Eminent Technology LFT-8c Robert Greene review

@ozzy62 Sorry to hear about your bass issues. I hope you can work things out. In the event you can't and need to move on, where are you located? Send me a PM if you like. I know someone that may be interested if your anywhere, regionally close.  

As for my midrange panels on the inside setup..even today (after listening for 6 hours yesterday) they still sound new and different. Big improvement. Of course, in my head, you seldom get anything for free. An improvement in one area often leads to a decrease in some other area. This is sort of a cardinal rule of optimization, regardless of what is being optimized. With the tweeter panels now on the outside I'm playing with increased toe-in. Originally (mids on the outside) I was using maybe 15 degrees of toe-in. I've now rotated the rear spike(I have the Sound Anchor three-spike stands) in two adjustments of 1.5 inches each around the front-inside spike. I can hear a difference. I think I'm going to settle in on the 3 inch rotation and see how things play out. 

I'm typically fairly skeptical about the accuracy of A-I on many topics as it often authoritatively makes stuff up. That said, I ran this mid panel outside vs inside and toe in question through A-I and it indicated the mids inside is often the preferred setup for most people. It described the resulting changes one would expect to hear pretty much spot-on with what I'm hearing. When I asked about the degree of toe-in to try it gave me suggestions (and results to expect) that ranged from the speaker axis crossing over behind me to crossing at my head to crossing 1-3 feet in front of me.  The room size wasn't specified, but directly on axis or crossing in front of me would be extreme toe-in for my room. I've tried on-axis in the past and didn't care for it. A-I suggested, in a sufficiently sized room (with plenty of set-back to the listening position) that the crossover 1-3 feet in front of me might be optimal. I have a hard time believing that, but it isn't an option for me anyway, so I'll let that go. 

As @douglas_schroeder mentions above, he was one of the first to do a deep look into the ET LFT-8 (both a and b versions). And I completely agree with his assertion that the LFT-8 is preferable to many of the current Maggies, and is as I have many times opined the greatest loudspeaker value (imo) on the market. While I haven’t heard the newer Vandersteens and therefore can’t comment on them versus the LFT-8, I prefer dipole planars over all non-planars. By the way, it was in the showroom of Brooks Berdan that I was introduced to the LFT-8. He carried the speaker as his choice of a magnetic-planar alternative to Vandersteens and Quads, both of which he also sold. 

Douglas’ post also includes mention of a very important issue: the problem of the comb-filtering inherent in the design of the speaker. For some reason Bruce Thigpen chose to run the midrange LFT driver all the way from 180Hz (great!) up to 10kHz, crossing over to the tweeter at that frequency via both high-pass and low-pass 1st-order filters in the crossover. The wavelength of 10kHz is only 1.35 inches. While the tweeter is mounted as close as possible to the LFT driver, small changes in head position in relation to the orientation of the LFT driver and tweeter can result in phase cancellation centered at (but not limited to) 10kHz, resulting in a deep null.

Most loudspeakers with a separate tweeter use a crossover frequency in the neighborhood of 2kHz, where the wavelength is 6.75 inches. That crossover frequency allows the speaker drivers to remain in phase to a far greater degree than does 10kHz, even with changes in listening position relative to the speaker. The greatest challenge with the LFT-8 to me is aligning the midrange and tweeter drivers so as to minimize comb-filtering (and the resulting phase-related frequency response irregularities), accomplished via toe-in. The speaker really is a single-listener design.

@bdp24 

the problem of the comb-filtering inherent in the design of the speaker. For some reason Bruce Thigpen chose to run the midrange LFT driver all the way from 180Hz (great!) up to 10kHz, crossing over to the tweeter at that frequency via both high-pass and low-pass 1st-order filters in the crossover. The wavelength of 10kHz is only 1.35 inches. While the tweeter is mounted as close as possible to the LFT driver, small changes in head position in relation to the orientation of the LFT driver and tweeter can result in phase cancellation centered at (but not limited to) 10kHz, resulting in a deep null.

What would this ‘deep null’ sound like, or rather what would be missing from the sound in this frequency area? And the way to compensate for this  comb-filtering effect is through toe-ins. This would better time align the midrange + tweeter to to listener. Am I understanding this correctly?

I asked because I have only use the speakers in a straight firing arrangement throughout. The thinking with my 11’ wide room is to place them as close to the side walls as possible, i.e. 8”. Would you recommend that I move the speakers further away from the side walls before I experiment with toe-ins. With the close proximity to side walls created other unwanted back wave interactions?

@ledoux1238 

There is a fair amount of misunderstanding about at what frequencies the various parts of music reside. 10kHz is WAY above what we hear as the upper overtones of most musical instruments and vocals, the highest overtones of brass cymbals being one exception. 10kHz is where we hear things like the "air" audiophiles refer to. So the 10kHz crossover frequency of the LFT-8 is not THAT destructive to the sound of music itself.

However, the phase cancellation of the LFT driver and tweeter relationship creates another problem: as does the width of the LFT (Linear Field Transducer) driver. With that width, the LFT driver starts beaming at a relatively low frequency, so the speaker’s total output into the room is different from what it’s on axis frequency response is. A Google search for an explanation of driver size to resulting beaming characteristics will explain it all.

Because the LFT driver’s radiation characteristics are so "beamy", the LFT-8 may be placed as close to side walls as any speaker on the market. All dipoles create a null to either side of the speaker, the result of the forward (positive) and rearward (negative) sound waves "wrapping" around the sides of the planar panel, where they meet and cancel (+1 plus -1 = 0). As the tweeter plays only the very highest frequencies (10kHz is barely audible to many musicians, what with their hearing loss), it’s output reflected off the side walls is not of great consequence.

To answer your question, yes, toe-in is used to minimize the comb filtering characteristics inherent in the LFT-8. Use a test CD with various frequency tones (Stereophile made a bunch of them), and rotate the LFT-8 to achieve maximum output of a 10kHz tone at your usual listening location. That orientation is where the LFT driver and tweeter are most in phase with each other. Remember when I said the LFT-8 is a single-listener design? 

It is because of the beaming characteristics of the LFT-8 that I suggest diffusion on the wall behind the speakers, rather than absorption. However, I believe that beaming is also at least partially responsible for the LFT-8 being perceived as sounding more "direct" that most others. It interacts less with the room than any other I can think of, for better and worse. I feel that way about all dipoles, and even the Rythmik Audio/GR Research OB/Dipole Subwoofer. The reason that sub---unlike "normal" subs---can be used with the LFT-8 is that it works up to the very high crossover frequency of 300Hz. The LFT-8 crosses over from the woofer to the LFT panel at 180Hz.

I don't remember where I came across this, but it useful in understanding various instruments and voices with respect to frequencies they deliver. Note the legend in lower right, fundamentals, harmonics..etc.

https://homestudiotutor.com/dfchart/