Multi-way OB

[offwego]

Going to see how far I can get using my 10 out studio interface with EQ's to make a 3-way crossover to play with different cross frequencies.

Got a MOTU Ultralite (Mk3), and it looks like it should be possible, and it has onboard EQs etc, so I THINK I should be able to set it up to take my analogue in from the Linn DS, and put it out on three pairs of outputs...

 

[scottjoplin]

When you listened to your simple OB set up, what did you like about the sound? Can you accommodate them in your room regards positioning? Have you delved into the Linkwitz website yet?

 

[offwego]

When you listened to your simple OB set up, what did you like about the sound? Can you accommodate them in your room regards positioning? Have you delved into the Linkwitz website yet?

I liked how they put sounds in the room - describing what you hear is always going to create more confusion than clarity!

I was also very impressed at how good it sounded considering what a butcher job I'd done...my thought was that if it sounds this good just thrown together, then there is something going on here that should be explored.

Currently they sound good, but I've got a dip where the '85 rolls off before the Rel rolls in. Most of what I'm reading on OB seems to be battling to get lows from a system that doesn't seem to want to do it (certainly vs a sealed box/reflex/etc design).

I got the '85s because I was looking for 125's to try in place of my 126's, which are in a box. The reason I like the 126's is that they do a similar 'in the room' thing that the 85's do, but the 85s in OB seem to do it much more.

I could just put the 85s in a little reflex box, but they sound so good as they are now, I'd rather just find a way to plug the gap with another driver.

I have some flexibility in placement, but again, if having the 85 OB, then having it supported down to 80ish hz by a boxed driver means that they can pretty much just be treated like any other stand mounter, then that seems a strong argument vs 18" drivers in 20" square footprint monsters - I guess my main thing is I've ever heard an OB full range system...I suppose there must be something in it for people to bother!

 

[offwego]

Yes re Linkwitz, but I tend to work best from first principles, so I need to really learn stuff before I can best manipulate it - it'll take time!

 

[scottjoplin]

I liked how they put sounds in the room - describing what you hear is always going to create more confusion than clarity!

This is actually a very good description of what OB can do. I asked because since it was a single full range driver you were listening to I wondered whether it was the pinpoint imagery that appealed. Often people describe the difference between open baffle and most other speakers as the difference between "they are here" and "I am there", it's largely a matter of personal preference.

The Linkwitz site is very in-depth and technical, this may help you get started a bit better Introduction To The Art And Science Of Open Baffle Speaker Building Article By Darrel Hawthorne Of Hawthorne Audio

 

[offwego]

This is actually a very good description of what OB can do. I asked because since it was a single full range driver you were listening to I wondered whether it was the pinpoint imagery that appealed. Often people describe the difference between open baffle and most other speakers as the difference between "they are here" and "I am there", it's largely a matter of personal preference.

The Linkwitz site is very in-depth and technical, this may help you get started a bit better Introduction To The Art And Science Of Open Baffle Speaker Building Article By Darrel Hawthorne Of Hawthorne Audio

Thanks - I'll add it to the reading list!

The 'your own design' page of the Linkwitz site seems to be pretty relevant...

Yes, I love the imaging of the FR thing...that was what was amazing when I tried the 126's in a box, and then the 85s in the cardboard OB seem to be taking it a step yet further.

 

[scottjoplin]

I think it is possible to have both the simplicity of a wideband speaker (coupled with a woofer to fill in the bottom end where, for a number of reasons the wideband [especially in OB] is going to struggle) and a dipole set up. I went from multiway bass reflex via coaxial, full range transmission line to my OB WAW set up which I have now, and for me it works well and is a good compromise. I would call Linkwitz's LX521 a no compromise design.

 

[offwego]

What is your current setup?

Some of the noises coming from these daft cardboard speakers are insane...the ability to put trumpets/saxophones/clarinets/etc in the room is just like nothing I've heard before. Considering that I sold fairly expensive HiFi for a while, I'm not talking just from having had basic kit before either...

