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Tap tones for archtop top-plate, help needed

PostPosted: Mon Jun 17, 2019 9:06 am
by Daniel Oates
I posted this on another forum but this forum seems more relevant to the guitar I'm building. I need a little advice regarding tap tones. I’m building my first full scale archtop guitar with a carved top and back. I’m using Benedetto’s book on building archtops for help. Currently, I’m working on the top and have got to the stage where I am ¼” at the bridge, tapering to 1/8” at the recurve. I’ve cut the “f” holes but I haven’t yet installed the bracing. Tapping the top at the nodes, I’m hearing a deep ringing tone around 94Hz.
But I am wondering if this is too deep and I may have gone too far. I just made an Audacity recording and did an analysis of the taps, but I’m having a hard time figuring out what I’m looking at. I’m getting peaks at 89Hz, 132Hz, 179Hz, and 221Hz, 237Hz, 285Hz, 334Hz, 382Hz, as well as others in higher frequencies. The 94Hz that I’m hearing does not appear as a peak. My question is, am I still on the right track? Are there target frequencies I should be aiming at, at this stage or should I wait until I get the bracing on? I can't attach the Audacity file , but here is a screenshot of the frequency analysis.
BTW, the top is a very tight grained bookmatched Engleman spruce that I bought back in 1994

Re: Tap tones for archtop top-plate, help needed

PostPosted: Mon Jun 17, 2019 9:35 am
by Barry Daniels
I assume that the top is not attached to the rims yet, correct?

Re: Tap tones for archtop top-plate, help needed

PostPosted: Mon Jun 17, 2019 9:50 am
by Daniel Oates
No, the top is still not attached to the sides

Re: Tap tones for archtop top-plate, help needed

PostPosted: Mon Jun 17, 2019 9:54 am
by Brian Evans
I never worry about the pitch of the tone, but about the quality of the tone. I look for a clean drum sound, with sustain and ring to it. The pitches will rise a lot when you brace it, and when you complete the box. The tap tones and air resonance of the box is what matters most, and only then can you "tune" the final graduation of the top. Again, all I ever look for even at that point is a clear, clean drum sound and even vibration over the top and back.

Re: Tap tones for archtop top-plate, help needed

PostPosted: Fri Jun 21, 2019 12:01 am
by John Clifford
I agree with Brian. Since every piece of wood is different, it doesn't make sense to try to force the top to produce a predetermined frequency spectrum. You're just trying to get that particular piece of wood to sound as good as it can. So how do you know when you're there? As Benedetto says, that knowledge can only come with lots of experience. I've built 9 acoustic archtop guitars and ukuleles, and I'm still feeling my way largely in the dark. The analytics are fun and interesting, but really not much help at this stage.

Re: Tap tones for archtop top-plate, help needed

PostPosted: Fri Jun 21, 2019 12:17 pm
by Alan Carruth
I've been replying to this thread on another forum, but I may as well chime in here too.

I've been using the 'tech' version of tap tone tuning, Chladni patterns, for a long time. It's not as intuitive as simple tap tone tuning, but it's much easier to teach, since it doesn't rely entirely on subjective information. It also gives more information. Since you can see each resonant mode shape it's easier to judge what's going on. It still takes some time to learn; beginners have trouble just finding modes to begin with, but once you get the hang of it, it's pretty straightforward. It does take longer to tune a plate this way that an experienced tap tuner would need. I do feel that with the extra information, and the ease of recording the results for future reference, that it's worth the effort.

Re: Tap tones for archtop top-plate, help needed

PostPosted: Sun Jun 23, 2019 12:46 am
by Randolph Rhett
As you must already know, the use of frequency histograms to tune a top was completely unknown to all the previous master builders. From Benedetto to D'Angelico, none ever saw a histogram.

To the best of my knowledge, the principal architect of using a histogram to tune a top is Trevor Gore. His book describing his method is less than ten years old, but has been enthusiastically received by many builders. That said, I have no idea if he has ever applied his technique to a carved top. I have never seen anyone build a carved instrument using Gore's book, but that doesn't mean it doesn't happen all the time. However, I suspect that if you want to use that tool you are either going to have to rely on Trevor or work it out on your own. I don't know of anyone else who can really help you.

Most master builders tuned their tops "by ear". Like so many crafts honed by years of trial and error it is very hard to explain precisely. It's a bit like using dance to figure out architecture. "Tap tuning" is a combination of things that are hard to quantify. They press on the arch to see its give. They hold it at several antinodes and tap to hear the tone. They listen for a richness in tone. They listen to a complexity of different pure pitches. They listen for volume. Experience and instinct tells them what to listen for, and when the top sound "right" they stop carving.

