Introduction: Building a Tapped Horn

A little background first: I’ve been actively involved in the audio community for quite some time, and have built many, many speakers and subwoofers. Full range, multi-way, front loaded horns, rear loaded horns, transmission lines, open baffles, isobaric, you name it. I don’t believe in fancy cables or mystical noise filters, just sound engineering along with repeated listening and measurements. The purpose of this instructable is to demonstrate a different way to build subwoofers, which has become my absolute favorite method for low frequency reproduction: the tapped horn.

The tapped horn is a relatively obscure subwoofer arrangement, only recently brought to prominence by Tom Danley. A tapped horn is unlike other horns, in that it uses the radiation from both the front and rear of the driver, and combines them at the mouth. This allows for many possibilities, including greater efficiency, smaller enclosure size, and deeper extension. One of the greatest benefits a tapped horn exhibits over other arrangements is lower excursion (the distance a woofer moves from rest). Because of the acoustic load placed on the driver, excursion is reduced, leading to increased maximum SPL and lower distortion.

My goal of this instructable was to build a versatile, affordable, small, and high performance tapped horn that someone with reasonable woodworking skills could assemble. Don’t just think of this as another common sealed or bandpass subwoofer tutorial, this is a much different realm, and is also significantly harder to build. It utilizes two 8 inch MCM 55-2421 drivers, which cost $28 each, and perform at a level of drivers costing magnitudes more. Add a sheet of plywood and few bits of hardware, and you have an excellent tapped horn subwoofer for $120. How much would a tapped horn cost commercially? Well, the most affordable tapped horn sub I know of is the TH-Mini, which runs about $1300 per piece. For less than 1/10th the price, you can see what all the fuss is about. Now, without further delay, let’s make some sawdust!

What would I do with a Shopbot? Tom Danley had hinted at offering a kit for a multiway tapped horn speaker, but because of limitations with time and a focus on the professional market, it is something that will not happen. However, he was very supportive of the idea, and offered to license the technology to a third party if they wanted to tackle it. If I were to get a Shopbot, I would pursue a licensing agreement with Danley Sound Labs to provide these kits at a very low price. It would be my way of giving back to the community for a Shopbot that I didn’t have to pay for. A Shopbot can precisely cut the complex angles and provide the precise alignment that is necessary for such an undertaking, not to mention do it at a rate that would allow production costs to be kept very low.

Step 1: Designing the Horn

I designed this tapped horn using three different programs: Hornresp (horn response), AkAbak, and Sketchup. All are free, but all have a learning curve, some of which are steeper than others. I won’t go into how to use these programs, as there are tutorials online if you are interested. If you haven't worked with speakers in the past, you will have to spend some time educating yourself on the various parameters and meanings. If you need any help, or want guidance on where to find info on using HornResp or AkAbak, feel free to ask questions in the comments section!

The arrangement of the driver is roughly similar to the TH-Mini, which has the driver on the bottom of the cabinet firing down into the throat. I designed mine in a similar fashion, as it works well in a cabinet this small. I’ve attached my Hornresp input file, as well as the Sketchup file with a 3D model. The cabinet is approximately 30”x30”x10”, but the 10” width can be increased to smooth out the response if materials and space allow. Please note that I did not follow the model I made in hornresp exactly. I changed the flare at the end to use as much space/wood as possible. The predicted response won't be far off, subtle changes so far down the horn have little effect.

When using a tapped horn, one has to be very careful about low frequencies that are out of the pass band, as excursion rapidly increases. An active high pass filter is what I generally use, but a passive high pass filter would work just as well. I'll explain later this in greater details later.

Step 2: Gathering Materials

The MCM 55-2421 can be found online here:

If you search for the 55-2421 on the MCM website, the price will be $38, but when I search for it in Google shopper, it is only $28. Not sure exactly why, but check to make sure you’re getting the right price. My total came out to $71.49 for two drivers including shipping.

Plywood: I used one 5x5 sheet of Baltic Birch ½” plywood, and one small scrap piece of ¾” plywood for the baffle the drivers mount to. You can use all ½”, I just had the ¾ on hand. Bought this from a local wood company for $44, and it’s high quality void free plywood. Try not to skimp here, and DO NOT use MDF.  A well braced plywood cabinet is superior to one made of MDF, and the additional stiffness is noticeable in both sound quality and overall strength. MDF also contains formaldehyde, which is used as a bonding agent, and cutting this stuff is nasty business. Even with a respirator, it will get everywhere in most non professional wood shops, and it is simply not a risk worth taking. For a unique look, bamboo plywood can also be used, which I have used in many of the speakers I’ve built.

Binding posts: I chose to use a Neutrik speakon connector ($2), as I like the cheap cost, secure connection, and fully insulated design. Feel free to use anything that works for your application.

Wire: I used some extra bits of 16 gauge stranded copper here, but any decent copper wire should be fine, given that it is at least 18 gauge.

Glue: I have a large bottle of Titebond II that I use for most woodworking projects, but will also use PL polyurethane adhesive. Achieving a good seal is very important in any horn design, and the expanding nature of polyurethane can be helpful in sealing up any potential voids.

$71.49 for the drivers, $44 for plywood, and $2 for the Speakon puts the cost at right under $120. Add a little more if you need glue and use a passive crossover.

