Integrating ribbons with woofers??

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Hello All:

I have been toying with the idea of going Trinaural eventually, and here is my predicament:

I have 6 30" Gold Ribbon Concepts planar/magnetic mid/tweeters that I would like to use one over the other for a total of 3 60" vertical arrays. Now, according to Jim Bongiorno(sp?), the developer of the Trinaural technology and processor, the three front speakers need to be identical and vertically symmetrical for best results. I was thinking about doing a Pipedreams style system with a woofer array next to the tweeter ribbons in a mirror imaged pair for the side speakers, but this makes for a big problem for the center. Do I double-up the woofer lines...one on each side of the ribbons...for the sake of symmetry, or go the Newform route and place a woofer under the ribbons?

So here are my questions:

Given a 300-400Hz 24dB/octave crossover, will using a tall ribbon over a single woofer cause imaging/ or other integration problems?

Are there any intermodulation issues when placing a woofer line next to a tweeter line?

Will there be lobeing problems if I go with the two woofer lines straddling the ribbons?

I have a stash of 6.5" Peerless mid/woofers I had planned to use in an equalized overdamped to critically damped sealed enclosure.

Thanks for all help/suggestions.

David
 
David,

I think you will get the best sound and imaging if you use a line array of mid/woofers next to the ribbon linesource. Any other configuration would have only one distance where the ribbon and an individual single woofer had the same SPL drop. You may try a "front bump" of wool between the ribbon and mid/woofers to block some of the cone air, but the planar drivers should be fairly robust. Round over the edges with about 4.5" radius, to reduce diffraction plus the "edgy" effects of the Gold ribbon...

For a cross over point of 300Hz the mid/woofers should be physically close enough to the ribbon to blend with cone speakers on just one side. Keep the ribbons on the end speakers on the inside baffle edge of the soundstage. Double the baffle width for the center speaker so the ribbon is in the physical center. I favor the sound stage of this arrangement over putting the ribbon between two columns of mids.
 
Thanks for the quick and thoughtful reply.

I had pretty much come to the same conclusions, but was seeking confirmation. I have come across D-shaped fiberboard tubes from Cubicon which may work well as enclosures but, unfortunately, they will need to be aprox. 20" diameter for the sides and 28" for the center...so much for decor...small sacrifice I suppose.

Thanks again,
David
 
My thoughts are a "bit" different..

Understand that line arrays like the pipe dreams are working from an acoustic deficit - trying to minimize comb filtering by limiting the "center-to-center" distance betweent the tweeter line and the midwoofer line. Additionally, there is also the matter of spl matching with an array as opposed to a pure line source. Finally, they are trying to achieve a so-called "infinite" line where most freq.s (except for those above 10kHz and below 50Hz) behave as a line source.

As to the first "deficit reduction" strategy - it is largely irrelevant in your case because you are crossing over so low in freq.. As you start moving past 1.5kHz comb filtering can be a problem, but below this freq. it almost never is..

The Spl difference is also irrelevant in your situation. The planars you are using are not that efficient and don't represent the "gain" that an array can have over a pure line. In this case then a line of woofers would be FAR more efficient than your planars and could only be effectivly reduced ahead of your amplification (i.e. you would have to use an active crossover and bi-amp your speakers.. or tri-amp if your using a subwoofer as well). (though I think that it is likely you intended use of an active crossover anyway - so this is probably moot)..

As to the "infinite" line approach for freq.s lower then 400 Hz? I'd see little benefit. At 400 Hz the length of the wavelength is almost 3 feet long. Factoring in boundry issues it is HIGHLY unlikely that there would be an audible freq. abberation between the line and mid-woofer unless your listening VERY closely to the speaker and well above (or below) the woofer. (i.e. within say 3 feet of the speaker while being above the woofer's center by about 3 feet - or traditionally speaking - standing right next to the speaker.) What you'll loose by not having a uniform low freq. line is a degee of freq. linearity as you extend lower in freq. due to room nodes, BUT low freq. vertical lines are not that advantageous anyway, (horizontal lines however can be, but thats another matter entirely..).

