My exponential Horn

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Just thought i would show you guys the horn i've been making, bit of a test project more than any thing. Thinking i've made an error using 0.5mm alluminum for the enclosure as the sound is gonna travel through it rather than down the horn.
Just have to see how she tests

PS does any one know how to plot SPL's for exp horns, have plotted using a program but i cant find out wht equations it uses. (wanna check responce for my self)

PICT0010.jpg

PICT0011.jpg
 
Looks interesting,

Personally I wouldn't worry to much about the sound traveling through the aluminium. To stop frequencies going through materials, the materials needs to be thick in relation to the wavelenght. That's pretty much impossible to accomplish.

Can you post the simulated response? And what is the compression factor that you used (it seems rather small)?

What program do you use? The SPL plot from Hornresp is known to be pretty accurate.

Mvg Johan
 
Rademakers said:
Looks interesting,

Personally I wouldn't worry to much about the sound traveling through the aluminium. To stop frequencies going through materials, the materials needs to be thick in relation to the wavelenght.
Mvg Johan


gday!
I'm not sure about aluminium. this looks like copper...
anyway damping factor, logarithmic decrement of metalls is..... not the best

Material E, 109 Pa Material E, 109 Pa
aluminium 70,0-71,0 spider's web 3
beton 14,6-23,2 rubber 0,9
tungsten 415 lead 16,0-17,0
granit 49 steel 200,0-220,0
iron 190,0-210,0 glass 50,0-60,0
kapron 1,4-2,0 cotton 12
brick 2,7-3,0 grey iron 115,0-160,0
ice(about t=-4oC) 10 silk 13
marble 56,0-73,0 wool 6
plexiglass 2,9-4,1 ebonite 3

that's why I use marble
An externally hosted image should be here but it was not working when we last tested it.
 
gday!
I'm not sure about aluminium. this looks like copper...
Yellow!

Hint: Read the first post ;)

anyway damping factor, logarithmic decrement of metalls is..... not the best
Not sure what those numbers mean exactly (aluminium 70-71 what?) but marble looks pretty close to aluminium.

As I actually think it isn't really relevant, my main question to you would be: Why do you think it matters and if so in what way?

Since a horn is a pressure/speed transformer, in my opinion it are those 2 factors the aluminium has to basically endure.

The fact that you have a cab made out of marble (nice btw) shows your intended use. But you should not forget there are more intended useable fields out there with other important factors then the ones that matter for your field.

Off course I am curious to find out what the intended use is actually :D


Mvg Johan
 
Its aluminium mate im sure of it (i built it), im gonna be using this for a sub 40-140Hz. Ill post some pics of wht it looks like now (fully closed up) it aint that pretty but it should do. Ill post my testing resultrs next week but im not sure if the aluminium is gonna flex under big loads, just have to wait and see
 
Rademakers said:

Yellow!

Not sure what those numbers mean exactly (aluminium 70-71 what?) but marble looks pretty close to aluminium.
Mvg Johan
sorry. it's come into view like cooper on the pic.
aluminium 70-71 (E*10^9 Pa)
but you're use very thin alum. therefore:

tiroth said:
Is that flashing? I think resonances could be damped (see: old theatre horns), but does it have sufficient rigidity?

it's shouldn't be rigidity
I made sub, cameras by thick marble for solve damping problem. it's better then lot of inflexibility partitions.
nad that's why I'm afraid your box is not integral, not solid, but it's very importance for sound camera. box must be noiseless
 
I'd be more concerned about compression in the throat area more than anything else. What is the throat area and what size drivers are those? Also you may want to add a strip of wood to the edge of the aluminum inside the chamber to prevent it from acting like a reed in a clarinet and start ringing.
 
I don't believe there is a relationship between wavelength and the thickness of the material as was previously suggested. This would suggest a 17m thick material to stop 20 Hz. I think in fact you would find that a 1m thick concrete wall would do a good job at stopping bass getting through!

If you consider a regular subwoofer box, the thinnest part will actually be the cone itself which is very thin, however, its shape makes it much stiffer. The key with bass is stiffness more than anything else. In fact below 100 Hz typically transmission through a material is determined by stiffness AND damping. With TL plotted vertically against frequency you will see peaks and dips, their height being determined by the damping. Aluminium is good due to its stiffness, but the box contribution due to low damping may be a problem.

