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Old 22nd May 2009, 08:19 PM   #5721
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One of the things that really bothers me about how the H1N1 swine flu situation is being handled is that little or no objective information is being disseminated about it. Instead, a set of drastic prescriptive measures is being implemented as if it were far worse than HIV. This puts me in mind of totalitarian North Korea's satellite/submarine launch, which they are still pretending to their captive people has been successful. This is very wrong, and the way the swine flu situation is being handled reeks of the same type of deceitfulness.
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Old 24th May 2009, 12:24 PM   #5722
mige0 is offline mige0  Austria
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Thinking about alternatives to overcome some limitations in OB designs I have come to the conclusion that we might have to leave pure OB to proceed one step further.

Using a roughly 30cm / 12" wide baffle the first deep notch after the baffle peak is around 1,5 kHz.

No big problem for a fairly big mid - like my 8" Jantzen due to its smoothly increasing directivity but already a severe limitation for the AMT tweeter with its very wide polar pattern at this frequency.

Net result is that there are three options left:

1. take a mid that has smooth response above 1.5kHz and increase XO to well above 1500Hz (my current 2.5kHz setup)
2. equalise the notch which will result in peaks off axis as polar pattern is non-CD here
3. leave pure OB and go the way of wave guiding for that particular freqeuncy range.


First of all I had to jump my mental hurdle of not having ever considered (well at least for a very long time) to use horns for home applications.

This thread really helped in this - and after having done the hardest step and also got some start-up help - thanks Jean-Michel ! I decided to do an experiment on dual hemisphere horns (or whatever the best name may be).

I've already built the first one mouth roughly 30 x 15cm (12"x7" ) and 15cm (7") deep.

Two of them identically attached to the front and the rear of my AMT and you get the picture.

Being in the process of building a second "dual hemisphere horn" speaker for stereo and not having done any sophisticated equalisation but lowered the XO point to roughly 1,5kHz my first audition is well - very promising.

Has this been done previously? Anybody who has any experience with such an "dual hemisphere horns " approach?

Michael

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Old 24th May 2009, 08:37 PM   #5723
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Here is a thread from Audiocircle 2007 discussing a similar subject: http://www.audiocircle.com/index.php?topic=36331.0 . JohninCR built front and rear waveguides for B200. I am afraid pictures are absent from AC at the moment so we cannot look at his efforts. But it was in the same direction that you are going now. Let us know the progress.

/Erling
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Old 24th May 2009, 09:40 PM   #5724
ScottG is offline ScottG  United States
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Quote:
Originally posted by skorpion
Here is a thread from Audiocircle 2007 discussing a similar subject: http://www.audiocircle.com/index.php?topic=36331.0 . JohninCR built front and rear waveguides for B200. I am afraid pictures are absent from AC at the moment so we cannot look at his efforts. But it was in the same direction that you are going now. Let us know the progress.

/Erling

Though physically similar, the result is *very* different between the two.

Presumably Michael has a high-pass filter that significantly attenuates the lower freq. response where the AMT driver (with waveguide) becomes increasingly "non"-directional. This method should substantially remove significant diffraction effects.

In contrast John's prototype will only add diffraction. (..it's a full-range driver operated lower in freq. than the waveguide's loading.)

*VERY* different results.

BTW, the Neo 8 (std. version) is exceptionally well suited to the method Michael is experimenting with (..though with a much larger waveguide(s) and a lower operating freq.). Something similar, (though better loaded), to this could be done with the several Neo 8s:

http://www.audio-consulting.ch/?Prod...aker:Rubanoide
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Old 24th May 2009, 10:11 PM   #5725
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As far as I remember the B200 was highpassed with an H-baffle housing a 15" bass below. But much detail was never given more than some photos. My remark was more in the spirit that 'nothing is new under the sky'. But I am certain, and also quite interested, that the approach can be a fruitful way of handling those bafflewidth - frequency - directivity issues addressed.

/Erling
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Old 24th May 2009, 10:51 PM   #5726
cuibono is offline cuibono  United States
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I tried building a 'waveguide' for my B200 after seeing JohninCR's, and the results were poor - the FR became substantially rougher, so I abandoned it. Ours were fairly different though, so YMMV.
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Old 24th May 2009, 10:57 PM   #5727
cuibono is offline cuibono  United States
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Quote:
Originally posted by mige0
Thinking about alternatives to overcome some limitations in OB designs....

What limitations, specifically?


Quote:
Originally posted by mige0


1. take a mid that has smooth response above 1.5kHz and increase XO to well above 1500Hz (my current 2.5kHz setup)
2. equalise the notch which will result in peaks off axis as polar pattern is non-CD here
3. leave pure OB and go the way of wave guiding for that particular freqeuncy range.