My own experience is that I was fortunate to be involved in an archtop building course for three years. There were 25 student per year, which in addition to my own guitars, means I have had the chance to hear the "tap" of more than 75 guitars. My experience was that at first the tops would sound like wood. Resonant, sometimes beautiful, but like hitting a piece of wood. Some people would stop there (carving is hard work). If it looked like a top that was good enough. Others would keep working with a violin plane. As the top was carved more and the recurve lightened it would suddenly sound like a drum. Not wood, but like a drum with a skin. That deep drum note would get louder and deeper until you wouldn't dare carve any more. Those were the best sounding guitars. What pitch? Dunno. How many overtones? Dunno. Could you carve too much? Apparently. One of my guitars sank and eventually cracked after I strung it up. But I bet it would have sounded great.

If I was building twenty guitars a year I have no doubt my ear would develop further and I would get an even better sense of what was needed beyond just making it sound like a deep and resonant drum. Perhaps I would even identify target pitches that I could then look for in a histogram. I would probably learn when it was a matter of thinning the arch and when it was better to thin the recurve. But I'm not, and am not likely ever to. So I'm content with my very limited "tap tuning" experience.

If the voodoo of "tap tuning" isn't up your alley, Alan's technique relying on Chladni patterns is the most objective and quantified method I know of for tuning carved plates, and the only one that doesn't rely on vague impressions built over a lifetime of experience. He builds on the techniques of some violin and cello builders, but mostly comes from his own rigorous experimenting over several decades. If you want to use his method, it is an invaluable resource that he has responded to you in two different forums. It is a relatively small investment in equipment, and he sells a DVD demonstrating his technique.

Otherwise, as one luthier said to me, "it's a musical instrument. If the parts make music, the whole should too. If taping the top doesn't make music, what makes you think pushing it with a string will."

Re: Tap tones for archtop top-plate, help needed

PostPosted: Sun Jun 23, 2019 9:11 pm
by Alan Carruth
Thanks Randolph, but I need to make a couple of corrections.

The video on 'free plate' tuning that came out a few years ago only covered steel string flat tops. I was the 'talent' if you will, but I don't sell them myself. I'm not even sure if they're still available. There is information on plate tuning for archtop guitars in the 'American Lutherie' articles mentioned in the other thread (1991-92, iirc), and those should still be available from the GAL.

As I think I mentioned on the other list, there are limits to what you can determine using a tap spectrum. In some respects your ear is better at extracting information from a tap than a microphone and a computer, although there's no denying that the info you get from the computer is more specific and exact within it's limits. Gore gives a lot of tips about interpreting the data you get and it's limits.

One of the drawbacks to using the Chladni method has been getting and using the necessary equipment. Signal generators are fairly common in electronic labs, but tend to do lots of things we don't need and can be pricy. I helped build few 'luthier friendly' ones back in the 80s, and assisted the late Dom Bradley in designing much nicer ones, but those sources are now gone. Recently Mike Mahar has put an Android app into beta test that pretty well covers the water front for 'free' plate tuning. It includes a signal generator, spectrum analyzer, a module that can print out arch templates, and ways to record and file the patterns you get using the camera on your tablet. You will still need a power amplifier (12-20 W), compatible speaker, and some odds and ends like foam pads and some sort of powder (sawdust from a table saw works well for some folks).

There's no denying that 'free' plate tuning using the Chladni method is controversial. Although a logical case can be made in it's favor, a rigorous 'proof' would be difficult. The same can be said of any other tonal quality control method I'm aware of; it's been said that they're all religions, in the sense that they tend to work well enough for the people who believe in them, and make no sense to anybody else. I will say that I've been using the Chladni method on my instruments for over forty years, and teaching it to my students. None of my hundred or so students has gone home with an instrument that sounded bad (some didn't look as nice as I'd like...) or that self-destructed from being too lightly made, and one of my students has gone on to make a very good name for himself as a Classical maker, based in some part on his sound (he also makes a really beautiful instrument).

One thing that I do differently from many archtop makers is that I don't 'fine tune' the sound by thinning the recurve after the instrument is together. I don't deny the possible utility of this method, but I also see it as carrying a certain amount of risk. It's easy to get the edge too thin, and end up with an top that collapses fairly quickly. My objective is to get the top 'right' before gluing it to the rim, and I believe that the Chladni method gives enough information to largely accomplish this. It's quite plausible that further fine tuning could bring improvements, though. I make a lot of different kinds on instruments, and not as many of any one sort as a specialist. Also, quite frankly, my hearing is not what it used to be. Also, and just as important, I've got a dog in the fight; I have every reason to be biased when I listen to my instruments and compare them with others, so I try to avoid making judgments. I will say I've gotten some very positive reviews from players who should know, and rely on them for input on improvements.

