Measuring top and back frequencies while voicing

[JamesO]

I was just going through the books to look for the process of measuring tops and backs while voicing. Build page 11-48, for example, gives a number of different measurements for a falcate braced top. Build 11-7 gives a rough target for a live back (rough because we know it needs to be four semi-tones higher than the top's eventual T(1,1)2.

In another thread, Trevor writes:

I don't do much tapping until the guitar is "boxed". I've never been able to make much of free plate tapping or tapping without the back, because so much changes when the box is closed. Once closed, you can hold the guitar pretty much as normal, albeit without a neck (or with the neck if you've got that far).

So, my question now is do we measure for anything while we're still in the voicing stage of either the top or back? I'm working on the back of my current project before moving on to the top, and I'd been measuring the long grain frequency of the braced back. Just realized this wasn't the correct method. It's all about the learning process though, right

Thanks in advance, guys.

[kiwigeo]

You're going through exactly the same learning process I've been going through with Gore and Gilet's books.

[Trevor Gore]

So, my question now is do we measure for anything while we're still in the voicing stage of either the top or back?

That's the point. You don't, because it tells you very little about how the panel will perform when boxed up, unless everything is the same from one build to the next (which it never is). It seems possible to get an idea of where the T(1,1)3 might land (provided you can place the T(1,1)1 and T(1,1)2 where you want them) by tapping on the back once attached to the sides but before the box is closed. Discussed briefly here. All the maths (measuring Young's moduli, the plate thicknessing formula etc.) get you in the ball park so that when boxed you are close enough to bring it to spec. Saves wasting time on "voicing" of free plates which mostly doesn't tell you much, especially on tops where bridges, plate and air coupling effects, side mass etc. etc. all change the resonant frequencies. Remember that you're working with a construction system here; if you don't follow the process (or have enough knowledge to know what changing the process is going to do) you're unlikely to get to your desired outcome. As has been said many time, everything couples, so you can't really consider components individually.

[JamesO]

Excellent. Thanks, Trevor. That actually makes it much simpler, too.

[collinsguitar]

Has anyone tried to temporarily attach the top and back plate to test coupling so that it will be easy to affect changes rather than make those changes through the sound hole?

[kiwigeo]

Has anyone tried to temporarily attach the top and back plate to test coupling so that it will be easy to affect changes rather than make those changes through the sound hole?

I've unintentionally done this. Top main frequency was too low so prized it off (glued on with hide glue) and beefed up the falcate braces. Unfortunately the braces were still not stiff enough (King Billy Pine with low Youngs Modulus) so I then replaced the top completely with a new top braced with sitka spruce.

[collinsguitar]

Another quick question. If a free plate is held in a fixture where the edges are constrained, is there any correlation to what the top does once it has been installed on the guitar itself?

And does anyone think that it would be possible to build a jig that mimics the internal sound chamber but allows the removal of a top and/or back plate so that individual components could be tested and adjusted before the guitar is assembled?

One more, has anyone measured specific mobility and collected data of the plate prior to attaching the bridge and comparing this figure to specific mobility with the bridge attached. Can this help predict a final outcome?

Thanks.

[Craig Bumgarner]

CollinsGuitar: Hi Michael, good to see you here. I attach the top first and measure specific mobilty of the top, the deflection and the peak monopole frequency T(1,1)2. I know from experience that I can expect something like 20Hz drop of T(1,1)2 after full assembly. With just the top on, I can play with the bracing and/or top thickness easily. This is especially helpful with the Selmer guitars you and I deal and those little sound holes. The top is more important than the back, so I favor putting the top on first to be sure I get this where I want it (for better or worse

You raise a good point, however, the back is important too and I've been spending a lot of time thinking about how to voice it properly before attaching, assuming the top goes on first. Because we can't just reach in a shave braces, we need a different work around. Temporary attachments have their appeal. Taping the back to the top is reported to work, I tried it once and I recall the results were closed to the glued results. The tape has to solid and complete Thinned hide glue and paper between the joint is a time honored method for making an easily reversed joint especially if you get it apart before it sets up rock hard. I've not tried this, but I plan to this weekend. Attach the back, get it where I want, remove it, put the top on, voice it, then put the back on and hope for the best.

I'm all for letting the math do the work. I need to go back to G&G and see if the math on the back can be applied to the a full size laminated back.

Getting accurate data using a jig to temporarily hold the plates is difficult because the mass of the jig significantly affects the voice of the plate. This is quite noticeable and Trevor talks about the effect of the mass of the sides on the top and back extensively in the book. Unless the mass of the jig and hold downs is very close to that of the sides, it probably will give you a good result.

[Craig Bumgarner]

Typo.... Unless the mass of the jig and hold downs is very close to that of the sides, it probably will NOT give you a good result.

[collinsguitar]

Thanks Craig!

So, if the mass of the jig was the weight of a set of sides, liners, neck and tail block, plus the additional side weight (approx 400 g as in side mass loading) it is possible to get something approaching the effect of gluing a plate to a set of sides? I will make an attempt and see how it goes.

[kiwigeo]

Thanks Craig!

So, if the mass of the jig was the weight of a set of sides, liners, neck and tail block, plus the additional side weight (approx 400 g as in side mass loading) it is possible to get something approaching the effect of gluing a plate to a set of sides? I will make an attempt and see how it goes.

