[hungrych]

Has anyone had any experience building Bookshelf/Monitor sized OB's for use on say, a desk? I'm gonna get some B200's soon and try it out myself but I haven't got around to it yet. Are there any particular sizes that do the best or tips to follow? Thanks!

[JLM]

hungrych,

Desktop monitoring using open baffles is not a good idea...

Small open baffles = no bass.  Unless you carefully add sides/wings (so as not to mess up the dipole effect of open baffle and start turning it into a ported design) you'll probably not get response below 250 Hz (middle C on your piano) which is way too high to properly crossover to a powered sub: 

http://melhuish.org/audio/baffle.html

If your desk is against a wall there will be insufficient space to develop the rear wave to produce the open baffle dipole effect and the delay between front and rear waves would be minimal, thereby creating a smearing between the front and rear waves (messing up detail and imaging).

The farther back you can get from open baffles the better they sound (as the front and rear waves have a chance to "gel").

Just start looking around here and at the Darkstar thread in the Red Wine Audio circle to get a handle and ideas for open baffle.

[Bob in St. Louis]

That's disappointing to hear. I was thinking along the same lines on my PC. Thought it could be done very cheaply (although not using something of the quality of b200s) with good results.
Maybe if nothing else, you'd loose the plastic box sound, granted the loss of imaging would ba a negative.

hmmm, idea is not dead yet in my mind. Just sad to hear JLM.

Bob

[konut]

You might want to search the Dark Star thread for info on the 'joint chiefs' design by JohninCR. If memory serves me, the pentagonal baffle is not much larger than the B200 itself, with different size wings, at 90 degrees, of different lengths. It could be described as more of an 'open box' design. This would extend the bass, although you'd still need a sub to fill in.

[JohninCR]

OBs work great at the desktop and I used some that way for over a year.  If you have a CRT monitor then magnetic shielding is important unless you can set the speakers well away, but then you really need to roll your chair back for better listening.

As far as I'm concerned virtually all OBs need bass augmentation, and a sub under the desk works fine.  I've used desktop OB's in a number of ways that you can get extra effective baffle for free.  Also, being in the nearfield gives you free effective width as well, because OBs have more bass in the nearfield due to 2 factors.  1.  The rear wave has travelled significantly farther as a % of the front wave travel distance, so the rear is lower in magnitude and has less affect.  2.  The separation distance (rear of driver to your ears minus front to ears) is greater due to the angles (except for U-baffles).

How do you get free baffle?  One simple method is using your monitor as part of the baffle.  Small baffles attached to the sides of your monitor is the easiest example.  Add an angled wing to the sides, and get surprising extension.  Another way to get free baffle is, if your desk has open space in front, set the speaker near the edge.  Then much of the rear wave will expand down below your desktop.  This can work even better than the floor, because you get boundary reinforcement for the front wave, but not for the rear wave.

Small drivers are the way to go for the desktop for size reasons and I find their sound gels better in a short distance.  2 drivers I've used for desktop OBs that work really well are the Fostex FE127E and Adire's AV3.  Both are magnetically shielded.  Without a sub, I'd go with the AV3 as its higher Q makes for fuller sound on the same size baffle, but it isn't as efficient, so it depends how loud you want to listen.
Another benefit of using small drivers is the test baffles.  You can literally make test baffles in a couple of minutes with cardboard, tape and a knife.  If you want to spend a couple of bucks, get some of that foam filled posterboard like architects use to build models, because it's more rigid than cardboard.  Then you can try some different things to see what you like best, and the final form wood baffle will sound even better.

Speaking of my Joint Chiefs, I do have a pair of mini-JC baffles that I haven't even tried yet.  They're not much bigger than a typical computer speaker, but they are equivalent to an 18" wide flat baffle in the farfield.  I designed them for surround use near my ceiling corners, but they'd probably do great at the edge of a desk.  I just haven't decided which driver to use and I don't want to mar the wood until I'm sure.

[PSP]

From Linkwitz' page:

There is a practical limit to the smallest size of an open baffle speaker, if it is expected to reproduce a balanced full range sound spectrum. The PMT1 represents this case. The speaker might be used for a stereo system in a small room or for side and rear speakers in an audio surround setup.

and, more on the PMT1:  http://www.linkwitzlab.com/proto.htm#PMT1

These are not exactly "small", but the question of how small you could make a dipole vs. how much bass one would sacrifice is a good one. 

Good luck,
Peter

[mcgsxr]

I played around with using the FR125S (available from either CSS, or DIY Cable) on an open baffle, as a center channel in my HT, just for fun.

The baffle was absolutely tiny, just large enough to fit the pair of drivers, and it was usable.  If it were mounted near the monitor, as suggested, and 2x the size I used, with a sub, I bet it would be fun.



Not perfect, but a fun experiment.  Great driver too, wonderful build quality, shielded, full range, cast basket etc.

