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Timbuck
10-09-2016, 12:27 AM
I don't usualy get involved with technical stuff like this..I just work to known dimensions..But I have noticed on almost all my soprano's that tapping on the bridge with my banana like finger before stringing up almost always gives the same tone note at around Middle "C" ..Could it be that this is the secret to building a good sounding uke..for instance if a uke had a taptone of "D" would it sound best tuned to "D" :anyone:??..also blowing across the soundhole flute style gives the same note.

Michael N.
10-09-2016, 12:48 AM
The soundhole will be the main mode or the helmholtz resonance. There is also a top mode, which may be the one that you get when you tap the bridge. I'm a little unsure on this stuff so I may have that wrong.
Some makers of guitars will target a fixed note for the helmholtz. On classical guitars it's usually in the F to G# region. I don't target it myself but by default they do tend to end up in that region. Then again F to G# is four different notes and I've heard very good sounding instruments with any of those notes, so I doubt that one specific note is the secret formula.

printer2
10-09-2016, 02:53 PM
There are at least two main resonances in our instruments, three if you have an active back. There is the top resonance which you can measure when you block the sound hole with something. The hole resonance of course shows up when you have it uncovered. Using a spectrum analyzer program and a cheap mic you can find these when tapping the top. The back resonance can be found with the sound hole blocked and tapping the back.

These three are where the most of the sound comes from. Notes that do not fall on these frequencies still excite these resonances and use them to transform the top movement to sound. The top and back resonances work like a speaker diaphragm, with most of the lower bout area moving in and out. The sound hole acts as a port in a bass reflex speaker. Generally you would want these resonances not fall on scale frequencies as when on the scale tones the energy gets sucked out of the string very quickly. The note would be louder but not last very long. In guitars they are called wolf notes, originally the term comes from the violin family of instruments.

The spectrum of a small guitar I built.

http://i406.photobucket.com/albums/pp142/printer2_photo/Guitar%20builds/Guitar%20Build%20No%203/Lapcat002acomp122-154219-254275309550643_zps095bda86.jpg

As you can see the top and the hole resonances are the big dogs on the block, the other modes (ways the top vibrates) are lower down but contribute to the loudness of the instrument. The violin community has don a lot of work learning how instruments vibrate, classical guitar people have done some also. Since ukes are built similar to a classical I would think the same properties apply but are just scaled up in frequency. Not much done for steel sting guitars though.

sequoia
10-09-2016, 05:47 PM
I love this stuff. I don't really understand it, but it has to mean something. Not how to make a better sounding uke necessarily, but how the darn thing actually works. Fascinating... Hey thanks Printer2, but I have a question; what are the cross and long dipoles again? Also, is this a composite diagram of all the resonances? Shouldn't there be three diagrams showing each of the the three states? Top resonance with hole open, top resonance with hole closed and then back resonance (hole open)? Sorry, if this is a stupid question.

Recstar24
10-09-2016, 06:00 PM
The air resonance is the top with sound hole open

TjW
10-09-2016, 07:37 PM
I love this stuff. I don't really understand it, but it has to mean something. Not how to make a better sounding uke necessarily, but how the darn thing actually works. Fascinating... Hey thanks Printer2, but I have a question; what are the cross and long dipoles again? Also, is this a composite diagram of all the resonances? Shouldn't there be three diagrams showing each of the the three states? Top resonance with hole open, top resonance with hole closed and then back resonance (hole open)? Sorry, if this is a stupid question.
No, the graph is a frequency sweep using a constant amplitude to excite the instrument. The line is its amplitude response at each of the frequencies. The various resonances can be seen as peaks in the response.

Dan Gleibitz
10-09-2016, 09:52 PM
I don't know. I've been reading a lot about this lately, and found people who swear that it's bunkum, others who are adamant that different parts of an instrument should be precisely tuned to specific frequencies, some who say it's about ensuring a range of overlapping responses, some who reckon it's useful for getting similar sounds out of dissimilar woods, etc. I have no idea, but I keep tapping and listening. It's surprised me how little wood needs to be removed from a top to deepen its tone.

There must be enough instruments on this forum to get sufficient data to determine some interesting (perhaps even useful) results, if anybody is inclined to make a poll.

For what it's worth, my favourite tenor gives a slightly sharp G when I tap the bridge. The tenor I'm building is at A#, but its currently got an open back and will get a bit more sanding before finishing.

Timbuck
10-09-2016, 10:36 PM
Well I'm no wiser than I was a the start..Here is a demo of tap tone playing :) https://www.youtube.com/watch?v=UeD8xNeNqzM
I tried this, and now my finger hurts.

mainger
10-09-2016, 11:13 PM
It's surprised me how little wood needs to be removed from a top to deepen its tone.