Based on the above, my suggestion would be as follows:

If you are going the route of traditional sealed or ported enclosures utilizing the 6.5 inch peerless woofers..

Use 4 - 6.5 inch drivers per speaker parralleled to the sides of the enclosure stacked vertically. In other words picture your planar line on top of a box (the width of which would be the same as your planar baffle) that would be about 16- 20 inches high (i.e. 2 drivers vertically stacked would equal about 14 inches + room for base and top "plates" for the box). On one side of the box you would see two drivers stacked vertically, and on the other side you would see another two drivers stacked vertically.

This scheme would provide a number of benefits:

1. because of complementry compound loading (with one side of driver's magnets facing the other side of driver's magnets), you'll lower distortion (both harmonic and IM related), because of balanced pressurization in the box and minimized cabinet "rocking" and vibrations.

2. because they are placed near the floor yet are parralleled horizontally and vertically they will interact with the floor boundry better than conventional methods at these freq.s.

3. because of the driver's off-axis radiation (i.e. the drivers placement on the "sides" of the speakers) higher freq.s will be attenuated in the foward plane (even at low spl's considering a slope as "deep" as 24th order).

4. you MAY be better off with repsect to the time domain because of the phase rotation (and corresponding time "lag") that typically starts to occur at around 3-400 Hz with most dynamic drivers. But getting the crossover integration "right" could take quite a bit of trial and error.
 
This is a layout that has quite a bit of appeal to me, and one that I had contemplated earlier...except that my variation placed the woofers further up in the box just above and below the horizontal center-line of the ribbons. This would be a very easy box to build, using (for example) 2' X 4' X 3/4" MDF side panels separated with 4.75" X 1/2" boards...if I go with 1 ribbon per line instead of 2. Assuming about an average of 4" depth for the ribbon backwave enclosure, I should have enough volume in this box to get Qt to about .5 with a lot of stuffing. Then build a base to keep the whole thing from falling over if bumped, and I should be off to the races. Thanks for reminding me of this option.

BTW, I do plan to bi-amp and EQ the bass response using a McIntosh MQ104, which also allows for limited parametric EQ.

What, in your opinion, is the best stuffing material for a sealed box? I want the woofers to "see" as much volume as possible.

Also, do you think that a smallish 4" deep fiberglass stuffed enclosure will be big enough to absorb the ribbon's backwave without overdamping it?

Lastly, just to satisfy my curiosity, what is your educational/professional background and could you describe your DIY efforts? I ask because your suggestions seem very thoughtful and (perhaps) seasoned by experience.

Best to all,
David
 
david yost said:
This is a layout that has quite a bit of appeal to me, and one that I had contemplated earlier...except that my variation placed the woofers further up in the box just above and below the horizontal center-line of the ribbons. This would be a very easy box to build, using (for example) 2' X 4' X 3/4" MDF side panels separated with 4.75" X 1/2" boards...if I go with 1 ribbon per line instead of 2. Assuming about an average of 4" depth for the ribbon backwave enclosure, I should have enough volume in this box to get Qt to about .5 with a lot of stuffing. Then build a base to keep the whole thing from falling over if bumped, and I should be off to the races. Thanks for reminding me of this option.

BTW, I do plan to bi-amp and EQ the bass response using a McIntosh MQ104, which also allows for limited parametric EQ.

What, in your opinion, is the best stuffing material for a sealed box? I want the woofers to "see" as much volume as possible.

Also, do you think that a smallish 4" deep fiberglass stuffed enclosure will be big enough to absorb the ribbon's backwave without overdamping it?

Lastly, just to satisfy my curiosity, what is your educational/professional background and could you describe your DIY efforts? I ask because your suggestions seem very thoughtful and (perhaps) seasoned by experience.

Best to all,
David


Even in a "far field" application (listening distance greater than 12 feet), center-to-center driver distance should not be more than one wavelength.. so at 400 Hz we have almost 3 feet in length BUT your array presents an effective driver seperation of more than 60 inches (or 5 feet +). Additionally the woofer will actually operate higher than 400 Hz (so its worse). However we need to "temper" this because of boundry issues where acoustic theory and acoustic reality "diverge" a bit. (Note though that IF you only use 1 planar* per speaker then it should be OK. See * for more info below on why I would extend the planar line to two planars for 60 inches..)

