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Another First Reflection Trap (FRT) Adventure : Quasi-Ambiophonics

I've been busy of late concentrating on another FRT configuration which is a pretty big improvement over those I've previously posted here about. The sketch below shows the concept. Sorry it's so busy, but there is a lot going on and I tried to capture the most relevant bits. Briefly: the FRTs prevent the left ear from hearing the right hand side tweeter, and vice versa by using parallax. As a result, it approaches ambiophonics in crosstalk cancellation of the critical high frequencies. It definitely is a one person, head-in-a-vise experience, but not only does it increase envelopment, it also makes the phantom center sound as clean as a mono signal from a single speaker, which is to me an important goal. In short, it is like having the absolute best features of headphone listening (the definition of crosstalk elimination) without the band-in-your-head phenomenon. Oh, it also has the effect of reducing the volume of the phantom center by up to 3 dB, which in the case of most recordings is probably a good thing. :) Best of all, you don't need to bring a mattress into the room, stand it up between speakers, and rest your head on its edge! I actually tried using wood to use MMGs in a textbook ambio setup a while back, but it failed miserably. My new setup, however, works! And no, no additional DSP, just FRTs. The FRTs also have the benefit of breaking up room nodes, making it a bit easier to EQ the room response "flat".

I use the Moody Blues' "Days of Future Passed" disk as a way of testing hall depth reproduction and detail retrieval. Specifically, the first track has some very low level harp and celeste glissandos in the first minute which I can clearly hear on my AKG 240 headphones but were inaudible on my Gunned MMGs. Up to now, I ascribed that to not having 1.x or 3.x Maggies, known for superior detailing. Well, guess what? Those glissandos are now clearly audible on my MMGs!

Here's the sketch of the layout:



Again, apologies for the busyness of the sketch. In order to clearly show the slight of hand in blocking crosstalk, I could only fit the left speaker details on the sheet. Maggies are ideally suited for this treatment, btw, since the tweeter is located on one edge. By placing the tweeters "in", a single 4 foot wide by 6 foot high panel can be placed so that, in nearfield (seated about 5 feet 6 inches away from the speakers, which are spaced a bit less than 7 feet apart) the left tweeter is blocked from being directly heard by the right ear yet the left ear has an unobstructed "view" of that tweeter. In addition, the rear portion of the FRT behind the speaker acts like all the other FRTs I've written about, blocking the first reflection off the front wall from reaching the listening position within the first 10 milliseconds. And by toeing in the MMGs as shown, side wall first reflections are eliminated as a concern. Yeah, this does nothing about rear wall early reflections, but my experience has been that if you manage the front and side early reflections, the sonic benefits are significant.

The presence of the FRTs changes the bass modes in the room; a massive suckout at about 80 Hz without them virtually vanishes, but a new one at 160 Hz pops in, although it isn't as deep. In my system, I'm using the Behringer DEQ2496 to "flatten" the response at the LP anyway, so the end result is a smooth response.

Here are some photos:



This shows the right MMG from the listening position (LP). Apologies for that blasted vertical shadow in the foreground. I hope you can tell that the entire grill cloth of the MMG is visible; this represents the view of the right eye (left eye closed) showing that the tweeter has an unobstructed path to the right ear.

Also note the cinder blocks raising the MMG, and the fact the MMG is shimmed to be almost perfectly vertical. I find a hint of tilt-back aids in fine-tuning the sound. YMMV, of course. Also note the other cinder block propping up the FRT to keep it from crashing down. :o



This pic represents the view of the left eye (right eye closed) from the LP. Note that a few inches of the left side of the grill cloth are hidden behind the FRT. This is where the tweeter is. Thus, the right hand speaker's tweeter is prevented from being directly heard by the left ear.



The LP :) Note the electronics easily assessable on the low table adjacent to the seat, and that the LP is about 36 inches from the rear wall. Also note the measurement microphone for flattening frequency response; I swing it into position centered at the rear cushion of the seat when in use. And no, I'm not a closet karaoke fan. :)



Another view of the cinder block propping up the FRT, and the Armaflex butyl (sic?) pipe insulator I use for edge diffraction effect elimination on the rear of the FRT. It's optional, but the other piece of pipe insulation between the MMG and FRT is mandatory, both to protect speaker finish as well as to keep the FRT from buzzing. The Armaflex does a good job of that.



Better view of the Armaflex protecting the MMG and isolating the FRT acoustically.



View from LP of the front wall mirror, showing that the MMG driver is not visible due to the FRT.



Sorry for the dark image; this is the side wall mirror showing the MMG edge on to the LP.

If you only try one of my many FRT configurations, make it this one. The improvements in phantom center and low level detail retrieval are well worth it.

MG-bert


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Topic - Another First Reflection Trap (FRT) Adventure : Quasi-Ambiophonics - MG-bert 09:06:54 10/5/13 (70)

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