size when you purchase it.  This will make it easier to handle and transport.  Another alternative is to get a Kreg Rip Cut and some Styrofoam insulation.

I am very fortunate because the guys at my local big box stores are very accurate when they make cuts.  But don't count on that.  Ask them to oversize the cuts by at least 1/4", possibly more.  Plan this out before you go pick up the plywood.  In my area they will make two free cuts, but if they guy is in a good mood he will do more.  The best advice I can give is to make all of the cuts of a given dimension at the SAME TIME.  That way if you are off by some small fraction (1/16" or less), it will not make any difference in the finished product. 

Here is an example of how you might break down the material. This assumes that you can get two free cuts at  your big box store.

• Start with the largest dimension is the top and bottom piece, at 22.25" (22 1/4").  Have the store cut a piece that is 22.5"x 48".  Then when you get it home you can trim it down to size to 22.25" Then you can set the saw to 17.5" and cut out the top and bottom.  That will leave you with a 22.25" X 12.875" piece of scrap.  Save that piece, you can use it for the port support later. 
• The next largest dimension is 21.75.   So ask for a 22" cut.  Again, trim this down to 21.75" at home.  Then you can make the two 21.25" sides.  When finished you will have two side peices plus a 21.75" x 5.375" piece of scrap.  This one will not be very useful.  But it may work great for the port rings, so save it.
• Now with the saw still set on 21.75 take the last piece (this one will be large you may need some help if you are using a table saw, at the very least you will need some rollers if you are using a table saw) and rip that down to 21.75.  Then set the saw to 15" and make three cuts for the double baffle and the back piece.
• With the saw still set to 15" take the remaining plywood and rip it down to 15".  If you want to grab the scrap from before and make the port brace do that now.  Set the saw to 19" and cut the window brace, then to 7" and cut the port brace and ridge brace.  The ridge brace was added after the fact and even though it did raise the tuning frequency it stiffened up the box quite a bit. 
  

Port Design

The ports are made from inexpensive flared port tubes available from parts express.  The tubes themselves are not long enough to reach the desired tuning frequency.  So extensions were fabricated to lengthen and connect the tubes.  The internal tubes are connected to a brace inside the box and the tubes are mounted using 5 minute epoxy.  You will have plenty of 3/4" scrap from your plywood to connect and extend the ports.  When you take into account the fact that 3/4" plywood is undersized by 1/32" then four layers of plywood rings will get you very close.  Here is a video showing the fabrication process and the mock-up:

 

 

Assembly

 

You can use any method you like to assemble the enclosure.  Keep in mind that the wood glue does all of the real work, any mechanical fastener that you use in the build does the same thing that a clamp would do, it holds things in place until you can   Typically I build enclosures out of MDF, and I just use wood glue and brad nails.  One of the goals is to show off that nice expensive birch veneered plywood, so you will need to take care that you don't damage the surface veneer.  Therefore clamps and glue will work just fine.  But I am going to make use of some additional tools.  As described here.

You can start by laminating your two baffles pieces together, before you make the speaker and port cut out.  If you go this route you will need a rather large rabbiting bit in order to recess the speaker, or you can forgo the speaker recess.  If you don't have the proper rabbeting bit you can cut the circles before you laminate the pieces.  Here is a video with some tips to help make that process easy.

 

 

For the ports I made several test cuts in scrap material in order to get the best fit, and I used a round-over bit to match the curve of the ports.  Make sure to align  port holes on the brace support with the port holes on the baffle.  To do this I clamped them together, made sure that the pieces were square to each other and drilled the pilot holes for my circle jig at the same time.  My original plan was to recess the ports, but that did not go as planned so I just inserted them in the port holes and epoxied them into place with 5-minute epoxy.  I then  up the ports, the port support and the window brace an connect them all together.  Here is a video that will hopefully shed some light on the process.