IMO, we're all trying to make better instruments more consistently. To the extent that a maker succeeds in this they're probably doing similar things, but may be taking a somewhat different path to get there. Very few really successful makers end up doing things that are radically different from what the masters of the past did, unless they've introduced a new material (such as CF or Nomex), or a technique, such as CNC, that improves precision and allows for more complex designs. The old boys may not have had our theoretical understanding, but they were not stupid, and knew their business. I think we do best when we respect the traditions, and use our modern knowledge to understand and maybe improve on their work.

Re: Tap tones for archtop top-plate, help needed

PostPosted: Thu Jun 27, 2019 11:19 am
by Daniel Oates
Thanks everyone for your help. Everyone here has a lot more experience than me and I am kind of taking a little advice from everyone as I am moving ahead.
I am at the point where I have glued the top to the sides and the braces are in place. I had some nice ringing tones before I glued the tone bars on, but attaching them killed them and raised the pitch. I then worked them down until the sound board became responsive again. After gluing the sound board to the sides, the top sounded dead again and now I am continuing to thin the top and the tone bars. The nice ringing tones are coming back but I think I still have a little way to go. None of the tones are as deep as they once were. I'm not sure if they can be. The top is flexing very slightly when I push down quite hard but I have no real idea of how hard I should push or how much it should flex.
I'm kind of feeling my way in the dark and I think I'm working in the right direction but it's difficult to know. Its a very tight grained spruce, so I think I can thin the top to about 3/16" at the bridge and still be OK. I'll let you know how it goes.

Re: Tap tones for archtop top-plate, help needed

PostPosted: Thu Jun 27, 2019 2:48 pm
by Alan Carruth
There are lots of different ways to do this, of course. The objective of the 'free plate' method that I use is to get the plates 'right' before you glue them to the rim, and then don't change anything. This may not be the 'best' way, and there are lots of other traditions that do work on things after the box is together, but it's worked well for me so far.

I will note that the 'free' plate modes are not very sensitive to removal of some wood around the edges. Scraping in the recurve is a widely observed tradition, including, apparently, by Stradivari. I'm leqary of doing too much of that, though, since you can weaken the edge to the point that the top will sink a lot in service, and there's not much you can do to fix that.

Dan MacRostie has done some work using deflection masurements, at least on mandos, and I'm pretty sure similar work has been dome with arch top guitars. There should be, at least, a 'no go' measure that tells you when you're getting too thin ; I just don't know what it is. I suspect that it would vary some depending on the exact arch shape.

Re: Tap tones for archtop top-plate, help needed

PostPosted: Fri Jun 28, 2019 7:45 am
by Brian Evans
Just remember that depending on neck angle, bridge and arch height, and tail piece geometry, plus of course string gauge and tension, you can have up to 50 lbs of downforce on the bridge. Try an experiment and see what kind of deflection you get with 50 lbs sitting on the top.

Re: Tap tones for archtop top-plate, help needed

PostPosted: Fri Jun 28, 2019 1:41 pm
by Alan Carruth
Back in the '90s (!) I got an e-mail from Joshua Gordis, an instructor at the Naval Post Graduate Research Institute. He asked if I had ever seen a change in the modal pitches of guitar tops when they were strung up. He had been working with loaded columns, such as truss members in helicopter frames. A column that is fixed at the ends will have a certain fundamental resonant frequency when unloaded. As a load is applied the pitch drops, going to zero at the load that will cause the column to buckle. The interesting thing is that the relationship between the load and the pitch is linear; if the unloaded pitch is, say, 100 Hz, the pitch will be 50 Hz at half the buckling load. This provides a neat non-destructive test of truss structures. There is a sense in which a flat guitar top can be seen as a set of parallel columns, loaded in tension behind the bridge, and compression in front of it. One might plausibly expect a pitch change in the fundamental mode of the top under load depending on whether tension or compression loading predominates.

I ran a quick test to see if this would hold for plates. I cut a disc of expanded styrene bead board, stretched a heavy rubber band (#109, iirc) around it, and tested the modes. Then the rubber band was swapped out for a broken one that was stuck on with double-sided tape, so that the weight would be the same but there would be no compression load. Sure enough, the one with the slack band had a higher pitch. When you think of it this is simply the converse of tightening a drum head, of course.

