OB one and Hello everybody!

Russell Dawkins · « **Reply #20 on:** 26 Jan 2012, 08:51 pm »

Given that the front of the tweeter diaphragms is horn loaded in both these cases, I imagine the greatest acoustic effect will result from the reduction of cavity and other internal resonances and their influence on the radiation from the front of the diaphragm. In both cases, the rear radiation would be substantially less than the front and probably making a very subtle contribution to the sound heard in front of the speaker, wouldn't you think?

gamgee · « **Reply #21 on:** 26 Jan 2012, 09:31 pm »

Quote from: Nate Hansen on 26 Jan 2012, 07:46 pm

I now see from your drawings that you do want to run the tweeter naked.....cool! You also mention maintaining constant directivity. I would say use as small a baffle as you can for your mid to maintain its directivity as high as you can, even then it will be beaming well before the tweeter, causing a narrowing of the directivity pattern before the tweeter takes over. You are going to use active eq right?

I'm also wondering about that horn on the tweeter. To the front of the speaker it will allow you to cross lower, but to the rear I think you might end up with a hole in the response due to the lack of that horn loading to the rear......

Well, it's not that I think it is really posible to make constant directivity speaker this way, I supose it will take more resources and knowledge than I currently have to do that...but I want to try and see how close I'll get. I already thought of mid without baffle but Edge simulation does not look too good...I should try to measure naked BG20 and see what I'll get.
About active EQ...I'll definitely use some kind of EQ in the end, but would really like to try and see how far I can get doing things only pasively. Kind of excersize in crossovers (god knows I need excersize). Some kind of separate amplifier for the woofers would largely simplify that task...
About horn/waveguide on the tweeter - I plan to ask Peter and get a pair of waveguides for the back too. Is that a good idea? I don't know much about those things...

gamgee · « **Reply #22 on:** 26 Jan 2012, 09:44 pm »

Quote from: scorpion on 26 Jan 2012, 08:32 pm

K + T speaker, it is a cone tweeter not a dome or ribbon or planar or anything else, just a plain old cone. The variation with BG20 as a midrange was in Nr 1 2009.

I find the 'backless' Beston measurements very interesting.

Have a look at GR-Research's Super V which has a coax hornloaded compression driver forward firing with the membrane naked at the rear:

It will be more or less the same here. You don't want the backwave to dominate the sound just give a subtle addition.
I agree with Nate to keep midrange baffle as narrow as possible for constant directivity and also let midrange and treble be totally physically separated from the bass baffle. Should be easy to fix judging by your sketch.

This also opens for vertical physical alignment for no phase difference between speaker units.

/Erling

Thank you, I'll try to find that issue (Nr 1 2009.)
Would you please elaborate on Beston "backles" measurements? I'm really curious to hear what you think. If only subtle addition from the backwave is needed than I'm already there (in email corespondence, Peter told me that with open back and waveguide in front, tweeter would be "quasi dipolar"...probably that is reason why ih the graphs he wrote "bi-polar").
So, you agree with Nate that I should get rid of the mid baffle alltogether?
If I understand correctly your last sentence, you want to say that getting rid of the mid baffle will make it easier for me to align acoustical centers of drivers and avoid tweeter reflection? That's all true, but I suspect BG20 cannot reach low enough without any baffle at all. I'll give it a try tomorow.

gamgee · « **Reply #23 on:** 26 Jan 2012, 09:52 pm »

Quote from: Russell Dawkins on 26 Jan 2012, 08:51 pm

Given that the front of the tweeter diaphragms is horn loaded in both these cases, I imagine the greatest acoustic effect will result from the reduction of cavity and other internal resonances and their influence on the radiation from the front of the diaphragm. In both cases, the rear radiation would be substantially less than the front and probably making a very subtle contribution to the sound heard in front of the speaker, wouldn't you think?

