Curve fitting?

[EMT]

I have read that one can make a vented loudspeaker sound the same as a closed box one by making the lower end response curve of the vented box match that of the closed box. Let's not argue at the moment about whether or not this is true. What I am curious about is how one would go about it. I imagine that loudspeaker design programs have an option to do this. But what are they doing? Changing enclosure size? Modifying length/diameter of the vent? Both?

[ThomasW]

Hi Ellen

Sort of hard to comment not knowing the source for the info you're using.

I'm certainly not aware of any program that specifically allows for the option you describe. Programs that model changing amounts of damping are somewhat beneficial. Obviously adding damping lowers the "Q" of any cabinet.

I'd start the process by using the lowest "Q" ported design possible for the chosen driver.

About a bzillion years ago when Jon and I first met, TL's were all the rage. See early 1970's issues of Speaker Builder for some of the first published DIY TL articles. People (us included ) played with all kinds of damping schemes for the lines. One of these included adding a 'charge' or wad of damping sometimes in the shape of a donut, directally behind the woofer. Note this was in addition to the normal damping used inside the line.

Ok now fast forward to the 21st century.....

I can't remember if Jon's M8a article (ported cabinet) went into all the little details of what is done to that speaker . So I'll rehash them here.

First by normal standards the port is oversized. Most people don't think to use a 3" flared port with a 8" driver

Second all the interior surfaces were lined with thick high wool content felt gasket material.

Third a pad of polyester batting was made by folding the material several times to create a pad roughly 12" square and ~3" thick. This is set over the woofer hole and then gently pressed into the cabinet so it sets directly behind the woofer.

As a result of these three things the bass from the M8aMKIV is very tight , very deep, and very detailed. It certainly doesn't 'sound' like a ported box.

So IMO the tricks are start with a low "Q" box, that's overported (much larger diameter port than the norm). Then line the interior with the felt gasket material, and finally add a pad of poly directly behind the woofer. The pad is the final 'tuning', so one should experiment with the amount of damping used so as to get optimal performance.




theAudioWorx
Klone-Audio

[EMT]

Hi Thomas,

Thanks very much for your reply.

I think I've heard it said by one or two of the popular internet direct subwoofer companies. I don't know for a fact that the loudspeaker CAD packages do it. I, perhaps foolishly, made an assumption about that. I guess I didn't think that someone would do that sort of thing thru trial and error

I missed the M8a series of articles when they were published. Will have to look into getting back issues.

E

[ThomasW]

I just remembered a series of threads several years ago on HTF; they culminated in Dan Wiggins posting some low "Q" ported designs that were created in LspCAD.

It seems to me that you participated either directly or indirectly in those threads. And if memory serves there were some box designs that at one point you had copies of. If you still have them 'reverse engineer' Dan's design and see what he did.

BTW, if you're referring to SVS; then yes, you bet trial and error do play a big part in the designs. One simple example is the spacing between downfiring drivers and the bottom/floor plate. TV made numerous experiments to determine a specific spacing that provided a small amount of free boost. Things like that are only ceated by people playing in the 'lab'




theAudioWorx
Klone-Audio

[EMT]

Thomas,

Now that you mention it, I do vaguely recall some rather heated discussions over at HTF about this. I can't imagine that I actually participated though. I might as well participate in a discussion about designing rocket ships for all the meaningful info that I could add to the conversation I'll have to go over there and see what I can find in their archives.

E

[Hank]

Yes, there were lots of entertaining, and some heated "discussions" between TV and DW in 2000. DW recommended my configuration: Tempest driver, 175 liter enclosure tuned to 17.5 Hz. Not really overported though, but sounds very good.

[ThomasW]

The threads I'm referring to weren't heated debates between anyone.

They were DW discussing and posting LspCAD files for low "Q" ported boxes. Some actually used some flexible black drain pipe rolled up inside the box to accommodate an extra long port length




theAudioWorx
Klone-Audio