At any rate, tests of flat top steel string and classical guitars showed no real change in the fundamental mode pitch with or without tension. Other, higher order modes, do show some small changes, which could as easily be attributed to local stiffness changes in areas of the top due to distortion. Back pitches, on the other hand, can rise a bit with string load: the drum head thing at work.

Arch top guitars are a different animal, and do show a drop in the fundamental pitch under string load. Simply going from slack to tight strings, without changing anything else, drops the top pitch a bit.

I would not be so bold as to suggest that the 'main top' pitch would drop to zero at the load point where the top collapsed, or that the relationship would be simple. Obviously one would need to do a fairly large experiment to suss this out. OTOH, I believe that a 'small' drop in pitch probably signals a 'safe' load, at least for the central part of the arch. Given that a wide recurve could be loaded in tension for some way in from the edge it's plausible that the top could sink around the rim even though the central part of the arch was sound.

Re: Tap tones for archtop top-plate, help needed

PostPosted: Fri Jun 28, 2019 2:24 pm
by Brian Evans
I have come to believe that for F-hole guitars, in particular parallel braced tops, the string load is carried between the neck and tail blocks. In fact, the more you can emphasize this the freer the top can be. On X-braced instruments I have been putting in a dowel as a brace directly between neck and tail block, under and not touching the top, so that the top can be freed of some of the string tension load. I've also made two archtops without F-holes. I look for any and all ways to remove pre-load from the top, including trying to minimize down-load from string break angle. You need some, you don't need much.

I think of it this way - take an unstrung guitar and touch the ball of a tuning fork to the top where the bridge would be. You'd get a loud, clear ringing tone. Now think of loading it up with 150 lbs of string tension and 35 lbs of dead weight on the top. You'd expect less from the tuning fork, wouldn't you? Pre-load... Long way from tap tones, i apologize.

Re: Tap tones for archtop top-plate, help needed

PostPosted: Sat Jun 29, 2019 5:55 pm
by Alan Carruth
Certainly minimizing the break over angle at the bridge makes a lot of sense. Six degrees may possibly be 'enough', and 12-15 could well be too much.

I used to use 'hook' tailpieces: they were wood, and 'L' shaped, with the lower arm of the L hooked over the back edge to hold them on. The idea was to increase the break angle, since the string path runs on a line from the bridge top to the pivot point. With the pivot being down on the side the strings can actually effectively go 'through' the top. I decided to check out the theory by inserting a spacer between the tailpiece and side to provide a pivot that could be moved up and down. With the pivot point just below the top edge the guitar sounded fine, and it seemed to work well as I moved the pivot down a couple of notches. However, at one point the sound simply died. Moving the pivot back up restored it. There certainly seems to be such a thing as 'too much' down bearing.

The pressure of the strings on the tailpiece pushes inward on the top, and should push the arch upward. It's at least plausible that you get the best sound when that upward force at the bridge is just balanced by the down bearing. I've seen variations of that argument made for violins. That's a dangerous path to go on, though; violins and guitars work much differently. Things that work on one don't always work the same way on the other. If there is such a thing as an 'optimal' break angle it might take a lot of work to find it, and I'm sure there are plenty of variables to consider. Failing a definitive solution, I've been keeping to the minimum break angle I can get away with, and so far that seems to be working OK.

Re: Tap tones for archtop top-plate, help needed

PostPosted: Mon Jul 01, 2019 2:14 pm
by Daniel Oates
Alan , that's something that never even occurred to me, but it makes perfect sense. So you're saying that it should be possible to achieve some kind of equilibrium between the downward pressure of the strings and the upward pressure caused by the compression from the strings and a shallower break angle would bring you closer to that. Would 6 degrees really be enough downward force to achieve enough volume? I'm also trying to figure out what kinds of stresses are on the back perhaps you can give me a better understanding of what is happening there. Also, in terms of top deflection, do you have any optimum numbers for a given weight of, lets say, 35lbs?

Re: Tap tones for archtop top-plate, help needed

PostPosted: Tue Jul 02, 2019 2:39 pm
by Alan Carruth
"Would 6 degrees really be enough downward force to achieve enough volume? "

Actually, as far as I can tell, break angle has no effect on volume, once you get to 'enough'. And you'll know it if you don't have enough. I've done a fair amount of work on this, and talked about it at some length on line, but I'll try to give a decent short summary.

First, the actual forces the string exerts on the top of the saddle are well defined, and depend only on the displacement angle of the string, the tension, and some of the characteristics of the string itself. The main force driving the top is the 'transverse' force: if the string is moving 'up and down' relative to the plane of the top it's pulling the top along with it. If the string is moving 'side to side' it's also pulling the top that way, but, for the most part, a guitar top can't move in that direction. There is also a twice-per-cycle tension change signal, but that's not a factor on archtops, and, besides, is much weaker than the transverse force for most strings (that's one place where the characteristics of the string come in).