But comb filtering isn't the only problem here - room node pressurization will also be a problem and the effectivness of such an arrangement (wide driver vertical seperation) will be even more dependent on the room its located in. (..the arrangement I suggested will effect the room the least over a broad coverage area for a monopole radiation pattern..)

(*true line radiation is dependent on listener distance and line length. With a line length of about 30 inches your line will start behaving like a point source at 12 feet (or more) listening distance just below 3 kHz. With a line length of 60 inches at 12 feet (or more) this is lowered to around 900 Hz. In otherwords the further you move away from the loudspeaker the more the line operates like a point-source. The effect? Remember that lines have a reduction of 3 db per meter and point sources have 6db per meter as a result you could have freq. non-linearity (or suck-out) in your midrange depending on how far away you listen from your speaker. At about 9 feet in distance the 60 inch line will start becoming effectivly "infinite", but not true for the 30 inch line - there you'll not achieve an effective "infinite" line at any reasnable distance from the speaker.)

In otherwords, don't go the route you've just suggested.

(Note though that IF you were suggesting a 2 planar unit WITH a "break" in the line where you are using your other set of mid-woofs i.e. "ceiling/line/mid-woof/line/midwoof/floor" - you will extend your line length lower in freq. (good), but you will also be breaking the line higher in freq. and cause comb filtering depending on the center to center distance of the planar drivers (bad). In this instance we will presume 8 inches (your 6.5 inch drivers + "fudge factor"). At a little over 1650 Hz comb filtering will occur (and get progressivly worse). In otherwords this is NOT a viable solution.)

As for stuffing...

I personally dislike stuffing because it typically adds acoustic resistance that gives the driver an "in-the-box" sound (..fixes bass "imaging" near, or "in", the speakers), and fine detail is typically lost as well. This seems to be less important as you go lower in freq. BUT at 400 Hz it still sucks. (Note the hybrid Apogee Centaur line had this "quality".) Now you can still use maximal stuffing and effectivly get around this problem by using laminar flow "regulation" near the driver's rear radiation. So what would I suggest for a laminar flow with such a compound arrangement..?

Hmm, well the driver's "smaller" diameters might allow for a large PVC "T" pipe extension IF the interior diameter of the "T" is greater than a single driver (..though note you'll still have compression because the "leg" of the "T" that will be "dumping" into the box will be the diameter of only one driver as opposed to the two drivers forcing air flow through it..). Unfortunetly though, the length of the "T" will determine the maximum width of the speaker - so unless the speaker isn't very wide then this will be uselees.
My suggestion then would be an 8 inch cardboard concrete forming tube (typically found at Home "Despot", and other such stores), cut to length to span the interior width of the speaker (sealed to the side walls with caulk or glue), stuffed with poly drinking straws (giving a honey comb cross section effect) and then making a wedge shaped cut into the center of the tube where the airflow will "dump" into the box. (..Obviously one tube per complimentery compound pair of drivers.) The straws should start just beyond the back of the driver's magnets. For better bracing you could ditch some of the center staws and brace the two drivers with a wooden dowel of at least an inch in diameter wedged between the back of each driver's magnet. Note that even at the "V"/wedge termination though I would still have as little stuffing as possible (say at least 3 inches of air space surrounding the wedges venting - perahps with a wire cage). The rest of the box then should have maximal stuffing of any hollow fiber poly stuffing (unless you are using an aperiodic vent in which case you'll need some air space surrounding the "vent"). The actual brand of the fiber isn't neccesarily indicative of quality - what your looking for is a fiber that keeps its hollow structure with a fair bit of compression from packing while still being structurally "usable" under air pressure. For instance natural down is probably the very best material for airspace, but it is unusable in a speaker because it tends to "crowd" around venting structures and "chuffs" if it isn't very tightly packed. I'd say look to Dupont fibers like Holofil and Quallofil - the long fiber variety (though there are newer fibers from them that might be better). Quilter's supplies should have these types of fibers for batting material in comforters OR typically for pillow fill (..which prob. would give you more material for your money). I really don't know what the fiber is like for Acoustastuf (which is commonly used for T-lines in speakers). I *think* the fiber is irregular in shape and crimped to resemble wool. If this is the case it would NOT be as good as a hollow fiber for use in a sealed or ported enclosure (but would be better in a T-Line).

