a departure from conventional back bracing

[mauna lisa]

I am currently building #14 and #15. Number 15 is solid body, so we won't go there.

But number 14 is acoustic. I dabble in alternate tunings. I started off taking the range of a standard guitar and a bass guitar and stacking them, fitting it all on six strings, with the strings going from .130 at the bottom and .025 at the top. I started with E then dropped everything a whole step down to D A E B F# D .
I then dropped A (with the low string at A0 on a piano, but had some issues, so I came back up to D A E B F# D . I found so much potential, that I tuned my 3 guitars (#8, #10, and #12) that I still keep, all to that tuning. But these guitars were built for standard tuning.

I feel this is a lot of tension (but they sing well), so I am building #14 with this alternate tuning in mind. I thought I would try an experiment. My previous acoustics were all built with the back bracing attached to the back, as per most acoustic guitars. Number 14 is a departure from that school of thought. I placed the two back braces with a space between them and the back. This will allow for a free floating back. I am excited to see if any differences might occur.

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Oh, and as another departure, all my guitars, except #1 are all headless.

Any thoughts?

Lisa

[Mark Hatcher]

Hi Lisa,

An active back gives a wonderful feel to the player as the guitar seems to come alive when the body vibration is so easily felt. Also, the player can get a feeling that they are enveloped in the sound because the back is broadcasting some of the sound. It's my experience that the base response can be extended as well with a properly set up back.

The other side of the coin-

An active back picks up more outside vibration so it's more prone toward feedback when amped. If the back is too flexible it actually robs response and turns it into, well, "phlop". If the back naturally vibrates too much in sync with the top additional bad things happen with wolf notes etc. Also, it's easy to make the argument that it robs forward projection.

Overall it has it's place and can be a wonderful experience to play and a good way too squeeze out a little more base. I'm a fan, but I have made some real duds figuring it out.

Hope that helps!
Mark

[arie]

looks interesting!. i like active backs although i do have some bracing on them, they're not floating. as mark states, getting them to not contribute odd tones and to not suck energy from the box can be some work. it's certainly a balancing act.

also, the player can make or break an active back. when they're standing the back is resting on the body (of the player), dampening the back. when they're sitting and can keep their body away from the back, the back can go to work. but if the player is a bit plus sized and can't keep their body away from the back no matter what they do, the back gets dampened.

one way to neutralize "body dampening" is to build a double back. basically two back plates separated by a small air gap in between them.

[Jim.S]

Originally Posted by mauna lisa

Any thoughts?

I have built one live backed guitar and have closed the box on a second and tweaked that back but left some room so that I can adjust the back once it is strung up. One thing that seems very important when making the live back is a method of tuning the back post construction (which also means you have to have a target and you have to be aiming at that target during construction). It is hard to see your pic but if you have braces on the back then you can then go in post construction and take the tops of those braces lowering their stiffness til you hit your target. So you start a bit stiffer than what you want and work down.

If you want some good info on making live backs you will find it in Trevor's book. Here is his website

Jim

[mauna lisa]

Hi Mark, Thank you for your very helpful post on this. I was not aware that active backs were actually out there. I'm sorry, but I spent a most of my time in machine shops working nights, so I didn't get out much. Heck, I'm still using a Zero Fret instead of a nut. More uniform individual string voicing on open chords has always been my excuse. I had an introduction to wolf tones and other resonance issues when I tuned the low string down to A0. Thank you for your help!

Hi Arie, Thank you for your help. I do like to stay clear of those odd tones. I don't mind the balancing act, I always did like a challenge. And thanks for the tip about body mass damping. I prefer to sit anyways. This guitar is for my own use and I don't think I am that heavy, yet. But summer is coming and I love ice cream, so anything could happen.

Hi Jim, Thanks for your help. The longitudinal braces towards the back side are about 3/8 to 1/2 inch away from the back, so I am shooting for a full floating back. There is also no center seam on the top or the back, to hopefully allow some good vibrations. There is bracing on the top, but nothing in contact with the back except for the lining and sides. Because of the modified X bracing for the top to help deal with extra string tension, there not much room for a sound hole, so I was thinking F holes, which might make post construction tuning a little different.

Actually today I started putting wood together for #16. This will be the same size and woods and tuning as #14. But #16 will have conventional braced back. I am hoping this might help give me something to compare it to. I have enough old redwood for another 3 acoustics, and I get bored with watching the glue, and/or urethane to dry.

I posted some more photos at blufolk on the guitars pages of today's efforts.

[mauna lisa]

I want to thank you all again for your very helpful posts. I am so very intrigued by the possibility of being bathed in my own music. And now I know what an active back is, also.

I have been spending a lot of time waiting for the glue to dry. So I a started on guitar #16. Sweet Sixteen. I will be the same as #14, but without the active back. That way, I can make a good comparison.

Thank You All Again,
Lisa