[rajacat]

Has anybody implemented the BSC in their Bi/dipole Omegas?

-Raj

[bprice2]

It is my understanding that Louis' bipole speaker accomplishes the same thing as implementing the BSC.  I wish I could offer more.   

[Duke]

Bprice2 is absolutely correct about the bipole Omegas, in my opinion.

I have built bipole speakers going back to the late 80's, though not commercially.     

You can think of the front baffle of a speaker as a 180 degree pattern horn, as it in effect focuses the radiation into a 180 degree angle (front hemishere) down to the frequency where the baffle edges are 1/4 wavelength away from the center of the driver (or, down to the frequency where the baffle is 1/2 wavelength across).   At lower frequencies (longer wavelengths), the baffle is physically too small to control the radiation pattern, and the energy starts to wrap around the cabinet.  This is commonly called the "baffle step", with the "baffle step frequency" being the frequency where the enclosure is 1/2 wavelength across, and the result of the baffle step is a theoretical 6 dB loss in on-axis sound pressure level in the deep bass relative to the midrange (above the baffle-step frequency).   It's not an abrupt drop - it's a gradual shelving down, and in practice it's more like 3 or 4 dB.

Now note that the power response (summed omnidirectional response) of the speaker is unaffected by the baffle step.   So whether or not to compensate for it is a juggling of tradeoffs - but that would be another post for another day. 

Back to the bipole.  The bipole very elegantly compensates for the baffle step in this way: At the frequency where the sound waves start to wrap around the front baffle, guess what - they also start to wrap around the back baffle!  So the wrap-around from the rear-facing driver compensates for the baffle-step rolloff!

Now unfortunately it's not perfect - there is a path-length-induced time delay, which will cause a notch in the on-axis response at the frequency where the rear-facing driver is 1/2 wavelength farther away from the listener's ears than the front-facing driver.  There are geometrical parameters that can be juggled to minimize this notch, and I can tell by eyeballing Louis's bipoles that he is using an appropriate geometry.   In addition, this notch is a phenomenon that looks much worse on paper than it actually sounds.  The ear really doesn't notice it.  However, if Louis ever submitted a bipole speaker to SoundStage or Stereophile, its on-axis anechoic frequency response would not look very good in the lower midrange region compared to other speakers.  When the day comes that there's a measurement protocol that correlates very closely with subjective preference, I think Louis's bipoles will score very high.  The on-axis anechoic frequency response is not an accurate predictor of subjective preference.

I can think of at least six different ways to configure a bipolar speaker.  Mirage used two of them, Definitive Technology patented a third, Omega uses the most simple and elegant, and then there are two more that I haven't seen anyone use in a commercial design yet.   

In the deep bass region, where the path length difference between the front and rear facing drivers is an inconsequentially small fraction of a wavelength, an Omega or Mirage style bipole gives about a 3 dB boost.  The designer can take advantage of this and tune the box lower than normal, resulting in fairly smooth and extended bass.  Now note that the rear-facing driver is eating up half of the wattage you're putting into the speaker, so this extended bass comes at the expense of efficiency to a certain extent.  Tradeoffs, always tradeoffs.

In my opinion, the bipole has some very desirable characteristics.  Dollar-for-dollar, it has to compete with considerably more expensive forward-facing-only drivers.  So its commercial niche probably lies with particularly high-value drivers, which Louis is very good at finding and getting the most out of. 

I used to be a dealer for Louis, and think very very highly of his designs.  He is getting performance out of fullrange drivers that I couldn't begin to duplicate.  If I wasn't off playing speaker manufacturer in my own corner of the playground, I'd still be an active dealer for him.

Duke

[bprice2]

Duke, nice reply.   Just curious though...does your explanation also apply when the speaker is in its dipole configuration (w/ front and back wired out of phase)?

[JoshK]

Very well written explanation Duke!  Right on the money. 

[Duke]

Thanks, bprice2.  My explanation does not apply to dipole configuration. 

With a dipole the front-firing and rear-firing drivers are out of phase, so their outputs tend to cancel rather than reinforce one another when they meet.   Baffle step pales in comparison to dipole cancellation. 

I'd be hesitant to equalize one of Louis's designs for dipole cancellation, as that eats up x-max very quickly.   I'd stick with the bipole configuration. 

Thanks, Josh.  Nice move you got going on there, by the way. 

Duke

[bprice2]

Thanks, Duke.  That's what I suspected.  I will let you know, however, that Louis' bipole speakers do in fact sound fabulous in dipole configuration...hollographic is the word that comes to mind.  But, I guess if I were to measure, I would find that the speakers produce a little less low-end in the dipole set-up.  Is that right?

[rajacat]

I find that my Hemp bi/dipoles in dipole configuration produce less but more clearly defined bass than in bipole. I believe that ZLS runs his Hemps in dipole and has also instituted the BSC in that configuration.

Raj

[bprice2]

I believe that ZLS runs his Hemps in dipole and has also instituted the BSC in that configuration.

That's interesting.  I'd like to know more about how that is working out for him.

[rajacat]

He likes it. I imagine he'll respond to this topic and share his findings.

Raj

[Louis O]

Hi Duke,

Many thanks for the wonderful post and you did a great job explaining the bipole and dipole configurations. Abig part of my philosophy is to keep as simple as I can and let the mechanical aspects of the designs do the work. This was the reason I wanted the design to be versatile and be able to be switchable. This way the speakers in dipole the bass would be a bit fuller and in dipole the bass was less, but the sound would be more focused. The hemp drivers have helped a lot to the low end output especially in dipole mode. When I built the first BPCs I was always focused more on the bipole, but now with the hemp drivers filling in more Bass I'm using dipole mode all the time.

the circuit works great but suffers in a little clarity and sensitivity. It's all a two way street and you always have to give something up to gain something else. I try to bend physics the best I can.

Thanks again,
Louis

[ZLS]

aa  Yes I have the BSC circuit, and I am running the Super 3 Bipole/Dipole in the Dipole configuration in the 4 Ohm configuration with the circuit in place.  I find the sound more focused in the Dipole configuration and smoother and more coherent with the circuit in place.  Even in Dipole the speakers throw a remarkable sound stage, and when the source is a live recording it is actually spooky how large the sound stage becomes. 
    I believe the sound the BSC circuit produces is a matter of taste and preference.  I prefer the sound of copper over silver, and coherence over detail.  I also believe, though I have no way of proving this, that the sound the BSC circuit produces is very much room dependent. 
    Either way the Bipole/Dipole speakers Louis produces using the HempTone Driver are very emotionally involving speakers.  Combined with the magic of single driver speakers generally, you cannot go wrong. 
Saylma Hayek-Bipole
Penelope Cruz-Dipole
    My answer is yes!