Is an identical center ALWAYS the best option? - AVS Forum
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post #1 of 24 Old 02-27-2011, 09:13 AM - Thread Starter
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I've seen mentioned multiple times that a center channel that is the same as your left and right mains is the best way to go if you can. The rationale given is that the center is often a compromise to make the drivers, sometimes with a smaller mid, fit in a horizontal array using an MTM setup. However I'm wondering if there are situations where a separate center channel is a better option (for reasons other than cosmetics).

I'm thinking of reasons like off axis response, horizontal dispersion, etc. One of my main viewing positions is outside of the left speaker, so I'm not always in the sweet spot.

My question is more of a general one, but for those that want specifics, I have NHT Classic 3s as mains, and can fit a third as my center. The Classic 3 Center is not the typical MTM setup though and is a very nice center from what I've heard. Drivers and sensitivity are essentially the same - although the bookshelf is rated a tad lower in FR.

Thanks
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post #2 of 24 Old 02-27-2011, 09:16 AM
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3 the same is best way to go.

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post #3 of 24 Old 02-27-2011, 09:29 AM
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Quote:
Originally Posted by EJD View Post

I'm thinking of reasons like off axis response, horizontal dispersion, etc. One of my main viewing positions is outside of the left speaker, so I'm not always in the sweet spot.

That is one of the important reasons for having an identical vertical speaker in the center.

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My question is more of a general one, but for those that want specifics, I have NHT Classic 3s as mains, and can fit a third as my center. The Classic 3 Center is not the typical MTM setup though and is a very nice center from what I've heard. Drivers and sensitivity are essentially the same - although the bookshelf is rated a tad lower in FR.

That is one of the properly designed horizontal 3ways with the mid and treble positioned vertically and with the spacing between the two woofers (and the mid) close, compared to the 800Hz crossover. It would be an excellent choice if you cannot accommodate another vertical speaker in the center. An identical center is ALWAYS the best option.

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post #4 of 24 Old 02-27-2011, 06:24 PM
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For seamless imaging, you can't beat 3-of-the-same.

That said, I personally prefer a center that's slightly stronger/louder than the mains, so I can hear voices over music/sounds. Of course, this can be done with speaker level adjustments, but I've also done it by upgrading to a larger/clearer center.

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post #5 of 24 Old 02-27-2011, 06:42 PM
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Quote:
Originally Posted by Florida_Gator View Post
For seamless imaging, you can't beat 3-of-the-same.

That said, I personally prefer a center that's slightly stronger/louder than the mains, so I can hear voices over music/sounds. Of course, this can be done with speaker level adjustments, but I've also done it by upgrading to a larger/clearer center.
Sorry but that makes no sense to me. If it is louder (by adjustment), you are compensating for a personal effect. OTOH, larger will make no difference, unless it is egregiously and disproportionately larger.

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post #6 of 24 Old 02-27-2011, 09:31 PM
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Here are some articles from couple of sources about various center channel designs as well as pros and cons of different center desgins/placement

http://forum.blu-ray.com/speakers/89...d-avoided.html

http://www.audioholics.com/education...peaker-designs


http://www.audioholics.com/education...hannel-speaker
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post #7 of 24 Old 02-28-2011, 04:05 AM
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Quote:
Originally Posted by avsnoob10 View Post

Here are some articles from couple of sources about various center channel designs as well as pros and cons of different center desgins/placement

http://forum.blu-ray.com/speakers/89...d-avoided.html

http://www.audioholics.com/education...peaker-designs


http://www.audioholics.com/education...hannel-speaker

Thanks for the interesting links.

One question I have, with regards to the center channel "sweet spot" is how narrow is the sweet spot angle, really? One of the articles mentioned the negative effect "as little as 30 degrees off the primary listening position..." Realistically, how many seats in your home theater are a full 30 degrees off axis? For me, with my couch ten feet from the center channel, the left and right couch cushions (3 feet off center) are 16.7 degrees off axis. I just can't imagine that there are many people who actually sit "as little as" 30 degrees off center, and expect no negative effect on the sound.

Much more noticeable, I think, is the difference in proximity to the left and right rear channel speakers, when sitting in my left or right couch seats.

