Bridge mass/frequency drop

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jeffhigh
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Bridge mass/frequency drop

Post by jeffhigh » Wed Mar 28, 2012 8:43 am

Looking at section 4.6.1.3 and Fig 4.6-3
The figure seems to contradict the text above which indicates that Glued on bridge mass will have the most (frequency drop)effect on X braced guitars as compared to classical fan braced where the added stiffness often fully compensates for the mass.
Am I reading this correctly?

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Trevor Gore
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Re: Bridge mass/frequency drop

Post by Trevor Gore » Wed Mar 28, 2012 9:27 am

It seems to read OK to me (but then it would, I suppose!)

Perhaps the thing to look at is the length of the line showing the standard deviation (just one standard deviation, + and - on the chart). For the classical guitars, 3 SD's out, which will cover most (~99.7%) of the population, some will have negative frequency drop (i.e. a frequency rise) whereas for X-braced guitars the standard deviation is much less, and consequently there is always a frequency drop. I don't say anything in the text (to my reading) about the magnitude of the drop. I'm mostly discussing the magnitude of the variability, which makes it difficult to give prescriptive numbers.

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Re: Bridge mass/frequency drop

Post by jeffhigh » Wed Mar 28, 2012 11:15 am

The chart Fig 4.6-3 is just a bar chart and shows no standard deviation lines that I can see.
It seems to indicate that for a given bridge mass, the X braced guitar will have the LEAST frequency drop after gluing on the bridge.

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Re: Bridge mass/frequency drop

Post by jeffhigh » Wed Mar 28, 2012 11:32 am

Is that faint line in the centre of the bar the +-1 standard deviation?

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Trevor Gore
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Re: Bridge mass/frequency drop

Post by Trevor Gore » Wed Mar 28, 2012 1:34 pm

jeffhigh wrote:Is that faint line in the centre of the bar the +-1 standard deviation?
Yep, you got it! (Might need the good, clean, glasses for that one!). Fortunately, I can still see it without visual aids. There are similar SD lines on some of the other charts, too.

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Re: Bridge mass/frequency drop

Post by Trevor Gore » Wed Mar 28, 2012 1:48 pm

jeffhigh wrote:It seems to indicate that for a given bridge mass, the X braced guitar will have the LEAST frequency drop after gluing on the bridge.
Yes, that's right. Typical X-bracing already has so much cross-grain stiffness (with the bridge plate as well) that the bridge contributes more to the mass than to the stiffness, so there is a fairly consistent, smallish frequency drop. On the other hand, falcate bracing and fan bracing have somewhat less cross-grain stiffness and, relatively speaking, the bridge can have a much greater impact, either way, depending on the mass and stiffness of the bridge.

Fig. 4.6-3 is one that every builder should produce for themselves by keeping their own records, because the possible variations in top stiffness and bridge design are quite large and everyone has their own ideas about what they like.

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Re: Bridge mass/frequency drop

Post by jeffhigh » Wed Mar 28, 2012 2:56 pm

Yes I am taking measurements on the way through for my current pair
What I am having trouble reconciling is if I look at the chart for a classical and see an average of 1.05 HZ per gram for a classical, which would imply 16hz average drop for a classical with a 15 gm bridge.
Is the statement that for a classical or flamenco the T(1,1)2 may be raised, an outlier situation rather than the norm?

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Trevor Gore
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Re: Bridge mass/frequency drop

Post by Trevor Gore » Wed Mar 28, 2012 3:23 pm

jeffhigh wrote:Is the statement that for a classical or flamenco the T(1,1)2 may be raised, an outlier situation rather than the norm?
Yes, for that sample. The main message for fan and falcate braced classicals is that it's very variable. This is partly because that sample is of guitars made to a variety of specs; Gerard's shape, my "classical" classical shape, my neo-classical shape, falcate and fan bracing. All the shapes mentioned have different lower bout dimensions and were pitched at a variety of frequencies. If you build basically to one spec. you should be able to achieve a much tighter spread than shown on the chart (except perhaps for the X-bracing). However, to me, it made more sense to put in all the data and show the spread so that readers are aware of the likely variation rather than me showing how tight a spread I can get using data from guitars of one spec. and have readers disappointed when they get a completely different result.

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Re: Bridge mass/frequency drop

Post by jeffhigh » Wed May 09, 2012 5:12 pm

Just glued on the bridge to my falcate "classical"
13.5g Koa/CF on a falcate braced top, no bridge plate
Pushed the main top resonance up 6Hz which is going to work well for me

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Re: Bridge mass/frequency drop

Post by Trevor Gore » Wed May 09, 2012 9:53 pm

The mass of the saddle and strings may well bring it down a bit, so beware of that if you're close to a good number.

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Re: Bridge mass/frequency drop

Post by jeffhigh » Thu May 10, 2012 7:26 am

Yes, at the moment I am about 4hz above my goal, Strings, saddle and tap plates (Yes I am going flamenco) and back brace tuning will bring it down.

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