Doctor Diffraction: A donut shaped high end loudspeaker.

4thtry

For this project I will be creating a high end 3 way floorstander using the Rival R176-P-08 paper cone 7 inch woofers and the Morel TM4055-8 tweet/mid's.  These woofers were used by Ben in his Fenrir speakers and by Javad in his Rivalries with good results.  The Morel drivers were used by Paul K in his Cornetta speakers (MWAF 2019) with very good results as well.

https://www.parts-express.com/morel-tm4055-8-2-midrange-and-1-1-8-tweeter-speaker-plate--287-038
https://rhythmaudiodesign.com/products/r176-p-08-carbon-pulp

My plan is to mount the TM4055 in the center of a large 4 inch thick donut style baffle using a 4 inch radius roundover along the top 80% of the baffle. My drafting skills are not the best, so the drawing does not show the actual roundover (sorry).   The radius will taper down to 3", then 2" and finally to 3/4" where it merges with the lower box.  The lower box will have a 3/4" radius roundover (not shown).  I am going to grind out the roundovers with my belt sander, forming the radius in stages using guide lines and cardboard templates.  JR did a good job of rounding the corners and facets on his small bookshelf speakers a few years ago, using only a beltsander.  Seems like a good plan and it should work as long as I develop a good spherical rhythm as I sand back and forth to reach my layout lines.   I will, of course, be wearing an M95 dusk mask and using particle board instead MDF to keep the dust down.

What I am trying to do here is to reduce diffraction ripple above 1kHz as much as possible.  Attached is a model of the expected diffraction using a square 13.625 baffle with 4 inch radius roundovers.  I used Jeff B's BDS program to create this model.  My baffle be round, but the results above 1kHz should be similar.  The remaining lower frquency diffraction bumps should be easy to handle in the xover (approx 800Hz).

I'll be picking up lumber next week and will stop back here from time to time with pictures. As soon as the cabinets are done, I'll be running up a full set of measurements. Stay tuned and thanks for reading.  Comments and feedback are always welcome. 

Wolf

So- Fenrir + Cornetta + Agenda = Fendetta? Works for me....

Ron_E

I've always been under the impression that putting a driver in the middle of a round baffle was not a good idea because the distance to the edge of the baffle was constant. https://trueaudio.com/st_diff1.htm  Linkwitz has some interesting diffraction measurements here. http://https//www.linkwitzlab.com/diffraction.htm

Ron 

dynamo

Bill continues his epic battle of diffraction started with his side towers! Love it!

kenrhodes

4thtry, I can fit this on my wood lathe and could turn this very quickly. It would be a round disc with a recess, you would need to fit it to your box. I have done similar with plywood and it looks pretty cool. Let me know if I can help.

4thtry

Ron_E said:

I've always been under the impression that putting a driver in the middle of a round baffle was not a good idea because the distance to the edge of the baffle was constant. https://trueaudio.com/st_diff1.htm  Linkwitz has some interesting diffraction measurements here. http://https//www.linkwitzlab.com/diffraction.htm

Ron 

Thanks, Ron.   I agree with what you are saying, but I think you are misunderstanding what I am trying to do here.  I am NOT putting a single driver in the middle of a round baffle with sharp edges.   I am putting two drivers, slightly off center, in the middle of a circular baffle that is transitioning into a spherically shaped baffle.  The huge round-overs make a big difference.    A circle begins to transition into a sphere when you make the circle and the round-over radius extremely large.  In addition, a spherical shape is superior to a conventional rectangular shape because it helps to reduce the diffraction ripple created by the top two box corners. 

4thtry

kenrhodes said:

4thtry, I can fit this on my wood lathe and could turn this very quickly. It would be a round disc with a recess, you would need to fit it to your box. I have done similar with plywood and it looks pretty cool. Let me know if I can help.

Thanks for the offer, Ken, but I have already cut the wood.   I thought about doing this on a lathe, but then I would have the problem of filling in the lower section where it connects to the lower box.   Also, the lower section edges slowly transition from the large to small round-over, and this cannot be done on a lathe.   If my belt sander approach fails, I'll let you know.   CNC is probably the only other way to do this.   

4thtry

Picked up some lumber: two 6 ft long particle board benchtops (Menards).  They are 2 feet deep and 1.14" thick (1-1/8" nominal).  I revised my plans.  I boosted the radius from 4" to 5-5/8" and changed the overall donut diameter from 13.625" to 17".  As a result, I ran out of lumber, so I will have to go back for more.

