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I have drawn up my 20Hz subwoofer horn in Google SketchUp 3D. This horn is more for home theater than for music. My smaller 40Hz loaded with B&C Speakers 8PE21’s is more musical and articulate. The 20Hz tapped horn is loaded with two Tang Band W8-740C woofers. These are high excursion heavy cone woofers. They have quite high Bl factor which makes them good candidates for horn loading for low frequencies. The woofers are arranged in a push-pull configuration with one woofer reverse mounted and wired in opposite polarity. This arrangement linearizes the excursion components of the woofers and cancels nearly all even order distortion.

The first picture is the side profile with the sides still off so you can see how it is folded. If you look over to the left top of the woofer chamber, you can just see where the throat opening is into the horn path.

The next picture is a closer look at the woofer chamber and horn throat. The entire horn is of the constant width design. There are no wired or difficult angles and tappers to cut.

The third picture is a look from under the horn. You can actually see the throat opening into the horn path from the woofer’s view point.

The last picture is looking into the mouth of the horn with the side panels on. In the finished horn, I have removable panels to access the woofers from each side. In this picture, the right side woofer would drop into the chamber with its magnet inside the chamber. For the left side, the woofer would mount with its cone facing the chamber and its magnet sticking out toward the horn mouth.

How does it sound? The best way to describe it is clean and subtle. If the gain on the power amp is adjusted properly, then it is down right acoustically invisible. It does not let its presence known until it is called on. It adds a very nice tactile sense to movies and enhances the experience greatly. I would have to say it is a success.

Physical size is 16” wide X 17” deep X 65.5” tall. The horn path length is just a hair over 19 feet.

Here is the HornResp predicted response and input parameters.

As I guessed, room gain in my place begins at 38Hz, so the actual response is completely flat out to 16Hz. Sensitivity is about 97dB with 1 watt input. It doesn’t take much power to get this thing going.

I would encourage anyone to try a tapped horn. With tools like HornResp, it is pretty easy to design a tapped horn.

Rgs, John

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