How about using multiple baffles, each one keeping its driver operating below the dipole peak?

Good luck with your dual horn.
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Old 26th May 2009, 09:42 PM   #5728
mige0 is offline mige0  Austria
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Took some time to do a decent measurement setup Sorry for the delay in responding to you input.
I'm usually not so much interrested in FR and polar patterns - but have - at least slightly - changed my mind during the course.

Thanks for the pointer to the other thread - very much appreciated - and as already mentioned - yes my aim *is* different but may lead to comparable conclusions.

I'm definitely not looking for constant directivity as a mayor goal but from an equalisation point of view its a welcome gift if we get it "for free".

What I want to achieve is an improvement to the behaviour of OB above the first peak.

Depending on the baffle width with respect to the driver size there is a relatively deep notch following the peak. There are more peaks and notches to follow but usually they get less severe further up the frequency range.

As outlined, if I equalise for good on axis response that notch will show up as dip in the power response.


First pix is for the tweeter in dipole configuration.

Click the image to open in full size.

This is a measurement not a simulation !
I haven't done excessive equalisation, as its not needed to show the important thing so there are some ripples left and above 18kHz there is no equalisation applied at all.

Next pix is for the same OB configuration measured at angles of 0 10 20 30 40 deg

Click the image to open in full size.

This traces ain't very specific for an AMT. Any 1" tweeter in an 30cm OB would perform roughly the same *if* it could be operated absolutely symmetrically (front to rear) as a dipole

If you look at the 1,5kHz behaviour you immediately see what I want to improve.
The peaks at 1500Hz are *not* due the baffle peak (this one is at slightly below 1kHz for that OB) . This peaks are an unavoidable "by product" if we want to get good on axis response for small speakers (tweeters) in a modest baffle.



With the next step we come to the solution I was looking for.
As the peaks and dips are caused by comb filtering we successfully can try to avoid that in the frequency range of interest by adding some directivity by a wave guide / horn.

Next pix again is on axis measurement but this time for the dual hemisphere horn

Click the image to open in full size.

Again boring flat 4th order Butterworth HP (though a completely different XO equalising of course)

Next pix is for the same double horn measurements at angles of 0 10 20 30 40 deg

Click the image to open in full size.

Pretty well behaved considering the high zoom range no?

What is most obvious is that we do no longer get the "stick out" 1500Hz in the power response though it's still there but at an attenuated level.

Telling form (fairly short term) auditing I'm very happy with the results.
Overall presentation of this kind of "dual hemisphere horn" has a very smooth but still crisp top end and no roughness or "in the face " pattern at all. Nothing of the excellent resolution of the AMT got lost.
If I wouldn't see the horn in front of me it could be a direct radiating speaker beautiful soundstage as well as far as can tell for now.
What more could I ask for?

Michael


PS
don't get lost in speculations on the 10kHz irregularity.
I was in a hurry and didn't seal the cables.






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Old 26th May 2009, 09:52 PM   #5729
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Quote:
Originally posted by mige0

This is a measurement not a simulation!
I haven't done excessive equalisation, as its not needed to show the important thing so there are some ripples left and above 18 kHz there is no equalisation applied at all.

...

This traces ain't very specific for an AMT. Any 1" tweeter in an 30cm OB would perform roughly the same *if* it could be operated absolutely symmetrically (front to rear) as a dipole.

If you look at the 1,5 kHz behaviour you immediately see what I want to improve.
The peaks at 1500 Hz are *not* due the baffle peak (this one is at slightly below 1 kHz for that OB). This peaks are an unavoidable "by product" if we want to get good on-axis response for small speakers (tweeters) in a modest baffle.



Thanks for the measurements, very illuminating - especially as it applies to the general case of all open-baffle tweeters and midranges.

I've been wondering what would happen if the front and rear horns were intentionally mismatched in size and cutoff, so the ripples of each would be smoothed out. Maybe this would just make a big mess. The application I'm thinking of would have a single cone driver driving the front and rear horns (push-pull), so the two horns would "see" each other impedance-wise. I'm not sure if asymmetric or symmetric horns would be better in a push-pull application.

P.S. The "mystery" flu - H1N1 or not - is finally all gone. Still catching up on my sleep, but energy levels are recovering.
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Old 26th May 2009, 10:02 PM   #5730
mige0 is offline mige0  Austria
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Lynn, no problem at all - my dual horn is exactly like that.

I know there is ongoing discussion about the impedance interaction of two horns at the same cone.

But for mine - I can look staight through the horn as the foil is pretty thin - beautiful yellow color when illuminated from the back...

So this experiment may also answer some of the basic questions floating around.
My guess is that it work at least as well as with one horn attached (though not tried)

Michael
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