Although it was surprising to hear that the driver configuration in the center channel would have a negative effect. It just hadn't crossed my mind before, so thank you for the articles.
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post #8 of 24 Old 02-28-2011, 04:26 AM
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I had a wall mounted plasma with 2 towers and a horizontal center. I also did some reading on front stage and tried the 3 towers across the front. To accomplish that I put up a ceiling mount for my plasma. I have had 3 identical towers now (albeit 2 companies) for around 2 years. To my ears and my families 3 identical towers sound a lot better. The front stage is "Seamless" panning across the front.

"We can complain because rose bushes have thorns or rejoice because thorn bushes have roses". - Abraham Lincoln
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post #9 of 24 Old 02-28-2011, 04:47 AM
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Center channel horizontal MTM lobing is overstated in those articles. Frequency and space between the drivers are major parameters of lobing; for this reason most small MTM either eliminate lobing (e.g. Energy cc-5) or limited to frequencies near the M/T crossover. However, both the right and left main speakers are sufficiently far away from the center channel to cause lobing at most of their overlapping frequencies above 500 Hz. If your listening area is against a back wall (as are most) there is lobing caused by the reflection of the sound from that wall, as well as reflections from the ceiling, side walls, and the wall behind the main speakers. And now, for the coup de grace--if your front speakers are all in a straight line (as are most) there is significant lobing (even for those in the center sweet spot) caused by the fact that the right and left speakers are further away from the listener than the center speaker. The majority of sound heard in a typical room is reflected – – causing lobing. This is why none of the lobing articles made tests in a typical environment with all five speakers playing at the same time. If they had, they would have found that MTM center channel lobing is overwhelmed and functionally neutralized by the dozens of other lobe producing speaker/reflection sources.
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post #10 of 24 Old 02-28-2011, 04:52 AM
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I have Ascend CMT 340's for L/C/R. The center speaker is the "SEC" version which has some mods to address being placed as a center speaker. Although the center is designed to be used horizontally, I've also tested it vertically. I've found no difference in sound in our primary seating positions. Our normal seats are no more than 15 degrees off axis max. We do have a love seat for extra seating that's closer to 30 degrees off axis. There's a small difference there, with a slightly diminished sound when the speaker is horzontal.

I'm thinking that it would be rare for people to sit more than 30 degrees off axis as much for the image as for the sound, therefore in most instances the problems with MTM centers would normally not come into play. Get a good quality MTM center and you should be good to go.

BTW, the Ascends are amazing. I recently upgraded to a receiver with multi EQ Audyssey and the difference is significant.
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post #11 of 24 Old 02-28-2011, 07:14 AM
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Quote:
Originally Posted by Kal Rubinson View Post

Sorry but that makes no sense to me. If it is louder (by adjustment), you are compensating for a personal effect. OTOH, larger will make no difference, unless it is egregiously and disproportionately larger.

Well, in my case, I replaced a Polk CSR (dual 5.25" drivers + .75" tweeter) with a Polk CS20 (dual 6.5" drivers + 1" tweeter). This provided stronger/clearer voices, even after lowering the center channel level a couple notches (from +4 to +2 db's; mains are at 0). The CS20 matches my mains well (which are also Polks with dual 6.5" drivers).

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post #12 of 24 Old 02-28-2011, 07:36 AM
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Quote:
Originally Posted by Florida_Gator View Post

Well, in my case, I replaced a Polk CSR (dual 5.25" drivers + .75" tweeter) with a Polk CS20 (dual 6.5" drivers + 1" tweeter). This provided stronger/clearer voices, even after lowering the center channel level a couple notches (from +4 to +2 db's; mains are at 0). The CS20 matches my mains well (which are also Polks with dual 6.5" drivers).

That's not really upgrade over your mains then, you essentially matched drivers same as your mains with CS20 upgrade and that is the general advice. Identical fronts, if not horizontal center but still make sure you have same drivers and ideally in WTMW over MTM.
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post #13 of 24 Old 02-28-2011, 10:56 AM
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Quote:
Originally Posted by Florida_Gator View Post

Well, in my case, I replaced a Polk CSR (dual 5.25" drivers + .75" tweeter) with a Polk CS20 (dual 6.5" drivers + 1" tweeter). This provided stronger/clearer voices, even after lowering the center channel level a couple notches (from +4 to +2 db's; mains are at 0). The CS20 matches my mains well (which are also Polks with dual 6.5" drivers).