Somogyi very clearly explains this by referring to the cube rule: the stiffness of a material is a cube function of its height and a linear function of its width. So, if you shave bracing or thin down tops, every Z you remove has a Z x Z x Z (Z cubed) effect on the stiffness at that point.

fungusgeek
10-10-2016, 02:47 AM
I understood from my engineering son, and I checked, that the strength of a beam (a brace is a beam after all) is proportional to it width and the square of the height, not the cube of the height.

printer2
10-10-2016, 01:49 PM
Here are the lower modes.

http://www.goreguitars.com.au/main/img_1275278348_15128_1276069361_mod_521_410.gif

Chladni patterns with tea leaves.

http://www.jameslisterguitars.co.uk/ListerGuitars/PublicImages/Chladni/2_4_tr_S.jpg

http://www.odycc.com/blog/wp-content/uploads/2011/12/Chladni-Mode-2-of-Spruce-Top.jpg

And how about a uke?

http://i1092.photobucket.com/albums/i410/808UpFront/Ukulele/Uke_zpscf20be68.jpg

Basically the resonance peaks will coincide with the pattern of the appropriate mode. In a nutshell.

mainger
10-10-2016, 01:53 PM
I understood from my engineering son, and I checked, that the strength of a beam (a brace is a beam after all) is proportional to it width and the square of the height, not the cube of the height.

I give it to you from the horse's mouth, though I cannot vouch for the accuracy of the horse's theory :D squared or cubed still mean that one should remove wood parsimoniously... But it would be nice to know which is correct. Any engineers here care to chime in?
94837

mainger
10-10-2016, 01:55 PM
:O
Beautifully clear illustrations of the principles. Mind blown.



Here are the lower modes.

http://www.goreguitars.com.au/main/img_1275278348_15128_1276069361_mod_521_410.gif

Dibblet
10-10-2016, 11:45 PM
I understood from my engineering son, and I checked, that the strength of a beam (a brace is a beam after all) is proportional to it width and the square of the height, not the cube of the height.

Strength (force required to break it) is proportional to the square of height, stiffness (force required for a particular deflection) is proportional to the cube of the height. Hence thin things will bend further than thick things before they break.

Rrgramps
10-11-2016, 04:06 AM
This is an interesting discussion; but I'm still at the starting gate of building a ukulele, and it's at the budding hobby level for me. Tap-tuning by ear may come later, but I would want more objective data to support my old ears. Like an R&D lab with working prototypes; hahah

Printer2 has started the R&D mechanics which illustrate the measuring that is helpful to visualize and predict what is happening when acoustic properties are modified. Modal analysis is another objective test that shows results too, but the instrument build has to be complete for a signal to be pumped through. Now, there are simulated modal analysis software packages that do not need a physical prototype.

Most of you probably have seen tap-tuning bunked and praised; usually before the top is braced, lowered and secured. Yet so many more controllable and uncontrollable variables are responsible for a good sounding, durable, AND good looking instrument. Those you who have been building for a long time probably have a handle on things already, and scientific study might just get in your way. That takes mastery of your skill sets, and a reasonable seasoning of artistic creativity. I think.

printer2
10-11-2016, 01:04 PM
Just for kicks, a coupled two oscillator model of the guitar.

https://newt.phys.unsw.edu.au/music/guitaracoustics/guitar_graphix/guit_2mass.gif

So you have a mass, the top, and the spring, which is the top's compliance and the air behind it acting as a spring. Then the mass of the air in the sound hole oscillating with the air in the box. They even have a three oscillator model with the back having its own mass and piston area. The three interact and they have found that the math seems to measure up with what occurs in a guitar.

And then there is the big 'so what?' Not like we are going to do measurements and calculations for every instrument we make. What I have got out of this is when a resonance is not where I want it, on a scale tone' I have the ability to move the resonances around by modifying the stiffness and mass of one or more of the elements. Well that is the theory of it. I have not done it yet, too busy trying to get the guitars I am making out to the people they are being built for. But one day...

sequoia
10-11-2016, 07:16 PM
This is an interesting discussion; but I'm still at the starting gate of building a ukulele,.

My advice: Don't over think things too much. Just start. All this talk about resonance and dipoles is great stuff, but when it comes down to it it is just wood meets glue. As for tap tone, it comes with a little experience. As in: That sounds pretty good and clear and I like it. But here's the rub: You gotta hear a set number of wood constructions before you get enough experience to know what is good or bad. In other words you have to establish a reference with your ear. It really doesn't take that long. I'm not there yet totally, but I get the concept. I used to think tap toning was bullshit. Not so. You just have to learn what to listen for. Not rocket science: Dead is dead and live is live. Doesn't take long to hear the difference.

Rrgramps
10-12-2016, 03:47 AM
Overthinking? Does it show? LOL

I thought I had tap tone tuning down pat 15 years ago, but the long hiatus since has found me avoiding the possibility of being error prone and dead wrong in my tracks. Procrastination has therefore set in as firm as rigamortis. But if I walk down the path of strictly theoretical or even overly subjective thinking, I will not enter the realm of physically starting. To fix that, I'm going to pry myself away from armchair coach position, and take the most important (for me) advice in this thread; what you said...

..."Just start."