# How do I design a back chamber for a horn speakers.

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#### djn

I can't seem to find any data on how to design a back chamber for a midbass horn. Is it driver dependent? Thanks in advance.

#### WithTarragon

I can't seem to find any data on how to design a back chamber for a midbass horn. Is it driver dependent? Thanks in advance.

I assume you are talking about honest -to-goodness front loaded horns. You are correct, the size or volume is driver dependent. The concept is called "reactance annulling". Unfortunately, my memory for the equation fails me at the moment.

You should be able to get the equation with a google search. If not, then have a look at the JAES article by Delgado & Klipsch (2000). It has been posted over at the Klipsch site

#### djn

Well, I have a BIG problem with all this. I can create anything and make it look beautiful in any medium....but I am NO math whiz. I read all the data and links you guys posted but it is all greek to me. I was hoping for something like 2+2=4

#### Helmuth

I can't seem to find any data on how to design a back chamber for a midbass horn. Is it driver dependent? Thanks in advance.

I played around with horn response and build some back-loaded horns.

My conclusion is that the closed volume depends on what resonant frequency you want of the driver. And see it as a closed box.

The resonance frequency is precise in the area where the back-loaded horn has its loading before cut-off. Then the driver is the most efficient in the horn.

Also when looking for a driver you only have to look for a driver witch will resonate around 70-80 Hz and so will work down to 50-60Hz. That are often bas-midrange drivers.

When you use horn response you can change the high cut off with the closed volume but it also effects the low end cut off characteristic.

So calculate it as closed box and try to get the resonance a bit above the cut off of the horn.

And when the horn has good loading the resonance will be suppressed and only where the loading is less a impedance peak appears.

This isn't a formula bit a simple explanation what happens in a back-loaded horn.

#### Helmuth

I can't seem to find any data on how to design a back chamber for a midbass horn. Is it driver dependent? Thanks in advance.

Midbas horn you mean a front-loaded horn I guess.

Then agian it is like a closed box and the resonance is near the cut off frequency of the mid horn. So you can change the way the horn falls off at the low end, the point the horn looses loading.

The force of the voice coil is F= B*l (B force magnet, l length winding voicecoil in the magneticfield). This force driver the cone with area Sd. The resistance of the hornloading and force of the driver will effect the extrusion and Spl produced.

Horn response calculates al this so one doesn't have to be a math expert to design a good horn only fill in the TS-parameters in horn response.

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#### WithTarragon

Well, I have a BIG problem with all this. I can create anything and make it look beautiful in any medium....but I am NO math whiz. I read all the data and links you guys posted but it is all greek to me. I was hoping for something like 2+2=4

Actually, it really is not complicated at all.

Have you done the google search on "reactance annulling". This, incidentally, is what was presented in the chart that was linked. Although, that introduced background that was perhaps confusing (it was also not needed for what you are trying to accomplish).

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#### Helmuth

Actually, it really is not complicated at all.

Have you done the google search on "reactance annulling". This, incidentally, is what was presented in the chart that was linked. Although, that introduced background that was perhaps confusing (it was also not needed for what you are trying to accomplish).

They knew it already a long time ago article from 1955.
http://forums.klipsch.com/forums/storage/3/869228/Reactance_Annulling_by_Plach_and_Williams.pdf

you wil play with this.

An externally hosted image should be here but it was not working when we last tested it.

speaker building is constant trying to stretch the low end with the enclosure. doesn't matter if it is a horn closed box or basreflex. The driver will somewhere resonate and create the cut-off frequency.

And a QTC of 0,7 is seen as most flat but it can go > 1 to have a more flat response with your horn or OB so we can play a bit with that.

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#### jamikl

djn, I think MJK has a paper about it on his quarter wave site. At least he did have it. There is also a small program called ML which uses the Keele equations
and some driver specs to output the information. If you Google ML you might find it.

The main idea seems to to have the Fs when mounted in the chamber equal to the horn cut-off frequency but as it usual to cutoff the horn above that frequency I am not sure whether it is the frequency that you use the horn at or the cutoff frequency that you size the chamber to. The usual is to build a
bit larger than calculated then to add solid packing in small blocks to reduce the size until the impedance is right or it sounds right. Don't stretch the low end unless you want it to sound like a horn, ie megaphone.
jamikl

#### djn

Well, this is interesting. I read up on back chambers and did all the calculations. What got me going on this was, one night last week, I thought that maybe my OB horns would sound better as a straight horn so I found two 6x6x6" shipping boxes and duct taped them to the back over the drivers. There was a huge WOW factor and I thought this was the way to go. After all the calculations and such, it turns out that the best size back chamber is .....6x6x6" So, I will make boxes out of MDF this week.

#### WithTarragon

Well, sounds like a small volume or maybe I am confused.

Is this a compession horn. IOW, is the throat smaller than the readiating area of the driver (cone)?

#### djn

It is a very small volume. No compression. the back plate is 6.5" x 5.5" and the fostex FE126e is a 4.5" driver. They were origanally OB, but the mid bass was way down compared to the highs so I am trying to even that all out. I have a big 100uf oil can cap in line on the poss side of the 126e so it is rolling off around 200hz. I have woofs to go up to 200hz. They are starting to sound pretty good. Everything I am doing is a backward engineered mod so everything is not according to hoyle.

#### CLS

Compared to ordinary wooden (plywood, MDF... ) boxes, a cardboard box is somewhat leaky. So it behaves more or less like a bigger one. Once you build an MDF box of the same size, some tuning (by the internal damping) might be needed.

That's for the loading side of thing only. Considering the cardboard box is leaky, there's some sound emitted from the surface of cardboard box and it's frequency dependent. It's probably no big deal, compared to the difference of open/close back. But if the wall behind the speakers is very reflective, then it might have some (subtle) difference.

#### djn

The wall behind the speakers is very reflective. When I run my big Heils, the room sounded very alive. I am picking up a bunch of scrap MDF to make the boxes today. I sure they will sound much better than the MDF.

#### pumpkin1

How to measure the chamber resonant freq?

I just completed a set of 77hz front loaded conical midbass horns. I don't know how to actually measure the chamber resonance to know where I am and to get to 77hz. I have a mic and rta, but am clueless as to the procedure. Any help would be appreciated. thanks

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