Kornbread

The old brick measure reminds me of the days I used to help dad.

That's going to be one top-heavy cyclops.    

4thtry

Wolf said:

So- Fenrir + Cornetta + Agenda = Fendetta? Works for me....

Too late.  Name is already "Doctor Diffraction".   For my next project, however, I was thinking a small 8" two way using up my restored "grab bag" RST28F's and the pair of SD215A-88 8"  woofers I bought from Craig.  With your permission, I would name them the "Zonkers".    Then we would have the Zingers, the Zangers, and the Zonkers.  How does that sound?

4thtry

 Kornbread said:

The old brick measure reminds me of the days I used to help dad.

That's going to be one top-heavy cyclops.    

I have two of these.  Picked them up from my Dad's workshop when he passed away 18 years ago.  I often prefer these over a standard tape measure.

Top heavy:  Yes, indeed.   I'm concerned about this and am hoping for the best.  I weighed the large donut on my bathroom scales:  30 lbs!!!!   I'm hoping that after I hollow it out, cut off the bottom, round the edges, etc., that I get it down to about 10 lbs or so.   Otherwise the speaker may fall flat on its face!!!!  

PWRRYD

4thtry said:

Wolf said:

So- Fenrir + Cornetta + Agenda = Fendetta? Works for me....

Too late.  Name is already "Doctor Diffraction".   For my next project, however, I was thinking a small 8" two way using up my restored "grab bag" RST28F's and the pair of SD215A-88 8"  woofers I bought from Craig.  With your permission, I would name them the "Zonkers".    Then we would have the Zingers, the Zangers, and the Zonkers.  How does that sound?

Hi Bill,
I'm not Ben but I think your name idea is fun.  I would be very surprised if Ben didn't agree.

Wolf

Zonkers- I love it!

Tom_S

4thtry said:

I have two of these.  Picked them up from my Dad's workshop when he passed away 18 years ago.  I often prefer these over a standard tape measure.

I inherited my Dad's too. I don't use it, but I sure remember him using it all the time.

jr@mac

I think a better idea for an informal IowaDIY "shootout" would be 8" 2-ways, all with a name that is a variation, permutation, or iteration of "Zingers"....

I reserve Octozaenger.

kenrhodes

The very first picture in this thread is how I assumed you would join the disc to the cabinet. That would help reduce weight and would be simple to cut on a band saw or even a handsaw. If the inside of the box had many layers it would make it so you could smooth out the transition without sanding through the corners. You could also mount the woofer higher in the box to keep CTC spacing close.  
  For future projects you might want to consider pink foam. It can be painted with acrylic paints and is both cheap and lightweight. Even hand sanding can remove material quickly. This might make it so you can model the idea and measure before going through the work of making it out of wood.

dynamo

jr@mac said:

I think a better idea for an informal IowaDIY "shootout" would be 8" 2-ways, all with a name that is a variation, permutation, or iteration of "Zingers"....

I reserve Octozaenger.

Dibs on Zoinkers! My upcoming 8” two way mtm needs a name 

squamishdroc

Cool build Bill! I am quite happy with my Rival build.

jhollander

I spent a bunch of time re-voicing my Limus build with the R176-P. I wasn't happy with the sibilance and ended up pushing down the breakup with a 6th order slope.

4thtry

jhollander said:

I spent a bunch of time re-voicing my Limus build with the R176-P. I wasn't happy with the sibilance and ended up pushing down the breakup with a 6th order slope.

Thanks for the heads up, John.  What was your xover frequency?  I will be targeting 800Hz or so to the 2" midrange driver.  If I run into problems, I can substitute my pair of Esoteric 7" woofers and give them a try.   The flange on the Esoteric is identical to the R176-P. 

4thtry

kenrhodes said:

The very first picture in this thread is how I assumed you would join the disc to the cabinet. That would help reduce weight and would be simple to cut on a band saw or even a handsaw. If the inside of the box had many layers it would make it so you could smooth out the transition without sanding through the corners. You could also mount the woofer higher in the box to keep CTC spacing close.  
  For future projects you might want to consider pink foam. It can be painted with acrylic paints and is both cheap and lightweight. Even hand sanding can remove material quickly. This might make it so you can model the idea and measure before going through the work of making it out of wood.

