Question about the CC shape in A126

[FlorianO]

While my build for A126 is still on ice -- after 3 weeks I still haven't got the drivers, they apparently got stuck in the customs -- I have a question about the shape of the CC in A126.

Since we need to extend the top back of the CC to get to the recommended 2.2 liters volume, its shape changes from that nice triangular profile to a stranger, elongated shape.

My question is if the _shape_ of the CC is important in any way.


TIA,

Florian

[ronc]

Yes it is. I dont believe in flat panels which reflect a wave back to the driver. My original idea was to reflect the wave thru an absorbant medium at the rear of the chamber.That or an object that would disperse the higher frequencies.
Simply line the rear of the extended chamber with 1/4"T balsa with a thin layer of felt on top. The area in the rear would be 1/4 the length of the chamber.

ron

[FlorianO]

Ron,

Many thanks for chipping in

Originally posted by ronc

Simply line the rear of the extended chamber with 1/4"T balsa with a thin layer of felt on top.

Only the top part ike in the attached picture of Dave's modified CC ? (With the corresp. volume correction, of course Didn't have a better image at hand) Or both that part and the bottom sloped part (i.e. the part sloped towards the exit of the CC) ?

The area in the rear would be 1/4 the length of the chamber.

*Ahem*.... Not sure I follow...You mean the lining should go for the whole width of that part (i.e. width of the front baffle) and for a length equal to 1/4 of the "max depth" of the CC (when seen from the front baffle) ?

[FlorianO]

Ron,

Since I got your attention, another quick question:

Another possible way of increasing the CC volume up to the recommended 2.2 liters is by extending the CC upwards like in the attached picture.

Any preferences compared to the above "elongated" CC ? Lining of the back of the CC in this case (as per above) ?

Thanks again,

Florian

[FlorianO]

Originally posted by ronc
The area in the rear would be 1/4 the length of the chamber.

Or like this, parallel with the front baffle ?

[ronc]

The first image is correct but all around the rear of the CC. The idea is to reflect as much of the wave as possible thru two different mediums. The first medium has a lower density than the second, this provides for a greater BW of attenuation as well as greater attenuation of the higher frequency waves.This is termed "progressive attenuation".
Example! Ever see the grouping of barrels on the median of an off ramp on a freeway? The first set of barrels are filled 1/4 with water,the second 1/2 full and the third full. This allows a progressive damping of the forward mass/velocity of the auto.

ron

[johninCR]

Florian,

My suggestion is that if you are willing to spend time tweaking, make the CC larger than recommended. I did that with my Frugel's, and found that not only does the shape and volume have a big affect on sonics, but so does what you choose to fill it with. Interestly, I discovered my favorite combination while I was exploring only different volumes. That included cans of refried bean paste as fill with only minimal foam rubber to ensure they didn't rattle together. Not only did it diffuse reflections nicely due to the shapes, but the cans were sonically dead but rigid, and I found angled pieces of plywood and mdf to result in sound more muffled and dead.

[FlorianO]

Ron,

Many thanks for the explanation

John,

Thanks for the advice...If I follow Dave's CC (first picture above) the CC volume computes to 2.9 liters. As pointed by Dave himself that is above the recommended volume (2.2 liters) and he had to stuffed it with corrugated wood. However, Ron disliked the idea of stuffing. OTOH lining parallel with the back walls of the CC seems to be it.

Wrt shape the choice is either extending the CC backwards like Dave did, or maintaining the triangular profile and extending it upwards. In the latter variant, if I extend it all the way such that the top sloped back of the CC becomes flush with the front baffle the CC volume gets up to around 2.0 liters. Getting up to 2.2 liters from there can be done in two ways:

- Adding the suprabaffle and accounting for the cut-out in the front baffle (added voulme is 0.177 liters for a circular cut-out of 56mm radius, like in Daves PDF).


- Additionally pulling the back of the CC backwards, like above.

OTOH Rons dislike for front panels reflecting the wave back to the cone this seems to discourage the idea of extending the CC upwards, and Dave's way may be better. Unfortunately I cannot find any information wrt CC shape in BLHs to get some guidance to what effects one should expect from either (sharp corners from triangular profiles vs more 90 degrees, rectangular-ish ones) but seems to be a matter of trial-and-error .

[ronc]

but seems to be a matter of trial-and-error .

No it is not. I do a wave form on the chamber and attempt to find the most effective ,simple way of attenuating the higher portions of the FR. The problem is to make it simple to the average builder.
There are two basic ways of wave attenuation.,dispersion and absorbing.If you study the wave pattern of the original designed chamber you can see that the wave fronts are reflected at two points, if there is a progressive attenuation at these points then the energy is reduced. The wavy wood is a dispersion, but if it was me the ridges would be of a random height and width between the ridges.On one of the designs , i believe it was the adjustable volume method, there is a styrofoam tube at the rear of the chamber where the energy is concentrated.
Now the most effective is to have all four sides of the chamber reflect the positive wave rearward and both diffuse and attenuate the wave. However now you are talking a complicated build.As has been stated by many before, a horn is a series of comprimises. I attempt to make the build as simple as i can any at the same time have good performance . The Dallas is a good example, its a very complicated build and i am sure if i had the Austin programming at that point it would have been a more simple design with the same or better performance. One of the more difficult design parameters is total use of the internal volume. The Austin has a very small waste volume at the mouth exit and again in the deflector, but otherwise the volume is totally used.

ron

( words of my old engr prof "the hardest thing you can do is to make something simple")

[FlorianO]

Ron,

When I meant that it seems a trial-and-error process I was thinking about the impression I've got from other builders, not the designer himself . IOW _I_ couldn't find material -- other than your own brief descriptions -- with insight in the design of the CC (and, by extension, the horn itself).

Your input confirms my interpretation that Dave's build was actually better wrt back reflections whereas extending the CC upwards had -- in my simplistic view of wave reflections -- had same type of back reflections towards the cone _but_ had one extra sharp (i.e. 45 degrees) corner at the very top where energy could concentrate.