CSS FR125 Bipole revisited..

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Bukem

Member
2008-03-07 1:51 pm
Dear All,

I have recently purchased as set of FR125's (initially to use these sealed with a BSC circuit and perhaps a notch filter as well).

Intrigued by the various offerings I have given this some more thought. I guess the primary reason to choose a full-range driver is it's coherency and simplicity of any filter network. Fewer components can only mean less components to affect the sound in a negative way.

Having read pretty much all the posts on the FR125 it seems that there's not a real consensus on driver loading. Bass Reflex seems to be the least popular, followed by horn/TL. Sealed and aperiodic seem to be most popular since horn/TL might emphasize CSS's impressive base response a tad too much. However there does not seem to be a "definitive" type of loading. I understand this also depends on the room..

Now personally I have a soft spot for sealed but I am drawn to the various horn designs. I might match it with a subwoofer in the future so I guess sealed is the way forward.

In order to work the benefits of full-range drivers I thing it is appropriate to use a second driver to solve the baffle step issues. Of course I could use a few passive components but If I follow this route I might as well build a 2-way speaker.

I'd like to do some more work on this type but before I move on I'd like to get a few things straight:


1) What's better for a bipole > a wider baffle or a narrow one. Somehow it seems that the width in this type isn't that important at all since the driver at the back in a push-push configuration sees the same width. Am I right or wrong here? Is it depth that is crucial?

2) To low pass or not to low pass on the rear driver..... Isn't it unnatural to run the rear driver with an inductor / low pass? What I am trying to say is that if the rear driver is rolled off early then surely there must be an unbalance below the baffle step region and the region between the baffle step and the roll off? The two drivers are partially summed aren't they - but the higher frequencies aren't hence the imbalance - right?

3) What are the advantages and disadvantages of having the two push-push drivers sharing the same physical volume. Are there advantages to be had by giving them both their own volume?

4) Is there a consensus on the ideal enclosure volume for the FR125 in a sealed config?

Many questions but hopefully interesting enough to pursue.

As a last point, one of the things that really keeps me occupied is what about using another two FR drivers (so 4 drivers in total per enclosure) but run these passively like a passive radiator. They'd need to be wired together to create a short with a rotary switch and a few resistors. This way if memory serves me right you can alter the qts of the drivers. Sonus Faber did this in the past with a KEF B139 at the back of an Extrema monitor loudspeaker. It even included a small heatsink to conduct the heat produced by the resistors. I know this effectively is a bass reflex enclosure but the possiblity to tailor the "passives" behaviour might prove interesting - both aesthetically and hopefully sonically as well.

Many thanks for thinking with me!

Bukem
 
Hi Bukem,

During my own testing I had approx 10"W x 9"D and the bipole effect was good.

Low passing the rear took 'life' away from the sound.
The rear driver should not be directly flat facing the wall though - the reflecting HF 'pings' into the room.

I cannot see any advantage for giving each driver its own cabinet; reflection paths to the rear of each driver would be shorter, whereas some damping material or reflectors could be used between sharing drivers.

Re additional 'bass' drivers.
Without tuning or long path augmentation you will not be getting really low bass, so why not fit something larger and choke passed, front and back near the floor, then maybe the entire cabinet could be resistively vented in order to minimise LF peaking ?

Cheers ........... Graham.
 

Bukem

Member
2008-03-07 1:51 pm
Graham Maynard said:
During my own testing I had approx 10"W x 9"D and the bipole effect was good.

Hi Graham,

Thanks for thinking along with me. You mention that you've used a nearly rectangular cabinet. Is there an aesthetic reason or is there a specific calculation you made to come up with this width/depth ratio?

Graham Maynard said:
Low passing the rear took 'life' away from the sound

Does that correspond to any measurements that you might have taken?

Graham Maynard said:
The rear driver should not be directly flat facing the wall though - the reflecting HF 'pings' into the room.

Makes sense, especially when the rear driver's HF is not rolled off prematurely..


Graham Maynard said:
Re additional 'bass' drivers.
Without tuning or long path augmentation you will not be getting really low bass, so why not fit something larger and choke passed, front and back near the floor, then maybe the entire cabinet could be resistively vented in order to minimise LF peaking ?

I'm not completely following you on this one. Using the FR125 as a passive radiator (albeight using resistors to alter the Q) is effectively turning it into a bass reflex system. What do you mean with choke passed, and surely you don't mean that I should mount the FR125 close to the floor...


Chrisb,

Are you refering to the horn with the side firing WR?


Any further thoughts re the "passives" idea?

Many thanks,

Bukem
 
Bukem said:
Chrisb,

Are you refering to the horn with the side firing WR?


I believe our dear friend is indeed referring to these:
 

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Bukem

Member
2008-03-07 1:51 pm
Hi Cal,

I like your boldness - just do it !

What did surprise me is that you've mounted the 2nd driver on the side of the enclosure instead of the rear. (not ideal in this design :clown: ) I suppose you lose the push - push mechanical argument but on the other hand you reduce the distance between the two drivers.

There have been arguments that the worst thing to do is stick two FR drivers on one baffle. Isn't your design more or less approaching a "one baffle for all" scenario? Apparently there'd be severe combing issues.

I presume your design could be used with FR125's instead of the WR + tweeter? Are you currently running the side driver "fullrange"?

