Dear Big Picture Big Sound,
I have an Onkyo TX-NR747 receiver which supports Dolby Atmos. On the back is a pair of speakers connections labeled "back surround/height." But if I use that for my back surround speakers, for 7.1 then where will the two height speakers go? Do I splice them into the same connection as the back surround speakers? The receiver is supposed to be 7.2 channels. I have two front channels, one center, two side surround and 2 back surrounds. So where do the two speakers for Dolby Atmos go?
The Onkyo TX-NR747 receiver supports seven main speakers plus one or two subwoofers (hence the "7.2" designation: 7 main speaker channels plus 2 subs). This means you can either do a traditional 7.1 or 7.2 surround system with side and rear surround speakers or you can do a five channel surround system (plus one or two subs) plus two height speakers. This is what Dolby would call a "5.1.2" Dolby Atmos system: five main speakers, one subwoofer, plus two height speakers.
Unfortunately your receiver does not support a 7.1.2 system which would include the seven regular surround speakers, a subwoofer plus two height speakers. There are receivers that support this, but they are a bit more expensive than that particular Onkyo receiver. It would not be a good idea to splice in height speakers with the rear speakers because then you'd be getting the rear channel sounds from the top speakers (or height information fom the rears) which would not sound very good. Also, depending on how you wired them, it could put additional strain on the amplifiers.
If you decide to go with the height speakers instead of the rear surround speakers, then be sure to specify this in the receiver's speaker set-up menu.
In my opinion, a 5.1.2 immersive sound system (five main channels plus a sub and two height speakers) is preferable to a traditional 7-channel surround system, in most rooms, particularly if you are watching Dolby Atmos-encoded content. Having that discrete height information coming from above really helps make the movie or music sound more immersive and enveloping.
If you do go with an Atmos speaker set-up using this receiver, then you should probably move the current "side" speakers to the back wall or somewhere behind the main listening position as these will be your main surround channels. For a 5.1.2 Atmos system, Dolby recommends putting the height speakers on the ceiling a bit forward of the main listening position, but you can also put them on the front wall close to the ceiling pointing down toward the listener and get decent results. You can also go with Dolby Atmos "height modules" which sit above your main left and right speakers and bounce sound off the ceiling for the height effects.
Good luck and enjoy your immersive audio!