Home Theater Forum and Systems banner

Status
Not open for further replies.
41 - 54 of 54 Posts

·
Premium Member
Joined
·
19,397 Posts
Re: DIY TC Sounds LMS-5400 18" + 2x18" PR 200L *new*

"No appreciable absorption effect" is in reference to credible co-efficient testing of different materials - as can be found in many databases. You can research/look up this data, or you can refer to Ethan Winer's RealTraps web site, which has(or had) a database of lab test measurements of different materials, including two foam vendors(Auaralex and Foam By Mail). Rod Elliot's site(sound.westhost.com) also has some testing of various material as it relates to absorption in a small midbass application, which the general results match up with known general absorption properties established by other testing.

The only appreciable effect the foam can be expected to have in your subwoofer application, is that it can in some instances, reduce the port(or passive radiator) output due to physical air-coupling losses that can be induced in the system alignment. However, this seems unlikely based on the relatively thin foam you used based on the subwoofer cabinet size. For foam to be effective at say reducing the long dimension half-wave modal resonance at bass frequencies, or 1/4 wave null effect based on driver to nearest boundary point; you would have to use a high grade acoustic foam, such as Auralex, in 5-6" thickness. Alternatively, you can use 4" of 6-8lb/ft^3 mineral wool board for similar absorption properties. It must be clear, though, that this amount of material will certainly result in measurable loss in port efficiency/output.

-Chris
I was looking for a link showing ""no appreciable absorption effect"" when used in subwoofers since that is what is being discussed here. I've searched and can't find anything. Since I have 2 retired Shiva sonotubes on hand, I may pull the foam out of one and see if there is any difference. According to what is being stated here they should perform the same.
 

·
Registered
Joined
·
75 Posts
Re: DIY TC Sounds LMS-5400 18" + 2x18" PR 200L *new*

There's a bit of a red herring here re absorption.

The original question I believe was about absorption at somewhat higher, audible freq, whereas most of the responses were about the low absorption coefficients at low freq.

In fact that's good, as absorbing it would just reduce the output we built the sub for.

"I would say that when the size of the enclosure goes above ~5 ft^3, stuffing the enclosure with "dampening" material becomes less and less effective and also less important."

Not what I found, see above.

Sure, you have to buy more, but you have to buy more of everything for a bigger sub.
 

·
Elite Shackster
Joined
·
1,468 Posts
Discussion Starter #43
Re: DIY TC Sounds LMS-5400 18" + 2x18" PR 200L *new*

The decrease in Fb of my 12 cu ft dual Avalanche 18 subs indicates >20% volume increase.

I used 3 layers of 2 lb/cu ft on all sides.
So you measured the Fb before and after adding the fiberfill? Is that a ported subwoofer? How did you measure the Fb?
 

·
Registered
Joined
·
75 Posts
Re: DIY TC Sounds LMS-5400 18" + 2x18" PR 200L *new*

Yes, I measured before and after, by freq of impedance peak across driver, w/a series resistor.

Though pricey, I use foam, not fiber, as I have found it to lower freq more, and easier to get it to stay in place.
 

·
Registered
Joined
·
316 Posts
Re: DIY TC Sounds LMS-5400 18" + 2x18" PR 200L *new*

An important reason to use stuffing is to increase the box effective volume.
Yes, but the subject I was discussing was not stuffing, but using a thin(1.5") layer of foam, which is not going to have a substantial effect such as if one used a moderate level of stuffing or lining with thick materials. Of course, in a ported/PR system, this is a double edged sword. Stuffing for effective volume increase is only feasible in sealed systems. Using enough material to increase volume by a substantial degree in a ported system will cause the port to lose appreciable output. Now, one could use this as a technique for a specific target tuning, as long as one realizes the inter-relationship of factors so that they can accurately target a desired response.

Not sure where you came up with "physical air-coupling losses", but the issue, which is much more likely with ports, is restricting the movement of air because of its high velocity and the distance it moves.
Here is an article on this subject:

http://www.geocities.com/kreskovs/Box-1.html

Another article on this subject

http://sound.westhost.com/articles/boxstuff.htm

-Chris
 

·
Registered
Joined
·
63 Posts
Re: DIY TC Sounds LMS-5400 18" + 2x18" PR 200L *new*

As TC/Audiopulse's PRs seem unobtainium now, and prolly $$$ anyway, the AE/CSS 18" PRs look like a decent replacement, especially given their extremely compliant suspensions. Any other large PR alternatives?

