Dipolar, Bi-polar & Monopolar Speakers

[HD MM]

For anyone researching a new set of speakers, this FAQ should come in handy in deciding between Di/Bi- Polar speakers for surrounds or a direct driving monopolar speaker.

Dipolar refers to speakers with drivers that are fired in two different directions and are in reverse phase causing a cancellation of sound waves in front of the speaker. This is usually done in rear speakers that are wall mounted. The front of the speaker is aimed at the listening area, which causes all of the sound to bounce off the walls before it is heard. This makes it almost impossible to determine where the speaker is, creating a true surround effect.

Bi-polar refers to speakers with drivers that are fired in two different directions, but are in phase causing an increase in bass output. I this type of speaker the drivers can be in the front and back of the speaker, side firing, or at 90º angles from one another. There are also speakers which function as both bipolar and dipolar. This can be adjusted using a switch.

Monopole is a type of speaker with all drivers facing one direction. Used for precise placement of sounds. Usually used in front and center speakers.

 

[HD MM]

From Polk's FAQ page....

Dipole and Bipole refer to speakers that have drivers on opposite baffles. To put it crudely, they squirt sound out of both ends. Our feeling is that this type of sound radiation isn't desirable for front channel speakers. You get a spacious soundstage but at the sacrifice of image specificity. Soloists sound as big as the room. Pavorotti may be a large guy but he's not 10' wide. Orchestras may be large but you should be able to precisely locate individual instruments within the group. If not done properly, dipolar or bipolar speakers may also exhibit frequency response problems due to phase cancellations (particularly true with narrow baffle designs). Polk front channel speakers use wide dispersion drivers to provide open, spacious sound-staging while maintaining pinpoint localization.

But the lack of image specificity of dipolar or bipolar speakers is just the ticket for rear channel use, especially in Dolby Pro Logic systems in which the surround channel is mono.

In a Dipolar speaker, the two sets of speakers are out-of-phase with each other, while the drivers are one side are pushing, the opposite side is pulling. The result is that there is a "null" or a dead zone of sound in the area along the 90 degree axis of the speaker (see illustration below). Why is that good? When properly set up, a pair of dipole speakers used as surround speakers will provide a very open, enveloping rear effects soundstage without allowing you to pinpoint the location of the speakers themselves. That's a good thing. But for all this to work properly, the speakers need to be positioned "in-line" with the listening position as shown on the illustration below. If you are sitting out of the null area, the effect is ruined. What if you can't or don't want to place your surround speakers and listening position as required? That's where bipoles come in handy.

In a Bipolar speaker, the two sets of drivers are in-phase with one another - both sides push air at the same time. The result is greater sound output where the dipolar speaker's null would be. Theoretically, a bipolar speaker approaches a 360° soundfield - it squirts sound all around the room. That's a good thing if you need to position your surround speakers behind your listening position or anywhere outside of the null area. Some people prefer the greater localization of bipolar speakers when used in digital discrete (Dolby Digital 5.1, DTS) systems.

 

[HD MM]

Example of a Di/Bi-Polar speaker.......

Example of a Monopolar speaker.....

 

[jayn_j]

And then there are hybrids like these, where the tweeter is set in a bipolar mode, and the midrange driver is a direct radiator.

http://www.klipsch.com/products/details/s-3.aspx

These would disperse the high frequencies, thus widening the soundstage, while still giving directional feeling. However, I think it is actually a cost and space saving measure.

 

[HD MM]

Question for Di/Bi-polar speaker owners....

Where do you find the best placement for a left/right surrounds in a 5.1 setup?

(Currently, I don't have the luxury to answer this question myself as I own monopole speakers).

According to THX, they recommend placing the surrounds to the left/right of the listening area. I assume they are running "di-polar" instead of "bi-polar"?

http://www.thx.com/home/setup/speakers/51.html

And additionally, I would've thought that in a Dolby Digital, DTS or lossless 5.1 audio setup, it would be better to place your surrounds to the rear of the listening area to avoid the "null"?

Responses from Di/Bi-Polar owners appreciated....

 

[jayn_j]

Question for Di/Bi-polar speaker owners....

Where do you find the best placement for a left/right surrounds in a 5.1 setup?

(Currently, I don't have the luxury to answer this question myself as I own monopole speakers).

According to THX, they recommend placing the surrounds to the left/right of the listening area. I assume they are running "di-polar" instead of "bi-polar"?

THX Home Theater 5.1 Speaker System Layout

And additionally, I would've thought that in a Dolby Digital, DTS or lossless 5.1 audio setup, it would be better to place your surrounds to the rear of the listening area to avoid the "null"?

Responses from Di/Bi-Polar owners appreciated....

I'm pretty much in the same situation where the room dictates the speakers. My Dipoles are directly r+l from the main listening area. They are above listening height as recommended. However, they are only a foot or so from the back wall because of the small room. Here is the picture from another thread:

 

[dfergie]

Informative... stuck thread...

 

[John Kotches]

I know this hasn't had a post in 2+ years, but I thought I'd point out this.

All speaker drivers are naturally dipolar radiators (front / back out of phase). Yes, that's right all. The cabinet in a traditional speaker absorbs the out of phase back wave. Flat panel speakers (electrostatic/planar magnetic/ribbon etc) are all built generally as dipolar radiators. These types of speakers (contrary to polk) can indeed image like a (insert expletive here) but only over a small area.

Also some surround speakers can be easily switched between dipole and bipole radiation characteristics. If not, you can DIY by reversing the polarity of connections to one set of drivers in the cabinet. This will convert from bipole to dipole or dipole to bipole.

Don't know if this is TMI, too late or simply unwanted but thought I'd add my nickel's worth to the discussion.

 

[jayn_j]

True, John, but most dipoles don't place the radiators 180 degrees from each other. Instead, they angle them into the room. Wouldn't placing them at 180 degrees create a serious null straight out into the room?

If you disassemble those Klipsch dipoles in response 7, you would find that there are a pair of tweeter horns, each placed at 45 degrees off axis, and a midrange aimed directly at the listener.
S - 20 Surround Speakers (pair) | Klipsch (BTW, these are not the speakers pictured, but they are a similar configuration)

Better dipole surrounds will have multiple midrange drivers, also placed at 45 degrees (see response 3)

 

[John Kotches]

jayn_j:

I don't think we're discussing the same phenomenon here. I'm discussing the fact that every driver physically radiates in a dipolar fashion... When the speaker moves forward the front hemisphere compresses the air while the back hemisphere gets a rarefaction. Ye olde equal but opposite reaction concept.

In a dipolar surround speaker (not to be confused with planars built sans cabinets ) still have back waves to deal with, and they are absorbed by the cabinet.