So, suppose you pluck by pushing 'down' on the string somewhere near the bridge and then releasing it. The string bounces 'up' as soon as you let it go, but it can't make an 'upward' angle greater than the 'downward' angle it started out with. The closer to the bridge you pluck the larger the angle you'll get from a given displacement, but it will take a larger force to get a given displacement. If you push the string down too far it hits the fretboard, so that sets another limit. As it turns out, it's pretty hard to get an angle anywhere near six degrees, so the string will probably never make an 'up' angle greater than that at the saddle. This means that so long as the break angle exceeds that the string will always stay in contact with the saddle throughout it's whole vibration cycle. So long as it does that all of the force will be transmitted to the top of the saddle, and thence to the soundboard.

What would happen if the pluck angle did exceed the break angle? The string would hop off the top of the saddle, probably briefly, once per cycle. For the time when it was out of contact the vibrating length of the string would be the distance from the nut to the tailpiece, rather than the nut to the saddle. This would introduce all sorts of 'interesting' new frequencies into the sound, and you'd know it.

Of course, you never only push the string straight down; there's always going to be some sideways motion too. This can cause the string to roll across the saddle top, which also introduces some spurious frequencies into the sound. A shallow string nocth usually keeps thing in line.

I performed a rather lengthy experiment some years ago on this. I used a mechanical plucker to drive the strings of a Classical guitar 'vertically' at a known point with a known amplitude, and recorded the sound. I 'plucked' each open string six times so I could make sure they were all the same. This was done in my 'semi-anechoic closet', and the sounds were recorded in the same way for each pluck on my computer.

The variables in the experiment were the break angle and height of the saddle off the top. I was able to tie the strings differently to get two different break angles with the 'standard' saddle height of 11mm off the top. One tie gave a 'normal' break of ~25 degrees, while the other gave a break of only ~6 degrees. Then a taller saddle was put in that gave a ~25 degree break with the low-angle tie, but a string height off the top of ~18mm (don't try this at home!). Thus I had two setups with the same string height and different break angles, and two with the same break angle and different heights.

The recorded sounds of the strings were analysed. Changing the break angle without altering the string height off the top made no difference in either the maximum amplitude or the duration of the sound, nor was there any measurable difference in the overtone series of any string. When the height off the top was increased the power was the same, as near as I could tell, but the overtone series changed.

On an arch top guitar the twice-per-cycle tension change does not seem to drive the top usefully, since the tension is actually taken up by the tailpiece. On a flat top or classical guitar the tension change tends to rock the top of the bridge toward the nut, and produce some sound. The force is not large, and it's not driving the top in an efficient way to produce sound, but it's there. The taller saddle fed a bit of energy into the top at even multiples of the fundamental of the string, so there was a bit more energy in the second and fourth partials of the string in particular when the strings were higher off the top. This would not happen on an arch top.

I also ran listening tests, using a 'synthetic strum' for each case. Again, nobody could reliably tell the difference when the string height off the top remained the same, but the break angle was changed. When the string height changed almost everybody could hear the difference.

The bottom line, as far as I can tell from the experiments that I've done, is that break angle is the only thing that matters on an archtop. You need to have 'enough', and six degrees or a bit more should do it so long as the strings don't skate off across the bridge top as they're being played. Too much break angle and down bearing force does seem to be a problem. I can't say what 'too much' is in any general sense; it probably varies depending on the way you made the guitar. The 'pop up' model of the string tension pushing up and balancing the down bearing is just one plausible way of looking at it, and almost certainly leaves a lot to be desired, if it's even remotely 'right'. So long as you're within the range between 'enough' and 'too much' I don't think the exact angle matters. That's a reasonably broad range in my experience.

Re: Tap tones for archtop top-plate, help needed

PostPosted: Tue Jul 02, 2019 4:09 pm
by Brian Evans
In the "isn't that interesting" pile o'trivia, that Benedetto style tailpiece lends itself to low string break angles very nicely. It has a little fulcrum pivot that raises the tailpiece off the top of the guitar, and if you place a little shim under that pivot, as one might do to stop the top from being dented, you can make that height adjustable. The string angle is the same as the angle of the gut that holds the thing on, and loops around the end pin. It's quite easy and unobtrusive to have the string plane 1/2" or higher at the rim of the guitar. A typical trapeze tailpiece has the string plane around 1/8" at the rim of the guitar.