Questions/ideas on variations on BSC bi-pole config

[DeonC]

Hi guys

I have two questions about possible alternative layouts and ideas for baffle-step compensation. The drivers in question will be the CSS WR125S (the 8-ohm version) and the CSS WR125ST (the 16-ohm version).

OK, here is my first question- how about mounting the second driver on top of the cabinet instead of behind (like in the normal bi-pole configuration). This idea is similar to what Keith Kidder used in his Sentinel Surround design (seeKeith Kidder's Sentinel Surround on the T-Line Speakers site). This is so that the speakers can be placed much closer to the rear wall than is normally possible with the standard bi-pole configuration.

The second question/idea is this- I am thinking of using the CSS WR125S (the 8-ohm version) on the front baffle, and the CSS WR125ST (the 16-ohm version) on top (or behind) as the baffle-step compensation driver. The idea here is that the CSS WR125ST will draw less power than the CSS WR125S, and therefore there will be less issues with it’s radiation affecting imaging, etc. So in a way it won’t be ‘too much of a good thing’ as the normal combination can be (as noted in Jim Griffin’s post on the subject- see Bipolar MLTL Speaker with FR125S/WR125S). This will also be beneficial in the top-loading idea as in my first question.

I hope to hear from all the gurus on the forum, and from everyone else (some of whom are gurus but are just too modest to admit it).

Enjoy,
Deon

PS. If there were discussions of similar ideas on the forum could you point me to them. Thanks.

[DeonC]

Come on, guys, no one to answer? The top-loading idea is not exactly revolutionary as Castle did it with their Howard speakers, and Dick Olscher also did something similar. Where are all the BSC experts? Help me out here.

Deon

[Landroval]

If the other driver is on top, the speaker is no longer a bipole. The top driver can be used for BSC, but it's hard to say how it should be filtered without measuring the setup.

[sreten]

Hi,

just my opinion :

Top mounting is a a bit of a halfway house.
You get more room drive and reduced but not eliminated baffle step.

The second high impedance driver idea :
Suffers from the basic flaw the second drivers power handling /
excursion capabilty is only half utilised, and introduces baffle step
again to rear mounting, and would dilute top mounting room drive.

3 sixteen ohm drivers probably a better way with 2 front 1 rear/top.
baffle step still problematical though.

/sreten.

[DeonC]

Hi again guys

I see how the idea of placing a driver on the top-panel of the cabinet ISO (ISO = In Stead Of) on the back panel would result in less that perfect BSC. The main reason for this suggestion is situations where the speaker needs to be placed close to the rear wall (think of WAV in your average household, but more on that later). In that case the rear driver would cause too many reflections and simply muddy up the sound. Couple that to the fact that I have read in various reviews of speakers with a top-loaded driver that such speakers are often described to have a very good soundstage, so I thought that this could be a possible solution. Those were the main reasons for that suggestion.

OTOH, you all however seem to be missing my point with the suggestion of the 16-ohm unit as the BSC driver.

3 sixteen ohm drivers probably a better way with 2 front 1 rear/top.

Hi sreten

I get your point and will have to consider it a bit further. However then we get the problem of comb filtering again.

I decided to add as bit to make my point a bit clearer.

First off I must say that I am not a fan of bipolar radiation. Sound bounced off the walls and other areas are absorbed and reflected differently at different frequencies, so that the music attains the ‘flavour’ of the listening room, ISO transporting the listener to the recording venue. IMHO this is totally WRONG. I believe that the real goal in audio is the ‘I am there’ experience, NOT the ‘They are here’ experience. My lounge is way too small to house even a 20-piece orchestra, never mind a 100-piece orchestra in full cry.

Now to get back to my idea. The theory of BSC states that the baffle step is 6dB, and I have no arguments with that, in theory. This situation will exist in a perfect anechoic chamber, but who’s lounge/listening room is built like said anechoic chamber. In practise the total baffle step is less due to various factors, but mainly due to room reflections, etc. In fact in the one article an active 3dB BSC circuit is suggested, and I contend that in real-life situations this is much closer to the ideal.

