�Explain, why �di-poles*� are not recommended for DD-5.1??�
(Note: The following explanation is based on my opinions only & understanding with direct radiator speaker designs, specifically, the high frequency drivers.  This is NOT an official statement, . . .it is just my best guess!!)
The easiest way to explain, why Dolby Labs has recommended "direct radiator" type designed speakers for DD-5.1, is to look at the physical high frequency dispersion attributes of those speakers.

If you were to graph
(see J520M Graph below) the frequency response of any well designed direct radiator speaker, . . .you would see that the high frequency signal would maintain it�s flat frequency from the 0 (zero) axis, all the way up to 20 kHz. 

Then, if you were to move the calibrated microphone to 15 degrees to either side of 0 axis, the high frequency would start to drop down from flat.  Next, @ 30 degrees, it would start to drop more rapidly, . . . .and then @ 45 degrees or more, . . . the high frequency response basically nose dives. That�s the laws of physics.
Types of DD-5.1 surround sounds:

When you hear an
totally isolated wide-band sound effects, (i.e. thunder, voice, metallic noises, musical instruments, etc.) in one of the individual rear channels, . . .because the direct radiator surround speakers are pointing towards your listening / seating area (not just the sweet spot) and it is above your hearing location, then that isolated wide-band sound effect should sound like it�s floating in front of that particular rear surround speaker, whether it�s being produced on the right side or left side.
Click image to see "Anatomy of HT" webpage
Once balanced to 75 dB, depending on the DD-5.1 mix, the effect can be subtle, or dramatic.  The advantage to Calibrating your HT speakers is that no matter how it�s mixed by the Sound Engineer, you will hear those wide-band effects the way he intended it to be heard while your viewing the DVD movie. As long you are within 30 degrees of the center 0 (zero) axis of the side speakers, . . .you will hear what�s been encoded there distinctly and clearly up to 20 kHz.
JBL J520M Speakers - Click to see "Anatomy of HT" page
Other advantages:
If the encoded
wide-band surround signal shares that sound effect with one of the front channels, let�s say front left and rear left, the sound then will sound like it�s in between the two speakers. Again, you will be within that 30 degree axis from your front speaker baffle and side speaker baffle location @ your listening / seating locations, obtaining the best possible frequency range that your speakers can produce with the wide-band (20 Hz - 20 kHz) effects! Physics!
What might be missed w/dipoles*:

Anyone with dipoles* speakers will never really hear the full impact & complete
360 degree flying pattern of Draco (Dragonheart) in the scene where he is discussing the Old Knight�s Code with Dennis Quaid.

Dipoles* speakers
diffuse the sound to either side, pointing the center axis of those angled speakers 30  to 45 (or more) degrees away from your listening / seating area, whether they are installed on the side or back wall.

When the sound is thrown to either side of the listener�s / seating area by the dipole designed speakers, . . .the high frequency center axis is over 45 degrees away from you!

Now, . . .based on the law�s of physics, what happens to that individual wide-band high frequency (20 kHz) effects at those extreme angles????  You get the picture!
(*DPL/THX specified 'dipoles', designed to diffused the encoded surround channels, to either side because the signal they were producing had limited band frequency range of 100 Hz - 7 kHz, and the signal was  �mono�!)
Bipole, dipole, monopole? from SMR HT Magazine website Bipole, dipole, monopole? from SMR HT Magazine website
Bi-polar design brief
Here's one reason why Dolby Labs recommends monopole vs. bi-polar . . .according to John R. Potis Jr. -- quoted below (from SMR HT Magazine - added: 2-5-01):

"Unfortunately, as in the classic "quantity VS quality" argument, all is not roses for the bi-pole. Creating the exacting, pin point image (the ability to close ones eyes while listening to music and point to each musician up on stage, both in terms of lateral spread and of front to back positioning on that stage) requires great precision on the part of the speaker. Timing and phase cues are everything. Sadly, with all the reflecting going on with bi-poles, many of these timing and phase cues are lost and the imaging specificity is lost."
Click to read SMR HT Magazine article - "Bipole, dipole, monopole?"
Bipole, dipole, monopole? from SMR HT Magazine website
PI - Nov 18, 1999
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