Suggetsions:

* use two layers of aluminium joined with liquid nails which will provide some damping as well as increase stiffness
* sandwich construction using bendable plywood inbetween two layers of aluminium bonded with liquid nails
* add bracing

Imagine the walls being shown as a 3D computer mesh - you will see a mesh created with straight vertical lines and horizontal curved lines. The curve gives stiffness in one dimension, but the straight lines could benefit from a smaller span ... hence bracing which runs horitontally.

Just out of curiosity I went and knocked on my subwoofers. The aluminium cone is actually quieter than the box, which is 18mm particleboard. It's probably related to the stiffness of the shape and the damping provided by the cone's suspension.
 
A little bit of topic:

I think in fact you would find that a 1m thick concrete wall would do a good job at stopping bass getting through!
Off course it does.

But if you would measure the intensity from the frequencies before going through a wall and again after, you would find that the intensity-drop from the lower frequencies is relative lower than the intensity-drop from the higher frequencies.

Lower frequencies take more energy to be generated then higher frequencies. The lower the frequency the more energy is needed to generated it and thus the more material is needed to stop/reflect/absorb it

I don't believe there is a relationship between wavelength and the thickness of the material as was previously suggested. This would suggest a 17m thick material to stop 20 Hz.

Since there is a relation between frequencies and the energy needed to generated it, it's only natural that there is also a relation between the frequency and the amount of material needed to stop it.

And a little bit back on topic:

With Hifi-speakers generally two methods are being used for making an enclosure sound "dead".
Using materials as stiff as possible. Using materials as heavy as possible (and off course these two methods combined). The reason heavy materials are effective is explained by the theory above (absorbing energy).
But even for low intensity sound as in a living room, very thick/heavy materials are needed to have effect.

Totally back ontopic:

That's why I tried to explain that the sound is going through the aluminum no matter if the material is 0.5 mm or 5 mm. The sound will go through it anyway but the pressure won't.
So when using lightweight materials like aluminum most important factor is stiffness (as we both agree with). Theory above rather explaining why it's in fact the key-factor in this case.

I also have a nice way offtopic explanation but I will save it for another time ;)

Personally I would just test it (with one side not glued but pressed) and if necessary, brace it in the middle with a brace made the same way as you made the sides.

Still wondering about the compression ratio tho. It does benefit the flatness of the response/ the low frequency response, but it could also put to much pressure on the cone.
A compression ratio of 2:1 is rather safe.

Mvg Johan
 
Since there is a relation between frequencies and the energy needed to generated it, it's only natural that there is also a relation between the frequency and the amount of material needed to stop it.

This is true where the only variables are the transmission loss (TL) and the thickness of the material. This breaks down when you start comparing different materials or different means of assembling them. You can get different enclosures of different materials and constructions and different thickness to have the same result in transmission loss (TL). Or you can even get a lighter material which is thinner to have a higher TL.

With Hifi-speakers generally two methods are being used for making an enclosure sound "dead". Using materials as stiff as possible. Using materials as heavy as possible (and off course these two methods combined).

Another very effective method, not so commonly used in speakers, but used more in sound insolation is constrained layer damping. Although a little more difficult to implement, it is often more practical than mass damping in that it requires more attention to construction detail and method without adding significant size or weight.

That's why I tried to explain that the sound is going through the aluminum no matter if the material is 0.5 mm or 5 mm. The sound will go through it anyway but the pressure won't.

I'm not sure I understand the distinction you are making but, sound IS pressure. It is a pressure disturbance which travels as a wave through a medium without transporting matter.

There are two key issues here:

1. TL through the enclosure
2. box coloration

TL should be as high as possible by means of damping and stiffness, mass also helps. Where you can't get a high TL, then it's preferable that the sound which is transmitted through the enclosure won't significantly add coloration.

I don't think this is off-topic, although a lengthy debate won't help.

What would be interesting to hear is what you would hear if the mouth of the horn were effectively blocked. Then you would hear the box contribution to the sound. It may be more than you expect.
 
Sorry for not repliying been very busy building the amp for this, the base plates are only small 450*450mm, the drivers are only 6.5". There were major construction problems on geting the second plate on so i was unable to test it before glueing (once it was on it werent coming off again) . Just going to test it and see how it performs fingures crossed th curved shape of the alli will give it enough stiffness. Thanks for all your comments :D
 
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