[MGbert]

Here is an update on my efforts to make the First Reflection Trap concept workable in a not-as-intrusive form.  The history of this saga can be found here: 

http://www.audiocircle.com/index.php?topic=107064.msg1097758#msg1097758

The latest (maybe last) version had me cutting one of the big 4 foot by 6 foot FRT panels in 2, making two 2 foot by 6 foot panels to use taming the front wall early reflections.  Then I realized that a simple way of dealing with the side wall reflections was to toe in the speakers so that the side wall reflection seen in a mirror is of the speaker edge on.  That way, since the edge is a null, there is no side wall reflection that makes it to the listening position too early.  After some more trial and error, here is the room layout which, in not just my humble opinion, sounds a whole lot better than any stereo has a right to sound in a 11 foot square space.



Here is a photo of the left MMG and the mirror view of the edge of the MMG from the listening position (LP):



Here is a view of the left MMG and the mini-FRT.  Note the black pipe insulation tube I use on the edge of the FRT to handle any edge diffraction issues):



As I intimated above, other Circle members have heard my system.  You can read their reactions here:

http://www.audiocircle.com/index.php?topic=118426.0

where the discussion veers toward electronics as well as placement/treatments. 

Compared to the prior iterations which made the shed feel like the interior of a Bose radio, this is easy to live with... and dirt cheap to try!  Just make sure that the toe-in places the null as shown in a mirror from the listening position, and that the FRT blocks and diverts the back wave so it takes at least 10 milliseconds (an extra 11 feet) of travel compared to the distance from the speaker to your ear to arrive.  Even though the floor plan makes it look like the rear wave reflections could get to the LP before then, I believe that th diagram is off due to scaling.  Using the mirror, I confirmed that the rear wave is aimed into the corner, where the reflections take many bounces before escaping, thereby lengthening the time.  There is bound to be more than one configuration which will greatly reduce the comb filtering effects in your room, which can affect the phantom center in a bad way.  Which is the whole point of using FRTs.

MGbert

[jult52]

MGBert - I've been re-reading your threads on the FRTs and have three follow-up questions:

1) Where does Josh fully present his idea that reflections <10ms must be eliminated?  I assume this is an old studio engineer rule-of-thumb.  I'm interested in the objective here - I assume it's the simulation of a concert hall.

2) In the drawings from your first FRT plan (other thread), it looks to me like you've eliminated all reflections, so that the listener only hears the direct sound wave from the Maggie.  Is that accurate?  In the drawing on this thread, I can see a number of vectors which would result in reflections <10ms.  (e.g. backwave from Maggies at a 150 degree angle against the backing wall.)  I must be missing something so please explain your objective in general terms.

3) Because treble dissipates faster than lower frequencies, sitting nearfield like you do would seem to accentuate higher frequencies. Does that happen in your set-up?  Is that one of the reasons you use the Behringer? 

Any comments are welcome - I'm trying to understand this and apply it to my room. 

Hope you are well.

[MGbert]

MGBert - I've been re-reading your threads on the FRTs and have three follow-up questions:

1) Where does Josh fully present his idea that reflections <10ms must be eliminated?  I assume this is an old studio engineer rule-of-thumb.  I'm interested in the objective here - I assume it's the simulation of a concert hall.

2) In the drawings from your first FRT plan (other thread), it looks to me like you've eliminated all reflections, so that the listener only hears the direct sound wave from the Maggie.  Is that accurate?  In the drawing on this thread, I can see a number of vectors which would result in reflections <10ms.  (e.g. backwave from Maggies at a 150 degree angle against the backing wall.)  I must be missing something so please explain your objective in general terms.

3) Because treble dissipates faster than lower frequencies, sitting nearfield like you do would seem to accentuate higher frequencies. Does that happen in your set-up?  Is that one of the reasons you use the Behringer? 

Any comments are welcome - I'm trying to understand this and apply it to my room. 

Hope you are well.

Hey jult52:

First off, doing OK; thanks for asking.  Glad to see you're trying this out.  Keep in mind this has been trial and error, so there might be some "try it and see" beyond "theory".  That said...

1)  I'm sure Josh358 will weigh in, but I think 10 msec comes from the Floyd Toole book "Sound Reproduction".  Haven't read it myself, but it's on order so I can become even more dangerous.     I've actually seen 15 msec online as the magic number, but in my space it was hard enough to squeeze out 10 so I made that a realistic goal.  And the significance of the 10 msec number is that a reflection of a signal less than 10 msec after that signal is not differentiated by the listener as an echo, but rather as a comb filtering of the original signal.  Comb filtering sounds pretty bad, reducing clarity.  Particularly to the "phantom center".  As a test: play a mono recording through just one speaker, then through both and compare the clarity.  In my cramped space, there is a definite degradation, but I find the FRTs help immensely to fix it.

2) For better or worse, I started out with the assumption that human ears will interpret early reflections from behind the listener to mean that they are near a back wall, vice mess up the main signal.  My experience has been that this is the case.  And yes, the angle as shown on my sketch does make it appear that some of the "trapped" reflections could make it to the listener within 10 msec.  Actually, in practice I used a mirror against the reflective surface of the FRT to ensure that, when I was standing in the front wall/side wall corner, I would see the rear surface of the MMG centered in the mirror.  Probably not the best sketch, since it does make it appear that the reflections are NOT aimed at the corner...     That's where the try it and see comes in.  My thought is that, once the reflection is aimed at the corner, it will take it a long time to finally escape... with much reduced amplitude.  In effect, I'm turning the MMGs into nearly line source mini monitors, taking room reflections (except rear wall and the reflections of the right speaker from the left wall, which is greater than 10 msec) out of the picture.

3)  As you can see from the Behringer RTA plot of the unequalized response in page 2 of this thread http://www.audiocircle.com/index.php?topic=118426.0, the big issue is midrange in my room.  In order to get the side wall reflection coincide with the side null of the MMG, the tweeter is angled to aim behind my head.  That helps tame the treble response, I think.

Please write again if you have more questions, or if these answers weren't clear.  Most of all, have fun!

MGbert