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.