Step 3: Cutting the Plywood

Here’s how I would cut the 5x5 sheet:

The large side panels are each 30” by 29 17/32”, so let’s start there. Cut the full sheet at 30”, you will later cut this piece in half to form the two sides. I didn’t have access to a full sizes table saw, so I used a circular saw and guide to cut the large pieces to size. I’m accurate to about 1/32”, which is acceptable for these larger pieces. I use a flushing bit on my router to clean things up at the end, but sandpaper could do the trick if you are limited.

Next, rip the remaining ~29 ⅞ piece into three equal widths, I made mine roughly 9.5” wide. Did this on a table saw, which makes things easy. This doesn’t have to be an exact width, but it is important to have all these internal panels the exact same. Otherwise, an internal panel might be slightly off, which could lead to leaks, not to mention a more difficult assembly. I generally try to use all the wood with minimal scraps, here’s the formula to get you close: 29 ⅞ by 3, minus the width of the blade *2. Go a little bit smaller, just in case.

Now that you have three 60” long pieces at roughly 9.5” wide, cut these pieces to length. Here are the lengths of the internal pieces:

Bottom and Top: 29 1/16”

Back: 29 17/32”

Front: 13 ½”

Inner pieces:

a: 22 29/32”
b: 18 15/32”
c: 14 ½”
small piece that joins baffle and bottom: 1-1 ¼”, depending on what thickness baffle is used

Driver baffle: 26” - NOTE- I used a seperate ¾” piece of plywood for this piece.

Step 4: Final Preparation Before Gluing

Now that all the pieces are cut to dimension, it’s time to sketch out how the internal panels fit to the side panels. I use a straight edge and pencil to sketch how things should look, using my Sketchup drawing as a guide. I just marked the interior corners of the horn path, and line up the pieces as I assemble the horn.

One thing that is worth noting is the position of the drivers. In my Sketchup drawing, I only placed them in an approximate location. The exact location of the drivers is important to how the horn operates. We want to maintain a compression ration of approximately 3:1 to 4:1. This ratio is the total area of drivers (each driver is 210cm squared, so 420 total) to area of the horn at the point where the drivers fires into the throat. So to maintain that ratio, we want to have the drivers centered at point in the throat where the area is around 100cm squared. This will change depending on how wide your cabinet is. If you cut your panels to 9.5” like I did (roughly 25cm), that would mean the drivers should be centered around the point in the horn that is 4cm tall. I simply test fit the baffle piece, measured the point at which the height was 4mm, and made a pencil mark. On each side of this point will be a driver, oriented as close together as physically possible. Don’t fret about this too much, as I’ve found these drivers can actually handle a variety of ratios without problems.

Drill the proper holes for your input connector(s) on the back panel, and the holes for the speaker drivers. I used a circular jig attached to a router to do the large speaker cutouts (7.125” hole for each), and used a roundover bit to smooth the transition from driver to throat (note: only do this on the side facing the bottom). The speakon connector is slightly larger than 7/8", I used a forstner bit to get the approximate size, and reamed it out with a file. I also drilled two small holes in panel A for the wires to connect the input connector to the drivers. Don't forget to seal these with some glue or caulking, we don't want any leaks between chambers!

Step 5: Gluing and Final Assembly

Now that everything is cut to size,  start gluing pieces together. I start with the front panel and top piece, and work my way back, constantly checking to be sure things are square. All internal pieces are glued to one side first, making sure to use plenty of glue to avoid any leaks. A brad nailer can make things go faster, but I just use wood glue and a little patience. If you cut the pieces with some accuracy, this should be a straightforward process. One all the internal pieces are glued in place, it's time to install the drivers. I use a set of heavy duty screws, but bolts and t-nuts are also often used.

When wiring the drivers, the amplifier you are using must be considered. Each driver has a rated impedance of 4 ohms, so you can wire in series for 8 ohms or parallel for 2. I went with parallel wiring, and have an amplifier that can handle low impedance loads. If you want to add a passive high pass crossover to prevent over excursion out of the pass band (don't ignore this, use either a passive or active filter), it will change depending on how you wire your drivers. In both cases, there is a capacitor in series with the drivers, and a inductor in parallel with them. Here are the values for each wiring scheme:

Parallel (2ohm load): ~1600uf capacitor, ~12.86mH inductor
Series (8ohm load):  ~400uf capacitor, ~51.4mH inductor

These values don't have to be exact, just try to get as close as you can. I've also provided the modelled excursion of the drivers at their rated power. These drivers have an xmax (maximum one way excursion) of 8mm. As you can see, as you go below the low corner at 40hz, excursion quickly increases. At 30hz, you are above the rated max excursion. A high pass filter greatly reduces the chances of damaging your drivers due to over excursion, so please use them!

Wire the drivers to the input connector, seal the holes with caulk/glue, and take a final look at your handiwork: once the other side is glued on, you won't be able to easily access the internal workings! Apply a generous amount of glue to the edge of all internal pieces, and clamp the final side in place.

Step 6: Testing

After patiently waiting for the glue to cure, place the sub in a desirable location (corners will increase the low end response even more), and connect to an appropriate amplifier and high pass filter. Slowly bring the volume up, as long as you wired things properly, you should be able to hear output. As you crank up the volume, you will soon see just how much. When modeling this in Hornresp, the predicted maximum output was a stunning 122 dB at 40hz! For two 8 inch drivers in a 130 liter cabinet, that's pretty impressive! 

I hope you've enjoyed this tutorial, please comment if you build this tapped horn,  feedback always appreciated!

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