Bipolar MLTL Speaker with FR125S/WR125S - diyAudio
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Old 1st August 2005, 02:57 AM   #1
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Default Dipolar MLTL Speaker with FR125S/WR125S

I recently completed a bipolar MLTL using the CSS FR125S as the front driver and the WR125S as the rear driver. The connection of the two drivers in each box is in phase so that a bipolar field (forward and rear radiation) is created.

The box duplicates Greg Monfort's (GM) design that he describes in his posting:

I built Greg's MLTL design that is the 45 Hz and +/- 1 dB. The drivers are located 14" internally below the top of the box. I lightly stuffed the top of the box (from below the drivers). The port tube is on the rear of the speaker and just above the bottom floor of the MLTL line. The drivers are positioned about 33.25" above the floor. I located the terminal plate in a cavity at the bottom of the enclosure. The cavity spaces the drivers upward to their listening position plus allows room to add mass to stabilize the box. Also any contouring components, if needed, could be located in this cavity.

The enclosure was constructed of 0.75" thick MDF panels for the baffle, rear, and bottom of the box with walnut top and side panels. Internally, three braces are used to stabilize the structure. The outside dimensions of the enclosure are 48" high, 7.5" wide, and 8.75" deep.

The sound of these speakers is very nice indeed with more bass than you would expect from the volume. The four drivers do a nice job in adding low end impact to your music. The frequency response of these drivers is very flat across the band and this design retains this benefit. More important these speakers exhibit the coherent nature that we have come to expect from quality full range drivers without a crossover.

The bipolar drivers in these speakers necessitate that you position them away from the wall--I'm using 3 feet in my room. Their bipolar nature of this speaker does two things that you will notice to your listening. First, you'll notice a little ambiance as sound energy is reflected from the back wall in the room. In this case ambiance is a good thing--more apparent during say a newscast--but with not an issue when you listen to music. Secondly, the soundstage is wider than you'll have with just a single front firing driver. One very minor disadvantage to the bipolar nature of these speakers is that the stereo image is a little more diffuse than with direct firing drivers. In my opinion having a wider sweet spot is an advantage in my book.

The FR125S full range driver does an excellent job across the frequency band and for most music you'll not miss any extra airyness that a separate tweeter might contribute.

Bottom line is that this is a very simple design yet it yields great results. A lot of bang for their cost.

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Old 1st August 2005, 06:08 PM   #2
RAW is offline RAW  Canada
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Looks good as usual Jim and love the cheaks as usual.

Now the big question.
Against the X92 what do you think of the FR!
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Old 1st August 2005, 07:32 PM   #3
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It is hard to compare the CSS FR125S as a driver directly versus the Jordan JX92S as my implementations are so different. My initial impressions are that the JX92S is more sensitive (say a couple of dB or so) while the FR125S has a smoother response in the upper octave than the Jordan. The Jordan may have a slightly lower F3 point on the low end but not by much but that would depend on how you design the box (bass reflex, MLTL, etc).

The bipole configuration by its nature eliminates the baffle step compensation issue so in practice the speaker implementation of a bipolar configuration would be more sensitive than one that needs a BSC. I am also impressed with the amount of bass that the MLTL has with the FR125S and WR125S. So much bass that only a bass head would wish for a subwoofer for listening to most musical programs.

Bottom line is that even with 2 FR125S and 2 WR125S drivers in these boxes the biploar speaker design would still be lower driver cost than a pair of JX92S drivers.

Thus what is not to like about this bipolar design? I guess you could fault the 4 ohms impedance or such but for its bang for the buck is high if your amp can cope with the impedance.

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Old 1st August 2005, 09:34 PM   #4
RAW is offline RAW  Canada
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Thanks Jim
I was wondering if you got to list to them in say a .25cft cabinet just for fun.

I am sure of the design you have done will have a few others looking this way as well but like you said the 4 ohm is the only low side.

Like the rear firing driver Dan did this on the Apex design and helps out a lot for larger mid presence.