As to my prof. background.. strangely I'm an attorney. I started with DIY'ing the typical mini-monitor, then onto the famous PE 69 cent wonders for a line array (combined with some film tweets), and most recently I'm experimenting with fullrange drivers coupled to closed termination rear-"horns" with aperiodic venting, combined with midbass dipoles (..and some T-lines subs I have yet to even begin). With a member search of this forum you'll see I have a variety of interests with speakers (and audio in general).
 
ooops, the planar dampening..

You don't have to enclose the driver. You could run it in dipole mode (and there are several "spreadsheets" available that would determine the theoretical baffle width required for 400 Hz). I'm not particularly fond of dipole mids to tweets simply because I use a sound processor that does not work well with it, however that doesn't prevent me from a recomendation of this format as long as the rear of the speaker is WELL (8+ feet) away from the "front" wall of your listening room.

If you want to enclose it then I'd suggest that the interior volume's wall's be painted with a silicone borscilate paint (multiple "coats" of the stuff). This will give you the volume you need while minimizing reflections off of the back of the driver's delicate diaphram - all without creating airflow resistance. The "easiest" source for such a paint is Tempcoat:
http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=268-232

If you want to save a bit and make your own I'd suggest this source:
http://www.hytechsales.com/insulating_paint_additives.html
 
One experiment............

My Apogee Full Range speakers have a 2" wide 84" long midrange ribbon covering 320-10,000Hz, crossed to a 0.5" wide 84" long tweeter and a 24" x 84" planar bass panel.

I experimented with using a 15" Lambda woofer in a 24" tall ported cabinet crossed at 320Hz in place of the bass panel. All Krell amps and crossovers. The vocal sound stage dropped to the floor. The soundstage was "just messed up" (I used test CD, and Jazz at the Pawnshop). SPL vs. frequency measurements at different distances also showed a point-source vs. line source SPL variation.




My current DIY ribbons are monopoles, and I use 2" of poly fill backed by 3" of Dow 703 fiberglass. I found it critical to always use a couple inches of open weave material behind the ribbon to minimize sound reflection to the back of the ribbon.
 
Perhaps I didn't do a very good job of explaining my layout to ScottG. To clarify, for a single 30" ribbon app: I am thinking of a box aprox 6" wide X 24" deep X 48" tall with a suitable base for stability. Ribbon tweeter to be mounted towards the top of the 6" X 48" face, woofers to be mounted 2 on each side of 24" X 48" faces up from the bottom at roughly 25" and 40" centers and as close to the edge with the ribbon as cabinet making will allow.

This will give me a very narrow and tall cabinet with the acoustic centers of the woofers maybe only 4-5" behind the tweeter plane. Think something along the lines of the Burmeisters and you would be close.

I have no experience with monopole planars, but my relatively small room doesn't allow placement too far from the walls, thus my preference (as of now) for sealed boxes. Also, my plan to EQ the bass relies on Qt~0.5 tuned to the high 40s sealed box characteristics.

I know that most folks would think adding up to 20 Db of boost at 20Hz would be folly when using 6.5" woofers, but I am assuming that with enough of them (12 altogether in this case between the 3 cabinets), and relying on some room gain and the fact that I mainly listen at non lease-breaking levels, I can get away with it.

Lastly, I am concerned about the changeover point between line source radiation and point source, but it seems to me that unless a true infinite line is used, all practical designs of this type will encounter this problem at some combination of frequency and distance. Am I wrong in this? Anyone know how loudspeaker designers usually deal with this phenomenon (hf contour controls maybe?)?

