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Old 20th April 2012, 01:47 PM   #31
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Thanks IG,

now for some "alternative" design for comparison ...

From left to right

A) B) C) "Making Of" a 35cm in length quarterwave tube with "resistive bypasses",
............only estimatedly positioned at potential area of max. pressure
............for the lowest resonance and the "near mouth pressure maxima" for
............the higher order modes.
............(Bypasses at throat and in the last third near the mouth)

D) K-Coupler style 37cm in length

E) Measurements of "bypassed tube" without stuffing
D) some measurements including stuffing (approx. 8cm in length between the bypass areas)

F) K-Coupler measurement from a setting before, not directly comparable due to signal level and
positioning, but same smoothing (1/6 octave) like E) and D)

All measurements "on axis approx. in line with the tube / K-coupler respectively


____________________

Of course the bypasses could be optimized in position and also a "mouth slot" in line
with the tube would be possible - but this kind of PP tube better keeps circular cross
section if slotted this way ...

Even the "resistive bypass slots" cause effective acoustical shrinking of the tube
(lowest resonance rises in frequency) as a (in my case unwanted) side effect, but
that is no surprise.

The last measurement did not hit the "most beautiful radiation angle" of the K-Coupler
style tube ... nevertheless the "bypassed" counterparts are able to keep up in terms
of bandwidth and ripple/smoothness.

In fact from these measurements the K-slot type does not look very well.

More precise comparison and optimizations would have to be done, but at this point
damping and "resistive slotting" seem more attractive to me to get a usable midrange.
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www.dipol-audio.de

Last edited by LineArray; 20th April 2012 at 02:16 PM.
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Old 20th April 2012, 07:06 PM   #32
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Quote:
Originally Posted by LineArray View Post
...
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.
...
Which of course argues for the strategy in the
Faerber design, i was talking so impolitely about before ...

If used for high frequencies only, long tube with relatively
short K-Slot slot seems a valid approach.

Unfortunately i am searching for usable midrange too.

@IG : Small diameter compared to wavelength surely is
preferable, as you stated before, to have acceptable dispersion
in highs.

For larger membranes needed for midrange maybe use a
compression chamber of moderate ratio, to adapt a
sufficiently small diameter tube.
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Last edited by LineArray; 20th April 2012 at 07:11 PM.
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Old 21st April 2012, 02:03 PM   #33
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A proposal for discussion:


|<-----dipole path length----->|

D-----------------------------D
D..................::::..................D
D...=======..::::..=======...D
D..................::::..................D
D-----------------------------D


D One driver at each side of the tube wired in antiphase

-- tube wall

.. porous damping material inside tube

:: increased stuffing ( e.g. foam plug)

== resistive slots

______________________________________

Goal would be having a wide range (mid- to high frequency)
dipole radiator without baffle in shape of a low profile rod.

For efficient LF radiation sufficient dipole path
length is needed.

But with larger separation of sound sources
also side lobes will occur at lower frequencies.


Resistive slots may be used to

- suppress side lobes

- damp the tube's resonances occuring now at N x lambda/2 (N odd)


Increased stuffing in the middle of the tube to

- (also) damp the tube's resonances occuring at N x lambda/2 = tube_length (N odd),
where velocity maxima occur in the middle of the tube


________________________________________

Would it be possible for the arrangement to be aligned in a way that

- LF operaton is like a dipole with moderate loss due to leakage
(aligned/compensated by dipole path length) ?

and at the same time

- towards higher frequencies a "smooth morphing" into two cardioids
(in form of resistance boxes) each radiating in opposite direction
takes place, having only moderate side lobes ?
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Last edited by LineArray; 21st April 2012 at 02:23 PM.
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Old 23rd April 2012, 01:30 PM   #34
IG81 is offline IG81  Canada
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Do I get it right, that your proposed arrangement would have the drivers working push-pull? Sharing the same cavity, when the drivers are "out of phase" for frequencies with too short a wavelenght, it might start to act weird, so that's where I'm guessing that the damping should be arranged in a way to prevent any ill effect.

IG
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Old 23rd April 2012, 05:27 PM   #35
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Quote:
Originally Posted by IG81 View Post
Do I get it right, that your proposed arrangement would have the drivers working push-pull?
...
Yes, i thought about doing so. Resonances should not be any different,
than using half of the tube (single side open) and only one driver.

Anyhow, i used the

- 35cm slotted one from above,
- single (rear) side open,
- completely stuffed (from driver to open rear end)

to mount the driver in conventional arrangement.

So some "tube modding" was used to make a usual quarter
wave tube virtually free from resonant conditions in midrange
and mount a driver to radiate conventionally ...

I guess all who followed this comic strip might agree,
that smoothness in midrange is not approached by any of
the "tube radiators" tested before.

Even in this state, the lower rolloff is quite acceptable and
should integrate well using a simple filter slope.
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Last edited by LineArray; 23rd April 2012 at 05:46 PM.
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Old 23rd April 2012, 08:45 PM   #36
IG81 is offline IG81  Canada
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That's a cool 300Hz-2kHz response. I assume 0 is along the tube's axis?

IG
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Old 24th April 2012, 10:16 AM   #37
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Yes along tube's axis.

Fs of the driver is about 160Hz, so the tube could even be
a bit longer.
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Old 24th April 2012, 10:25 AM   #38
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Geometrical length including the tin adaptor is about 40cm.

With that rather tight stuffing used the tube seems to be
effective down to about 250Hz, which would propose an
effective length of 0.3m assuming c=300m/s
(which might be approximatly right inside the stuffed tube)

or

0.34m if using c=340m/s (when using sound in air for
estimation of effective length)

To have the tube effective down to Fs of the driver the
tube could even be extended to 0.53m effective length,
about 0.34 x 1.6.

But depending on how to use it, that length may be somewhat
impractical, because the new tube would have to be about
0.64m long geometrical, protruding most woofer cabinets in depth,
when keeping it straight.
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Last edited by LineArray; 24th April 2012 at 10:52 AM.
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Old 24th April 2012, 01:16 PM   #39
IG81 is offline IG81  Canada
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The size factor makes it preferable as a midrange reproducer, but since this is the fullrange forum, we can also explore how low that will play. It won't be doing much bass in any case, but it might meet a superwoofer decently.

IG
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