 

 

That video also shows how I used pocket holes and pocket screws to connect the internal parts, as well as the process for the window brace.  The top, bottom, and sides overhang a bit.  This was an important part of the design as helped hide most of the plywood edges.

Finishing

The goal of the finished product was to hide as much of the plywood edges as possible.  This was done by paint the back, bottom, and the bottom edge with black duratex, and spray painting the subwoofer recess black.  Then iron-on edge banding was used on the top and the front of the sides.  The entire thing was finished with Polycrylic. 

*This site uses affiliate links, if you click on a link then we may earn a small commission on some purchases.  As an Amazon Associate we may earn a commission on qualifying sales. 

 

What is a Bandpass Subwoofer?

What exactly is a Bandpass Subwoofer?  Let’s take a look and see!  Along the way we will dispel some popular myths about bandpass subwoofers.

 

Forth Order Bandpass Subwoofer Enclosure

The image above shows a speaker enclosure with the subwoofer mounted on an internal baffle that divides the box into two separate airspaces.  One of those chambers is sealed, one is ported.  At first glance this seems like an odd configuration.  How does it make any sound if the speaker is inside of the box?  The answer is simple once you understand that a port is just a speaker made out of air.  As the air in the ported side of the enclosure is compressed and decompressed it causes the air in the port to move back and forth and the port produces the sound.  A common myth is that the subwoofer driver makes the sound and the port “let’s the sound out”.  That is not accurate, what you are actually hearing is the sound waves produced by the port itself.

DIY 4th Order

It turns out that there are many different ways to configure a bandpass enclosure.  The simplest, the one shown here is known as a 4^th order.  This article will only focus on 4^th order enclosures.  The image at the left shows the internals of a 4^th order bandpass that I built a few years ago. 

Why would you want to use a bandpass subwoofer?  The answer is simple “efficiency” which is code for “loud.”  A bandpass enclosure has the potential to be much louder than the same subwoofer in a sealed or a ported enclosure.  This is, of course, not an absolute statement.  From my experience modeling, designing, and building speakers a ported subwoofer can often play just as loud as bandpass subwoofer.  But in general, if you want to get the “most” out of a subwoofer then bandpass is the way to go.  They pop up quite often in car audio SPL competitions.  

 

Mackie Professional Bandpass Subwoofer (click image to view on Amazon)

 

This is where one of my favorite myths about bandpass enclosures pops up. 

If you do some searching online you will learn that there is a “magic” ratio that you need to use to build the “best” bandpass enclosure.  This ratio is 3:1, meaning that the ported chamber has three times the volume as the sealed chamber.  This is bunk.  One only needs to crack open Vance Dickson's Loudspeaker Design Cookbook to discover a lot of very complicated formulas, none of which have anything to do with ratios. Next, what do we mean by “best”?  Best in this context means loudest.  Check out the following screen captures from WINisd showing the response for three different bandpass designs. 

 

Three Bandpass Response Functions

Dayton Audio High Output Reference 10"

These graphs are for the Dayton Audio High Output Reference Subwoofer.  It is an awesome driver with an cool aluminum cone.  What you will notice is that the 3:1 (blue) is louder than the 2:1 (Black), which is still louder than the 1:1 (Red).  You will also notice that as the ported chamber gets larger, relative to the sealed chamber, that the bandwidth (i.e. the passband) gets smaller.  The 3:1 box is a “one note wonder”, also known as a “burp box.”  The idea is to play a single frequency, the one where the box peaks, and throw two or three times the rated power at the subwoofer (cooking the voice coil in the process).  You then pop the driver out, throw in your replacement driver and re-cone the original driver. 

 

This leads us to the second myth.

Bandpass subwoofers sound bad.  Not at all.  In fact Bose, a name that many** associate with sound quality, patented a very complex bandpass design back in the 1980’s.   A 3:1 bandpass sounds like utter crap.  But, a bandpass enclosure can be tuned to sound absolutely amazing.  The other contributing factor to this myth are the cheap pre-fabricated bandpass subwoofers designed for car audio.  These often have flimsy plexiglass viewing windows and fancy lighting.  They look flashy, and they can play loud, but they use cheap drivers and sound terrible.

