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Old 27th December 2007, 09:56 PM   #1
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Default Tapped Horn for Dummies

Over in the tapped horn thread, I posted a cheap and easy to build tapped horn. I decided to break it off that thread, to keep all the details in one place.

Here's the post:

http://www.diyaudio.com/forums/showt...36#post1383036
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Old 27th December 2007, 10:03 PM   #2
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Here's a couple of pics of me putting it together:

Click the image to open in full size.
Gluing the end caps

Click the image to open in full size.
Believe it or not, I had all the sonotubes AND the woofers just sitting in my garage
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Old 28th December 2007, 03:00 AM   #3
judtoff is offline judtoff  Canada
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Thanks for posting, I read the post in the collaborative thread and found it interesting. Keep us informed!
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Old 6th January 2008, 07:19 PM   #4
zobsky is offline zobsky  India
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I thought your were going to put this together in an hour

Seriously though, let us know when you have this project up and running, .. I have two of those MCM drivers working in different "Tuba" horns.
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Old 10th January 2008, 07:48 PM   #5
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Quote:
Originally posted by zobsky
I thought your were going to put this together in an hour

Seriously though, let us know when you have this project up and running, .. I have two of those MCM drivers working in different "Tuba" horns.
I built a clone of the Autotuba too. That's a fun sub box. Used it in my old Accord for about six months. When I bought a new one I made a new sub, using the same driver, but tweaked it so that it would play deeper.

Because the Autotuba's response falls so quickly, it has a lot of "slam" and efficiency, but lacks the deep bass that carfi guys know so well. You might get the best of both worlds if you used EQ to "cut" the midbass though. I was a bit concerned about frying the driver, because I was feeding it about 800 watts! (it's rated for 100 IIRC
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Old 10th January 2008, 08:00 PM   #6
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Bad news - I had second thoughts about the viability of this, so I went back to the drawing board.
Good news - I solved the problem.

Here's how I did it.

Anytime you're building a horn, you're juggling a bunch of variables. If you want efficiency, you have to sacrifice extension. If you want to make it smaller, the response gets ragged. You can smooth the response with careful use of a taper, but that makes the box harder to build.

Building horns is tricky.

In the design I posted, with a single woofer, I felt the response was too ragged. It was easy to build, but I was worried it wouldn't sound good. I tried fixing that by reducing it's efficiency. Reducing the efficiency smooths the response of a horn. I reduced the box efficiency by adding a second driver in the same horn, basically halving the airspace for each one. That worked well, but the efficiency wasn't much better than a bandpass.

Then I tried a trick Dan Wiggins recommends. I added a resistor to the circuit, which raises the QES (and the QTS.) That worked great! By raising the QTS FROM .24 TO .60, the response of the tapped horn was much smoother. Of course efficiency suffers, but I'll take smooth response over efficiency any day! Another bonus is that you could adjust the QTS value by changing the resistor value. Even better, this gives me an easy load for my amp to drive, which is particularly important since I intend to use three or four subwoofers (per Geddes.)

The resistor value is five ohms; I intend to use two 10ohm resistors in parallel, which you can get from any ol' Radio Shack.
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Old 11th January 2008, 06:03 PM   #7
Eva is offline Eva  Spain
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Consider modifying your amplifier to simulate 5 ohms output impedance.
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Old 19th July 2008, 08:29 PM   #8
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In case anyone's curious, here's how you model the size of the resistor.

http://www.diysubwoofers.org/dipole/design.htm

To increase the Qts to the target value, we can use a series resistor Rs, and calculate its value as follows:

Qes'=Qts'*Qms/(Qms-Qts')
Qes' = 1.75*7/(7-1.75)
Qes' = 12.25/5.25
Qes' = 2.33

Rs = Re*(Qes'-Qes)/Qes
Rs = 2*(2.33-1.30)/1.30
Rs = 2*1.03/1.30
Rs = 1.6 ohms

As we plan to drive the system with 100W of power, assuming 10:1 differences between average and peak levels, we can use a 10W or greater resistor for Rs.
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Old 19th July 2008, 09:00 PM   #9
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Here's the math for figuring out the resistor with the MCM speakers. In a nutshell you need a 5.67ohm resistor - I just rounded down to 5ohm since they're easy to find at

First, here are the specs of an MCM 55-2421:

John Krutke posted these specs a few years back; I have personally measured half a dozen of my woofers, and they're in sync with his measurements.

Fs=30.95 hz
Qms=12.2863
Qes=0.2365
Qts=0.2320
Vas=23.88 l
SPL=86.59 1w/1m
Re=3.4
Le=2.42
BL=13.15
Xmax=16mm P-P

To increase the Qts to the target value, we can use a series resistor Rs, and calculate its value as follows:

Qes'=Qts'*Qms/(Qms-Qts')
Qes' = 0.6*12.28/(12.28-0.6)
Qes' = 7.37/11.68
Qes' = 0.63

Rs = Re*(Qes'-Qes)/Qes
Rs = 3.4*(0.63-0.2365)/0.2365
Rs = 3.4*0.3935/0.2365
Rs = 5.657 ohms
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Old 19th July 2008, 09:17 PM   #10
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Here's my original post, from months ago. NOTE I've modified it to reflect the new design, which is 25% longer. In all other respects, it's identical to my original proposal.

This is my design for a tapped horn using the MCM 55-2421, which I still believe is one of the best horn drivers available. The fact that it's $25 is a bonus.

The goals of this design were different than what a lot of you guys are going for. This one is optimized to be as small as possible. It's also INCREDIBLY simple to build. It's a eight foot tall sonotube, with a plywood divider running down the middle. There isn't even a taper! Because it's efficiency is lower than most tapped horns (about 96db) you don't get a lot of ripple.

I think this tapped horn would be great for car stereo too, but you'd have to come up with a novel folding scheme. (Unless you can figure out a way to squeeze an eight foot long subwoofer into your car!!)

So here's how to build it:

1. Get a 8ft tall sonotube that's 10" across
2. Get a piece of plywood, cut it to 10" x 90"
3. Cut out a 7.5" hole for the woofer. The location of the hole is very important. The centerpoint of the hole must be 18" from the end of your 90" piece of plywood.
4. Put the two MCM woofers in the hole
5. Put the plywood divider right inside the sonotube
6. Cap one end of the sonotube completely. Leave a 5" gap between the end cap and your piece of plywood
7. Put a cap on the other end that covers up one HALF of the sonotube
8. That's it!
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