 

[Juhazi]

Linkwitz's site is a bit challenging for learning, I want to recommend these two sites for multi-way dipole planners, by Rudolf Finke and John Kreskowsky

Dipolplus - Alles über offene Schallwände Mostly in German, but a summary in English
Tech Lots of basic stuff and practical considerations

With WAW/FAST 2-way and wide baffle you must just forget these principles and do it quick and dirty and stop thinking! (conscious provocation)

 

[Hydrogen Alex]

Great post .... Really great!

I've been experimenting with open baffle and 'frame type speakers over the past few years. Here are some tidbits that might be helpful for someone who is embarking on a new design and wants to get the best sound possible.

1. Less is More (*except see #2)
When it comes to baffles, less is more. That is to say that you will get the best off-axis response when you use minimal or no baffle. This is one reason why SL's LX521 is shaped like it is - minimal baffle. He showed in a recent presentation why large baffles with a small driver mounted more or less centrally is bad, acoustically speaking.

Make the baffle dimensions as close to the diameter of the driver as possible for even off-axis response. You can even use no baffle at all, the "nude driver" system. It's most useful for frequencies above 800-1k Hz where even "minimal baffle" is still too much baffle.

The downside of little to no baffle is a 6dB per octave loss at lower frequencies, below the "dipole peak" (the first peak in a dipole's response). The peak is at about +6dB WRT the inifinite baffle SPL, and below this it will drop off a 6dB/oct. When the driver is small this peak can be above 1kHz - for a 4" driver this might be as high as 2.5kHz if the driver is used "nude". This causes so much loss at lower frequencies (e.g. by 200Hz) that the driver's maximum power input or driver excursion cannot sustain output relative to the SPL near the dipole peak. There is just way too much dipole cancellation! But the best way to solve this is NOT to make the baffle larger.

A better solution to this problem is to use more and different sized drivers. Each driver can then operate in a band that is closer to the dipole peak and not too far above and below it. Again, this is one reason why SL's LX521 is 4-way. When you try to construct a system from a small full-ranger and a woofer, you have to make the baffle large so that the fullranger dipole losses are not too large at low frequencies. This leads to the off-axis response issues I mentioned above, which many builders don't seem to know about or care about but if you want your system to sound good, it's important to not go overboard with a huge (planar) baffle. Thus we are back to small baffles, large dipole losses, and the need for some kind of driver to fill in the hole between the (sub) woofer and the fullranger. This "hole" is often spanning frequencies from 150Hz to 500-750Hz. A large (e.g. 15" or 18") low Xmax, high sensitivity pro driver is perfect for this application. Because of the size of the driver cone, just put it in a planar baffle not much wider than the driver frame and you are done. Most 18" pro driver that are not designed to be a dedicated subwoofer can reach 500Hz.

2. Load that BASS!
The exception to the rule of "less is more" comes at the lowest frequencies, that is below 150Hz. Remember, we are designing a DIPOLE system so we need a dipole (sub) woofer. Otherwise the radiation pattern will change from (mid) driver to (sub) driver and it will be difficult to blend them properly. At the lowest frequencies you want a LONG front-to-back pathlength to reduce dipole cancellation as much as possible. But because the frequencies are low and wavelengths are long, the problems with large baffles do not apply here like they do for high frequencies. You can even get away with folding the baffle info a U- M- or H-frame if you know how the internal resonances of these structures will influence the response. When you fold the baffle into these shapes you are forming a short transmission line, which has a 1/4 wave resonance. You can only use the 'frame BELOW about half this resonance frequency. The resonance causes a null in the response, and the response starts falling off into this null at about half this frequency below a peak that will also form. Useful depths (one side depth) of a 'frame are from 8-16 inches, which corresponds to bandwidth up to a max of about 275Hz and 125Hz respectively.

We want to make the losses for our subwoofer as low as possible, so we might as well make the 'frame as deep as possible so that where the response is rolling off into the TL resonance null is about where we want to cross over to the next (e.g. mid) driver. This determines the "depth" dimension of our 'frame.