Good idea.   Do you mean the pink insulation foam sold in 4x8 sheets in 1 or 2 inch thicknesses?   Did not know you could paint or sand this material.  

My drafting skills are not the best.  On the first picture, I simply created a box and then I created a donut.  I merged the two drawings, which makes it look like donut is cut out.  My plan is to cut a flat spot on the bottom of the donut and bolt it to the top of the box.   I might cut the corner out on the top of the box to make it look better.

4thtry

dynamo said:

Dibs on Zoinkers! My upcoming 8” two way mtm needs a name 

Cool!   Wish I would have thought of that.   Zonkers is pretty good too.  

4thtry

I forgot to mention one other thing about the Olson paper, diffraction, and cabinet shape.  Here is a link to the original paper:

http://www.aes.org/aeshc/pdf/how.the.aes.began/olson_direct-radiator-loudspeaker-enclosures.pdf

My speaker will be a 180 degree hemisphere; not a true 360 degree sphere.   As referenced in the AES paper, the diffraction for the hemisphere shape is actually smoother than the sphere above 2kHz.   The spherical shape is superior below 2kHz.   Olson did not mention why this occurred, but I think this happens because sound waves follow the surface of the sphere, traveling all the way around back to the front side again and this, in turn, is picked up by the microphone as HF ripple.   With a hemisphere, the sound waves diffract at the exterior sharp edge which causes higher lower frequency ripple instead (based on the size of the hemisphere). 

Olson only measured out to 4kHz, but I think these measurement differences may be important, because we are very sensitive to phase differences in the high frequency area.  

jhollander

4thtry said:

jhollander said:

I spent a bunch of time re-voicing my Limus build with the R176-P. I wasn't happy with the sibilance and ended up pushing down the breakup with a 6th order slope.

Thanks for the heads up, John.  What was your xover frequency?  I will be targeting 800Hz or so to the 2" midrange driver.  If I run into problems, I can substitute my pair of Esoteric 7" woofers and give them a try.   The flange on the Esoteric is identical to the R176-P. 

Good point, I was using the Tymp Corundum at 1,200 and moved it up to 2,000. Anywho the R175 response was pushed 50 dB down at 6,500

4thtry

I revised my donut drawing again to remove more material; now down to 18 lbs. each.  Before gluing up the rings, I plan to chamfer the internal ring edges at 45 degrees to get rid of even more material.  18 lbs. is still way too much.  I'm hoping the big 5.625" radius round-over will get rid of another 4 lbs.   

**Note on TM4055:  If you use this tweeter/mid in a project, do NOT cut a 4.76" mounting hole as suggested.  Cut a smaller 4.5" hole and then add "bump-out" style clearance "ears" for the chambers.  As shown in the photos, I used a 1.5" diameter bump-out for the tweeter chamber and a 3" diameter bump-out for the mid chamber.  Otherwise, if you cut a 4.76" hole, the mounting screw holes will be too close to the edges.

hifiside

Layered mdf gets heavy! I found that out on my Tubb’s project. A large Salad bowl and fiberglass?

dynamo

4thtry said:

dynamo said:

Dibs on Zoinkers! My upcoming 8” two way mtm needs a name 

Cool!   Wish I would have thought of that.   Zonkers is pretty good too.  

The name is yours if you’d like it Bill!

4thtry

 dynamo said:

4thtry said:

dynamo said:

Dibs on Zoinkers! My upcoming 8” two way mtm needs a name 

Cool!   Wish I would have thought of that.   Zonkers is pretty good too.  

The name is yours if you’d like it Bill!

Nah.  You get the credit for that one.    I'll stick with Zonkers. 

4thtry

Glue-up, templates and guidelines:

Kornbread

That looks like a ton of sanding.  Have a chainsaw?   

4thtry

 Handheld 3x21 Rockwell belt sander.  Picked up new 50 grit and 120 grit belts today.  Yes, indeed, lots of sawdust.  Should be fun!!!  

The plan:    I will be clamping/mounting the donuts in my 45 degree angle sled and bringing the edge down to the guidelines in a total of 5 stages.   Double checking the round-over consistency along the way with the templates.  When I get close to the final round-over guidelines, my plan is to glue sandpaper to my templates and do the final sanding by hand.