It seems everybody agree on this one - open back drive with waveguide in front will definitely have different (and lower intensity) radiation from the back - for the time being I don't see this as something to worry about, I'll try to get another pair of waveguides to experiment with and see what happens, but I don't think that will have big impact on overal performance. I still have bigger fishes to fry (for me at least), like definitive baffle design and biggest fish of all - crossover.

gamgee · « **Reply #24 on:** 26 Jan 2012, 09:55 pm »

And I wanted to say thank you Nate, Erling, Rusell for your help. It's apreciated.

scorpion · « **Reply #25 on:** 26 Jan 2012, 10:00 pm »

Baffle first, not baffle-less but perhaps no more than 25 cm wide for the midrange. It will depend upon if you plan to go active with EQ or passive.
Passive you might need a wider baffle to marry mid and bass at best. But perhaps you can EQ also with a passive setup.

Acoustic centres ypou could align I think also with a mid baffle.

I think the backwave contribution will be strong enough to make an impact for the overall dipole effect.

Regarding the Bestons I would like to inquire if they running dipole could be crossed lower than stated before.

/Erling

gamgee · « **Reply #26 on:** 26 Jan 2012, 10:29 pm »

Quote from: scorpion on 26 Jan 2012, 10:00 pm

Regarding the Bestons I would like to inquire if they running dipole could be crossed lower than stated before.

/Erling

I would like to know that too, but if I remember correctly, Beston did not investigate that at the time (april 2011.). Maybe they did in the meantime, I'll ask ('cause I'm not really willing to risk mine to find out how low they can go).

gamgee · « **Reply #27 on:** 31 Jan 2012, 12:26 pm »

Regarding crossover point lower then 2500Hz recomended for regular RT002A, Peter says not to do it since driver is not designed for such use and could get damaged.
Regarding directivity, he sent me their data (for regular RT002A).
Here it is:

Could someone help me decipher?

gamgee · « **Reply #28 on:** 31 Jan 2012, 12:31 pm »

And here is impendance comparison RT002A regular/dipol...

gamgee · « **Reply #29 on:** 1 Feb 2012, 02:34 pm »

I've been playing...
This is a sketch of hybrid OB/slot loaded woofer - I need to do some more experimenting to see if it's possible to shift woofer response lower without too much sensitivity/cleanliness penalty. Hopefully, cavity in front of the woofer is small enough to resonate outside of usefull range...(cca 30 cm diameter translates to 1143 Hz wavelength).

gamgee · « **Reply #30 on:** 1 Feb 2012, 02:43 pm »

Tweeter is "hanging" on console with soft elastic rings - allows for precise aligning of tweeter with mid driver and a bit of vibration damping. Peter from Beston is going to send me another pair of waveguides for RT002A to make it symetrical. Semicircular foam cushion should help with difraction...

gamgee · « **Reply #31 on:** 1 Feb 2012, 02:47 pm »

Sand filled magnet mount, folded baffle side amd mid baffle.

Nate Hansen · « **Reply #32 on:** 2 Feb 2012, 05:44 pm »

Very cool design, it will be interesting to see this come together (you are going to keep us updated, right?

)

Those directivity plots don't really tell you much IMO since they're showing monopole behavior, forward output only. You'll have to measure it yourself as a dipole. A standard frequency chart will be fine, with measurements at 0*, 15*, 30*, 45*, and 60*.

gamgee · « **Reply #33 on:** 2 Feb 2012, 08:40 pm »

Quote from: Nate Hansen on 2 Feb 2012, 05:44 pm

Very cool design, it will be interesting to see this come together (you are going to keep us updated, right? )

Those directivity plots don't really tell you much IMO since they're showing monopole behavior, forward output only. You'll have to measure it yourself as a dipole. A standard frequency chart will be fine, with measurements at 0*, 15*, 30*, 45*, and 60*.

Oh I'll keep you updated allright, no worries about that. But I would like to get as much input and discussion, comments and critique from you guys, since many of you are quite accomplished in this loudspeaker art/science. OB field is still new and so exciting, I'm reading and reading and there's no end...every time I decide it's time to finally draw a line and finally MAKE something, I find some new info, consider my project in light of this new info and start changing things...I probably have at least 20 sketches and many variations...it's going on like that for few years...
I got tired and frustrated of that vicious circle, I WANNA MAKE the damn speaker already!

