Ask S&V

There is a situation in home theater where you can find dipole speakers useful. In a room large enough to get 6 feet or more of distance from any speaker to the closest seat, I would suggest using direct radiating speakers. But in rooms where you have a listening seat that is 5 feet or less to any given speaker, a dipole speaker can deliver the correct "amount" of sound, but it will not be too loud for the nearest seat as it would be with a direct radiating speaker. This can be useful in the side surround position in narrow rooms. In "short" rooms that have limited depth and little space behind 1 row of seats, dipole speakers for rear surrounds can be useful for, again, delivering the correct "amount" of sound into the room without that sound being loud because it is so close to the listener. Most height channels should be far enough from listeners that direct-radiating speakers will usually be the best choice, but a situation like a basement system where the finished ceiling is 7 feet or even a little less... you may find direct radiating speakers too easy to localize while dipoles would spread the sound out enough that they would "disappear" into the room better. So, to recap---if you have 6 feet or more from any given speaker to listening seats you care about delivering balanced sound, direct radiating speakers are the best choice. At speaker positions that have to be 5 feet or closer to the nearest listener who you wish to NOT have the sound from that channel seem to be too close or too loud, using dipoles (left and right, even if only 1 side is 5 feet or closer) can provide a better balanced sound. If you only care about your main seat having the best sound... let the measurements from your ears in that seat to each channel be your guide to whether you might get a better experience with dipoles in some channels, and direct radiators in most channels.

Are ML Electrostatic speakers considered dipoles, and if so would it help to place some kind of sound reflective material directly behind the electrostatic panels?

It is important to understand, as Al has explained, how and why dipoles were recommended to be used in tn the early days of surround sound. Basically with a dipole, you get more or less complete cancellation to the direct side of the speaker. When oriented so that the side of the speaker is pointed at the listener the listener hears no direct sound from the speaker and just hears the indirect sound that that comes from the front and rear of the speaker into the room. In effect the listener hears all ambiance.Later surround technologies such as Dolby Digital and Dolby Atmos want very specific information contained in each channel directed and heard directly by the listener. If you were to orient dipole speakers with one of the driver arrays pointed directly at the listener, you would be getting, more or less, direct information, minus the bass that would be cancelled. When using a dipole like a Martin Logan, with the front of the speaker pointed at the listener it will be providing direct information and will work just fine. The sound coming out the rear will provide a small amount of added ambiance, but this will basically not interfere with things. Again, when a dipole is oriented with the side facing the listener, you end up hearing basically all ambiance and no direct info. A bipolar speaker, unlike a dipole, has the front and rear drivers in phase with each other, rather than out of phase. It is in someways similar to an omnidirectional speaker with some drop off to the sides but not full cancellation as you get with a dipole. Bipolar speakers normally would be oriented with the front drivers pointed at the listener and work fine as surround speakers in all surround setups.

Also, many dipole surround speakers are sold with a mode-switch. You can setup these surrounds as dipole, bipole, or direct radiating. This way, you could use direct radiating mode when nobody is close to one of these dipole surrounds, and only change it to dipole mode if the theater is "full" and someone is sitting close to one or more channels. But if the room is large enough... direct radiating is the way to go for most systems.