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Old 23rd October 2007, 09:45 PM   #11
Wizard of Kelts
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Default Re: Re: Closed Cell Vs. Open Cell Foam-Which For Damping Loudspeakers?

Quote:
Originally posted by myhrrhleine

see mfg. specs.
Which specific spec should I look for? I am not an engineer, so I do not know the terms. Acoustic cutoff frequency?

If there is a specification dealing with which specific frequency at which various materials start passing frequencies at a progressively reduced rate, I would be very appreciative if someone would let me know.
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Old 24th October 2007, 12:29 AM   #12
FrankWW is offline FrankWW  Canada
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This doesn't answer your last question but is a TL application. The stuff looks like mattress foam (which is open cell).

Click the image to open in full size.

PMC are not chopped liver.

Lets see if I can get something more useful to look at:

Click the image to open in full size.

Click the image to open in full size.
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Old 24th October 2007, 01:20 PM   #13
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The few really good TL lines I heard were all dampened by lonhair wool spun out very finely.
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Old 24th October 2007, 05:00 PM   #14
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by myhrrhleine

Believe what you want.
read the specs!
Hmmm.......

And then completely misunderstand them or apply them to a
situation where they are not relevant. it seems you do understand
the original question - but I admit it is not worded precisely.

Closed cell foam cannot be used for this application and if it was it
would be very poor and basically only reduce the CSA of the TL line.

The PMC shows the basic principles of damping higher frequency
modes whilst keeping a lower damped bass path open.
Another alternative is to only stuff the first 1/3 or so of the line.
Yet another is strategically placed damping at the maxima points
of the line modes you want to suppress.

The original question is bit woolly as to what output exactly is to
be damped, if its the line terminus output then the best way of
achieving this is mass loading the terminus, i.e. a port at the exit.

Sound passing through the walls is a completely different kettle of fish ....
And pretty much irrelevant at the frequencies bieng discussed ....

/sreten.
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Old 24th October 2007, 10:50 PM   #15
Wizard of Kelts
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Frank W:

Thank you for the pics and the advice. Mattress foam is open cell? Worth a shot. Cheap enough, for sure.
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Old 24th October 2007, 11:02 PM   #16
Wizard of Kelts
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Quote:
Originally posted by marcelnl
The few really good TL lines I heard were all dampened by lonhair wool spun out very finely.
Marcelnl:

Thank you for the advice. However, I think the TL's you heard, while assuredly nice sounding, went for a different design goal than I am. While the usual damping materials, including longhair wool, will produce a nice sounding Transmission Line, the volume of the TL will be much greater than a corresponding bass reflex using a similar driver and having a similar F3.

If you take a well designed bass reflex, then design a TL of equal volume with a length approx a quarter wavelength of the bass reflex's tuning frequency, you will get a Transmission Line that has equal bass output to the bass reflex but which will have huge peaks and valleys in the response from 200 Hz or so on up. I would like to locate a damping material which will cut down those upper resonances from 200 Hz on up but leave bass frequencies alone. If I can do that, I would have a speaker equal to the bass reflex in size and bass output, but more suited to tall skinny enclosures and with a port size far to big to create objectionable port sounds, (Grey Rollins terms it "Port-Noise Complaint" "
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Old 24th October 2007, 11:11 PM   #17
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Quote:
Originally posted by sreten


The original question is bit woolly as to what output exactly is to
be damped, if its the line terminus output then the best way of
achieving this is mass loading the terminus, i.e. a port at the exit.


/sreten.
What I want to do is line the walls of the TL. I do not want to mass load the line, I want the port, (or terminus), to be the same cross section as the line. I would also like the line to be untapered, with the closed end and open end having the same cross section area.
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Old 24th October 2007, 11:21 PM   #18
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Quote:
Originally posted by sreten


Yet another is strategically placed damping at the maxima points
of the line modes you want to suppress.

/sreten.
Where on the line would these maxima be?

I don't have Martin J King's software on this computer, but I would like to pose a hypothetical to try to get the idea.

Suppose my Transmission Line is a straight, untapered line of 84 inches, with the woofer 1/3 of the way down from the closed end. Suppose it has big peaks in the terminus output at 210 Hz, 400 Hz and 750 Hz. At what points along the Line, (counting the closed end as the starting line), would I stuff this enclosure?
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Old 25th October 2007, 04:19 AM   #19
FrankWW is offline FrankWW  Canada
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I've got more time now.

Remember there is more than one kind of mattress foam. There's the memory foam and also the cheaper kind that doesn't conform as well.

A less "dense" foam which is cheap is the kind used for protecting instruments and cameras - it's what they put in Pelican cases - you can buy it on Ebay. It might be useful in your sort of application as well. It's very close to, if not the same stuff, as the DJ supply companies sell as acoustic foam.* (Any thing sold for acoustic application has a higher price).

I'm not sure exactly what the aim of the PMC designer was and they aren't really very forthcoming at the website. Do note, though, that the foam is glued. And glued only to one side of the labyrinth.

I'm quite sure that if the foam is not glued it's likely to have a considerably different effect because some sound will transmit between the foam and the speaker wall. Also, if the passage is very constricted in width then the foam is likely to have some real effect on the speaker excursion.


*Note there are three kinds of foam"

Closed cell.

Open cell. The cell walls have holes in them like sponges do.

Reticulated cell. The cell walls are blasted right out and all that remains is the cell intersections and edges. The stuff used for instrument cases looks like it's mostly reticulated - if used in significant thicknesses it does give some attenuation of higher frequencies. I experimented with it - stuffed a midrange (300 - 300 Hz) horn with it and got about 10 dB attenuation but that was through a length of about 20 inches. Low frequencies would go through it like grass through a goose unless you used quite large amounts, but then, the stuff is cheap if you buy it as sold for packing application.

I took part in a brief if some what unrelated discussion last winter which I never followed up on but you might find it interesting:

A little working miracle?
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Old 25th October 2007, 08:50 AM   #20
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by kelticwizard


What I want to do is line the walls of the TL. I do not want to mass load the line, I want the port, (or terminus), to be the same cross section as the line. I would also like the line to be untapered, with the closed end and open end having the same cross section area.

Quote:
Originally posted by kelticwizard


Where on the line would these maxima be?

I don't have Martin J King's software on this computer, but I
would like to pose a hypothetical to try to get the idea.

Suppose my Transmission Line is a straight, untapered line of 84 inches, with the woofer 1/3 of the way down from the closed end. Suppose it has big peaks in the terminus output at 210 Hz, 400 Hz and 750 Hz. At what points along the Line, (counting the closed end as the starting line), would I stuff this enclosure?

Hi,

Well lining the walls of the TL has the biggest effect on
transverse modes and will effect length modes equally.

for a discussion on damping placement see :
http://homepage.mac.com/planet10/TLS...Pro9TL-Mk2.pdf

for a discussion on TL's in general see :
http://www.quarter-wave.com/Theory/Alignment_Tables.pdf

Nothing indicates that lining the walls only or no taper is the best
choice for what you want to do, as MJK indicates its a question of
compromise.
Using a port as an acoustic low pass filter seems to me an option.

/sreten.
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