Bipolar speaker configuration ... need info! accoustical pros and cons ?

[JinMTVT]

My understanding of loudspeaker is coming together slowly!
Thanks to all of you ..

Yet after a few hours of search, i never found the answer to a question in here or elsewhere.
what i found is the "marketing" reasons why they use this arrangement, but never found the accoustical reasons.

So i would like to know what exactly are the benefits of a bipolar
"push-push" arrangement of drivers in an enclosure.
I've been using Mirage speakers for a tad over 3 years now,
all 3 of em beeing bipolar "omnipolar" systems.

Else than having way to many relfections in a small room
I have not been able to point the other advantages.
So that is what i would like to learn tonight from you

all in all.. what are the pros and cons ( and the implications toward deisgning the systems ) of Bipolar arrangement of drivers.

Also would appreciate pointers on the design itself,
sub, mid and high freq if there is differences or problems associated with each Vs bipolar

Beeing here with you guys ,

compare for me Bipolar and Dipolar configuration in terms of
accoustical performance

Thanks again all for your time and efforts !!

 

[hunter audio]

hi

amongst other benefits -push pull reduces distortion

acts as a better piston to push air

and increases power handling of the drivers

suranjan

transducer design engineer

 

[navin]

i think we are talking push push here though.

dave (planet 10)'s site has a horde of info on push push (esp TLs).

u do get reduced distortion. in some desings the rear woofer is used to compensate for BSC so this also improves sensitivity.

also for the bass you get twice the ammount of volume or air displaced and hence a bit more low bass SPL capability.

 

[SY]

Depends on the design. Bipolar operation can give a polar pattern which is closer to omni, if that's what's desired. It can have a psychoacoustic effect that some will find pleasant due to increased reverberant energy and a degree of comb-filtering of in-room response. Woofers can be braced one against the other to reduce reaction forces. Using bipolar in selected ranges can do things like equalize reverberant energy (e.g., the rear tweeter in some Snell designs) or to compensate for baffle spreading loss.

Like everything else, it's a design choice with tradeoffs, not some sort of panacea.

 

[paulspencer]

I think most will agree that dipolar midrange is not a good idea for main speakers. It is probably more suitable for surrounds. The reason is that you are at the mercy of the room. The room will most certainly colour the sound and add its own signature. As mentioned, some may actually like this effect, but it is less accurate.

Bipole vs Dipole. Dipole will have less bass due to cancellation. For midrange, you might consider dipole with two sets of drivers facing in opposite directions for a surround speaker. You might also try this with bipoles. You can switch quickly by chaning the wiring.

However, for main speakers it is better to look into open baffle if you want dipole. This has a very different interaction with the room. With bipolar you are at the mercy of the room due to the way the signature of the room is imparted to the sound. With dipoles, one of their main strengths is the way they reduce room interaction, although it's not quite as simple as that.

 

[Variac]

It seems like the bi-pole speaker would always compensate for baffle step. Dave thinks so I believe.... Although I haven't seen measurements to prove this.

To rephrase another coment: Dipole will have less bass than bipole IF it isn't in an adequately sized open baffle (or in a very wide box the same size as the required open baffle I guess?)

 

[JinMTVT]

By baffle step, what do you guys mean exactly?
i am not familiar with the term ... thanks

 

[paulspencer]

It seems like the bi-pole speaker would always compensate for baffle step. Dave thinks so I believe.... Although I haven't seen measurements to prove this.

To rephrase another coment: Dipole will have less bass than bipole IF it isn't in an adequately sized open baffle (or in a very wide box the same size as the required open baffle I guess?)

The idea that a bipole would automatically have BSC would be true if the absorption if its output resulted in -3db above the point at which 3db bafflestep occurs.

re: dipoles and bass it depends on what you are comparing to. Even with a baffle suitably large, the output will at best equal a sealed box, but this will be less than a vented box. This is what fequal indicates, the point at which output is equal to a sealed box monopole.