Regards,

Bukem

(EDIT: typo)
 
Hi Bukem,

The side firing drivers seem to offer the most pleasing combination I tried. It allows for them to be pointed in or outward depending on the distance between the speakers and it acts like a mechanical BSC. Both drivers are running full with only a small cap and light padding on the tweet as I really hate taking a low sensitivity driver and sucking further life out of it. I couldn't even run it as a 2.5 way with both drivers on the front. It just didn't work. The Scottmoose designed enclosure goes a long way to making this a very enjoyable speaker. Combing is not a big issue at all as the side driver is so far off axis that it doesn't seem to have a detrimental effect. It's a big box but to get these drivers to really shine, you have to do something. I'm glad they finally found a home. For a while there I was beginning to wonder what all the fuss was about with them. Now I know.
 

Bukem

Member
2008-03-07 1:51 pm
Hi Cal,

That clears things up a bit.

Presuming it can be an all FR125 system then this is a serious contender. I have a few thoughts about how I could possibly rearrange the horn mouth so I get a smaller enclosure. I love your design but I don't think my wife is quite so sensitive to the benefits....

If you imagine the two horn mouths which are now situated at the top and bottom and place one on the left hand side and the other one on the right hand side you'd slash 2/3 rd of the height and triple the width of the enclosure. Provided the volumes etc of the cavities is respected do you reckon something like that might work?

I'll be away the whole day tomorrow so I won't be able to explain further until sunday. I'll try if necessary to draw what I mean in cad.


As another contender I would still like to explore the possibility of a front to back bipole with possibly another two units as a "Q-tune-able passive radiator".

Does anybody else have some further thoughts on this? I somehow feel that we need to think a bit further then just slap a driver into an enclosure. People also tend to avoid using two drivers in a tandem arrangement with a small sealed cavity in between to reduce box Vb. It also evens out non-linearities. I'm just trying to get the best type of sound instead of being driven by expenditure.

Regards,

Bukem
 
Hi Bukem,

Reason for shape - convenience, but worked well.

No measurements, only listening.

FR125 is not going to do much bass without LF displacement garbling the rest, so why not let other drivers come in near floor level and help at LF where otherwise the FR125s would be rolling off.
I don't mean woofers, but maybe more widerangers or mid-bass with a choke in series.

If as you suggest other drivers are used passively as 'resonant phase changers' the main drivers will still be the only drive source at LF.
Additional parallel connected drivers at LF need not rely upon cabinet or passive resonance.

Cheers ....... Graham.
 

Bukem

Member
2008-03-07 1:51 pm
Graham,

My intention is not to use another "active" bass driver. The reason why I kept going on about using the FR125 as a passive is just to explore another type of "loading".

People have explored different types, from sealed to horn, from aperiodic to bass reflex but never ever do people to experiment with using a fully working driver as a "passive" unit. Furthermore, by doing so you have the opportunity to play with the resistor values to alter the behavior of the "passive" unit.

There are a few of my original questions in this topic which have been left unanswered. Hopefully somebody manages to explain to me.

1) BIPOLE - What's best and why: Narrow and deep baffle OR Wide and shallow baffle OR Wide+deep OR Narrow+shallow......

2) Consensus seems that the rear or side driver should be run full range - possibly with a cap to pad the highest frequencies. However, I would like to find out why this is. General comments are that "the life is sucked out"...surely the objective MUST be to fill in the baffle step "gap" left by the front driver?


I cannot see any advantage for giving each driver its own cabinet; reflection paths to the rear of each driver would be shorter, whereas some damping material or reflectors could be used between sharing drivers

Why wouldn't there be an advantage. The mechanical vibrations passed on to the enclosure are the same for the front and rear driver. Wether there's a shared volume or not. The only thing that happens by dividing the volume into two, is that the one driver has no possibility to alter the other drivers' cone movement - surely that must be a HUGE advantage?

Cal,

Do you have any thoughts about sticking the mouth to the left and right of the driver? Or even to the side? (reducing the height of the enclosure by 2/3rd) ...More or less the idea like in the attached drawing. In this case I have positioned the 2nd driver at the back although a design with the driver on the side could be done as well. It's not a very good "copy" as for the length and path of the cavities but this can be done. I just want to know whether a reworked design like this might work.


Many thanks,

Bukem
 

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Hi Bukem, I'm not the expert here. I rely on others knowledge and I do the subjective listening part.

If i were to do it, I would consider cutting the box in half rather than thirds. That way the construction would be easier as the exit from the compression chamber is in the center as it sits now and you basically be just folding it in two. If you were to try it your way, you'd need others to guide you through that.
 

Bukem

Member
2008-03-07 1:51 pm
The way I look at the current enclosure style is that it's a partial bass reflex enclosure + a horn. The slot + the channels running up to the horn throat look like a traditional vent to me. Perhaps it even mimics an aperiodic type of vent.

At the end of the vent is the horn. I presume the slope angle and width/depth of the horn is fixed.

So basically, what I would like to know, can I shape the vent part in any way I like and stick the horn bit at the end of it? If that's the case then I'll do a few proper designs.

Can't wait to find out ! :D

Bukem
 
I would imagine you could reconfigure it as you wish. My thought was that the exit from the compression chamber is in the middle now so it would be rather easy to construct a side by side Calhoun. I wonder if the figure you posted might end up being a bit boxy?

Here's the shot again of the boards laid out before construction.
 

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