FE206E enclosure

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Onur,

I wish I could hear all the different designs, but there aren't any DIY meets here in Costa Rica and the only horns I've ever heard are PA junk. I have a pair of 206's and want to try them in a horn, but I want my initiation to horns to be a good one. That leaves me with trying to gain enough information to make an informed decision about which design to try.

I like the layout of your 206 horn and I believe it would make a fine looking speaker. My concern is the short horn length and the use of a large compression chamber as the low pass filter to generate deeper bass, essentially through some horn loading bandpass output. Can you please explain the difference in character between RLH's with large vs small compression chambers ?
 
horn length and rear chamber volume

1/8 tuning is another alternative. I have used it in my horn design. My main concern is the acoustic resistance seen by the cone of the speaker unit. When you make the horn longer you increase the acoustic impedance effecting the cone, also, when you build a small rear chamber, you get the same result. If you think horn lenght and rear chamber volume are two functions, they have an intersection, which we can call an optimum. While one is getting smaller and the other one longer, they intersect at a certain point. And when you examine the acoustic resistance at that point, you see that it has reached to a minimum. That is how I decide the lenght and the volume. I didn't think in terms of acoustic filtering though.
 
My understanding is that the Fostex horns (OK, I know that strictly they aren't 'real' horns, but let's not get bogged down with semantics) use right angles for 2 major reasons. 1) It allows a greater length for a given box-size 2) the edges are there to act as diffusors. (I'm not convinced that curves make an audible difference, except in pycho-accoustics, but diffusors certainly do) That's most likely why the people who smoothed the internal edges were unhappy -they hadn't realised that they are a vital part of the basic design. Better still, they make construction quick and simple (for a horn). Fine with me.
Best
Scott
 
not a secret I suppose !?

Friendly, I am not good at maths, the good point of this is that, I am aware that I am not good at maths :) In order to close the gap I go for the secure way and play trial-error. God knows how many enclosures I have built and measured before I found a corrolation between the simulated results and my measurements. (Just to give you an idea, I have built over 60 enclosures, these are the ones that I have recorded, there are ones which are not recorded) My predictions from those experiments can be wrong, however, since they are based on pure observations.

On the other hand, please visit this link,
http://www.yildiz.edu.tr/~ilkorur/speaker/fostex_fe127e.htm

Here you are going to find both simulation graphs and measurement results. In both SPL graphs, if you put the peaks and dips aside, you see that the enclosure reaches to 60 Hz. The impedance curve is different since I have added more absorbing material then I have entered to the simulation software. This enclosure came up in 2 hours time. I have measured the T/S of the driver, placed those variables in AJ-Horn, fiddled with the knobs for a while and then generated the dimensions in AutoCAD. The result was and is very good. I can say that it came out to be just as I expedted or maybe even more. What I am trying to say is that, I am very much familiar with AJ-Horn now, I can carry the same simulation graph to real life. So, you can look at those graphs and think that they are very close to real life test results.

/Onur
 
Onur,

For your 206 horn, what do you think about a "<" shaped brace centered behind the driver? It would serve 2 purposes. Of course add some extra rigidity to the compression chamber, but the main benefit would be to totally eliminate direct reflections from the flat rear of the chamber back to the cone. Since I've become an open baffle convert, reflections back to the cone are of great concern to me.
 
rear radiation pattern of driver

I wish I could show you my simulation result but I am working on to code a *.avi output of my simulation code.

All I can tell you is, midrange or the frequencies, which are causing reflections in the rear chamber and radiating back to the listening room from the thin cone of the speaker unit, are originating from the sides of the driver. Therefore it is better to cover the side panels with absorbing materials rather then rear panel. Also, rear panels of these enclosures are far away from the driver unit as compared to more problematic ones.

As for the "<" shape, the wave length of low frequencies are very long and that "<" brace is invisible for those, according to my oppinion.
 
this is my point of view

I have solved this question in my mind in this way. Think of horn throat. The acoustic resistance must be low in order to radiate through it. When there is a standing wave (resonance or reflection) occurs inside the horn, the throat resistance increases and you can not radiate energy. The energy which couldn't be radiated to the horn streses the rear chamber and the driver causing distorsion? No ?
 
