18" 90-deg OS WG coaxial (design) - diyAudio
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Old 4th September 2014, 03:27 PM   #1
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Default 18" 90-deg OS WG coaxial (design)

Still have to figure out a few things, but this design is pretty close. The compression driver mounts onto the back (currently sized for the DE250) and the screws go in from the front. Once the screws are tightened, the hole caps go in to restore the seamless profile.

I've run the motor through FEMM and it looks like I can get at least 0.8T through the gap using N40-grade disc magnets. The center pole hides a copper sleeve. What helps is that xmax has to be limited to +/- 0.5 mm (or some other arbitrarily low figure) to curtail modulation of the compression driver's output, so there are basically no excursion-dependent non-linearities. In case you're wondering, this MUST be used on top of another 15"/18", crossed 4th-order at 250-300 Hz to minimize excursion.

The basket will probably be made out of a few aluminum shafts connecting plates together, sort of like how Audio-Technology does their Flex Units.

Not quite sure what to make the cone out of. Paper is probably the easiest method, but the issues of shape tolerance and mediocre stiffness concern me. I'd like to try a sandwich cone out of either magnesium or carbon fiber, over Rohacell, but both of those materials I'm not familiar with.
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Old 4th September 2014, 03:44 PM   #2
gedlee is offline gedlee  United States
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Ambitious! Even I would not attempt to actually make a driver. It is incredibly difficult to get anything even remotely useable without lots and lots of engineering and resources.

But Good Luck! You will need it.
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Old 6th September 2014, 06:31 AM   #3
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More pics up here:

18" 90-deg OS WG coaxial - AVS Forum
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Old 6th September 2014, 11:25 AM   #4
xrk971 is online now xrk971  United States
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Neat ideas and ambitious but I think you idea for a ring of Nd slugs and a paper cone can work. You may want to check out the 5MR450NDY driver that has a huge Nd motor and a stiff paper cone and suspension. I use it in a horn and it like the sound. More info here:
PRV 5MR450-NDY for FAST applications

Maybe an OS horn can be coupled with this driver in a larger throat?
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Old 6th September 2014, 01:24 PM   #5
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Aside from being a waveguide, is there any advantage/disadvantage to that deep of a cone profile?
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Old 6th September 2014, 02:30 PM   #6
CLS is offline CLS  Taiwan
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A deep straight cone tends to be stiffer than a shallow one. But when it breaks up (up to a certain frequency), it's probably more severe, thus a higher and narrower peak. And it's slightly heavier and inferior in dispersion.
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Old 6th September 2014, 07:31 PM   #7
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Quote:
Originally Posted by CLS View Post
A deep straight cone tends to be stiffer than a shallow one. But when it breaks up (up to a certain frequency), it's probably more severe, thus a higher and narrower peak. And it's slightly heavier and inferior in dispersion.
In terms of dispersion, I don't know if there has been any reports that indicate off-axis response is significantly different for a deep cone than a shallow cone, as long as the cone breakup has not yet come into play.

Hopefully, the cone won't break up until at least 1500 Hz, which is about an octave above. The first resonance mode can be shifted up as well, by adding stiffening ribs to the rear of the cone. Very easy to do after assembly, with this type of construction.
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Old 6th September 2014, 08:40 PM   #8
gedlee is offline gedlee  United States
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The analysis that I did in my book implied that if the cone is rigid, i.e. no waves moving IN the cone body, then the effect of a deeper cone is to widen the response. If there is motion within the cone than anything can happen - it will depend on the specifics.
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Old 6th September 2014, 08:41 PM   #9
gedlee is offline gedlee  United States
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Quote:
Originally Posted by CLS View Post
and inferior in dispersion.
What does that mean?
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Old 7th September 2014, 12:18 AM   #10
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What about adding a flare to the cone 2" or so from the surround, like a radius termination on a proper waveguide?
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