Although it pains me to disagree with a fellow Gator, given the same quality, smaller MTM center channel speakers are better than larger ones, because the Ms are closer together – – thus minimizing lobing (which as I noted above, is overstated, but if it can be minimized, it should be). The Energy cc-5 (MTM) eliminate lobing entirely because of their staggered crossover frequencies and how close the Ms are to each other. I have an Energy RC mini center MTM with 4 1/2 inch Ms. Audyssey terminates them at 80 Hz. However, at 80 Hz, bass is omnidirectional, and provided seamlessly by my Energy RC–50 mains. If the bigger Polks sound different from the smaller center channel, then there has to be a quality difference in the drivers.
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post #14 of 24 Old 02-28-2011, 11:11 AM
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Quote:
Originally Posted by Florida_Gator View Post

Well, in my case, I replaced a Polk CSR (dual 5.25" drivers + .75" tweeter) with a Polk CS20 (dual 6.5" drivers + 1" tweeter). This provided stronger/clearer voices, even after lowering the center channel level a couple notches (from +4 to +2 db's; mains are at 0). The CS20 matches my mains well (which are also Polks with dual 6.5" drivers).

I didn't look up your speakers, but it sounds like the new center is more than 2 dB more efficient than the speaker it replaced, so that it is in fact louder than the one it replaced with the volume control at the same place. An effect we can achieve by raising the center channel level without replacing speakers. I have no opinion, though, on how hte new speaker compares in technical proficiency (is is it inherently clearer) or match with your L and R speakers.
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post #15 of 24 Old 02-28-2011, 11:46 AM
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Quote:
Originally Posted by pmalter0 View Post

Although it pains me to disagree with a fellow Gator, given the same quality, smaller MTM center channel speakers are better than larger ones, because the Ms are closer together - - thus minimizing lobing (which as I noted above, is overstated, but if it can be minimized, it should be). The Energy cc-5 (MTM) eliminate lobing entirely because of their staggered crossover frequencies and how close the Ms are to each other. I have an Energy RC mini center MTM with 4 1/2 inch Ms. Audyssey terminates them at 80 Hz. However, at 80 Hz, bass is omnidirectional, and provided seamlessly by my Energy RC-50 mains. If the bigger Polks sound different from the smaller center channel, then there has to be a quality difference in the drivers.

pmalter0: How about number of drivers and their sizes, if one doesn't have space for 3 identical towers or monitor/bookshelf type speakers, matching or having exact same number of drivers in MTM center would be better overall? For example Monitor Audio RX6 and RX Centre, both have exact same drivers (number of drivers and their sizes/type/material of course cross overs would be different) but the Centre is a two and a half way MTM design, both mid/bass are pretty close or at least no spacing between left mid/bass and tweet and then tweet and right mid/bass...
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post #16 of 24 Old 02-28-2011, 12:11 PM
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Quote:
Originally Posted by avsnoob10 View Post

pmalter0: How about number of drivers and their sizes, if one doesn't have space for 3 identical towers or monitor/bookshelf type speakers, matching or having exact same number of drivers in MTM center would be better overall? For example Monitor Audio RX6 and RX Centre, both have exact same drivers (number of drivers and their sizes/type/material of course cross overs would be different) but the Centre is a two and a half way MTM design, both mid/bass are pretty close or at least no spacing between left mid/bass and tweet and then tweet and right mid/bass...

If the center of two speakers is less than one wavelength apart, then those two speakers will act as a "coherent source", and will not have interfering/reinforcing
wave interactions. For example, if the centers of your two midranges are 9 inches apart and the lower crossover is at 1400 Hz, all the speakers in your center channel will act as a coherent source and not produce lobing (other than all the other lobings caused by other factors I discussed earlier). But even if they are not close enough to constitute a coherent source, the fact that they are close to one another means that lobing will not become significant until you are 20 or more degrees off center; and even then the lobing will likely be subsumed in all the other lobes created by the dozens of reflections and left/right main speaker lobes (which, as I discussed before, will affect all listeners if the left/right mains are not "arced" (same distance from the center listener as the center channel)).
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post #17 of 24 Old 02-28-2011, 02:19 PM
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Don't know nuthin' about "lobing", but I do know my Polk CS20 (with less power) sounds better then my Polk CSR (which is the "matching" center for my mains).

