Originally Posted by farok
I help run the sound system at my church and we are looking to move away from the portable speaker cabinets on tripods such as what a DJ typically uses and put some speakers in the ceiling instead. There don't seem to be many topics about this sort of configuration, but the members here seem to have the knowledge on the subject, so here goes. Anyway, the reasons are feedback from sound bouncing off the rear wall and going back up towards the stage, the hope for more balanced sound from anywhere in the room, and also in part for aesthetics. I've been looking through the forums for some time, but haven't been able to find the answers to a few questions I have and hope the folks here can help. Hope I got the post in the most appropriate forum.
As background, the room we are using is roughly 40' x 60', not including the stage, which is on one of the 60' walls (but which does not need speakers -- we have monitors for the stage already). The ceilings are 9' high, maybe slightly higher. The majority of what we send over the system is speech, but we occasionally have the need for CDs, vocal performances, etc. -- nothing terribly loud in any case, but should sound good. Today we run from the mixing board to a feedback removing and equalizer dbx box to a 240W amp (rated 120W per channel at 4 ohms), and then to each of the two large speaker cabinets at the front of the room (one per amp channel). Cost is also a factor in any changes, as we don't have much in the sound budget (you can guess by the size of the room we are a small congregation), but can save over time if needed since the current system does work. Also since the system is working, we're just in the planning stages -- nothing needs to be done in the near future.
I have the opportunity to get four pair of identical in-ceiling speakers for a very low cost. We borrowed and tested one pair in the room, one on each amp channel, and they sound pretty nice for our needs (we might need a subwoofer at some point). These are 8" speakers rated at 80W (160W max) at 8 ohms. If I am doing my math right, with that test setup we could have driven a max of 60W per speaker because of the impedance mismatch (120W at 4 ohms for the amp but 8 ohms at the speaker), right?
So, would these eight speakers, wired in two 4-speaker series-parallel groups (one for each amp channel, resulting in an 8 ohm load per channel) be able to be adequately powered by our current amplifier? My guess is no, as if I'm doing my math right, we would only be sending 15W to each speaker, four times less than when we ran our test with the single pair (and to get adequate volume during that test we were running the amp around 50% max). How large an amp should we look into if we need to upgrade?
Is there any benefit to running the two series-parallel groups in parallel on one channel to result in a 4 ohm load (and save the other amp channel for something else such as a subwoofer or remote speaker such as in the nursery)? From what I've read, this is harder for the amp to drive, but results in more power to the speakers, assuming the amp can drive the load (which our amp is rated to do).
Maybe I should have asked first, but does the set of eight speakers, mounted in the ceiling and positioned roughly equidistant throughout the room make sense? We're thinking two rows of speakers, about 10' apart, with about 10' between each speaker. This would leave about 15' to the walls on either side or in the rear and front, which should be enough to prevent sound from bouncing off the walls too badly or hitting the stage and causing feedback. Thoughts on this configuration?
Last, I feel that outputting mono is sufficient for this setup, as we rarely have a stereo signal for what we are dealing with, and even more rarely does it even make sense to output stereo. In today's setup, if we output stereo, only the middle of the room can tell, otherwise the speaker closest to the listener drowns out the other channel. On the other hand, one of the other sound guys really would like to have a stereo capability, and so has considered alternating left and right channel speakers throughout the configuration. Any opinions if this makes sense and if anyone feels there is any value in this configuration?
I think that's it for now. I appreciate any thoughts the members have.
Hope everyone has a great Christmas this week!