Please read
https://forum.blu-ray.com/speakers/6...rs-part-i.html.
Why are Bipole/Dipole Speakers Preferred in a Small Home Theater Room?
The most important effect of Monopole speakers is localization and imaging. This may be a desirable feature for the front speakers, but the localization of the surround channel sound in the listener's head can be very distracting. They may work if the listener is seated exactly on the center line between two identical surround speakers. Direct-radiating surround speakers are not very desirable if you are sitting off-center and close to one surround speaker.
As THX has pointed out, commercial movie theaters use quite a large number of speakers for each surround channel. Even with discrete surround information, an array of speakers like this do not create a distinct direct sound. Moreover, although these speakers are direct-radiating monopole speakers, the fact that they are placed very high combined with the large size of the room and their long distance from the audience makes it possible for the sound to become more diffuse than a similar array of speakers would create in a small home theater. Because monopole speakers radiate sound in only one direction, anyone sitting closer to one monopole surround speaker than another would only hear the closest speaker. The effect become too “in your face” and very distracting from the movie.
A Bipole or Dipole surround speaker, however, radiates sound to the front and back of the room and creates more of a diffuse sound. The viewer hears the surround effects from many different directions and cannot pinpoint their source. In addition, the more ambient atmosphere created by bipole or dipole speakers have the advantage of not calling attention to the speakers, even if a viewer is sitting closer to one side. This allows the individual to pay more attention to the movie.
When Should We Use Bipole and When Should We Use Dipole Surround Speakers?
Where you sit in your home-theater room determines to a great extend where you will place your speakers and what type of surround speaker you should use. The other factor that may determine the type of surround speaker you should use is your taste.
Dipole Speakers:
Dipole speakers[/B] have a very diffuse surround effect and turn everything into a spacious environment, even if that was not originally intended by the sound engineers. The small room effect cannot be effectively simulated with dipole speaker, because they scatter so much sound around they always sound spacious.
Use Dipole surround speakers under these conditions:
- When you want to create a more diffuse and spacious surround environment, in which you can not easily pinpoint the surround information happening around you.
- If you can install them on the walls exactly on the same line as the listening area as in the following diagram:
Source: Polk Audio
Dipole surround speakers radiate sound to the front and to the rear of the room and create a null area exactly in their middle area.
You must sit in the null area (along the 90-degree axis of the dipole speakers) to receive maximum benefit from the diffuse nature of bipoles; enveloping you in totally indirect sound.
If the speakers are placed outside of the listening area (to the rear or to the front of the listening area), the effect is significantly degraded.
Bipole Speakers:
With Bipolar surround speakers you have the best of both worlds, without some of their disadvantages. Bipole speakers can have the direct (localized) effect and intimate environment of the direct-radiating monopole speakers and at the same time, the spaciousness, wide open, and enveloping environment of the Dipole speakers. They are not as “in your face” as direct speakers and at the same time they do not create as much spaciousness as dipole speakers. They also do not have the bass roll off of Dipole speaker and have the advantage of more placement flexibility. In a 5.1 setup, Bipole speakers can work both on side walls or rear wall. Ideally, in a 5.1 setup, they should be placed on the side walls slightly behind the listener and a couple of feet higher than the listener’s head.
Because the Bipole drivers work in-phase, the result is a greater sound output (almost like a 360 degrees soundfield) than with out-of-phase dipole configuration. That can be a good thing if you want to place the Bipole speakers anywhere outside of the listening area on the side walls or behind your listening position. The Bipole setting also adds some channel separation to your mix and can create more localized sound scape. Some people may prefer this localization when they are used with discrete surround channels.
What About Quadpolar and Tripolar Surround Speakers?
The Quadpolar and Tripolar surround speakers marketed by some manufacturers are essentially variations on the Bipolar speaker arrangement. Although they have drivers emitting sound in multiple directions, these drivers are connected in phase.
How High Should the Surround Speakers Be Installed?
The side surround and rear surround speakers should normally all be installed at the same height and about 2 to 3 feet above ear levels when you are sitting down. They should not be installed too far above the ear level. Otherwise, coherence of the front to the surround field is lost. Unfortunately, in many cases, people have to place the surround speakers too high because of space limitations in their rooms.
Do Direct-Radiating Monopole Surround Speakers Have a Place in Home Theaters?
You should use monopole surround speakers under the following conditions:
- If the surround speakers have to be installed in the rear top corner of the room due to room limitations. In such a situation, the Monopole surround speakers may work better as the drivers of bipole and dipole speakers will be too close to the side walls and shoot directly into the walls.
