[atharva]

I'm attempting my first simple OB. The baffles are 22x18" in WFW configuration with a single FR and two 15" woofers on each side, bi-amped and using MiniDSP 2x4HD for active crossover. The speakers are 8' apart and I'm 10' from them. The room is 14x19x8.

There are a few questions I have.

- Larger drivers are supposed to beam. Will an 8" driver be fine to use? I don't want a few inches of sweet spot.

- How do I decide the crossover point? Do I find the F3 of the fullrange from the stated SPL or do I use the F3 as calculated from the baffle step loss i.e 4560/baffle width in inches?

- Can the baffle step loss be corrected with a low shelf eq? Should it start at or before F3? And can I use a 6dB filter?

- In MJK's article about calcuating the depth of the U/H Frames he says.. "The depth was selected to push the first quarter wavelength resonance above the desired crossover frequency of approximately 200 Hz." What exactly does first quarter wavelength resonance mean? Appreciate if someone explain this a bit. Also which resonant frequency he is referring to and why do we need to push it?

- How much above the crossover frequency should we aim for? e.g say if the crossover is 140Hz then what is good?

- If I read correctly it appears that a driver cannot do much below its Fs. MJK's document on H/U frame shows how you can get some bass extension at the cost of lower Spl. So is it ideal to get a driver like the AE Dipole15 that has a Fs of 21Hz?

- I assume the bass driver quality is as important as the FR?

- Is it possible to get tight bass from an OB or does one need to go servo?

Thanks!

[Early B.]

With so many basic questions, you may want to re-think building an OB speaker right now OR start by building a proven model as a learning opportunity. You'll also need a frame of reference (i.e., visit someone with an awesome OB setup) so you'll have a general idea of just how far or how close your builds come to what a well designed OB speaker should sound like. 

[atharva]

Hi Early B.

Unfortunately there aren't any good OB setups nearby. I prefer learning while experimenting so I built my OBs with PRV15W1000V2 woofers and FaitalPro 3FE25 FRs. I've played around with the crossover and also tried some EQ'ing. I'm impressed by how they sound. They are more "fun" and have a 3D sound compared to my commercial BLH speakers. The proven designs (MJK, Linkwitz, Manzanita, Betsy, Troels etc) I came across were based on drivers that are not locally available (In India). So I decided to take the plunge with a rather simple FAST/WAW OB.

I'm hoping these questions will help others too. Very frequently I've seen misconceptions being cleared up when very basic questions were asked and debated. BTW, even links to good material is fine.

Here is a pic of what I've got. It's still work in progress. The ones at the back were on loan from another DIYer.

[Tyson]

I'd suggest changing the speaker so its a MWW design instead of a WMW design.  Having that midrange driver on top of the speaker and closer to ear height will improve the sound, IMO.

Also, you have the midrange driver placed in the exact middle of the board, that's bad from a diffraction standpoint.  Offset the driver so it's different distances from the top, bottom and both sides (ie, it's as non-symmetrical as you can make it), this will give you better response and much better imaging because edge diffraction will be randomized.   

Here's what I mean (but in a box speaker, obviously).  Look how the mid/high frequency driver is offset, this is what you should shoot for:

[atharva]

@Tyson, the whole frame is tilted so the tweeter fires directly at you. Regarding avoiding edge diffraction, yes I plan to try that.

[atharva]

So I've been reading up on this subject and have found some answers/articles that I found useful and may be helpful to others.

- Larger drivers are supposed to beam. Will an 8" driver be fine to use? I don't want a few inches of sweet spot.
Apparently there is no exact math but the beaming starts at freq whose wavelengths equal to the diameter of the driver. You can read some stuff here... http://www.troelsgravesen.dk/Beaming.htm

- How do I decide the crossover point? Do I find the F3 of the fullrange from the stated SPL or do I use the F3 as calculated from the baffle step loss i.e 4560/baffle width in inches?
Found this series on this very forum (I know, I should have checked) https://www.diyaudio.com/forums/multi-way/189847-introduction-designing-crossovers-measurement.html

- In MJK's article about calcuating the depth of the U/H Frames he says.. "The depth was selected to push the first quarter wavelength resonance above the desired crossover frequency of approximately 200 Hz." What exactly does first quarter wavelength resonance mean? Appreciate if someone explain this a bit. Also which resonant frequency he is referring to and why do we need to push it?

I was lucky to stumble upon resonance of air columns as I hadn't thought of the H-Frame as a cyclinder closed on one end. If you go to http://hyperphysics.phy-astr.gsu.edu/hbase/Waves/opecol.html#c1. Actually this site is great as it explains a lot about acoustics in general. Just click on the links at the bottom to see the mindmap style navigation.

- How much above the crossover frequency should we aim for? e.g say if the crossover is 140Hz then what is good?
As MJK wanted to cross at 200Hz, he chose the dimensions that pushed the first resonant freq to 262Hz. My guess that the intention is to avoid resonance effects  so it MIGHT depend on the slope chosen. i.e if we have a steeper slope like 24dB/Octave then we could cross closer to the resonant frequency. Again the above link to designing crossovers might help.

Feel free to correct me if I'm wrong!