The Kolbrek Midbass Horn

This is a description of the midbass horn I built around 2001. It has been superseded by other designs, but I include it here as a reference. The text is a bit old, but please bear with it...

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Background

After I was bitten by the horn bug, it was a short journey to my own horn loaded system. After making the styrofoam midrange horns, of course there was something that wasn't there: The midbass. I had a subwoofer, but it operated just to 150Hz, and the midrange horns down to 250-300Hz. Almost an octave of good bass was lacking. There was only one thing to do about it: Make a good LF system. I was not quite sure if I should use an Onken approach or a horn, but horns would give the best performance, IMHO. Since I had a subwoofer, it would be silly to throw it out and substitute the Onken enclosure for it, since the sub could go deeper.

I will not go into detailed horn theory in this article, it can be found here. The horns were designed for a 10" driver coupled 1:1 to the throat. I used an exponential curve, with a flare frequency of 80Hz. With a pure exponential expansion, it became 80cm long, 1/4  WL at 100Hz. This meant that the horn started to roll off at about 90-100Hz.

The expansion

The vertical expansion is approximated in two steps. The first is 25cm long and quite straight, the second is 57cm long and expands more.

I made a table of the calculated dimensions in a spreadsheet, after using the horn expansion calculator at the page above. Here it is.

Construction

First of all, draw the horn expansion on a template. I used paper, but cardboard would be better. There are two parts to it, the 25cm part (A) and the 56cm part (B). This is straight, along the horn axis. In the table above, the dimensions are calculated for every cm, straight. You can use the straight length for part A, for part B, multiply by 1.1.

When the template is made, draw 4 equal parts of each on a piece of plywood or particle board, at least 19mm thick. Cut it out, and then make a jig for it:

The jig must measure 55 cm wide at the bottom, and 24 cm wide where the sides end. Hold it tight with clamps until the first pieces of the  expanding side are glued and nailed to it, see picture. This is important: Always use a lot of glue!

You can use the same jig to make part A. Put it on the top and fill in the spaces between the sides and the jig with pieces of wood until the distance at the top is 18.6cm. When you have finished making the jig for part A, take it off and proceed with part B.

Proceed by glueing and nailing 3mm plywood to it like this:

Isn't 3mm plywood too thin? Without further damping, it is. There are several ways to damp the horn walls. I used the good old method of reducing the Q of the vibrating sides. When they are stiffened, the resonance frequency will increase faster than the damping, and maybe become more noticeable. Maybe I would be unlucky enough to get the resonance at a frequency where the midrange horns operate! Instead I covered the sides with foam rubber, with braces to hold it in place. The braces were screwed to the particle board sides through the foam rubber and plywood. The result was sides which were almost deader than the particle board sides. It looks like this:

TIP: use the braces at the "throat" as feet to stabilize the horn when it stands on the floor, so it will not tip backwards.

The A parts are simple. Place the B-part on  the jig and make the A-part fit the dimensions. Then put on two 6mm pieces of plywood on each side to approximate the curve, like this:

Then put feet on them, so that they match part B. Make the feet long enough, so that there is no "bump" at the transition between the two parts. When watched from the mouth, it should look like this:

You can see that the feet are glued onto the horn from the sides, not from the back.

To increase the efficiency, I added a 20 cm long conical segment to the horn. It should not be difficult for anyone to calculate this. The cross section is square,  it starts at 10x10 cm and ends at 18.5x18.5cm. The image below shows the horn from the side, with front and rear chambers. The front chamber is 1.8 litres, and the rear chamber is 20 litres. The cross-section of the front chamber is 18x18 cm. I have not used ordinary speaker terminals, but rather solder tags, so I can hardwire my system in the future for best performance.

I have also added spikes and a pair of planks under each horn to keep everything in place.

The effect of the extra 20 cm and the front chamber is a flatter and more extended response. The simulated response gives: -3dB bandwidth is now 82-550Hz, compared to 86-450Hz without the added parts. The efficiency is 1.6dB higher, but the slope is slightly steeper at the low side, and quite a bit steeper at the high side. See Fig. 9. This is simulated in David McBean's excellent program (version 2.0). Newer versions are available here: http://www.hornresp.net.ms

For the driver I use the Eminence Delta 10. Compared to the 50 year old no-name drivers I have tried, the horns are more efficient and go deeper. The bass is good and powerful. It's no problem listening to them without a sub, because even if they do not go very deep, they move a lot of air, and the bass is pure and clean. It is easy to get addicted to horn bass...