FR125S in open-baffle - diyAudio
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Old 24th August 2005, 08:35 PM   #1
robertG is offline robertG  Canada
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Default FR125S in open-baffle

While calculating proper enclosures for the FR125S, I mounted them on open baffles, just to break them in.

Now I'm not so sure I'll continue calculations! These drivers are quite exceptional in OB configuration.

The baffles are not optimized in any way, just plywood pieces I had in the basement (around 24 x 36 in., with the drivers slightly off center to stagger the acoustical short-circuit).

Bass is kind of awesome (for the driver size), which is a bit surprising considering the smallish baffle size. High Qts seems to be responsible for the great performance.

Sound is not perfect though, the baffles need to be very stiff and dead, so I'll get some decent 2 in. MDF panels with braces.

Anyone looking for good quality sound should look at these. They are quite inefficient (compared to my Fostex horns), but seem to be very happy with some SS muscle (I drive them with a small Bryston amp).
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Old 24th August 2005, 09:45 PM   #2
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Default Re: FR125S in open-baffle

Quote:
Originally posted by robertG
The baffles are not optimized in any way, just plywood pieces I had in the basement (around 24 x 36 in., with the drivers slightly off center to stagger the acoustical short-circuit).
The baffle diffraction step for 24 x 36 in. is around 190Hz if I'm not mistaken.

What other things in an OB affect the bass responce than the size of the baffle?
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Old 24th August 2005, 10:04 PM   #3
robertG is offline robertG  Canada
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Default Placement!

Assuming the driver is centered, there would be 12 inches to the right front, 12 inches to the left and same on the back side, which gives me a short-circuit happening at 283Hz...

The bass I get is much lower than that and is room related. For example, with the baffle on the floor and touching side walls, I get a much lower cancellation (or half the cancellation, since the baffle is acting - almost - like an infinite baffle).

Another thing that would affect cancellation would be the addition of "wings" on the back of the baffle. And also distance to rear wall will affect cancellation frequency.

Speaking of cancellation, as is, it happens at 90 degree relative to front/back axis, so it will be noticeable at various degree, depending on the distance from the driver, baffle from side walls, baffle from back wall and distance between baffles.

A very simple design, very hard to optimize in a given space...
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Old 24th August 2005, 10:06 PM   #4
xcortes is offline xcortes  Mexico
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Quote:
Sound is not perfect though, the baffles need to be very stiff and dead
I'm not sure that´s true. My understanding is that an open baffle stores no energy and thus the construction material is less relevant than in other designs. But I may be wrong.
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Old 24th August 2005, 11:42 PM   #5
robertG is offline robertG  Canada
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It's not the open baffle that stores energy, it's the material it's made of. For example, the test baffles are made of .5 in. plywood.

The plywood vibrates a lot, acting like a huge membrane.

There are two ways to kill those vibrations:

First way is making the baffle very very heavy, so the cone energy takes the path of least resistance (by transferring energy to air), second way is to make the baffle out of a dense and inert material such as thick MDF, so the vibrations will be tuned lower (due to the dampening of the material).
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Old 25th August 2005, 02:24 AM   #6
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RobertG,

You have a misconception about how OB bass works. Don't think of it as a short circuit, because then you'll start applying baffle step principles which are irrelevant. With a flat baffle OB there is no baffle step because when the front wave wraps around the baffle to the back, the rear wave wraps around the baffle to go forward.

What is relevant is the added distance the rear wave must travel to get to your ears compared to the front wave travel distance, and the resulting phase relationship of the 2 waves.

The important forumlae are:

Fe (Fequal which is the point where the OB bass output is equal to that of a large sealed box). Fe = .17 X 343 / D where "D" is the additional distance travelled by the rear wave. In your example "D" is 12". This gives you an Fequal of 191hz. Below this point bass will roll off at an additional 6db/oct in addition to the driver's natural roll off in free air use.

Above Fe, at approx (3 x Fe), you will reach the point where the added distance is equal to 1/2 wavelength and the 2 waves are now directly in phase so the output will be +6db. This climb to +6db is not linear and is about +4db in the first octave. In your example, this won't really work out because with the size of your baffle the frequencies are radiating only into half space, causing things get more complicated due to reflections.

Another thing to keep in mind is that the larger portion of the 360 degrees around the driver that the floor is a boundary, the more bass you get. Also note that baffle wings can greatly increase the travel distance for the rear wave using the same amount of wood (ie you make the rear wave travel backward then forward to the baffle plane before the remaining distance is equal to the front wave), however, as you narrow the speaker using wings you give up some of the floor benefit. Lastly, wings change the radiation pattern away from pure dipole, but the effect is not dramatic unless you use very deep wings and then you have cavity resonances to deal with as well. FWIW, not having you flat baffle OB exactly in the center of the room changes the radiation pattern away from pure dipole as well, so IMHO the dipole purists with their huge flat baffles are barking up the wrong tree.

I do believe you are absolutely correct about the FR125 making a great OB driver and I can't wait to get my hands on some to try it out. The efficiency and 25W power limitations may force me to wait until I can afford an OB array with them. In the meantime a pair would make a great set of OB speakers for my computer rig.
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Old 25th August 2005, 09:47 AM   #7
Rudolf is offline Rudolf  Germany
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Quote:
Originally posted by johninCR
With a flat baffle OB there is no baffle step because when the front wave wraps around the baffle to the back, the rear wave wraps around the baffle to go forward.
John,
I appreciate the in-depth explanations you are giving and I do subscribe to almost every statement you make in your post - except the above which simply isn´t true IMHO.

Full cancellation of baffle step is achieved by a Bipole. Dipoles on the contrary enhance the baffle step issues. You really should simulate some baffle with Tolvans "EDGE" program and switch the Open baffle checkbox between OB and closed. Then have a look at the response diagram.

To understand exactly what happens you need to read Linkwitz´explanation in http://www.linkwitzlab.com/faq.htm#Q8 (Took me three times reading before I grasped what he was talking about )

Just my 2c

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Old 25th August 2005, 01:50 PM   #8
robertG is offline robertG  Canada
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Default Off topic...

This is so amazing.

You start one little thread, like a bottle in the ocean, and then you start receiving posts from Germany, Costa Rica, Helsinky, Mexico, Canada...

Thank you all for feedback.

Anyway, I probably won't keep the FR125S in OB, my listening room is simply too small to fit adequately the panels, and to have them away from walls, where I can fully enjoy their merits.

So, goodbye OB and hello TL or BR.
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Old 26th August 2005, 06:34 AM   #9
chrisb is offline chrisb  Canada
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Default can you say Bipole?

alluded to in above thread,

If you can afford the cost of pair of WR125 for the rear drivers, you should definitely consider a bipole MLTL. You could call it BFBSC (brute force baffle step compensation)

Tim Forman has already done the math, and Dave Dlugos has published drawings (below)

Click the image to open in full size.



I've recently built 2 pairs of these ( the old MDF vs baltic birch shootout) , ( as well as the Mini-Onken monopole BR design on Dave's site ) and the sound very good indeed.

The straight MLTL is a pretty simple box to build, and while a bipole does require more real estate than a monopole, I've found in my last 4 or 5 projects that it's well worth the effort.
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Old 26th August 2005, 08:48 AM   #10
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chrisb,

from what I can see it seems a very neat drawing.
Could you post it as a thumbnail so we (I) can see it in full?

Thank you in advance

E&E
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