Conical Midbass Fronthorns

After some back and forth I decided to build some alternative midbass horns to my 150Hz Tractrix to experiment with. The initial idea was to build some horns which have a lower cut off frequency.

Considering the space we've got within our living room, I decided that the horn shall not exceed 1m in depth (including the back chamber) which lead me to a flare frequency of 77Hz. A pair of Electro Voice EVM15L I bought some time ago were planned to be the driver for these horns. Since the EV does not have the 'highest' mass roll off, only a unfolded horn was a possibility. To reduce the amount of mouth reflections I choose a mouth width/height ratio of 1:1.6. The throat was chosen to be square. The size of the throat is based on Keeles calculations as explained in Bruce Edgar's Show Horn article (refer to download section). Initially I wanted to apply a hyperbolic exponential flare with a m-factor of 0.6. I simulated the predicted response of this horn and of several approximations (including a conical version). All these variants had the same throat and mouth dimensions as well as the same length. They differed only in the shape of the flare (bend vs. 2-step approximation vs. conical). The results of the simulation were very close to each other. A discussion on this topic on AA confirmed be that the conical approximation of the horn shall not be a bad choice (see discussion and simulations here).

In this place I have to admit, that all this would not be possible without the work and the help of Dr. Bruce Edgar and other inmates of High Efficiency Speaker Asylum forum. Thanks a lot to you guys!

 

Design Basis Summary

 

Building the Horns

img-1

First I cut the boards very rough from sheet of 40mm MDF with a circular saw. Than I prepared two templates for pattern sawing to cut the pieces very precisely. The pattern is the thinner board on top which was simply nailed on the board to cut.

 

img-1

With the pattern, cutting is very easy and quick (This guy in the dust is me).

 

img-1

Here You can see how pattern sawing works out. The saw blade only cuts the thick MDF board. The template will just be touched by the blade. The saw fence is adjusted in such way that its left edge is aligned with the left edge of the saw blade.

 

img-1

A first look

 

img-1

This is shed where I do the cutting and turning.

 

img-1

I was happy to see how good the pieces of the flare fit together.

 

img-1

Our daughter had a lot of fun crawling into the mouth and exiting throuth the back chamber.

 

img-1

View into the backchamber with speaker gasket.

 

img-1

Speaker installed. The volume will be adjusted later on, so that the first impedance peak will be at 77Hz. May be the amount of foam rubber stuff need to be adjusted.

 

img-1

Size comparison to the tractrix fronthorns.

 

img-1

Entire system just set up.

 

 

 

Traxtrix Midbass Horns

pic01 pic02

 

Design Goals

You might ask why this guy used the Tractrix contour for his midbass, where everybody knows, that a horn of such geometry runs out of gas way before reaching fs. You're right it's true. However, my idea was to design a horn which should sound wise match the Tractrix midrange as good as possible. That's why I decided to use Tractrix instead of a hyperbolic (m<1) or exponential (m=1) contour. Actually, in my room, these horns measure 140Hz to 700Hz within a range of ± 2dB. Below 140Hz these horns rolls off with approx 12dB/oct.

pic03

Although the EVM 12L has been discontinued by EV, you can get 'as new' units by buying blown ones on eBay and then let them recone at your local pro gear dealer.

 

pic04

This is the jig which was necessary to buit the inner part of the horn.

 

pic05

Here I just transferred the contur of the marked the flat sidewalls.

 

pic06

This is the second jig I needed to precisely cut the flat sidewalls of the horn.

 

pic07

Here I clamped together all four flat sidewalls in such way, that I could sand down the edges for more precision.

 

pic08

This is there I glued the two flat and the two curved sidewalls on the jig.

 

pic09

And another view of the jig.

 

pic10

Here is the horn after removing from the jig.

 

pic11

A detailed view of the edge of the horn, with the transition of the flat ond the curved sidewall.

 

pic12

Here I just added the speaker baffle on the horn, after I cut a groove in it to obtain clerance to the cone of the driver.

 

pic13

The same thing in another view.

 

pic14

To stiffen the curved sidewalls, I glued some more wood on them.

 

pic15

This is what it looked like before closing the box.

 

pic16

Frontview.

 

pic17

And another one.

 

pic18

I thought it could not make thinks worse when adding some foam.

 

pic19

My little workshop.

 

pic20

Some nice paint job.

 

pic21

for the second one as well.

 

pic22

The backchamber w/o speaker.

 

pic23

The speaker sitting in the backchamber.

 

pic24

Here the backchamber was closed.

Right after completing them I did some first listening test. I was just so curious if all the work paid off. The sound is open and clear, no nasal or horn-like sound.