There will still be dipole losses at the lowest frequencies. These add on top of the driver's own high pass response (around its resonance frequency). The size of the 'frame also influences this end of the subwoofer response. It turns out that the deeper you make the 'frame the more the mass of air within the frame couples to the moving mass of the driver. When you add mass to a driver, Qts goes up and Fs goes down. Any increase in Qts is useful in a dipole woofer system because by and large the driver's own Q is relatively low (e.g. 0.5 or less) with the exceptions being "high Q" OB specific drivers. With an OB even a Qts of 1.5 is not "too high", so in this case higher is always going to be better in terms of the amount of bass energy that the 'frame can generate per Watt of input power. In fact, I find that using an H-frame maximizes this effect because there is air loading on BOTH sides of the driver. Thus I suggest that you use an H-frame type system for your subwoofer.

The last piece of the puzzle for a 'frame is how WIDE to make it. The width influences the Q of the peak in the response just below where the response is falling off into the TL resonance null I mentioned above. It also has some influence on the overall efficiency of the 'frame system and the amount of airload. When the H-frame is wide, the Q will be low. This makes it relatively easy to employ simple filtering to create the crossover and accommodating the peak is not a problem. The classic example of this is Martin King's Eminence Alpha-15 subwoofer that he describes in his 2008 (?) paper comparing planar, U-, and H-frames with this woofer. He could simply use a second order low-pass filter to both flatten the response and, along with the natural rolloff into the TL null, cross over to the mid driver. He used a ratio of about 2-to-1 of WIDTH-to-DEPTH and this worked well with only slight peaking at the crossover point to the mid for the H-frame. I suggest this ratio, or even a bit more if/when possible and when the size can be accommodated in your listening space.

So, in summary, load that subwoofer with an H-frame. The crossover point to the mid should determine the depth of the frame. Note that the depth (in inches) of 8-16 inches is on EACH SIDE of the H-frame, e.g. the total depth dimension is 16-32 inches, front-to back, plus the thickness of the baffle supporting the driver. Make the width about twice the one-side depth, or a little more (more is better here), so if you use a one-side depth of 8 inches the H-frame will be 16x16 inches internal height and width. In this case you would get roughly a cube measuring around 18-20 inches (external dims) on a side (accounting for thickness of material used to construct it.

3. In Conclusion...
From the above lines of reasoning we can conclude that at least a 3-way system is required. This is comprised of an H-frame subwoofer, a large format pro driver "midrange" from 200-700Hz in a minimal baffle, and other driver(s) covering higher frequencies. A fullranger can be used, however, better would be to sue a small (e.g. 4-6") driver nude plus a dipole tweeter such as the B&G neo 3PDR crossed over around 3.5kHz or as low as the tweeter can support the output requirement. This results in a 4-way design, however, the added complexity does improve the response and the tradeoff is worth it if you are striving for the highest sound quality in your loudspeaker designs.

These design criteria are what I currently use to design my own OB/dipole systems.

Thanks for this, its spot on!
Cheers
A.

 

[Elemental]

I highly recommend the Mini DSP, just added one to my OB "science project." It takes this whole discussion about crossover points and throws it out the window. Just pick a point and see if it works... experiment.

 

[offwego]

I highly recommend the Mini DSP, just added one to my OB "science project." It takes this whole discussion about crossover points and throws it out the window. Just pick a point and see if it works... experiment.

I need to go back and have another look at their site - I remember looking at the amp plates a while back and thinking WAW (at that point all in boxes).

Can't remember whether there are any three way options on the processors though, and ideally I'd like top, middle, and bottom

Hopefully my MOTU will approximate it well enough to at least play with rough crossover points whilst I work out what the hell I'm doing!

 

[scottjoplin]

What is your current setup?

Some of the noises coming from these daft cardboard speakers are insane...the ability to put trumpets/saxophones/clarinets/etc in the room is just like nothing I've heard before. Considering that I sold fairly expensive HiFi for a while, I'm not talking just from having had basic kit before either...

My first experiments with a point source were car coaxials in very large sheets of cardboard, they weren't really dipoles due to the size of the sheets, but I liked the image they produced. Anyway, after much mucking about my current set up is Jordan Eikonas in the VTL cabinet http://www.ejjordan.co.uk/PDFs/Eikona_VTL.pdf which I built with a removable rear panel, which is how I'm using them now, they are crossed over at 100Hz to 4 12" woofers in two U-frames.

