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Old 9th April 2008, 09:12 PM   #11
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Location: alsace
Quote:
Originally posted by Elbert


Not quite sure what to think about foam surrounds...

Sure, the foam surrounds on the old SEAS 13" woofers I'm gonna use for the project were totally disintegrated and therefore replaced with new ones...

But ar new foam-surrounds as bad as the old ones? Should think that it was now common to ad some sort of UV inhibitor to the formulation?

But if not, some sort of UV proofing sounds like a good idea, but what to use??

Haven't exactly got the boxes ready and waiting for these drivers, it s a fairly long term project..

right now I'm gonna have to decide on the enclosure philosophy..

As I understand, a closed box (about 10L) will give the best transient response, whilst a vented enclosure (about 15 L) will reduce cone excursion and thereby distortion... hmmm...

suggestions are welcome!

Formulation of new foam surround should handle time better than old ones; but why not mail to Troels Gravesen for advices in this matter, he treated his new HES design: quote from his site: "Modern foam surrounds come with built-in UV resistance and with this driver we have further added UV protection from applying a thin black polymer coating."
http://www.troelsgravesen.dk/JA8008.htm
Using T/S parameters to calculate the middriver load is relatively unappropriated because very low energy remains at fs if you cut the driver at 200Hz. Therefore you shouldn't reach its x-max IMO.


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Old 9th April 2008, 09:55 PM   #12
Elbert is offline Elbert  Norway
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Ah, a good idea to contact Mr. Gravesen!

but then again, I'm not entirely sure if it is UV exposure alone that breaks down foam surrounds... could be other ting like general long term environmental exposure and sheer "fatigue" caused by milions of cycles..?

But then again, the midranges in the boxes from which I'm getting my woofers (they had a completely disintegrated surround) has a foam surround, but WITH some sort of coating.. and they were ok. Allthough i suspect the foam as such had deteriorated, it seems like the polymer or elastomeric coating has at least kept things sticking together!

I agree with you regarding calculating with TS parameters. I pretty much made the same assumption my self, but in my loudspeaker desing cookbook (by Vance Dickanson), I read that if the midrange could be crossed over as high as two octaves above Fs, a vented or transmission line enclosure was recommended, and that a closed box was more appropriate for a driver where only one octave separated the lower cut-off from the Fs.

Hmmm.. guess i will have to investigate a bit here...

But nevertheless, your point about cone excursion not becoming an issue anyway at 200 Hz (i intend to use 24 dB/ oct electronic filtering as well) seems very valid!

Just thinking this is now becomming an Norwegian-French design with SEAS woofer and tweeter and a Focal midrange... per haps a SECAL?
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Old 9th April 2008, 10:22 PM   #13
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Quote:
Originally posted by Elbert


I agree with you regarding calculating with TS parameters. I pretty much made the same assumption my self, but in my loudspeaker desing cookbook (by Vance Dickanson), I read that if the midrange could be crossed over as high as two octaves above Fs, a vented or transmission line enclosure was recommended, and that a closed box was more appropriate for a driver where only one octave separated the lower cut-off from the Fs.

Hmmm.. guess i will have to investigate a bit here...

But nevertheless, your point about cone excursion not becoming an issue anyway at 200 Hz (i intend to use 24 dB/ oct electronic filtering as well) seems very valid!

Just thinking this is now becomming an Norwegian-French design with SEAS woofer and tweeter and a Focal midrange... per haps a SECAL?
I wouldn't worry about the type of loading because with your 24db/oct filtering you will be more than 50db down at fs!
"SECAL" prononciation is close to "CY-CLE" (frequencies)
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Old 10th April 2008, 05:12 PM   #14
Elbert is offline Elbert  Norway
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True enough, but apart from cone ecursion, Wouldn't the loading of these drivers influence the response further up in the frequency range?

Interesting coincidence the pronounciation of "SECAL", perhaps it is quite appropriate then!
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Old 10th April 2008, 07:07 PM   #15
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Quote:
Originally posted by Elbert
True enough, but apart from cone ecursion, Wouldn't the loading of these drivers influence the response further up in the frequency range?
No. Except inner dimensions allowing standing waves...or not, as for any enclosure. Nothing else to relate to the chosed load principle.

And yes, SECAL wouldn't be a bad name, some intensity in the prononciation, no?


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Old 10th April 2008, 08:10 PM   #16
Elbert is offline Elbert  Norway
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Well, that's a clear answer!

Building an enclosure that discourages standing waves should be possible..

In one thread Mr Krutke stated that for a midrange not reproducing frequencies near the Fs, a sealed enclosure was just fine, and that the larger enclosure volumes with plenty of stuffing tended to give the best result.

So perhaps a 10 L closed box with plenty of stuffing?

and in addition, perhaps lining the enclosure with this stuff:

http://www.armacell.com/www/armacell...e?OpenDocument

Came across some bits of a black foam material on a construction site once, both bits of sheet and tubing. Turned out they used the stuff to insulate water pipes and other water and ventilation stuff. The material seemed very heavy and mushy and had an extremely "dead" feel to it.. As you can see from the link I posted, a special variant for acoustic absorption is available. Since this is a product used in construction and contracting, it should be priced thereafter.

Also considering to use this to cover the fronts of my speakers to reduce early reflections and absorption.
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Old 10th April 2008, 08:49 PM   #17
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The more your inner dimensions increase, the more the standing waves can appear at lower frequencies.
For me almost any dampening material is efficient enough depending on its density and amount. Trials are the best way to get a well balanced speaker even if it's less important for a middriver than a woofer. A good link (in german):
http://www.visaton.de/de/forum/pc_bedaempfung.html
http://www.visaton.de/de/forum/pc2_bedaempfung.html
You could also try bull pack. At least one well regarded french commercial speaker designer (Christian YVON_Apertura) seems to use such stuff. Personaly never tested this.
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Old 10th April 2008, 09:13 PM   #18
Elbert is offline Elbert  Norway
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Well, standing waves is a problem no matter what when it comes to midrange enclosures due to the short wavelength..

Of course reducing the size of the enclosure rises the limit for the lowest standing wave, but one can not hope to raise the lowest standing waves above the upper midrange frequency limit as this would make the enclosure to small to even fit the driver!

I guess the best way out is to design an enclosure with few parallel surfaces, thereby distributing the standing waves over a broad spectrum, and adding enough damping material so as to reduce the magnitude of these standing waves below any significance.

Interesting links by the way, to bad its 15 years since I had German in school (and not very good at it...), but I'll give it a try!
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Old 11th April 2008, 05:31 AM   #19
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Quote:
Originally posted by Elbert
Well, standing waves is a problem no matter what when it comes to midrange enclosures due to the short wavelength..

Of course reducing the size of the enclosure rises the limit for the lowest standing wave, but one can not hope to raise the lowest standing waves above the upper midrange frequency limit as this would make the enclosure to small to even fit the driver!

I guess the best way out is to design an enclosure with few parallel surfaces, thereby distributing the standing waves over a broad spectrum, and adding enough damping material so as to reduce the magnitude of these standing waves below any significance.
Right. I suggested relatively small inner dimensions because the density and amount of damping material is less and less necessary as frequencies increase.


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