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The folly of a horizontal centre channel speaker - RP-250C measurements

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Dear all,

I good friend has built an amazing home theatre, actually its the best I've ever experienced.


It has an all Klipsch front end - he was sold after hearing my KG-5.5's:D, in ceiling speakers for side and surround and five subwoofers.




Pioneer receiver, Behringer DCX-2496 for sub optimisation and a pair of Crown XLS...




Two tapped horns for freight train levels of impact...




The weakest link however is the centre channel.  This is nothing new of course, others have written about this elsewhere.


Nothing against the quality of his nice RP-250C. When sitting on axis, i.e. right in front of the RP-250C the vocals are crystal clear. However when on his outboard seats, I personally find dialogue intelligibility goes off a cliff. 




I took the opportunity over the weekend to quantify the off axis performance of his front stage, including the matching RP-160M's to see what the measurements might say about why dialogue is so difficult to catch when not in the two money seats.


Using Omnimic I have a stand I've made to do measurements of bookshelf sized speakers. This sits on top a turntable with 10 degree markings for accurate off axis measurements....





The first thing we did was measure the impedance of the RP-250C. We use this to set the measurement level to approximate 1watt at 1 meter.


The enclosure has a sticker on the back that says 8ohms, I'm thinking Klipsch made a typo here?!!!…. :emotion-14:





The following measurements are all taken in room, at 1 meter, with 5ms gating at 2 volts with no smoothing.


First here is the complete 0 - 90 degree horizontal measurements....




Here is just the 0 - 40 degrees which to help clarify the problem....




What we see here is that the RP-250C is only able to perform well in the critical 1-2khz region either on axis (0 degrees) or out to 10 degrees off axis. Not much then. Once 20 - 40 degrees off axis we can see here the suck out that is occurring.


 If we take this same set of measurements and create a Dr Geddes style polar map we can see more clearly the problem. First here is an ideal polar map lifted from Dr Geddes white paper on controlled directivity...





Here is the RP-250C. I've added in the lines to show the extent or lack there of the sweet spot from 2khz all the way down to around 500hz....





What happens when we measure the vertical off axis response....





The irony of the poor horizontal performance is that the RP-250C has just about the best vertical off axis performance I've ever seen outside of a synergy horn....




Polar is stunning, I'd say solid performance across the board out to 40 degrees off axis....




Certainly the RP-250C looks better than the RP-160M when measured in the horizontal, which itself is no slouch, even if it is a bit missing from 1 - 2khz itself and just a bit hot above 10khz....




Traditional horizontal for the 160C...




And another impedance typo? 160C also rated as 8ohms!??? 😉




So the moral of the story here is that a traditional centre speaker with the horizontally laid out drivers is a really poor option for precisely its intended purpose - delivering crystal clear dialogue to all viewers.


Ideally a center channel speaker is a perfect match of your left and right, or at least a speaker with vertically arranged drivers. Three RP-160C across the front standing on end, certainly not lying flat as intended, would make an outstanding front LCR stage, provided you could keep your ears right in the sweet spot vertically!


Cheers :emotion-22:


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Thank you for the plots showing it.  This is precisely why if you want a wider sweet spot, you don't buy speakers are horizontally arrayed.  Shifts at crossover causing cancellation and other issues with the frequency response.  Much the same thing as an MTM when you are out of the flashlight positioning of the center driver, everything else collapses.

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7 hours ago, Maz4bz said:

The irony of the poor horizontal performance is that the RP-250C has just about the best vertical off axis performance I've ever seen outside of a synergy horn...

This observation of course brings up the question of why most companies don't produce an MEH series of horns (now that the patent has expired), or at least produce a four-woofer center (except a configuration having no transmission line back-end and instead having a center horn-loaded HF compression driver):




or even just a coaxial-loaded center:






This approach does seem to make a lot more sense from an in-room listening point of view, especially when considering home theater (HT) requirements for more than one listener, spread laterally as your example room shows.


It's always seemed odd to me that audio engineers are forced by their marketing organizations to produce less-than-logical designs and features on their loudspeaker offerings (such as bi-wiring and horizontal D'Appolito center loudspeaker designs). 


PWK used to educate his customers on proper in-room placement and why, for instance, horn-loaded bass is so superior to direct radiating, and three-channel stereo is superior to two-channel (i.e., avoiding the "fundamental flaw of stereo", the following from Toole):





Have you had experience listening to multiple-entry horns (MEH, a.k.a. Synergy™), especially being able to walk around the listening room without any difference in timbre or soundstage?



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I'm very familiar with Bill Waslo and his "small syns" thread, as well as his SynergyCalc spreadsheet to help determine wood sheet cutting layouts for DIY MEH designs.


I see people that will tolerate a large presented area of direct radiators in frontal view, but curiously, large-mouth horns of the same presented area--like a full-range MEH--are not considered acceptable by them. That's why I presented the four-woofer configuration as an example.  I personally wouldn't use such a configuration myself.  Instead, I'd use an MEH or folded horn with horn mouth size that can control polars down to ~200 Hz, because I've found that the directivity in the mid-bass region is extremely important for speech intelligibility, etc. and avoidance of modulation distortion "muddy" sound.



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Your DC resistance measurement is going through the crossover coils into two parallel 8 ohm woofers, so that looks about right.  The tweeter is 8 ohms, above the crossover, and the woofers are 4 ohms below the crossover.  It's a trick to keep the levels similar; tweeter is high efficiency horn, woofers get +3dB by having two of them, and another +3 dB by having half the impedance/drawing twice as much power.  Would be fun to see the impedance curve through a frequency sweep.

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no MTM here...co-axial tweeter in Zu Audio cube..makes an excellent center channel and as sensitive as the La Scalas....


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