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Old 18th April 2012, 05:39 PM   #21
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Some investigations concerning selective stuffing/damping ...

Curves taken at

- about 1m distance
- about 30 degrees above the tube's axis (at the slot side).

(that means falling slope even in "unstuffed" setting)

"Stuffed throat" means about 10cm in length of the tube from
driver to beginning of the slot, not "pressed hard" but not
"loose" either.

In my view, stuffing such a device is essential to get balanced
output, like in any quarterwave design.

Of course highs are muffled by doing so, maybe the
"covered slot" setting can be refined if highs are essential.

Here the "stuffed throat" condition is the only one giving
a quite acceptable midrange from say 300Hz to 3Khz.

A lining of the tube has not been tested yet.
Attached Images
File Type: jpg K-Coupler_SelectiveDamping_01.JPG (194.4 KB, 185 views)
File Type: jpg K-Coupler_SelectiveDamping_02.JPG (189.9 KB, 183 views)
File Type: jpg NoStuffing.jpg (563.9 KB, 182 views)
File Type: jpg StuffedMouth.jpg (595.2 KB, 173 views)
File Type: jpg StuffedThroat.jpg (603.2 KB, 158 views)
File Type: jpg CoveredSlot.jpg (679.1 KB, 47 views)
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Last edited by LineArray; 18th April 2012 at 06:04 PM.
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Old 18th April 2012, 06:00 PM   #22
IG81 is offline IG81  Canada
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Stuffed throat is not bad at all, ~2.5 octaves of useable bandwidth.

IG
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Old 18th April 2012, 06:09 PM   #23
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Yes, and i do believe one can lower the cutoff
by resizing it (longer, modified slot), without affecting
the upper cutoff too much.
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Old 18th April 2012, 07:28 PM   #24
IG81 is offline IG81  Canada
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Here's a French test of 3 types of tubes as waveguides, straight-cut, elliptical and K-slotted. I can't help but think the K-Tube got the short end of the stick in more than one way though. The cut itself is barely more than a shallow half-ellipse and the whole thing is quite short.

Fichier d?origine: Acousticaltube.pdf

IG
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Old 19th April 2012, 10:40 AM   #25
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Thank you for posting, i have already seen this experiment
on the web.

I think if one was to compare the shape of cutouts/mouths
one should align for same lambda/4 lowest resonance.

That means the elliptical would have to be significantly longer
then the "straight" cut and the K-slot even (much) longer.

So the base quarterwave resonance would be a useful kind
of "norming" for comparison of impedances etc.

But here the elliptical and K-slot types are even much
shorter than the straight cut, thus comparison does not
reveal anything surprising IMO.
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Last edited by LineArray; 19th April 2012 at 10:43 AM.
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Old 19th April 2012, 01:23 PM   #26
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Using a phase plug like shown on the pics.

The phase plug reduces the cross sectional
area of the K-tube's throat from about
16 cm^2 to 7 cm^2

Inner tube outer diameter approx. 30mm
Outer tube inner diameter approx. 45mm

The inner tube is sand filled and closed
at both ends

A piece of kneading mass forms the compression
chamber at the inside end of the inner tube.

Compression ratio for given driver is near 2

The "phase plug" inner end has been brought
as close as possible to the diaphragm, without
the diaphragm touching the plug at low frequency
movement (should be about 1mm distance).

Comments after coffee break ...
Attached Images
File Type: jpg Foto1331.jpg (294.5 KB, 59 views)
File Type: jpg Foto1332.jpg (504.6 KB, 49 views)
File Type: jpg Foto1334.jpg (424.1 KB, 37 views)
File Type: jpg PhasePlug_Dist80cm_20DegOffAxisAboveSlot.JPG (190.8 KB, 60 views)
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Last edited by LineArray; 19th April 2012 at 01:27 PM.
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Old 19th April 2012, 02:24 PM   #27
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Variation of the pressure chamber volume ...

The phase plug including pressure chamber has little
effect at low frequencies, despite distorsion being
lower without compression chamber near the lower
cutoff of the tube.

For the top end it is important to have the chamber
volume as small as possible.

Nevertheless in the current device there seems no
benefit from the phase plug.
Attached Images
File Type: jpg PhasePlug_Dist80cm_20DegOffAxisAboveSlot_ChamberVolume.JPG (185.6 KB, 53 views)
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Old 19th April 2012, 04:15 PM   #28
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Now a 30cm long straight cut tube ...

A) Stuffed throat (approx 10cm) vs. unstuffed, on axis
B) Stuffed throat (approx 10cm) vs. unstuffed, approx. 20deg off axis

C) 30cm straight vs 38cm K-Slot both unstuffed, Caution: Conditions and
levels not identical, just for qualitative comparison


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What are the effects of the K-Slot compared to straight cut ?

- effective acoustical length shrinks > lowest quartewave resonance rises in frequency

- higher order resonances have lower Q, dampening effect similar to stuffing

- little/no effect on overall efficiency

- little/no effect on radiation/dispersion of high frequencies with wavelengths
comparable to diameter of tube


Some personal conclusions:

1) K-slotting may be used to complement damping by porous materials due to resonances of low
to mid order numbers.

2) It is questionable, whether noteably shrinking the effective acoustical length of the tube
by "K-slotting" is worth that potential benefit, since an effectively shorter tube tends to have
worse modal overlap.
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Last edited by LineArray; 19th April 2012 at 04:36 PM.
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Old 19th April 2012, 04:48 PM   #29
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3) Length and (strategic, local) damping of the quarterwave tube
are the main factors for the usable low frequency extension.

Moderate K-slotting (not too long slot) may allow for reduction of
porous material to be used and from this point of view increase
efficiency at HF a little, if the tube was stuffed already.

But there is no significant effect on high frequency radiation as such.
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Old 19th April 2012, 07:56 PM   #30
IG81 is offline IG81  Canada
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First time I see a phase plug on a K-Tube. That's some pretty involved testing there!

Subjectively, the 1" format K-Tube really does seem to do a good job at dispersing HF. Listening to a mono Karlson topped with a K-Tube makes for a wide sound despite mono.

I'll see if I have data anywhere on this or maybe I can measure some.

Keep up the work, there's not many folks playing with these and they can sound good.

IG
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