I think it would be difficult to mimic the physical properties of a set of sides with a jig.....you'd basically end up making a set of sides. I do recall an article in GAL Big Red Book series where someone was tap testing tops fixed to a jig...the jig was basically an external mold. Ill try and find same and PM you.

[Trevor Gore]

Michael, give Brian Burns a call. He builds flamenco guitars but has a way of taping the back on so he can test/trim things before he glues it all up. He reckons taping the back on has much the same effect as gluing it on. I'm sure he'd be happy to explain how he does it.

Re: your other questions, I've never got anywhere trying to "tune" free plates. I'm pretty sure no one else has either. They are very poor predictors of how the full coupled system will perform. Basically there are too many variables to control which makes trying to do it impractical.

[collinsguitar]

So after thinking about this for quite some time, it seems that an approach to the falcate type soundboard that the braces could be attached with the CF tow underneath and glued to the rim.

According to Brian Burns, the back can be temporarily stuck on with binding tape with a good approximation to a glued back, so coupling will be present.

With an easily removable back, the soundboard brace heights could be adjusted and tested in a coupled mode, and when the target is very close, the CF tow could be added on the top of the brace.

I suppose the only area that would be compromised would be under the upper transverse brace.

Does this sound like a way to get to the target top frequency?

Michael

[Jim watts]

I think the problem there is that by adding the cf tow, it's adding weight and stiffening the braces. I'm guessing it stiffens by a significant amount as the tow is on the outside of the brace.
Probably a really good experiment though to see if it works out.

Brian says you can just tape the back on?

[collinsguitar]

Jim, Brian Burns told me that when taping a back on it was fairly close to the result of gluing on the back, well, at least it only raised the back mode by about 5%. So it seems to work for a temporary attachment so the guitar can be opened and the bracing adjusted.

In any case, it would be nice to know an approximation of how much stiffness is added by the CF tow once it is glued to the top of the braces. It seems that you could glue them and test it before attaching the back permanently.

I think I will make a go of this and see what the results are.

If anyone else has any experience with this approach, I would love to hear about it.

[blackalex1952]

Craig Bumgarner wrote:

I attach the top first and measure specific mobilty of the top, the deflection and the peak monopole frequency T(1,1)2. I know from experience that I can expect something like 20Hz drop of T(1,1)2 after full assembly.

Craig, in your post regarding "testing good examples" viewtopic.php?f=33&t=5470&p=63880&hilit ... ity#p63880
this 20Hz drop doesn't seem to be reflected in the table when comparing the uncoupled top peak with the main top peak? Or is that because of the back air and top frequencies repelling each other? Im confused now...Ross
ps: Just re read some of the old post-

If anyone would like to see the FRCs on any of this, let me know, I have them all.

I would love to see those, if you are still keen to share.Cheers!

[Colin B]

According to Brian Burns, the back can be temporarily stuck on with binding tape with a good approximation to a glued back, so coupling will be present.

Can I ask what you use as binding tape? I assume this is a self adhesive tape, but not a strong tack?

[mqbernardo]

Brian says you can just tape the back on?

Hi Jim. Easy to check for yourself. On his site Brian shows very good correlation.I did it twice, but I only have results of my last guitar at hand:

No Back: Air x Mono 203 Dipole 260 Tripole 364
Back taped: Air 100 Mono 223 Dipole 244 Tripole 355
Back Glued: Air 100 Mono 223 Dipole 236 Tripole 349


This was a Engelmann top, amazon rosewood, 7 fan plus 2 closing bars classical. No back means top plus sides and neck. I wasn't´t expecting the dipole and tripole frequency drop. I didn't´t measure the taped back frequency as all the tape glued would make it pointless IMHO.

[Trevor Gore]

Yes, Brian tapes the back on and tests his guitars like that. But he has a few different steps to what's in "the book".

You can see a pic of a taped on back here: http://brianburnsguitars.com/my-process (bottom of page). If my memory serves, he glues the bridge on before completing the finishing so he can get a good measurement. Ideally you want a saddle in, too. I think he then removes the bridge for finishing.

If you go through all the videos on the page above (I haven't, yet) there may be more details. Looking at the brown paper tape he is using to tape the back on, it resembles the binding tape that some of the large lutherie suppliers sell.

[Jim watts]

Good to see this again and Brian's videos, thanks for posting that Trevor.
I curious if there would be any use in adding a Q calculation to one of the taps as done in Trevor's book. If so which one. Or can we just compare the stiffness and weight and get a similar kind of thing (I think so) ?
Seems easy enough, any thoughts?

[Trevor Gore]

I was hoping someone else would chine in...

Jim, do you mean adding Q in on a panel tap or a guitar box tap? You seem to be inferring a panel tap. If that's the case, the first thing to do is to determine what a Q measurement is: at a single frequency - a resonance governed by the panel dimensions, if so which mode or modes? Or are we looking for some sound spectrum absorbance chart, which gives the damping variation over some frequency range, which is likely more meaningful, but how would that be measured and distilled into a single number to combine with other parameters?

I've always found damping a problematic topic, for the many reasons discussed in the books. The doesn't mean it's not important, but it's hard to determine what aspects are important and how important.

[johnparchem]

I am not sure it is Q, but when I took a voicing class from Jeffery Elliott a Portland Oregon classical guitar builder, he taught that he consistently thicknesses all of his top spruce panels for evaluation. Then on tapping he wanted to hear it ring for 5 seconds. It seems like the sustain would be related to Q. He was just using this as a binning test, use or put aside.