[konut]

Speaking of my Joint Chiefs, I do have a pair of mini-JC baffles that I haven't even tried yet.  They're not much bigger than a typical computer speaker, but they are equivalent to an 18" wide flat baffle in the farfield.  I designed them for surround use near my ceiling corners, but they'd probably do great at the edge of a desk.  I just haven't decided which driver to use and I don't want to mar the wood until I'm sure.

Are you familiar with this site?  http://www.zaphaudio.com/smalltest/    Might have some useful information for your purposes.

[hungrych]

OBs work great at the desktop and I used some that way for over a year.  If you have a CRT monitor then magnetic shielding is important unless you can set the speakers well away, but then you really need to roll your chair back for better listening.

As far as I'm concerned virtually all OBs need bass augmentation, and a sub under the desk works fine.  I've used desktop OB's in a number of ways that you can get extra effective baffle for free.  Also, being in the nearfield gives you free effective width as well, because OBs have more bass in the nearfield due to 2 factors.  1.  The rear wave has travelled significantly farther as a % of the front wave travel distance, so the rear is lower in magnitude and has less affect.  2.  The separation distance (rear of driver to your ears minus front to ears) is greater due to the angles (except for U-baffles).

How do you get free baffle?  One simple method is using your monitor as part of the baffle.  Small baffles attached to the sides of your monitor is the easiest example.  Add an angled wing to the sides, and get surprising extension.  Another way to get free baffle is, if your desk has open space in front, set the speaker near the edge.  Then much of the rear wave will expand down below your desktop.  This can work even better than the floor, because you get boundary reinforcement for the front wave, but not for the rear wave.

Small drivers are the way to go for the desktop for size reasons and I find their sound gels better in a short distance.  2 drivers I've used for desktop OBs that work really well are the Fostex FE127E and Adire's AV3.  Both are magnetically shielded.  Without a sub, I'd go with the AV3 as its higher Q makes for fuller sound on the same size baffle, but it isn't as efficient, so it depends how loud you want to listen.
Another benefit of using small drivers is the test baffles.  You can literally make test baffles in a couple of minutes with cardboard, tape and a knife.  If you want to spend a couple of bucks, get some of that foam filled posterboard like architects use to build models, because it's more rigid than cardboard.  Then you can try some different things to see what you like best, and the final form wood baffle will sound even better.

Speaking of my Joint Chiefs, I do have a pair of mini-JC baffles that I haven't even tried yet.  They're not much bigger than a typical computer speaker, but they are equivalent to an 18" wide flat baffle in the farfield.  I designed them for surround use near my ceiling corners, but they'd probably do great at the edge of a desk.  I just haven't decided which driver to use and I don't want to mar the wood until I'm sure.

Awesome! I think I may give the fostexes a try first but tell me how the mini-JC's work out.

I can't seem to find the original posts about the JC design, I searched to no avail. Does anyone have a link? Thanks again. 

[JohninCR]

Here's a pic of some proto-types.  The big ones are Hawthorne 15's.  The center pair are B200's and the small baffle is an FE108Esigma.  The secret to the pentagon U-baffles is the shape of rear panels to eliminate resonances resulting from the pipe and direct the HF portion of the rear wave to up and to the center.  The bottom of the cabs are an expanding pathway connected to a slot immediately behind the driver.  Only very low frequency info enters the slot (like a Helmholtz bass trap), but instead of being dissipated, it exits the rear mouth resulting in a limited horn type transformation and a much longer rear wave travel distance (greatly enhanced bass extension).

You've got me thinking about the FR125 in a mini OB-RLH.  They have good Xmax and may feed the RLH portion quite well.  It would have the same proportions as the JC OB-RLH (the cab with the 15's), but minaturized.

[hungrych]

So how are the dimensions for the wings and baffle caluclated exactly?

[JohninCR]

So how are the dimensions for the wings and baffle caluclated exactly?

For what driver?  The depth dimension and shape of the rear edge of the pipe determine everything soundwise.  I've been very lucky by backing into the depth dimension starting with the driver's Fs and IB response, and then setting a realistic Fequal point.  Contact me directly for dimension details, as I'm not ready to make them fully public.

The pentagon is actually quite easy with a table saw.  Use the driver or a same sized plate laid on a piece of cardboard to decide on the widths of the 5 side panels.  Make 5 identical panels and cut the edges at 54 degrees instead of 90.  Do a test run with small scraps first to be sure the angle is set perfectly.  Lay the panels on the floor edge to edge, and tape them together (all laying flat).  Flip it over and put glue on the edges to be joined.  Then pull the outer pieces up together and tape them.  That's a little trick I learned from Terry Cain, the master speaker cabinet builder.  No nails, no screws...you just need accurate cuts to make good solid joints this way.

[Danny Richie]

My first thought was a pair of FR-125's.

The way to make it work though is to mount them one over the other and put the lower one in an optimal ported enclosure while leaving the top one in the open baffle.

Then connect them in series, but shunt the lower woofer with about a 62uF to 75uF poly cap. The larger the cap the lower the crossover point.

This is actually a series network between the two of them.

You still might have to add a notch filter to the upper woofer if the dipole effect peaks up the response somewhere in the 500 to 900Hz range.

It might make for a killer little computer speaker.