Newbie questions regarding alpair active OB 2 way

[jamesd88]

Hello all,

After much reading I think I am almost ready to embark on my first open baffle pair of speakers.

I just have a few questions regarding design possibilities that I am hoping some people have had similar experiences with and can shine some light.

Being from Australia my choice of drivers isn't huge (without paying a huge premium), so I would like to take advantage of darcheraudio's reasonable prices on the Alpair range (in particular the alpair 10 - gen 1 I believe, though).

My thoughts are - the alpair FR deals with all frequency from about 200hz upwards (exact crossover point and types to be determined from testing) and using an eminence driver for bass duties.

Currently I'm tossing up between two possible designs:
1) 1x 15" eminence beta and 1x alpair 10 (or 12, seeing as the eminence may need to be imported anyway) on a baffle 450mm wide at the bottom and tapering to about 250mm at the top.

2) 2x 10" eminence alpha and 1x alpair 10 on a rectangular baffle about 280mm wide.

Visually both would be ok - though scale drawings of the second option are a bit more appealing. Should I perhaps offset the fullrange driver - like in the MJK FR + alpha 15a plans?
Symmetrical just looks a lot better to my eyes haha.

The crossover will be a miniDSP, however I would like to minimize digital bass boost and similar where possible.

Any advice is greatly appreciated.
Thanks in advance!

[planet10]

Used as a mid-teeter, the extra size of the A12 is unneeded and in general you will get better performance from the smaller driver. Personally i'd go all the way to A7.

You definitely don't want to mount the driver in the centre (a recipe for macimum ripple).

Amount of bass and EQ needed will be determined by baffle width and Q of the driver. Alpha will need less EQ (none if done rigt) vrs beta.

You would do well to just copy the trapezoid MJK baffle of Jim Shearer but with the different mid-tweeter

dave

[jamesd88]

Thanks very much for the reply, very interesting indeed.

I was assuming the larger driver would be able to cross a bit lower to the woofers - I was hoping to keep most/all the vocal frequencies to the FR driver.

The alpair 7 would save on cost too which is always good. A clone of the MJK project would be easy enough however I have read a few places that the baffle width should be less than twice that of the midrange driver.
I guess the offset would allow this on one side, but does it affect imaging?
Also, and I know I'm just being vain, but I think the smaller FR looks a little bit funny on such a huge baffle in the MJK design.

Also, any views on the 2x10" bass drivers vs 1x15"?
The Nao and the Orion use 2x10" woofers, however with a great deal of custom EQ it seems, whereas the MJK project uses just a single 15".

So many options.

Thanks again,
James

[planet10]

Originally Posted by jamesd88

I was assuming the larger driver would be able to cross a bit lower to the woofers - I was hoping to keep most/all the vocal frequencies to the FR driver.

The alpair 7 would save on cost too which is always good. A clone of the MJK project would be easy enough however I have read a few places that the baffle width should be less than twice that of the midrange driver.
I guess the offset would allow this on one side, but does it affect imaging?
Also, and I know I'm just being vain, but I think the smaller FR looks a little bit funny on such a huge baffle in the MJK design.

Also, any views on the 2x10" bass drivers vs 1x15"?
The Nao and the Orion use 2x10" woofers, however with a great deal of custom EQ it seems, whereas the MJK project uses just a single 15".

Amoungst others i have built the MJK baffle. It works pretty well. Still none of these have swayed me to think about an OB in the main system. Something about them always struck me as not quite right. A couple recent papers i've read on OBs explains at least some of that.

The small baffle width comes from the goal of trying to maintain dipole behavior over as large a range as possible.

The best OB i've heard was really closer to an IB -- 16 foot wide baffle.

As i understand it, the baffle size really determines where the XO needs to go.

The big Eminence or the Goldwood certainly give a lot more bang for the buck. I may have one more OB in me, as i'm sitting on 8 10" OB drivers.