 

A flashy, but otherwise terrible enclosure.

 So how do you design a bandpass enclosure so that it sounds good?  The answer is tricky.  There is a reason why most subwoofers use a ported cabinet.  Designing a bandpass enclosure that can outperform a ported enclosure is a real challenge, building the enclosure is also a challenge.  The stars have to align.  You need the right driver in the right box for the right application, and no amount of cleverness can ever overcome Hoffman’s Iron Law. 

 

 

But this is a place where we learn new things and challenge ourselves, so let’s go over some basics.  The best common sense explanation of bandpass tuning that I have been able to find is on JL Audio’s website.  I am going to break it down here into it’s three most important components.

 

The sealed portion determines how low the subwoofer will play.

This is of course very dependent on the driver, some will play lower than others.  But, for any given driver if you want to hit the lows then the sealed side needs to be large.  There is, of course, a tradeoff.  If the box is tuned low you will have to worry about power handling.  Always remember that the air in the enclosure is part of the subwoofer’s suspension and smaller boxes have a stiffer suspension.  Stiffer suspensions equate to higher power handling.  This is one of the reasons behind the magic 3:1 ratio.  That small rear chamber means you don’t have to worry about mechanical failure when playing loud.  You will melt the voice coil long before you hear the subwoofer make any mechanical noise from over-excursion or hitting the back plate.  The car audio SPL crowd will typically place the magnet in the ported side of the enclosure so that they can smell the voice coil heating up and pull back on the power before cooking the coils. 

The ported side determines how loud the subwoofer will play.   

Again, this is the justification for the 3:1 ratio.  A big ported side will be very loud, but there is a tradeoff.  That tradeoff is called the “passband,” the passband is how the bandpass enclosure got its’ name.  Consider the following diagram:

The Passband of a Bandpass Subwoofer

You can clearly see that the bandpass enclosure plays a range, with a high-end cut-off and a low-end cut-off.  It is not unlike the crossover that you apply to your midrange speakers, it does not play highs, it does not play lows.  The crossover allows a narrow bandwidth to pass through to the speaker. A passband.  As the ported side gets bigger the passband range gets narrower, giving us a one-note wonder.  As we make the ported section smaller, we get more music out of the box, but the music gets quieter.  The other problem is a thing known as the passbrand ripple.  Consider the strange “dog ears” in the diagram.  There is a peak at the upper and lower frequencies, and there is a dip in the middle. That is going to sound bad. The difference between the peaks and the valley tend to increase as the passband widens.

Port tuning is counter intuitive.   

The port tunes the ported chamber.  But the port tuning is based on the sealed chamber.  The port should be tuned to the center of the passband and the center of the passband is determined by the resonate frequency of the sealed side.  The resonate frequency of the sealed side is determined by the subwoofer’s free air resonance (Fs), the stiffness of its’ suspension (Vas) and the volume of air in the sealed side (Vb).  Based on this simple formula:

 

Adjusting the volume of the sealed side will adjust the center of the passband, as the sealed portion volume (Vb) gets smaller Fc and thus the center frequency of the passband gets higher, a larger box and the center of the passband gets lower.  This formula also gives us some insight into selecting the driver.  If a low tuning is desired then look for a woofer with a lower Fs and a lower Vas.  If you do not tune the port to the center of the pass band you will exaggerate one of the dog ears and flatten out the other.  This will sound like garbage.  Here is what that looks like:

A Poorly Tuned Bandpass Response Function

There is a whole lot more to it, but this is supposed to be a brief, non-technical article.  So, I will stop here!  If you have enjoyed this article consider becoming a patron on Patreon, or you can always drop me a $5 or $10 tip via paypal:  paypal.me/diyaudioguy

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