By baffle step, what do you guys mean exactly?
i am not familiar with the term ... thanks

The baffle gives boost to the output above a certain point. For a typical tall floorstander with a 8" wide baffle, output above ~500 Hz will be +3db ... this is a gradual not a abrupt "baffle step." This point can be determined by the formula:

f-3 = 115/ baffle width (metres)

thus you need a baffle of 1m to get this f-3 point to 115 Hz.

Mostly this is dealt with in the crossover, where the midrange and treble above this point is attenuated. However, an alternative way with say a tweeter and a midbass is to add an idential midbass but roll it off above a certain point, which yields +3 db to the bass and lower midrange. This is called a 2.5 way design.

Here is a good article to describe it more fully.

 

[planet10]

It seems like the bi-pole speaker would always compensate for baffle step. Dave thinks so I believe.... Although I haven't seen measurements to prove this.

An externally hosted image should be here but it was not working when we last tested it.

By baffle step, what do you guys mean exactly?
i am not familiar with the term ...

http://www.t-linespeakers.org/tech/bafflestep/intro-bds.html
http://www.t-linespeakers.org/tech/bafflestep/index.html

dave

 

[Variac]

Well, I guess I've seen measurements now!!!

 

[paulspencer]

Dave,

I assume that on your site with that link (#6 brute force approach) you are essentially talking about a 2.5 way speaker where the .5 woofer is placed on the rear of the box?

When bipole is mentioned I usually think of a speaker that has the same response on axis front and back, like a dipole but not wired with reverse polarity.

 

[planet10]

I assume that on your site with that link (#6 brute force approach) you are essentially talking about a 2.5 way speaker where the .5 woofer is placed on the rear of the box?

This could be thot of as the right way to do a x.5 way speaker, hiding the phase anomalies of the x.5 driver in the shadow of the enclosure. Not really a bipole, but often differs only by the filter on the back.

When bipole is mentioned I usually think of a speaker that has the same response on axis front and back

Officially that is the case... but most of the time i am dealing with a FR as the primary driver, any tweeter on the front just helps with a bit of sparkle at the top.

like a dipole but not wired with reverse polarity.

usually a dipole is achieved by having an open baffle with a single driver, a bi-pole is really a different beast. The 2-driver dipole for surround use is purely an anomaly from the short-comings of Dolby Prologic and really should not be considered too much when talking about dipoles -- the major examples of which are electrostats and large electrodynamic planars like magnepans or BG ribbons.

dave

 

[paulspencer]

usually a dipole is achieved by having an open baffle with a single driver, a bi-pole is really a different beast. The 2-driver dipole for surround use is purely an anomaly from the short-comings of Dolby Prologic and really should not be considered too much when talking about dipoles -- the major examples of which are electrostats and large electrodynamic planars like magnepans or BG ribbons.

dave

Yes, I'm familiar with open baffle and have had some lengthy discussions re OB dipoles ...


I'm curious about your comments on surround dipoles. Do you see no other good use for these speakers? What about surrounds for DTS? Are you saying they are n't a good approach? As I see it, surround dipoles are used in this way as the *front* and *rear* waves (in this case the front wave from each set of TM) are used with equal importance given to each ... therefore an open baffle isn't suitable due to the effect of the magnet and basket assembly on the midrange of the rear wave.

 

[planet10]

surround dipoles. Do you see no other good use for these speakers?

I think surround sound is a moving target... all the surround schemes you see today will likely be superceeded by something that really works. It will likely require full-speakers that are all the same.

dave

 

[JinMTVT]

i thought that bipolar smaller woofers were used for surrounds
at something like 90 angle from each other
was told it is for wide dispersion of high hz as it should be at the rear ..

i have to say that i am not a fan of surround sound at all
never been impressed by any systems i've heared
and let me tell you that there are quite lots of those
at the Montreal's "Soud and Image" festival ..