The FR in a .25cft cabinet as we have done is a good small compact speaker that will fill the needs of most as you have said.

Good work Jim
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Old 23rd January 2006, 04:39 PM   #5
DeonC is offline DeonC  South Africa
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Hi Jim

I am also considering Greg's design, but I would like to build a triagularly-shaped speaker (to be used as rears in an HT set-up). For a triangular enclosure the two short sides will be ~9.35 and the long side ~13.22 long (all dimensions internally, and the short sides at 90 degrees to each other). I am also considering using a CSS WR125ST (the 16-ohm version) as the BSC driver (ISO the WR125S), but with the long side of the enclosure facing the wall, it will be mounted on the one short side and the FR125 on the other. My reasons for using the WR125ST is explained in my post Questions/ideas on variations on BSC bi-pole config. What do you think of the idea. I also wonder what would happen if you substitute a WR125ST for the WR125S you have in your current desing whether it would not reinforce the good points and reduce the weakness you descibed (loss of imigaing intergrity) as there will be less energy reflected off the rear wall. Just a thought.

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Old 24th January 2006, 03:17 AM   #6
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I suspect that you received several good replies in the other thread. The bipole effect would be best if the two drivers are arranged on opposite sides the enclosure--preferably a perfect 180 degrees difference. Otherwise you would not achieve perfect baffle step 'cancellation' that you desire from the bipolar arrangement.

You can use the WR125ST as the rear driver but again a little less than perfect results. It likely would work OK but a little less than a perfect bipolar effect.

The key with the bipolar imaging is that you can adjust the amount of ambience enhancement by the distance between the front wall and the rear driver on the speaker. Frankly, I like the imaging improvement you can get as it can broaden the image space. Thus, I suggest that you adjust the ambience for your room and taste by speaker postioning. You'll like it--I promise. Very nice bass from small speakers and good sound to boot.

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Old 29th January 2006, 10:28 AM   #7
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Default Re: Dipolar MLTL Speaker with FR125S/WR125S

Originally posted by Jim Griffin
The box duplicates Greg Monfort's (GM) design
It should be noted that GM's design is for a single driver & Jim has used 2... that Jim's box still turned out well is a testiment to how flexible an ML-TL is.

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Old 29th January 2006, 11:03 AM   #8
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Old 29th January 2006, 02:02 PM   #9
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Default Measurements On Bipolar Speaker (On axis near field plot)

Dave (Planet 10) stated:

"It should be noted that GM's design is for a single driver & Jim has used 2... that Jim's box still turned out well is a testiment to how flexible an ML-TL is."

Let me reply:

From other posts by GM and by comparison to similar designs by others, I believe that GM's cross sectional area and length values are for two drivers. The remaining question is whether his port parameters are for a single or dual driver. Perhaps GM can chime in to clarify his box and port parameters.

Regardless of the above questions, the really question is just how well does this speaker that I built performs and whether the bass response is flat.

I have attached a near field on axis frequency response plot for my speaker. Note that the near field plot is only good up to about 700-1000 Hz so ignore the data above those frequencies. Furthermore, the true low end response of the speaker needs to be combined with near field port data which I will include in the next message in this thread.

The on axis performance plot shows a relatively smooth response within its validity limits. The on-axis data rolls off (starts below 80 Hz) on the low end and shows a dip in its slope below 50 Hz. The near field port data in the next message shows a peak in this same area. The port data will fill-in this on axis dip. Thus, the total response of the speaker is the acoustical combination of the on axis response and port response and it clearly creates performance below 50 Hz.

Bottom line is that this speaker performs very well as built and I would recommend that others would be successful in building it as well.
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File Type: jpg nearfieldweb.jpg (12.0 KB, 960 views)
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Old 29th January 2006, 02:03 PM   #10
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Default Measurements On Bipolar Speaker (Near field port data)

The near field port data is shown below.
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File Type: jpg portnearfieldweb.jpg (12.2 KB, 944 views)
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