cosed box Frequency response falcate OOO

[JurgenV]

Hi,
it´s me again. This time again some frequency spectra. I finally managed to close the box of my falcate steelstring OOO version, trimmed the back and top and couldn´t wait to see the spectrum. Here it is:

My peak detection excel sheet finds 7 peaks in the range from 100Hz to 250Hz from which I identify the T(1,1)1 as 106Hz, the T(1,1)2 as 191Hz and the T(1,1)3 as 241Hz. Compared to the example in the book my top frequency is a bit low while the back is a bit too high. To change the latter one shouldn´t be a problem but I don´t know if I will still be able to reach the 180Hz for the top which I aimed for. The question would be if I have to change that to 170Hz or if its still possible to reach the 180Hz. Any recommendations from your side guys?

I already started with the bridge. I tried the walnut carbon fibre approach but didn´t manage to bring the weight under 23gr. I made two bridges and both ended in the same region (23.2 and 24.xx gr) actually more or less the same as some prefabricated rosewood bridges I had still lying around
As the humidity again rose to 50+% I couldn´t close the x-braced OOO but as soon as I can continue to work on that one the next frequency spectrum will follow.

Juergen

[JurgenV]

Ok,
finally closed the x-braced OOO as well. As I promised here´s the next spectrum:

Actually I had to shave the braces because the top frequency had been quite high. I think I miscalculated the height of the braces. Next time I should take more care to check the wood properties.
Had been interesting how the peaks were shifting (or not shifting). In addition it helped identifying the fully coupled back peak which was the only peak in this region not changing at all. The open box (sides and back) had showed a peak at 221Hz and the value of 225Hz seems to fit quite well.
So for the x-braced OOO I have
T(1,1)1 = 104.7Hz
T(1,1)2= 206.3Hz
T(1,1)3= 225Hz

Up to now no edge thinning done (hopefully tomorrow) and if I compare that to the values Trevor recommends for a OOO (180Hz/226) nothing too far away. Let´s see where I am after thinning and after the binding.

Juergen

[JurgenV]

Ok, I finished the binding yesterday during the night. Hmm actually it had been morning when I finished (1:30am). I should really think about doing something else instead of tinkering with guitars...

Ok, here are the results for the x-braced OOO:

T(1,1)1 = 99.5Hz
T(1,1)2= 193.8Hz
T(1,1)3= 226.4Hz

I´m wondering why the back peak so small compared to the other two (almost non existent). With both guitars I aimed to get a life back and I used the radial back bracing design from the book. Looks like I did something wrong here. I think with these values it should be possible to reach the 180. Let´s say the bridge will just act as added mass with a sensitivity of around 0.3Hz per additional gram then I would end around 186 with my 24gr bridge. By adding some weight to the sides I should easily adjust the T(1,1)2 to 180Hz (hopefully )

And here the falcate braced OOO:

T(1,1)1 = 98.5Hz
T(1,1)2= 187Hz
T(1,1)3= 238.5Hz

If the average value for given in 4.6.1.3 for medium falcate steel strings is valid for this one as well then the T(1,1)2 will drop by at least 10Hz (not taking in account the saddle and bridge pins) as the bridge has a mass of about 23gr. So it looks to me I will have to go for the 170Hz. Which in addition means the T(1,1)3 has to drop to 214Hz. Hmmm, what will happen with the relative amplitude if I do this? Will the back peak shrink to the proportions I have in the diagram from the x-brace OOO?
More important: As far as I understood the T(1,1)2 will drop if I drop the T(1,1)3... depending on how much I can get into trouble if the T(1,1)2 get´s below 170.
Any advise from the guys using this approach for longer time? Otherwise I will wait for an answer by Trevor.

Thanks for reading
Juergen

[JurgenV]

ohh. Correction: x-braced OOO:

T(1,1)1 = 99.5Hz
T(1,1)2= 202.7Hz
T(1,1)3= 226.4Hz

I forgot to say that the binding data was taken before scraping the binding and before sanding. After the final sanding there are further changes in the spectra of both guitars. I put in a graph which shows the changes from fresh out of the mold to final sanding

x-braced OOO:
Values after sanding:
T(1,1)1 = 98.1Hz
T(1,1)2= 199.2Hz
T(1,1)3= 226.4Hz ???

Actually now the peak I originally identified as the back peak seems to shrink even more to non existence.

There are also some changes for the falcate braced OOO after sanding:

after the final sanding:

T(1,1)1 = 96.1Hz
T(1,1)2= 184Hz
T(1,1)3= 235Hz

And I promise there will be no further posts with spectra. At least not before someone can give me advise on what to do

Juergen

[Trevor Gore]

Sorry for the delay in responding. I've been overseas.

A couple of things:

Remember that the T(1,1)3 is the back response as seen in the top, rather than just the back response. So if the top is too stiff, which may happen with X bracing, you might not see much by way of the T(1,1)3 because of that.

The T(1,1)2 on the falcate will definitely be below 180Hz with a 23 gm bridge. I would suggest you use a lower mass bridge to be sure you don't drop below 170Hz. It seems like you lost a lost of stiffness when finish sanding and cleaning up. I think I mention in the book (somewhere) that there is very little allowance for finish sanding and that the target thickness given by the formulae are the target final thicknesses.