So, all comments are more than welcome, I want to get over with theoretical part and start making some sawdust...
Sorry, I needed to take that of my chest...

gamgee · « **Reply #34 on:** 6 Feb 2012, 12:30 pm »

Quote from: gamgee on 1 Feb 2012, 02:34 pm

I see now that I made a mistake - ommited quarter wave frequency..."(cca 30 cm cavity diameter translates to 1143 Hz)"... which translates to cca 285 Hz quarter wave frequency. That means I should cross slot loaded driver lower than that (285 Hz), right?

gamgee · « **Reply #35 on:** 10 Feb 2012, 10:39 pm »

Sorry for the delay, first i got ill, then I got lazy..

Ok, so I tried to do some measurements of BG 20 and open back Beston. Note the words "I tried" - that's because I honestly have no clue about how much those can be trusted. Measurements were "conducted" in highly improvised environment - aluminum ladder in the middle of the room, drivers suspended on string at height of cca 150 cm from floor. Microphone is DIY WM61A connected to laptop mic input. Software is HolmImpulse, mic is on axis, 5 cm from the driver. Amplifier voltage was not measured so measurements are comparable between themselves only.

gamgee · « **Reply #36 on:** 10 Feb 2012, 10:48 pm »

This is RT002A dipol in same conditions, only difference is that input signal is starting at 1500 Hz in order to protect the driver.

Could someone lend me a hand with these? I could post screenshots of the HolmImpulse if needed. Mic is made as per instructions on Gainphiles page
http://gainphile.blogspot.com/2008/11/cheap-and-accurate-speaker-measurement.html

Both graphs are made with "Complex smoothing" option (frequency dependent time window) 1/6 octave smoothing.

gamgee · « **Reply #37 on:** 11 Feb 2012, 09:12 am »

This is dual 12" bass drivers (Tesla and SAL) on 60x96 cm with small (15 cm) wing on one side, connected in parallel (I know, impendance!), measured on axis with upper driver (Tesla), 10 cm distance.

gamgee · « **Reply #38 on:** 11 Feb 2012, 09:38 am »

This is what I get adding up all three responses in HolmImpulse...

So, what this gives me? If this measurements are any good (grain of salt? a handfull is more like it

), it does not look bad at all. High response rolloff could come from free air measurement, open back configuration or mic not being up to the task? (I read somewhere that WM61A is rolled off in highs...)
I don't know, so please, do chime in, I need all the help I can get...

Nate Hansen · « **Reply #39 on:** 11 Feb 2012, 03:33 pm »

Ahh progress!

Ok, a few things about your measurement setup. Off the floor like you've got is good! But 5 cm from the driver isn't showing a large enough distance for the dipole behavior to develop, which is why the measurements are rather flat. With no eq applied, a driver in dipole operation like that will measure anything but flat! You will have a peak roughly corresponding to the width of the driver (or baffle), and a 6db rolloff below that. I go about 1.5 meters to see what sort of dipole compensation I will need.

Your setup in Holm isn't quite right. What you've got with the complex frequency smoothing is a smoothed in-room response, and you want to see how the drivers behave without the influence of the room (at least for the mid and tweeter, I measure the woofers in-room from the listening position). You want to change the "distance" display on the x-axis of the impulse to "time". To do this go to Options at the top, and go to where it says 'Impulse Axis' in bold. Click on 'Impulse as time (millisec)'. Now click 'Options' next to your measurement, and click the 'Impulse time-window (gating)' spot. At this point, look at your impulse measurement again (this is what Holm is actually measuring, not the FR. The FR is derived from the impulse). See that dotted line that crosses over the x-axis? That sets the gate, which cuts out the room reflections, depending on where you have it set. For me, with the same basic setup conditions as you, I use 4ms. Now you should have a workable frequency response!

For some more basics, JohnR has a good guide here: http://www.hifizine.com/2011/03/refining-a-4-way-open-baffle-speaker-minidsp-2x4/

He uses a miniDSP, REW, and fuzzmeasure here, but the basics apply. You might also want to check out the free (demo, but fully usable) version of ARTA. I use both ARTA and Holm to make sure I'm on the right track.