BTW, thanks everyone for your input so far,
David
 
david yost said:
Perhaps I didn't do a very good job of explaining my layout to ScottG. To clarify, for a single 30" ribbon app: I am thinking of a box aprox 6" wide X 24" deep X 48" tall with a suitable base for stability. Ribbon tweeter to be mounted towards the top of the 6" X 48" face, woofers to be mounted 2 on each side of 24" X 48" faces up from the bottom at roughly 25" and 40" centers and as close to the edge with the ribbon as cabinet making will allow.

This will give me a very narrow and tall cabinet with the acoustic centers of the woofers maybe only 4-5" behind the tweeter plane. Think something along the lines of the Burmeisters and you would be close.

I have no experience with monopole planars, but my relatively small room doesn't allow placement too far from the walls, thus my preference (as of now) for sealed boxes. Also, my plan to EQ the bass relies on Qt~0.5 tuned to the high 40s sealed box characteristics.

I know that most folks would think adding up to 20 Db of boost at 20Hz would be folly when using 6.5" woofers, but I am assuming that with enough of them (12 altogether in this case between the 3 cabinets), and relying on some room gain and the fact that I mainly listen at non lease-breaking levels, I can get away with it.

Lastly, I am concerned about the changeover point between line source radiation and point source, but it seems to me that unless a true infinite line is used, all practical designs of this type will encounter this problem at some combination of frequency and distance. Am I wrong in this? Anyone know how loudspeaker designers usually deal with this phenomenon (hf contour controls maybe?)?

BTW, thanks everyone for your input so far,
David


OK, now I understand - you will likely not have dipole radiation.. i.e. you'll have an enclosure for the planar that is in the box (seperating the out-of-phase signals of the planar and mid-woofs). So then yes you could have the woofers seperated vertically (but moved farther back on the side panals) without breaking the line length IF you wanted to go the 2 planar route.. (in fact you could place the mid-woofs pretty much where ever you wanted so long as they don't interfere with planar's enclosure.)

As far as 4 6.5 inch drivers for electrically assisted bass freq.s.. Remember that you reduce your excursion by half for a doubling of surface area (so 2 drivers have half the excursion of one, and 4 drivers have half the excrusion of 2). So your not in as bad a shape as you might think provided you don't have high spl's at very low freq.s.. The only real problem will likely be 2nd order distortion at those low freq.s that are intrinsic to the drivers - (i.e. you might have "doubling" where the 2nd order dominates the primary).

As far as the line vs. point source problem:

yup - I'm concerned too. Most designers get past this problem by increasing the line to where it extends down to 600 Hz for a given distance (i.e. Magnepan, the large Apogees and Analysis loudspeakers). (at around 600 Hz, depending on the room's bondries, you will start aproaching the "infinite line".) Others just design their speakers to work properly only within a limited distance (the shorter Apogees and Analysis, Newform Research, etc.), and most of the population is none-the-wiser as to this limitation. Because you are DIY'ing your own speakers for a relativly set distance known to you I think it IS something you can do BUT you'll likely need to be more flexible with your crossover point to perhaps allow for increased output from your midwoofs higher in freq. to counteract the spl loss for a certain distance. The alternative is padding down the higher freq.s of the planar (though I think this is unacceptable for a number of reasons not the least of which is that eff. will take a huge drop.) Again though, IF you want to extend the planar as low as possible (say below 1.5 kHz) then I would strongly recomend using 2 planars per loudspeaker. This could however be a moot point IF you have a suitable EQ. for your crossover that would allow you to boost the midrange to compensate for any loss related to line length vs. distance.
 
LineSource said:
One experiment............

My Apogee Full Range speakers have a 2" wide 84" long midrange ribbon covering 320-10,000Hz, crossed to a 0.5" wide 84" long tweeter and a 24" x 84" planar bass panel.

I experimented with using a 15" Lambda woofer in a 24" tall ported cabinet crossed at 320Hz in place of the bass panel. All Krell amps and crossovers. The vocal sound stage dropped to the floor. The soundstage was "just messed up" (I used test CD, and Jazz at the Pawnshop). SPL vs. frequency measurements at different distances also showed a point-source vs. line source SPL variation.




My current DIY ribbons are monopoles, and I use 2" of poly fill backed by 3" of Dow 703 fiberglass. I found it critical to always use a couple inches of open weave material behind the ribbon to minimize sound reflection to the back of the ribbon.


