S12 OB - Returning to true OB

[gainphile]

This is the measurement of baffle with no structural wing (my reason for wing is not for bass extension but physical support). It's somewhat similiar to the Phoenix, just a bit narrower.

No structural wing:




With wing:




Immediately we can see the typical smooth response of narrow open baffle. 6db drop is also clearly formed as well as the dipole peak. The smoothness ends between 1khz-2khz (first null) where the response widens, shown as bunched up lines.

With no wings, this occurence occurs a bit higher, perhaps 100 or 200hz or so. This is expected because the wings created some extra path length.

More interesting is the difference in directivity. The wings actually made the response wider. The no-wings have more directivity. It is also very important to observe here that both have smooth response up to the first null. Also the wing provided a somewhat smoother response after 1khz.

I think I'm quite happy to find that the difference between having structural wing and no wing is quite negligible.

[gainphile]

I also took some measurement on back-to-back tweeter setup which is a common approach for a typical orion-ish setup. I haven't done careful measurements before knowing that it would be bad anyway and is the achilles heel of such setup.



It's pretty bad and the response is all over the place.

When walking around the speakers with the tweeters playing pink noise I hear null from the sides. But it's different. The null is abrupt as shown in the last line. I'm not sure whether the null is due to dipole cancellation or simply the directivity of the tweeters.

I hope one day to replicate mige0's dipole tweeter setup and see.

[Rudolf]

Originally Posted by gainphile

This is the measurement of baffle with no structural wing

Very interesting results, indeed.
Could you give us a picture of the back of the baffle too? Just to have a better idea how those 4 cm strips are placed.

Originally Posted by gainphile

I also took some measurement on back-to-back tweeter setup ...
I hope one day to replicate mige0's dipole tweeter setup and see.

I know that this is mainly semantics, but mige0's dipole tweeter is no tweeter dipole. If the distance of the two dipole sources is much longer than the wavelength, both sources get de-correlated. In fact it would not make a real difference whether they would be dipole or bipole. There will be directivity because of the waveguide, but hardly any dipole loss IMHO.

If you want tweeters with real dipole characteristics, they have to get as small as ever possible.

[JandG]

Have you tried running that mid driver free-air on top or on side with tweeter below & measure that..? maybe you could just remove & mock it up with existing baffle ..?
Joe

[gainphile]

Originally Posted by Rudolf

Very interesting results, indeed.
Could you give us a picture of the back of the baffle too? Just to have a better idea how those 4 cm strips are placed.


I know that this is mainly semantics, but mige0's dipole tweeter is no tweeter dipole. If the distance of the two dipole sources is much longer than the wavelength, both sources get de-correlated. In fact it would not make a real difference whether they would be dipole or bipole. There will be directivity because of the waveguide, but hardly any dipole loss IMHO.

If you want tweeters with real dipole characteristics, they have to get as small as ever possible.

No worries. This is what they look like from the back, and there's nothing special about the wings. One potential problem is resonance. I tapped on them using small mallet and I thought I can hear them radiate sounds. Perhaps some bracing will be needed (a triangular stuff every 20cm or so).




About the tweeter region, I'm not too fussed whether the ideal tweeters are operating as dipole or not, as long as the polar response is smooth. Something like Geddes' plots, only front to back. Am I thinking correctly?

[Rudolf]

Originally Posted by gainphile

No worries.

Certainly not.
I was just curious, because your first picture did not show any evidence of wings whatsoever.

About the tweeter region, I'm not too fussed whether the ideal tweeters are operating as dipole or not, as long as the polar response is smooth. Something like Geddes' plots, only front to back. Am I thinking correctly?

As I said - just semantics. Of course you are 'thinking correctly' if you go for best CD, regardless of dipole or not.
Indeed I´m not sure whether the other aspects of dipoles would be compelling enough for me, if there would be no built-in CD.

[ErikdeBest]

just a small question: what does CD stand for? (I don't think it is compact disc...)

[Davey]

Constant Directivity.

Dave.

[gainphile]

Dipole Active Crossover



Much of the functions are clearly explained at Linkwitzlab website. What is unique is the need to compensate early rolloff of the Seas L21RNXP. The midrange exhibit some coloration due metal cone breakup harmonics and this would be the same even if the more expensive W22EX is used. It is not always audible and only appears in certain recording material.

Seas L21RNXP is an excellent unit aside from the normal metal resonance, however they have been discontinued. Seas L22RNXP is the drop-in replacement. Forget exotic drivers such as the W22, I may as well build the Orion. Why use metal cones then? Well the audible transparency of those drivers is unmatched. They are pistonic until that breakup arrives.

Hivi K1 tweeters exhibited very low distortion and excellent dispersion when crossed at 2.2khz or above with LR4. The tweeters could easily be substituted with something else, as long as the units have excellent distortion and dispersion. With those criteria it's hard to beat the cost of K1 ($12). Better tweeters would have allowed me to cross lower and help reduce midrange breakup coloration, but not much. Ideally 8" metal cones like L21/22 or W22 should only be used below 800hz.

[gainphile]

The side panels are not to increase acoustic path length as normally done by dipoles with "wings". They are for structural reasons, and generally the narrower the better. Although they are very narrow indeed (4.2cm) the midrange section should not be obstructed. I have observed audible midrange coloration if the side panels are simply extended through the midrange height, although I do not see it in the measurement. This could be a very high Q resonance which are hidden by diffraction effects in measurements.

The cavity for rear tweeter must be blocked, otherwise it forms a resonator. Stuffing the cavity using polyfill only helped marginally. It has to be angled to minimise diffraction issues with the midrange.