(actually 2, as i will probably repurpose the MJK baffle with some no cost drivers, and give them away)

None of the OBs i've heard image nearly as well as a pair of full-on miniOnken.

dave

[DQ828]

I would go with the 15" driver & consider putting it in a H of U frame.

The width of the baffle (and or depth of H/U Frame) will determine where the baffle step/dipole peak will occur. With MiniDSP you will be able to drive the the bass to a higher crossover point if you like.

Traditionally the woofer crossover would be kept below the dipole peak, but I'm not sure this is an absolute.

Ideally you would build a simple prototype & then measure the response using HolmImpulse or equivalent, but of course that would require buying more stuff.

You might consider the Neo 10 as the full range driver. I have shown the response of my drivers taken outdoors from 1m on axis, no crossover or EQ applied & in prototype. The Neo 3 looks a dream for a full range driver but you wont get the SPL you need.

[jamesd88]

Hmm, very interesting indeed.

It's somewhat confusing as their seem to be two schools of thought regarding the baffle width (and also the Qts of the driver).

I have just finished reading the notes on the music and design website and they insist on a baffle (at least for the midrange) no wider than twice the driver diameter.

Without having a chance to test the theories against each other without taking the plunge and buying the drivers I think I may have to do just that.
I guess I can always put an alpair into a miniOnken if I don't like the OB and turn the bass drivers into... Something... haha.

Now just to decide on the 2x10" or 1x15" and source where from.

Thanks again,
James

[MJK]

The hardware trade off between narrow and wide baffle is as follows.

Narrow baffle ---> multi-amp and active crossover, also additional filters.

Wider baffle ---> can be single amp and passive crossover if drivers are selected carefully

I find the design of a wider baffle system to be much simpler, I am listening to an Alpair 6 paired with an Eminence Alpha 15A at the moment and it works very well. I am biamping and using an active crossover so I can try a variety of other small full ramge drivers (Fostex FF85K, Fountek FR88EX, Alpair 6, Alpair CHR-70, and a few more in the fiuture). I believe the Alpair 6 / Alpha 15A could be a passive system if I settled on these two drivers.

Both approachs work, it just comes down to how complex you want to make the design and how much hardware you need to run the system. Wide baffles work.

Martin

[John Bsuch]

Hello all.

Martin, as usual, has provided a key, insightful observation.

OB, like other loudspeaker designs, given enough properly applied engineering, money and effort can be made to work over a very broad spectrum. No pun on "spectrum"! You can make good or bad designs in either passive or active, on narrow or wide baffles. However, as Martin points out, with certain combination's it can be a lot easier to obtain a realizable, usable outcome.

Generally, in the world of OB, the wider the baffle, the greater the operating efficiency. Some times this can be a bad thing, it all depends on the interaction of the remaining elements of the total design.

As for the max mid range baffle with rule, (no more than double the driver diameter) in of itself, it means little. Asymmetrical driver placement on baffle faces is common, usually for response smoothing reasons. If your mid range is mounted on a 4' wide baffle, place it off vertical center closer the edge. No big deal. There is an optimum distance, however many design factors would come together to determine what is best. OB woofer or woofers tend to be large and usually crossed over below 500 Hz. So the distance from woofer to mid range center is not critical. It is instructional to do some card board mock ups to see what sounds best. You may be very surprised!

[ra7]

I can offer some experience with the Alpair 6 crossed to the 15Alpha. The crossover was 150 Hz, fairly low for the Alp6. It performed admirably, given the tough duty. It ran out of excursion pretty quickly, but it did sound nice.

I would advise choosing a higher efficiency driver than the alp6... or, in other words, one with a larger cone area. It can be done with the alp6 given a higher crossover point, but I wouldn't do it that way.

Cone area is everything on an OB.

[MJK]

I am using the following crossover frequencies.

Eminence Alpha 15A - 2nd order low pass at 200 Hz

Alpair 6 - 2nd order high pass at 400 Hz

I am using the wider baffle geometry to create an acoustic hump that fills in the region between 200 and 400 Hz for both drivers. Works well without any excursion problems.