Better speaker? Yes. Larger drivers? Yes. Louder/fuller/clearer with less power being sent to it? Yes. Lobing? Beats the sh!t out of me!

The CSR was (is) a decent speaker, and it matched well. However, I guess what I'm saying is, I wanted a little MORE out of my center speaker. While the CSR is supposedly "sound/timbre matched" with my mains, overall, the CS20 sounds better (probably because of the larger drivers). I should point out though, that it's not just louder; it's also deeper/richer/fuller/clearer (again: it's a better overall speaker -- from a higher-level Polk line).

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post #18 of 24 Old 02-28-2011, 03:21 PM
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Quote:
Originally Posted by pmalter0 View Post

If the center of two speakers is less than one wavelength apart, then those two speakers will act as a "coherent source", and will not have interfering/reinforcing
wave interactions. For example, if the centers of your two midranges are 9 inches apart and the lower crossover is at 1400 Hz, all the speakers in your center channel will act as a coherent source and not produce lobing (other than all the other lobings caused by other factors I discussed earlier). But even if they are not close enough to constitute a coherent source, the fact that they are close to one another means that lobing will not become significant until you are 20 or more degrees off center; and even then the lobing will likely be subsumed in all the other lobes created by the dozens of reflections and left/right main speaker lobes (which, as I discussed before, will affect all listeners if the left/right mains are not "arced" (same distance from the center listener as the center channel)).

This phenomenon is not "all-or-none" but graded. I have always regarded the criterion for sufficiently benign interference to be a lateral displacement of less than 1/2 the wavelength of the common frequencies given that the crossover frequency is used and that is only the beginning the roll-off at whatever the order of the crossover is. So, for a 9inch separation (center-to-center), 1400Hz is way too high to be benign.

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post #19 of 24 Old 02-28-2011, 05:48 PM
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How about 3 MTM Center speakers mounted vertically? Worked out well for me:



I have a new audio rack and better wire routing, so I will update this picture. I really recommend people try wall mounting their flat screen and pedestal mounting vertical center with a slight back-tilt to get the woofer/tweeter dispersion to ear level. I think it sounds much better.
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post #20 of 24 Old 03-01-2011, 05:21 AM
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Quote:
Originally Posted by Kal Rubinson View Post

This phenomenon is not "all-or-none" but graded. I have always regarded the criterion for sufficiently benign interference to be a lateral displacement of less than 1/2 the wavelength of the common frequencies given that the crossover frequency is used and that is only the beginning the roll-off at whatever the order of the crossover is. So, for a 9inch separation (center-to-center), 1400Hz is way too high to be benign.

I am glad you are on this thread, I have wanted to have this discussion with someone of your credentials. I am aware of the one half wavelength rule. However, that rule assumes a point source located in the center of the speaker; that may work for a non-pistonic paper cone speaker; but I don't believe it applies for today's pistonic speakers. For example, let us assume that we have a pistonic 6 inch cone speaker mounted on a 12 inch baffle; assume further that we have 13 microphones(placed 1 inch apart) in a line across the center of the speaker 1 inch from the baffle board, numbered one through 13, so that microphone number seven is located directly above the center of the speaker. If we initiate a sound pulse, the point source rule assumes that the sound will reach microphone number seven first followed by six and eight, five and nine, etc.; and this would be true for a non-pistonic speaker. However, with a 6 inch pistonic cone speaker(concave) the sound pulse would arrive first at the microphones closest to the cone – microphones four and ten.

That the one half wavelength rule doesn't apply to pistonic speakers has been confirmed by Energy speakers. Energy has been on top of the lobing issue for quite awhile now; their RC–LCR speaker was the perfectly designed horizontal anti-lobing speaker. However, recently they discontinued the LCR, and adopted their "coherent source" tweeter/midrange modules across their entire line. If you look at their coherent source modules you will see that the center of the midranges and tweeters are located within one wavelength of each other at their crossover points, not one half wavelength. Hence, I backed up my theory with Energy's expertise, technology, and (most importantly) money.