 

[Ap]

From what I have read on surround sound, the rear speaker should be in fact placed at the sides of the room. (Just as it is in the cinema) As this is not easy, the idea of a dipole came into play, which created a more diffuse sound so as to reduce the localisation.
With regards the bipole / Mirage concept,
For my current project I will be adding a second rearward driver (probably a dome mid) with a variable resistor to attenuate the output. I think bipole does create a lot of depth & size (albeit due to comb-filtering). If it were reduced in level to the front drivers I think you could have the best of both worlds.

 

[JinMTVT]

with the mirage case..
( as most bipolar loudspeaker would be i guess )

the speakers have to be placed far from rear walls or else you will get serious stereo imaging problems since reflections from the rear drivers will arrive way too fast ..this is one problem i had with the OM-9 when they were in my small "untreated" room ...
way to many reflections from all over the place, creating a warm but not really what i am looking for sound ..

i don't know what would your idea do to this,
if you reduce the output of the rear driver, you are also changing the cancellation thing happening inside the enclosure nah ?
wich one driver's wave overpowering the other's...

but i am more interested into learning what is happening inside than outside the box as far as bipolar concept...


talking about bipolar/dipolar stuff..

ESL beeing dipolar drivers, what problems do we encounter when dipolar drivers are working in free space?
and please i need to understand how big the baffle needs to be to prevent the rear wave to reach the front ..
is it 1 wavelentgh? 1/2? and do we calculate the total frontal area of the oudspeaker or doest the measurement start from each side of the driver's end?



if the baffle is large enough for no wave to reach other side intact, and the driver is far from side walls and front wall, i guess that secondary reflections would be controlled pretty easily nah ?

 

[paulspencer]

Some people are very hard to impress!!!

I think surround is a lot of fun, and some time later I'd like to build some monopole surrounds with dipoles - like M&K's "tripole surrounds," assuming that my experiments confirm that it's the way to go ...

But I think 90% of our enjoyment of the theatre experience is the big screen, the stereo channels and the sub, so I'm working on them first as I can do them all diy (including the projetor). Going to surround can actually cost as much as a diy projector, sub and mains!

 

[JinMTVT]

Some people are very hard to impress!!!
..... Going to surround can actually cost as much as a diy projector, sub and mains!

ahaha well i've hear quite a lot of dam good stereo setup at the hi-fi shows in here !!! so quite often in surrond setups all the quality of the components is down since cost as to matter at some point, and it just brings down all the experience..

my guess is that a well treated rooom with really good quaity stereo setup + sub is really hard to beat for movies..

and it is true that 5.1 and + setups are quite expensive
and unless you are running a really good processor with seperate high quality DACs ( or a good computer setup like mine ;p )
you ought to loose some quality ... but hey
it doesn't matter as much when watching a movie as when reproducing a quality recorded music..to me at least

SPL takes an important place in HT setups!

 

[SY]

ESL beeing dipolar drivers, what problems do we encounter when dipolar drivers are working in free space?and please i need to understand how big the baffle needs to be to prevent the rear wave to reach the front ..
is it 1 wavelentgh? 1/2? and do we calculate the total frontal area of the oudspeaker or doest the measurement start from each side of the driver's end?

Keep in mind that the back wave/front wave cancellation isn't an either/or, it is a gradual and approximate phenomenon. Because baffles are rectangular, this spreads the loss frequency out even a bit more. So, yes, you can use 1/2 wavelength as a rough guide to cancellation from the effective baffle width, but it's going to be only an approximation; there will be cancellation above and below that frequency, it's just the midpoint of the cancellation curve. An approximation to effective baffle width is the square root of the driver/baffle area, but this will not be a good approximation as the baffle gets farther and farther away from being square. If you haven't looked at the stuff on Sig Linkwitz's web site, I'd recommend it.

Now, all of this is lovely for free space, where the back wave obligingly goes away after the first order cancellation. But stick that speaker in a room and everything changes. One of the dipole gurus will no doubt have some software which will enable you to calculate the free space response for a given baffle geometry more precisely, but in the end, you'll still have to measure the results and eq to your particular situation. So don't be afraid of back-of-the-envelope calculations and order-of-magnitude approximations.