 

[planet10]

I could just put the 85s in a little reflex box, but they sound so good as they are now, I'd rather just find a way to plug the gap with another driver.

Don’t be afraid to try that. http://p10hifi.net/FAL/downloads/uFonkenWK-1v0-plan-130312.pdf

We 1st tried the FF85k OB — we were worried that we couldn’t sell any at the price we needed to get once treated — it didn’t take long to realize HF performance was in the same realm as equally expensive tweeters.

We preferred them in a box (the precursor to the one linked above), and that box will give enuff bass to meet your REL.

Youwant as much of the voice as possible to come from the FR, crossing too high loses too much.

dave

 

[offwego]

Don’t be afraid to try that. http://p10hifi.net/FAL/downloads/uFonkenWK-1v0-plan-130312.pdf

We 1st tried the FF85k OB — we were worried that we couldn’t sell any at the price we needed to get once treated — it didn’t take long to realize HF performance was in the same realm as equally expensive tweeters.

We preferred them in a box (the precursor to the one linked above), and that box will give enuff bass to meet your REL.

Youwant as much of the voice as possible to come from the FR, crossing too high loses too much.

dave

Shall I crawl back into 'Full Range' where I belong?

 

[planet10]

WAW's natural home is in the FR forum. This thread can be moved.

dave
diyAudio Moderator

 

[perceval]

Sorry, been away to my first audio show here in Taiwan... Didn't expect some of the big names here, like Kharma. Was fun. No OB whatsoever though...

If you have the Motu interface, then you can experience EQ and XO with either Motu 's own software or JRiver.

Then decide later if you'd like the miniDSP's own product and software.
That's how I started. With a Presonus interface (which I don't recommend due to the company's abysmal customer service) with JRiver, then a MiniDSP to help with easy trials of XO points.

You can get the 2x4 HD only, for the woofer and FR, then use a sub plate amp for the sub that you could turn on or off for night listening.

The sub plate amp will have its own XO control, and usually a small boost built in at about 25Hz.
It's pretty easy to salvage some plate amps from dead subwoofers, and second hand eBay and the likes, before buying something new.

That's the easy way to get your setup. And in the mean time, just try it with the MOTU.

I have a 15" woofer and a TB FR 8" and cover flat from 40Hz to 18kHz, well, more like a 5dB down curve. Very nice and room filling sound.

 

[diyaudnut]

I've been experimenting with open baffle and 'frame type speakers over the past few years. Here are some tidbits that might be helpful for someone who is embarking on a new design and wants to get the best sound possible.

1. Less is More (*except see #2)
When it comes to baffles, less is more. That is to say that you will get the best off-axis response when you use minimal or no baffle. This is one reason why SL's LX521 is shaped like it is - minimal baffle. He showed in a recent presentation why large baffles with a small driver mounted more or less centrally is bad, acoustically speaking.
.................
2. Load that BASS!
The exception to the rule of "less is more" comes at the lowest frequencies, that is below 150Hz. Remember, we are designing a DIPOLE system so we need a dipole (sub) woofer. Otherwise the radiation pattern will change from (mid) driver to (sub) driver and it will be difficult to blend them properly. At the lowest frequencies you want a LONG front-to-back pathlength to reduce dipole cancellation as much as possible. But because the frequencies are low and wavelengths are long, the problems with large baffles do not apply here like they do for high frequencies. You can even get ....................
3. In Conclusion...
From the above lines of reasoning we can conclude that at least a 3-way system is required. This is comprised of an H-frame subwoofer, .....................

These design criteria are what I currently use to design my own OB/dipole systems.

very instructive post. Thanks.

 

[Juhazi]

WAW's natural home is in the FR forum. This thread can be moved.

dave
diyAudio Moderator

I disagree here!

The headline is "Multi-way OB"
- Multi means many and many is considered more than one, usually 3 or more!
- OB means "open baffle", no box

Please don't be stuck with the WAW, just because someone is suggesting/using a FR driver in a multi-way OB speaker!

 

[scottjoplin]

I don't think he is, he wants to use a sub as well, so presently he seems to be veering towards a OBSAWAW