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xbraced back with responsive system?
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=23255		 Page 1 of 1
Author:  Jody [ Thu Jul 30, 2009 1:44 pm ]
Post subject:  xbraced back with responsive system?
I am wondering if anyone has successfully used the X braced back with the "responsive back" type of build, I just braced the back for my third , and I am looking at it ,thinking, it seems like alot of mass in the center of the back . thanks jody
Author:  Alan Carruth [ Thu Jul 30, 2009 4:34 pm ]
Post subject:  Re: xbraced back with responsive system?
I have been X-bracing backs for years, using various patterns. At this point, I'm not sure it offers a lot of advantages, but it can be 'different', and maybe that's a good enough reason to do it.

If you think about it, all of the energy that drives the back has to get there from the strings through the top. It's either communicated via the air in the box (which I think accounts for most of it) or through the sides.

Generally speaking, the back is a less effective sound producer than the top. It's heavier, for one thing, and it's facing the wrong way. Thus most of the energy that the back takes out of the top is wasted.

The exception to this is in the low frequency range, where the back can work together with the top to pump air through the soundhole at the 'main air' frequency. To get this working it's best you need to tune the 'main back' resonance to be about a semitone higher than the 'main top' mode pitch. Any closer risks 'wolf' notes of various kinds.

I find that when I use X bracing on the back it ends up getting trimmed down pretty low. But even if it doesn't I don't spend a lot of time worrying about added mass on the back. Once you get past that 'main back' pitch, it seems to me that more weight on the back is probably as good thing. It all depends on what sound you're going for, though.
Author:  Bill Hodge [ Thu Jul 30, 2009 8:42 pm ]
Post subject:  Re: xbraced back with responsive system?
I just closed up a body today similar to a D-28 in size that I put double "X" bracing on the back. The X on the lower bout is similar to the way Howard Klepper does his and the Upper bout has an X similar to that used be Michael Bashkin. Not sure how it's going to work out but I did do as Al suggested regarding tuning the back a semi-tone higher than the top. Unfortunately I forgot to take a pic once I finished carving the braces before gluing the back onto the rim. duh I do have a couple of pics though of the unshaped and the preshaped braces before the final carving was done. (see below) I'll let you know once I get to the point of actually stringing it up how it all works out. I used standard Scalloped "X" Bracing on the top (Sitka), and the sides are 1/4 sawn Black Cherry with a Mesquite back. This is going to be an odd puppy too because this client wants a 6 in line headstock. :shock: Still trying to come up with a design for the headstock that doesn't look like something robbed from an electric and doesn't take away from the classiness of an acoustic. :?
Attachment:
Dbl-X-Back-Brcg-Uncarved_Sm.jpg
Attachment:
Dbl-X-Back-Brcg_Sml.jpg
Author:  Jody [ Thu Jul 30, 2009 10:30 pm ]
Post subject:  Re: xbraced back with responsive system?
Cool! Alan thanks for taking the time to reply, and giving a vote of confidence! Bill, thanks to you too, and it looks like an interesting( to say the least) project you have going on there! Jody
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