[JurgenV]

Thanks Trevor.
I will have a look at the two bridges and see if I can get the weight a bit further down. The density of the walnut I used is around 655 kg/m3. Somewhere you posted a range of 550-650kg/m3 for the walnut you are using, so that would be at the upper end. But at the moment I don´t have anything with a lower density available. Just a blank of the same walnut and rosewood blank left
Looks like I really underestimated how much I would remove by cleaning everything up. Next time I will take that into account. Still... if I can reach the 170Hz it should be ok. But what about the T(1,1)3? At the moment it´s around 235Hz. For T(1,1)2 at 170Hz I should have to drop it to 214Hz but at the same time this should drop the T(1,1)2. Would it be possible to check how the T(1,1)2 changes if I drop the T(1,1)3 by adding mass to the back? That should give me an idea what to expect. Hmmm, while writing this I seem to remember you mentioned that somewhere


Concerning the x-braced OOO: the specific mobility should give me an idea if it is too stiff. If necessary I can shave the braces a bit more. Hopefully without making the top too weak. And again this would change the top frequency. ...

A bit off topic:
It had been a lot easier not knowing these things. You just wondered why the guitar didn´t sound like the last one even if you didn´t change the dimensions. Now I don´t have this excuse of ignorance anymore. I really thought a long time if I should buy your books Trevor. I even manged to get hands on the only openly available example of it from a university in Bavaria to see if the mathematics are making any sense (a long time ago I had some semesters of physics) but at the end the money had been well spent. Even if not everything works for the first time trying to do it your way I can already see that I learned a lot. And there are certain things where your methods from part 2 are a lot easier and better in result than what I did before. So again thanks a lot for writing this book and to being active in this forum and answering our questions.

[JurgenV]

Did the test with the added mass to the back and ended up with a drop of 2Hz in T(1,1)2 when I adjusted the T(1,1)3 to 214Hz. So I think the lighter bridge could do the work.

With the x-brace I´m a bit confused. I get a value that seems too high. The deflection is quite high according to my jig. As I changed it some time ago and didn´t check if it still works fine I think I should invest some time before believing those values. Actually it shouldn´t be complicated to build this thing. But looks like I introduced some play...

[JurgenV]

Actually this is up to now the longest time it took me to finish a guitar. Problems at the company when they had been firing 500 people and then when the rest had to do the same work as before. Building some additional jigs. Girlfriend breaking up with me also didn´t increase the mood for guitar building (at least in the beginning) BUT finally I´m done
Here they are:

As for the frequency responses: the falcate OOO is quite near the target values from the book with
T(1,1)1= 95.4Hz
T(1,1)2=170.5Hz
T(1,1)3= 214.8Hz

For the x-brace OOO I´m not sure if I should adjust the frequencies to the same set but I think I leave it to the new owner. Except Trevor tells me there is a big problem with the frequency response as it is.
T(1,1)1= 98.6Hz
T(1,1)2=180.1Hz
T(1,1)3= 218.5Hz

I had the guitars test played by my guitar teacher here in Bavaria, by my guitar teacher in North Rhine Westfalia (both professional musicians for many years) and as he war very interested in the results by my wood supplier.
Result: All agreed on the falcate OOO being the guitar they liked better from the two concerning the sound. By the way that didn´t mean my test players didn´t like the x-brace OOO but one of them said that he tried guitars up to 4000€ in a big music shop and the falcate OOO sounded "more alive". So it looks like I didn´t do everything wrong. That means from now on I can build guitars and aim for a certain sound instead of wondering if I can repeat the good results from the last build. In the beginning I really wondered if the books are worth the money I paid but I´m finally convinced. sounds like one of those fake testimonials in advertisement

I think I will start a new project soon and test the falcate style on a classical like John Parchem. And this time I will try the Bolt on Bolt off neck system. Let´s see how that works out

PS
They even liked the intonation even if I really had some trouble with that as my compensation calculations didn´t really work out. I had to correct the compensation via the spreadsheet for correction of an already finished guitar. (Remark: if someone can provide me with a set of data I can use to test my calculations I would be grateful ).

[johnparchem]

Very nice, my favorite steel string's spectrum graph is very close to your falcate 000. It compares well to any guitar I have heard, at least for my use as a fingerstyle guitar. The x braced guitar spectrum looks good to me. I built a few falcates with 180 Hz top and was very happy with them. I bet the x brace guitar sounds good as well. What differences are you hearing between the falcate and your x braces 000.

[Dave M]

Well done Jurgen for getting there with analysis along the way. We have been following with interest. This is not easy stuff. I have a falcate braced box on the bench at the moment and have high hopes of it given that the others I have built have worked so well. I have not been nearly as conscientious as you with measuring the spectra.

As John has said it would be interesting to compare the falcate with the x brace.

Dave

[JurgenV]

The answer is not easy. Depends strongly who you ask.
In the mean time I had someone else try both of them and this guy preferred the x-brace a lot more than the falcate. How do you describe sound with words. I always had problems with that.

Both sound quite open, you don´t have this boxed in sound. But the falcate sounds more evenly distributed in frequency. And on of the guys decribed the difference as " the falcate sounds like the whole top is contributing to the sound while the other one doesn´t, is more located in the lower body". I´m not sure it helps with what you want to know but that´s all I can offer

I already started with my classical with falcate bracing but I will start a new thread for that.