- If the rear or side walls are at least ten feet or more away from the listening area, Monopole speakers can be used. If possible, the Monopole speakers should be directed toward the front of the room and not toward the listening area so that they can create a more diffuse effect.
If your room is too small and you are forced to install two Monopole surround speakers very close to the listening area, make sure they are not pointed toward your ears. Use the following diagram as a guide:
- The most ideal situation for monopole surround speakers requires a very large room with multiple surround speakers on the side walls and on the rear wall. The surround speakers should specifically be placed apart at certain distances to de-emphasize the "directness" of the speakers and they should be as far away as possible from the listening area. They should not be directed toward listening position (similar to the way multiple speakers are placed along the walls in commercial movie theaters).
It is true. I have lost my mind. 
What If My Room Has an Unusual Shape and I Do Not Know Which Type of Surround Speakers are the Best?
If you are in doubt, buy Bipole surround speakers. They offer the most flexible placement and have some of the advantages of Dipole and some of the advantage of Monopole surround speakers without some of their compromises.
Can We Mix different Types of Surround Speakers in a 7.1 Setup?
Excellent
Excellent
Very Good To Excellent Depending on your Taste for Spaciousness of Dipoles
Good, But Only If the Room is Very Large
Corner Placement May Work Better With Monopole Speakers, But Should be Avoided If Possible
Excellent If the Room is Large and You are Not Afraid of Getting Kicked Out By Your Other Half 
Very Good
Strong Possibility of Phase Cancellation and Should Be Avoided
Direct radiating monopole speakers produce a very localized sound (i.e., you know where it is coming from) as the sound waves are emitted only from one direction (from the front of the speakers). In contrast, Bipolar/Dipolar speakers have two sets of drivers, usually one in the font, and one in the back of the speakers. The two drivers are either in phase with one another (Bipole) or out-of-pahse with one another (Dipole). The sound is coming out of two directions and also reflected against the walls, making a less localized and more of a diffuse sound. A pair of Bipole or Dipole speakers can do the job of multiple direct-rating monopole speakers found in commercial movie theaters.
This is a quote from Sir Terrence, an Insider at Blu-ray.com and an audio engineer working at Disney:
Quote:
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I don't think the studio's mix with any assumption on playback speakers. The guidlines for re-mixing or re-purposing soundtracks for DVD release specifies that monopole speakers be used. But I think that is largely because most control rooms for mixing are pretty dead acoustically and dipoles or any reflecting speaker will not sound very good under those conditions.
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How About Timbre Matching Between the Surround and Front Speakers?
Timbre matching for the front three speakers may be important. Some people even dispute that and consider timbre matching a bit overrated. There are far too many people who use a different brand of center speaker and are very happy with the results.
As a general rule, timbre matching between the front and surround speakers is not an important issue. However, if you consider this issue to be important, you may not be able to get good voice matching between the Monopole front speakers and Dipole surround speakers because of the out-of-phase nature of the Dipole speakers. You may have a better chance of timbre matching between Monopole front speakers and Bipole surround speakers because the speakers are in phase and will not work against each other.
Quote:
Timbre:
Definition according to American National Standards Institutde (ANSI): The combination of qualities of a sound that distinguishes it from other sounds of the same pitch and volume.
Timbre is also known as tone quality or tone color. It is sometimes described as sound quality. The term quality in this context refers to how a sound comes across to a listener as the general character of the sound.
In music, timbre is the quality of a musical note or sound or tone that distinguishes different types of sound production, such as voices or musical instruments. More specifically, timbre enables the listener to identify the source of the sound (the particular musical instrument making the sound) and according to some musical type such as brass or string instruments. For example, timbre is what people use to distinguish a trumpet from a saxophone if both instruments are playing notes at the same pitch and loudness. No amount of equalization will make the two instruments sound the same.
Pitch:
In music, pitch is the perception of the fundamental frequency of a note. Pitch is something perceived by the human ear, as opposed to frequency, the physical measurement of vibration. Pitch of a sound is determined by the frequency of vibration of the sound waves reaching the ear: the greater the frequency, the higher the pitch.
Pitch can be rank ordered on a scale from low to high and loudness can be arranged on a scale from soft to loud. We generally correlate pitch with fundamental frequency and loudness with intensity.
Timbre is different than pitch and loudness and cannot be reduced to one-dimensional scale. It cannot be related with any one physical dimension of a sound. We cannot say high timbre or low timbre. This makes the definition of timbre very vague.
Timbre and equalization are different by definition and no amount of equalization will have any effect on timbre. Equalization simply increases or decreases the level of certain frequencies. After equalization you can still distinguish between the speakers.
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A Guide to Bipolar, Dipolar, & Direct-Radiating Monopole Surround Speakers (PART II)
Hybrid Mode