Phil
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post #21 of 24 Old 03-01-2011, 06:01 AM
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Quote:
Originally Posted by pmalter0 View Post

I am glad you are on this thread, I have wanted to have this discussion with someone of your credentials. I am aware of the one half wavelength rule. However, that rule assumes a point source located in the center of the speaker; that may work for a non-pistonic paper cone speaker; but I don't believe it applies for today's pistonic speakers. For example, let us assume that we have a pistonic 6 inch cone speaker mounted on a 12 inch baffle; assume further that we have 13 microphones(placed 1 inch apart) in a line across the center of the speaker 1 inch from the baffle board, numbered one through 13, so that microphone number seven is located directly above the center of the speaker. If we initiate a sound pulse, the point source rule assumes that the sound will reach microphone number seven first followed by six and eight, five and nine, etc.; and this would be true for a non-pistonic speaker. However, with a 6 inch pistonic cone speaker(concave) the sound pulse would arrive first at the microphones closest to the cone - microphones four and ten.

That the one half wavelength rule doesn't apply to pistonic speakers has been confirmed by Energy speakers. Energy has been on top of the lobing issue for quite awhile now; their RC-LCR speaker was the perfectly designed horizontal anti-lobing speaker. However, recently they discontinued the LCR, and adopted their "coherent source" tweeter/midrange modules across their entire line. If you look at their coherent source modules you will see that the center of the midranges and tweeters are located within one wavelength of each other at their crossover points, not one half wavelength. Hence, I backed up my theory with Energy's expertise, technology, and (most importantly) money.

Phil

Actually, there are two rules involved here: 1) the one half wavelength rule, and 2) the point source rule. My first thought was to state the issue in terms of the wavelength rule; but it is really better and more accurate to state it in terms of the point source rule - that sound emanates from a point source in the center of the speaker. As my discussion above demonstrates, this is true for a non-pistonic paper speaker, not true for a pistonic speaker. The issue is more accurately stated as a timing issue: how far apart in time do the sound waves from the two speakers reached the listener's ears. The point source rule assumes the sound pulse reaches the edge of the speaker cone sometime after the center of the cone; as we have seen above, with a pistonic speaker, that is not true. Hence, you can apply the one half wavelength rule, if you don't apply the point source rule. I have stated the issue in terms of one wavelength from the center of the speakers; it is probably more accurate to state it as one half wavelength from the edge of the speakers; the two are very similar in practical application. I used what I thought was the simpler explanation, but now with this detailed explanation it is better to use the one half wavelength from the edge of the speaker calculation.
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post #22 of 24 Old 03-01-2011, 06:34 AM
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I concur with your analysis but it ignores the major point I was making about crossover slopes. If the slopes are of the usual 2nd order configuration, there is plenty of output from each driver only an octave down and that is why I use the 1/2 wavelength criterion, from the edge or, even, center-to-center, as a "rule of thumb." I say "rule of thumb" because I am addressing potential purchasers/users and there are other variables (such as how pistonic the cone really is) that cannot be assessed outside of a test lab.

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post #23 of 24 Old 03-01-2011, 07:19 AM
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Quote:
Originally Posted by Kal Rubinson View Post

I concur with your analysis but it ignores the major point I was making about crossover slopes. If the slopes are of the usual 2nd order configuration, there is plenty of output from each driver only an octave down and that is why I use the 1/2 wavelength criterion, from the edge or, even, center-to-center, as a "rule of thumb." I say "rule of thumb" because I am addressing potential purchasers/users and there are other variables (such as how pistonic the cone really is) that cannot be assessed outside of a test lab.

Well played, sir. Let me limit my conclusions therefore, to the following: if one is using Energy (or equivalent pistonics) speakers, and Energy (or equivalent slope) crossovers, you can consider the speakers to constitute a "coherent source" which will not produce lobing at any offset angles, if the speakers edges are within one half wavelength of each other (or centers are within one wavelength) ;-)
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post #24 of 24 Old 03-01-2011, 06:51 PM
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Quote:
Originally Posted by pmalter0 View Post

Well played, sir. Let me limit my conclusions therefore, to the following: if one is using Energy (or equivalent pistonics) speakers, and Energy (or equivalent slope) crossovers, you can consider the speakers to constitute a "coherent source" which will not produce lobing at any offset angles, if the speakers edges are within one half wavelength of each other (or centers are within one wavelength) ;-)

Point, set and match.

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