sanity check on Gainphile S11-StigErik fusion OB

[twest820]

I'd like to throw together a quick and dirty open baffle as a double check before proceeding on to more involved projects. The speaker would be a prototype to give me some in depth listening time with an OB to see what I think of dipole sound after living with it for a while. Regardless of the conclusion I reach the speaker's going to be a throwaway after a month or so of listening, so the emphasis here is quick and easy to build on the cheap. I don't want to buy drivers, so I'd like to reuse the drivers from my current two way box speakers---Vifa BC25TG19 tweeters and paired 6.5" OEM woofers that are quite similar to the Vifa XT18WH09. This limits the total woofer Sd per speaker to 258 cm^2 (both woofers combined) and excursion to about 5mm.

Driver wise, the result is rather similar to Gainphile's S11, though he's got one big woofer where I've two. I've been following StigErik's Beyama thread and, while I don't have the Sd to go baffleless, back of the envelope numbers from Linkwitz's dipole SPL suggest I could get ~80dB at 75Hz with a minimal baffle and a listening distance around 2m. Enough for my evaluation purposes and I'm curious to hear how reducing the baffle around the tweeter influences the sound. This led me to the S11 and StigErik inspired design in the attached images (the dimensions are in inches and the Edge curves are successive steps off axis for the tweeter and woofers, starting on the tweeter axis and going out to 30 degrees off axis horizontally). The round top is the tweeter mounting flange and, as you'd expect from a round object 100mm in diameter there's a bad diffraction suck out at 6600Hz. I have measurement gear and a biamp with a PC crossover. EQing the dipole rolloff and the suck out is just an MLS run followed by some mouse clicks.

This is my first OB build so before I start cutting MDF I thought I'd put the design out for more experienced eyeballs to look at. Constructive feedback appreciated---I know it'd be better with a bigger baffle, three or four way with bigger woofers and so on, but the point of this exercise is to quickly check that I like OBs enough to invest effort in building a better one. Note that the drawing is rough and doesn't include finishing touches such as chamfered mounting holes for the woofers. The panel that holds the tweeter would screw into the upper woofer and tweeter flanges.

[StigErik]

Interesting project!

Your tweeter will not be dipole, so you might as well build a wider baffle for the tweeter to avoid the diffraction your simulation is showing. It will be a problem with that large dip there.

[Rudolf]

Originally Posted by twest820

The round top is the tweeter mounting flange and, as you'd expect from a round object 100mm in diameter there's a bad diffraction suck out at 6600Hz. I have measurement gear and a biamp with a PC crossover. EQing the dipole rolloff and the suck out is just an MLS run followed by some mouse clicks.

That is basicly wrong. If you EQ the suck out for 0 deg, you will get large peaks off axis. This suck out is an acoustic phenomenon that can´t be cured by EQ. There are only acoustic 'solutions' to it:
1. Getting a smaller tweeter will reduce the baffle size and move the suck out up in frequency. But there is no tweeter that will push the suck out out of your hearing range
2. making the baffle wider does not avoid diffraction. But it will 'smear' that suck out over a wider frequency range - possibly making it look better in diagrams and easier on the ear. A big hump in the power response will still exist.

[twest820]

Thanks! I agree it's a compromised design. If this were a production project I'd choose different tweeters or mill the tweeter flange so it's not constant distance from the acoustic center to produce the diffraction smearing Rudolf mentioned. But, as it'll be a short lived throwaway speaker, I'm willing to EQ it for a narrow sweet spot and spend the five minutes it takes to redo the EQ if I move the listening position. I didn't want to bias the initial response, but since you two OB masters have zeroed on what seems to be the only issue that's a potential showstopper, the specific question I have is whether there's too much compromise to the tweeter radiation pattern for it to be worth building as a quick and dirty project. If so I'll extend the baffle to spread the null.

Since Parts Express has the tweeters on buyout I'd purchased an extra pair for a vented pole piece project or just to have as replacements when the original tweeters age. So I actually have the ability to do the usual back to back dipole tweeter mount. The diffraction null on the back wave will be about twice as deep as for front wave and the time alignment will be poor, but it's only a few minutes with duct tape and soldering iron to throw to give rear tweeters a try. It would be low quality data but I'd probably learn something about what diffraction nulls sound like.

[Rudolf]

twest820,
Now I understand what you are trying to achieve.
IMHO: Fuzzy effects lead to fuzzy learning. Better have some notable effect to see/hear clear results.
So I would stick to your narrow baffle and EQ to your taste. Tell us whether, were and how any resulting off axis peak can be heard and if it is annoying or not.

[twest820]

Will do; be a few weeks as I have to get a router, fix up my circular saw, and so on. Depending on how it goes I might try throwing on a baffle extension to break up the null.

[gainphile]

Hi Twest, sorry for not replying earlier. I've been in the remote part of java island for the past few weeks. Living audio system-less but enjoying the acoustic gamelans of the locals here. Amazing.

I have no experience with single or multiple 6.5" speakers as dipoles to reproduce such low frequency. But When I used 5" they simply were not able to.

I have tried "triangular baffle" and at that time I thought they sounded more smooth. However I do not have data to back it up since I had no measurement tools. They definitely looked better. The S11 uses rectangular baffle as I think the tweeter region is still an unsolved problem with dipoles and is their achiless heel.

[twest820]

Hey, no problem. Sounds like a nice trip!

Waiting for a sufficiently dry day here to haul the drivers outside and take raw measurements sans baffles. Don't think a single 6.5 would be very happy; the pair should do OK as together they've about the same Sd as a single L21 (xmax is comparable).

I've measurement gear but of course don't have any OBs to measure yet. I've found good agreement between Edge and published data, though.

[twest820]

So, like all good projects, this one's drifted some and I've ended up running the drivers nude and magnet mounted. Good stuff. And I can spend the money for the router on 15PW5s instead.

[StigErik]

Very cool magnet mounting there! (Too bad that dont work with neodynium magnets...)

How does it sound ?