Onur,

I have been looking at your response curves for your two FE-206E designs. A couple of things don't make sense to me so I thought I would ask some questions.

First your TL design :

1) I assume that AJ-horn is calculating into 2 pi space. if this is correct you will have almost no usable bass response once you add some baffle step loss (say 2 or 3 dB in a room) to the already falling bass output below 500 Hz. Why try and tune this design so low?

2) Assuming you have designed a TL, where are the peaks in the impedance curve due to the higher harmonics?

3) I assume that there is no damping fiber in your line based on your double humped impedance curve, why is your SPL response so smooth and not a very ragged response due to peaks and null associated with the line resonances?

4) Why are there not more nulls in the driver displacement plot?

Second your BLH design :

1) This design looks a lot like the Replikon horn discussed a few weeks ago, a big back chamber coupled to a short horn that acts like a hybrid bass reflex/horn design. This is concluded from the double humped impedance curve. So I ask again, where are all of the ripples due to the higher modes?

2) With your large coupling chamber, how did you eliminate the standing waves in this volume? Standing waves will have a significant impact on the driver's SPL response and the output from the horn mouth.

3) With your over under horn mouth arrangement, did you account for the floor loading of the lower mouth and not the upper mouth? Are you concerned about the two horns seeing different mouth acoustic impedances and how it will impact the SPL response from each?

These are questions I am addressing in my own designs as they are evolving. Since you are posting response plots based on a commercial design tool, I assume thet you have answered some of these same questions. I would be interested in your experiences.
 
I'll try my best :)

Hi Martin

Let me start in order and begin from your questions about TL design.

1) What I have understand from your question is, I will have an already falling response in the low frequency band and am I not making the case even worse by tuning the enclosure to a lower frequency then I should and causing a relatively higher slope. I have tried this once or twice before. The reason completely depends on my listening tests. What you get in return of such a tuning is a good low end feel. As if you are listening to a open baffle enclosure. On the other hand you know the setback about some open baffle designs that they do not have a punchy mid-bass character. As I have described on my web page, this design is aimed to go low, as low as possible in return of powerful mid-bass.

2) I have examined the early versions of your software and in those versions, I can not quite recall but, you didn't have any option for terminus placement. Those nasty peaks can be tamed by changing the distance between the terminus plane from the plane, on which the speaker is mounted.

3) This is indeed tricky. I use a remarkable feature given in AJ-Horn for that. AJ-Horn lets you simulate the enclosure placed at different locations. What I do is, I first simulate the design under half-space conditions, then I switch to corner conditions. In two different responses you see that the dips become peaks and peaks "sometimes" become dips. What you are trying to achieve is the response you get in the middle which is flat considerably. The two different simulations show you on which peak or dip has to be played with. After you build the design you try finding that spot where the enclosure gives a flat response.

4) Honestly, I do not have a clue :)

As for the Back Loaded Horn design;

1) This problem has surfaced in the Bass List some while ago but the question was,bass-reflex port with a exponential flare. Since some simulation software give very decent results under that setup. I start from where they begin but end up somewhere different I suppose. As far as I am concerned, what I do different is, I select the throath area according to my simulation results. This gives me a perfect starting point since when modelling a horn by means of electro-acoustic elements the dimensions can be very deceiving. You may reduce an opening to zero radious and still the model can show irrelevant data. What I do is, I collect data in time domain from two points (please see the picture). One, in front of the speaker unit and start of the throath. Then I apply fft to those data and see if the distorsion levels are low.

2) If you examine my other design (Singular, which I have built for my AER MK1s) there is a brace on the symetry axis of the speaker unit. This geometry is a result of FEM simulation. The optimised shape, reduces the standing waves, reduces side panel vibrations and reduces the amount of absorbing material which you have to use inside the rear chamber.

3) You are right, I have to mention that this is a custom design which is ment to be rised up. The reason is to improve imaging by lifting the speaker center to the 1/2 height of the listening room. Therefore different loading conditions wouldn't apply to this enclosure. But, when it is used as a floor-stander the conditions will change though.