To continue my argument I will get back to Jim Griffin’s design. In his article Jim states that the speakers should stand at least 3ft off the back wall. That is exactly the type of situation I was trying to address by suggesting that the rear driver be made a 16-ohm unit as opposed to the 8-ohm unit on the front. That way the rear driver will have 3dB less output than the front. Over and above the reason given above for this (3dB vs. 6dB total BSC), there is another consideration that I think merits this change. You see I know of very few wives that would allow a speaker to stand 3ft into your average living room or lounge area (the dreaded WAF, or SAF, or whatever you want to call it). The only guys I know that can afford such luxuries ITO placement are guys that have their own dedicated listening-rooms. Add to the fact that the in the application I have in mind for those speakers, if I placed them 3ft from the rear wall, the speakers would be 1/2 ft behind the listening position (me behind my desk). The most I would be able to pull them out into the room would be about 1ft, and them I’d have to toe them in by about 45 degrees. This is do-able, but the suggested 3 ft is totally out of the question, for obvious reasons.

So, now to get back to my suggestion. I still think it is a much better idea to use a 16-ohm unit (the CSS WR125ST) ISO an 8-ohm unit (the CSS WR125S, which ITO TSPs is very close to the CSS WR125ST) on the back panel for BSC (with all due respect to Jim Griffin for whom I have a lot of respect as a designer and audio thinker). This will allow a more correct level of BSC to be added, as well allowing the speaker to be a lot more flexible ITO placement in the listening room. Well, IMHO that is. Comments (and please be gentle )?

Enjoy,
Deon

[sreten]

Hi,

real life BSC is 6dB for speakers placed clear of walls.
Usually centred around 200 to 400Hz, range ~ 3 octaves.

Decent spacing to all walls also means room gain comes in
the bass and the the speaker can be bass aligned to this.
It also means smoother driving of all room resonances.

you have to get pretty close to a wall / corner to reduce
BS to 3dB and this messes up all sorts of things, but I'd
say you'd need 3dB for wall mounting and designing a
speaker for near wall mounting is perfectly reasonable.

I don't agree a half power driver on the rear will give
correct BSC in normal rooms / free space placement.
And a half power driver will only give ~ 2dB of BSC,
whilst a full power rear driver gives the full 6dB.

It also prevents full cancelation of forces between
the opposed drivers - a very powerful technique.

BTW your listening position indicates near field not
far field conditions. BSC is different in these cases.
Most near field speakers have no BSC.

/sreten.

[planet10]

I'm a bit late to this thread... but anyway.

I saw the driver on top 1st in the Linn Isobariks (althou later saw plans of implementations that predated it using lowthers). The Isobariks were a very musical speaker.

This configuartion is usually used up against (or real close to) a wall. Up against the wall, the box will not have baffle step -- the wall acting as an extension of the bafflr so that the driver only ever sees 2pi steriradians.

The idea of using a 16 ohm driver + an 8 was 1st suggested to my by Bob himself, so you are in good company... i myself had at one time considered a triangular speaker with the front facing panel with 1 single driver, and the other 2 rear/angled panels with 1 each of the same driver wired in series (and those wired in parallel with the front driver). That one never did get built.

dave

[thalis]

I also believe that baffle step comp. most times shouldn’t be calculated at the full 6dB, except if someone prefers listening too much low output, and off course up against the wall no such compensation is needed. Since you will be placing the speakers near the back wall, and also since you will be in the near field listening to them, I also find your solution elegant at first thought, but thinking of it more I don’t understand why you shouldn’t mount it in the front baffle since you are also concerned with the first reflections;-)) Mounting the driver in the front will give you better mid-lows in any case, as I believe. Furthermore, if you choose to use the same 8 ohm driver and not the 16ohm, front-mounted, gathering some more dB, you will be able to chose a more sensitive tweeter as well and apply ‘some’ baffle correction afterwards, with in box and in room measurements via the crossover. You will end up with the same money given but with a speaker considerably more sensitive and most probably better as well. Well, the only shortcoming will be that it won’t be ‘special’ or ‘different’ as you (I suspect) define it now:-)
Regarding the possibility of the 16 Ohm driver, bear in mind that you will likely have to charge each driver with its own internal air volume.
Regards,
Thalis

[Paul W]

Deon,
One thing you may want to do is simulate response of the physically separated drivers with LspCAD or another tool to make sure the wavelength related nulls and comb filtering are appropriate for your overall design. I once considered building a bipole woofer module...until I found a strong notch right in the middle of the passband.
Regards,
Paul

[DeonC]

Thanks guys. This is getting more intersting that I could imagine. A lot to consider...

Deon