Right now, trying to keep a small box, and allowing for some FR drop from 50Hz down to 20Hz anechoic, 180l with 2x2500g PR, 15.3Hz, appears to be a decent overall design? Some output sacrificed for the size, probably, but should still be a lot better than sealed...

Oh, and this is actually for the 18" Ultra, which spec-wise looks really close to the 5400.

Never having dealt with PR or vented modeling before, any opinions welcome; thanks!
 

·
Registered
Joined
·
999 Posts
Re: DIY TC Sounds LMS-5400 18" + 2x18" PR 200L *new*

Never having dealt with PR or vented modeling before, any opinions welcome; thanks!
I like using UniBox for modeling passive radiator based subwoofer designs. :T

But it does require you to have Microsoft Excel.
 

·
Registered
Joined
·
63 Posts
Re: DIY TC Sounds LMS-5400 18" + 2x18" PR 200L *new*

I like using UniBox for modeling passive radiator based subwoofer designs. :T

But it does require you to have Microsoft Excel.
Oops; yeah, my numbers were from UniBox. Didn't run the equations myself. But, if anyone's interested, I'd appreciate a sanity check, Unibox or otherwise.
 

·
Registered
Joined
·
105 Posts
Re: DIY TC Sounds LMS-5400 18" + 2x18" PR 200L *new*

This excusion chart looks very close to what the Acoupower 18" driver and these PRs model like. You really need to make sure there is a HPF inserted near the tuning frequency because things get out of hand very quickly with the PRs at and just under tune. (with maximum power anyhow)

Sounds crazy, but 4 PRs per driver would give so much more excursion headroom. Higher cost immediately comes to mind, but compare the costs of 2 TC Sounds VMP's @ $250 each and the cost of 4 AE PR's @ $129 each. $500+shipping vs. $516 plus shipping. Not much of a cost difference if you were able/willing to buy the TC PRs. Making a box would be more challenging. bottom firing active driver and a PR on each side of a square comes to mind. Just an idea i had floating around in my head...

Dr V

edit : After a little modeling, it appears the necessary HPF used to curb the active drivers excursion below tuning also takes care of the PRs no matter how many you have. Maybe 4 PRs would indeed be a waste of money?
 

·
Registered
Joined
·
63 Posts
Re: DIY TC Sounds LMS-5400 18" + 2x18" PR 200L *new*

Thanks for the thoughts, Dr. V. What PR mass and tuning did you finally end up with? Might have missed it, but didn't see it in your thread...
 

·
Registered
Joined
·
464 Posts
Re: DIY TC Sounds LMS-5400 18" + 2x18" PR 200L *new*

Ilkka,

Can you tell me a little about setting up the equipment and microphone to test these cabinets.







With the opposed drivers,is there a a certain place to set the mic? Does it just go to the sides of the drivers?

Thanks,

Robert
 

·
Registered
Joined
·
7 Posts
Re: Subwoofer Tests Round 5, 6th of October 2007, Test Summary ***READ ME***

Here's the natural (i.e. no HP filter or EQ) of the DIY TC Sounds LMS-5400 18" + 2x18" PR 200L. It indicates that the woofer could use a little bit enclosure volume when using such a low tuning frequency. It goes slightly against any simulation, but hey, that's why I take these measurements. So if you can, I would go with full 10 cu ft per woofer. That will boost up the area around the tuning frequency and give you more output down there..
Ilkka,

Subwoofers measured ground plane in the near field - each face of the woofer that projects sound must be measured at the same distance, and each curve summed and averaged to get the an accurate picture of it's LF performance.

If you can repeat your measurements, I am betting you will find it has more LF output than the initial measurements suggest - and will more closely follow "predicted" curves.

________
dB


(theBeast)
 

Attachments

41 - 54 of 54 Posts
Status
Not open for further replies.
Top