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Theo Nikolakis

Dipole speakers create a diffuse soundfield with two sets of drivers, one firing forward and the other firing backward out of phase with each other. This results in a null region along the central axis of the speaker, which is where you should be located for the proper effect. These speakers are designed to be placed to the sides of the listening position, but they would probably work if they were mounted directly above the listening position.

If you can't mount the dipoles directly above the listening position, bipoles might be better. Like dipoles, bipoles include two sets of drivers that radiate sound in more or less opposite directions, but a bipole's drivers operate in phase, so there is no null region along the central axis. Bipoles are still diffuse, but somewhat less so than dipoles. Some in-wall/in-ceiling surround speakers, such as the Atlantic Technology IWTS pictured here, let you select dipole or bipole operation with a switch.

The least desirable option for movie surround sound is to have direct-radiating speakers in the ceiling. This type of speaker has one set of drivers, making it relatively easy to localize where the sound is coming from. The surround channels are normally mixed with the expectation that they will be played on the sides of the room, not from above, so hearing them from above is not ideal.

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