/Onur
 

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Dimensions

Please be informed that I haven't examined the worksheets, therefore I will hand you the parameters in terms of AJ-Horn.

The perpendicular distance between speaker plane and the mouth plane is -20 cm.

Total Cross-sectional area of the mouths is 2024 cm square
Total Cross-sectional area of the throats is 80 cm square
Length of horn is 100 cm (each)
Rear Chamber volume 35 liters
The horn is an exponential one

I would be very glad if you could possibly share the output of your simulations, for a good comparison.

/Onur
 
Re: Dimensions

Onur said:
Please be informed that I haven't examined the worksheets, therefore I will hand you the parameters in terms of AJ-Horn.

The perpendicular distance between speaker plane and the mouth plane is -20 cm.

Total Cross-sectional area of the mouths is 2024 cm square
Total Cross-sectional area of the throats is 80 cm square
Length of horn is 100 cm (each)
Rear Chamber volume 35 liters
The horn is an exponential one

I would be very glad if you could possibly share the output of your simulations, for a good comparison.

/Onur

That should probably be enough for a rough sim. I'll get back to you shortly
Best
Scott
 
Onur,

You are right that the software available form my site treats the driver and mouth/terminus as coincident. You are also correct that having them seprated in space will help the response. My newest version of the software removes this assumption. I have never seen a case where the response is as smoothed out as your SPL traces show, there are always peaks and nulls due to the higher harmonics.

But let us assume that your SPL curves are correct since I have not analyzed your geometry. The standing waves in the pipes exist regardless of the placement of the driver and mouth/terminus. These stading waves will influence the driver displacement plot which in turn shows up in the impedance plot. The total lack of any wiggle, dip, or peak in the displacement or impedance plots is very confusing. Something is not making sense.
 
The standing waves in the pipes exist regardless of the placement of the driver and mouth/terminus. These stading waves will influence the driver displacement plot which in turn shows up in the impedance plot. The total lack of any wiggle, dip, or peak in the displacement or impedance plots is very confusing. Something is not making sense.

I agree with you.

Even in my numerical solution I can see some unwanted peaks besides my fundemental tone, under +/- 1 dB. (In my numerical solution I calculate the transfer function by sine sweep, and use rectangular windowing for FFT) Those are not present in AJ-Horn. I can not comment on these since I am not equipted enough to measure them, yet. I think Version 7.0 of ClioWin can generate the transfer function by two mic. method. I have to upgrade my 6.5 and run some tests with it before I can understand what is going on. Otherwise it would be almost impossible for me to examine the problem in terms analytical mathematics.

/Onur
 
Whatever method you use to calculate the impedance or displacement plots, the answers shpuld be the same if the linear model assumptions are valid. As long as you are still using small signal inputs, measured and calculated impedance plots have always matched very closely when I correlate simulations and test data for a given design. With no stuffing this works extremely well when validating a model. The method of calculation should not impact the results. Even if you are no longer in the small signal world, I think that the standing waves will show up on your impedance plots as significant peaks and phase changes at predictable frequencies.
 
Just another point

The length of my line and horn are 1/8 th of the wavelength of the drivers' fs. I mostly get those peaks or dips when I select the length bigger then 1/8 th of the wavelength. Besides, the frequencies doesn't change by this but the amplitude of the peaks do depend on the throat area.

Anyway, I have reached to these conclusions by trial and error method and I mostly used the feature in AJ-Horn which I have described in my previous posting. There is a certain point while placing your speaker at the corner where the response beclomes flat. Please see the picture below. The two curves belong to different placements. One close to the corner and the other one belongs to a response away from corner. You can see the difference between the dips and peaks. This gives me the idea that, between these two locations, there is a flat response.

On the other hand the Horn Design is immune to placement error or the placement of that design is not critical, since it gives almost the same results under both placement conditions, that is why I regard that as a better design.
 

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Sorry it’s been a couple of days –work's been a pain.
This is what MathCad predicts based on the information you've given, and a few guesses based on these and the drawings on your site. For a more accurate model, we’ll need a few extra things: the exact area of the closed and the open ends of the chamber, the depth of this chamber, and stuffing density (if applicable).
 

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