Most designers that utilize a hybrid (line + mid-woof) will either drop the freq. lower OR slant the baffle (provided the driver is facing forward on the front baffle) so that the mid-woof essentially "aims" upward at least 15 degrees (which should cure the problem in a single woofer application). As to the difference in SPL variation over distance, yup its still there - it just that you have an additional 3db to work with because of the boundry reinforcement from the floor (as opposed to the single driver higher on the baffle - not as opposed to a line array of mid-woofs). Overall - yes the midwoof array is preferable as long as you have the cash, the space, and the ability to attenuate that line if pairing with a less eff. line for the higher freq.s - (and thats a lot of "if's").

Question:

What was the slope you used?

and

How was the driver "aimed" (relative to the listener)?
 
Hi Scott,

The 3-way Krell crossover has switches for 6/12/18 db/octave Butterworth per stage. Two stages can be ganged together for steeper slopes, but then the Xover drops to a 2-way. The original Apogees used just 6db/octave, and I changed this to 18db/octave to keep the midrange ribbon under control. Big improvement! You can also plug in custom modules for each amp that compensate for the baffle width or bass boost, but there are just 6 discrete class-A amps per channel to play with.

When I substituted the Lambda TD15S I tried 18db, and 12+12=24db, and 18+18=36db slopes. The 18db slope sounded best. I naturally played with reversing the polarity of the woofer and physically moving it forward/back to see if I could hear time alignment....which was easy to detect even at 320Hz. THIS LAST EXPERIMENT MAY BE USEFL TO DAVE........

Hi Dave,

There is a great deal of directional information at 300Hz that mounting the midbass speakers on the two sides of the cabinet will likely adversely affect the sound stage. The only side mounted woofers that sounded right to my ears had crossovers down at 80-100Hz. I suspect a search on side mounted woofers would find some good information. There are also simulation programs that will allow you to model this driver arrangement. With a 300Hz crossover, I still think you will get the best results with a line array of midbass next to two stacked ribbons with cones facing forward for direct wave launch.

Tall linesources are very nice for the real world. You can sit, stand, dance with your sweetie, lay on the foor, and the sound is always good.
 
LineSource said:
Hi Scott,

The 3-way Krell crossover has switches for 6/12/18 db/octave Butterworth per stage. Two stages can be ganged together for steeper slopes, but then the Xover drops to a 2-way. The original Apogees used just 6db/octave, and I changed this to 18db/octave to keep the midrange ribbon under control. Big improvement! You can also plug in custom modules for each amp that compensate for the baffle width or bass boost, but there are just 6 discrete class-A amps per channel to play with.

When I substituted the Lambda TD15S I tried 18db, and 12+12=24db, and 18+18=36db slopes. The 18db slope sounded best. I naturally played with reversing the polarity of the woofer and physically moving it forward/back to see if I could hear time alignment....which was easy to detect even at 320Hz. THIS LAST EXPERIMENT MAY BE USEFL TO DAVE........

Hi Dave,

There is a great deal of directional information at 300Hz that mounting the midbass speakers on the two sides of the cabinet will likely adversely affect the sound stage. The only side mounted woofers that sounded right to my ears had crossovers down at 80-100Hz. I suspect a search on side mounted woofers would find some good information. There are also simulation programs that will allow you to model this driver arrangement. With a 300Hz crossover, I still think you will get the best results with a line array of midbass next to two stacked ribbons with cones facing forward for direct wave launch.

Tall linesources are very nice for the real world. You can sit, stand, dance with your sweetie, lay on the foor, and the sound is always good.


At 400 Hz a 6.5 inch woofer is essentially omni directional, this is not the case for a 15 inch woofer. In otherwords you should not have a problem with a properly implemented side firing 6.5 inch midwoof at this freq. (and slightly above it). I absolutely agree though that correct driver "time" alignment is very important (and not just for summation at cross point). (..your suggestion is a wise one.) In fact time is more detectible as freq.s decrease whereas spl is more detectible as freq.s increase.
 
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