Originally Posted by DarqueKnight
The sound of my HT system is accurate enough that... What suggestions can you offer for improving sound performance.. (that) would be generally applicable to the issues discussed by the OP.
You got it right that a CC must keep up with L/R speakers in sensitivity and power handling, but your approach belies a partial understanding HT requirements. Every time a sound is emited by 2 sources, the waves will interfere. This interference will be in the form of peaks and nulls that vary with location in space. Whenever the two sources are an integer number of wavelengths apart, they will add. Whenever a half-integer apart (0.5, 1.5, 2.5, etc.), they will subract. If the signals are of equal intensity, the peaks/nulls can be -60dB deep.
You've chosen a CC that's not designed to be a CC (per Polk), and then used two of them, side-by-side. I will wager that you gave "size" a very high priority as the only thing I like about your set-up is the looks. Too bad it's not a painting...
The key issues are:
- your LSi9's are a vertical MTM, with wide mid spacing and an assumed high crossover frequency (Polk doesn't say).
- MTMs have an advantage in sound field uniformity and reduce room interaction when used vertically.
- Unless designed otherwise, "reduced room interaction" when vertical becomes "deep nulls a little off axis" horizontally
- this only occurs above a minimum frequency determined by driver spacing, up through the crossover to the tweeter.
But, since you have two toppled MTMs, you get this problem at all frequencies, varying only in the scale on which peaks and nulls occur. To really understand, read this article:
and follow both of the links to prior articles, showing the actual data. Also note that their "raised tweeter compromise" discussion ignores the critical variable of crossover frequency. This is likely what your CC's individual FR looks like:
Cheap MTM as CC.jpg 74k .jpg file
Hoppefully, this gets your attention. Focus on the deep off-axis nulls around 1KHz (crossover's a 2.4K, so what's above 2.4KHz is the tweeter... a second disaster here.) That's if you only has 1 of these as your CC. Duals won't improve things. Still happy?
Here's the polar response of my sideways MTM CC:
ModulaIV2PLR1SS.jpg 76k .jpg file
Before you comment on the strong, tri-lobe pattern, read the key - it's at 15KHz. Every dome tweeter does this at high frequency. The other blue line is 1KHz, very close to the 1.1KHz crossover frequecy. Combined with a 9" driver spacing, lobing isn't there, just a slight off-axis dip. 90dB sensitivity (I use the in-wall design, as should you) with 120W nominal power handling makes for a very respectable center channel.
Please understand that 5 years ago, I had a little Polk MTM CC like the bad example above, an "upgrade" from using the TV's built-in speakers (yes, 2 speakers, with all those wonderful interference effects intact). Better, but it didn't sound good, and I like making things, so I entered DIY. Next was a toppled MTM, but a big enough one to keep up with a pair of 10's. I learned there that timbre had several dimension, the most obvious being tonal balance - I used a full baffle step conmpensation design intended for freestanding use, and it was boomy. A different crossover fixed that (no-BSC or "in-wall" design), allowing meaningful timbre comparisons.
Fast forward a year and it's now an full LCR of MTMs, center still sideways, and on-axis, I don't notice anything change as object pan across the field, or with pink noise. But it's still harder to understand dialog than I'd like, so (after more years pass) I spring for the proper design-intent sideways MTM, at a significantly greater cost due to use of a very special tweeter that is both a) small enough to permit a C-C spacing of 9" and b) crosses cleanly at 1.1KHz (Fs below 500Hz)
My wife noticed the improvement. It's one of the few positive comments she's ever had about speakers... dialog clarity is immensly better. With common mid drivers, the majority of timbre hasn't changed, and since it's all voiced by the same designer, overall voicing is very compatible with the vertical MTM's.
... and it fits on the shelf. We all have external requirements to sound quality!