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Does anyone here measure the frequency components while free plate tap tuning? I'd be interested in learning more about what has been done in this area.

No.

I only listen for sustain over the lower bout of the soundboard.

And usually I'll hear a bottom end rumble as well as some high frequency component
in the tap tone but it's the decay that interests me - not specific frequencies.

Cheers

Bob

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Bob Connor
Geelong, Australia

I know that Al Carruth often records a standardized 'tap tone' and puts it through some software to get a frequency spectrum.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

I am just getting into the "tap tone" craze. I just bought the Benedetto book and it sort of describes it. I had never done this while the guitar was appart. In fact, I always waited until the top was glued on and I would hum/sing into the sound hole and listen for resonance that way. Though I now see it would be much to late to change anything anyway.

I have about 10 different old violin tops in various states of being carved. Most are probably 20-30 years old. Last night I went down and performed some tap testing on them just to get aquainted with the sounds. I was quite surprised to here all the differences in tone from one top to the next. Even the boards that were the same wood and the same carve had completely different tones. Some bright and high pitched, others warm and lower pitched. Then of course the ones that were more completely carved had tones generally lower than those not as far along the process. Finally the the hardwoods like Maple had a complety different tone and repsonce than the soft and lighter woods like Spruce.

According to Benedetto's book, the actually "note" has limited or nothing to with how the guitar will sound. (I am not sure that is completely true...but will see). His book mentioned that it is much more important to get the particular wood you have in your hands to sound as best it can. If you do that, then the instrument should sound very good. He comments that even poor grade woods can sound fantastic if you take your times and tap-tune the top.

Anyway, I am just ramble'n but it seems timely to read this.

Joe

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Joe

On some sets of wood, one side will have a different tap tone, and holding it 'upside down' can yeild a different tap tone also. I have sets with "bass sides" and "treble sides".

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Here is what a Parlor Guitar is for!
http://www.youtube.com/watch?v=oEa8PkjO6_I

Is it the length of decay, or something else that you're listening too?

Here's a bit of detail about the measurements I've started to do.

First I thump the braced top and record the sound. I repeat this a few times holding it in different places. Here's a chart showing the amplitude of a thump vs time.


Then I convert to the frequency domain so I can see the resonance peaks.


I like to pick out the peaks and plot them as dots.


Then I take periodic "snap shots" and add them to my dot chart. This shows how each mode decays with time. Dots that are closer together vertically show a frequency that has less decay than dots that are more spread out vertically.

John,

So what does this tell me about this wood verses others? By doing these tests, can someone determine which guitar, once completed will have the most ring or sustain? Can you use this process while tuning the top? Thouht would be once you top reaches the longest decay time, will it start to go the other direction? I mean, if you could use this to help tune a top, would you continue to work the top until it no longer gains decay? (or would it continue to gain decay time until it was too thin to work effectively on the instrument?

Just wondering.

Joe

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Joe

That's the idea. It remains to be seen how useful it is. I can't imagine why it can't be at least as useful as tap tuning- but I could be wrong.

My hope is that it provides a way to understand and share information about tap tuning. It's very rare to find a description of tap tuning that can be understood at all, let alone accurately, by the person reading it. Now I'm not talking about how or if what the person is saying actually makes a guitar sound one way or another- that's a whole different story. I'm just talking about understanding what is being said. For example, if someone says they shave braces until there is no clear fundemental we should be able to describe that as not having peaks in the spectrum that are so many dbs higher than other peaks in the spectrum.

Now some folks use Chladni patterns as a way to get and communicate more information about plate tuning. Looking at the spectrum of the tap tones can compliment that technique. For example, the spectrum makes quick work of finding the frequency of modes. The Chladni pattern can then show what mode a given peak in the spectrum looks like. Chladni patterns give a lot of information about the shape of the various modes. The tap spectrum gives information about the relative amplitude of the various modes.

Although it would be nice to be able to relate the numbers and shapes to sounds in a guitar that's not yet possible- as far as I know. The general idea of how these techniques are used, including tap tuning, is that you learn as much as you can about the characteristics of the parts of a guitar as you are building them. Then, if and when you end up with a guitar that sounds the way you want your guitars to sound you build more by building guitars with parts that have the same characteristics as the parts of the "good" guitar.

Agreed John and this is admittedly one of the holes in the available information for aspiring guitar builders.

When I set out to begin to learn how to use tap tuning or more specifically in my case to learn to voice tops to assist me with voicing them I could not find information that I could understand anywhere. Then one day I PMed Lance and he helped me make a personal break through that got me started. The next step for me was getting a copy of John Mayes "Advanced Voicing" DVD and that went a very long way in helping me out further.

But most of all my own understanding and the methods that I use these days were gained by spending a lot of time bracing and attempting to voice tops and practice tops. I made 3 practice tops with inexpensive wood solely for the purpose of learning and this included exploring under bracing and learning where that fine line may be.......

Perhaps the reason why tap tuning or voicing is seemingly difficult to learn, although it need not be, is that it is as much of an art as it is a science. As such it needs to be experienced first hand, over and over again, and in short order the rings that you start to hear will start to make sense to you.

Attempting to understand voicing or tap tuning out of the context of actually experiencing it by doing is IMHO impossible.

You're right, experience is necessary. Tools can help too though. It's so easy for your mind and ears to play tricks and get in the way of what you think you're hearing. I wonder how many people are actually looking for the same thing when they're tap tuning.

I can't remember spending much time holding a plate to my ear and tapping when I was first learning how to build although I'm sure I must have tried it some. I think it was just too primitive (didn't give enough information) a method for my teacher and also too hard to teach to students. It was hard enough to get the idea of what was happeing when you could "see" what the plate was doing.

I find it curious that there's so little information available about the dynamics of the components of guitars and so much about everything else. Maybe because it's fairly recently that the tools needed to study this are readily available.

Now I see where you're going with this.

I strongly suspect that there are several things we could usefully look at; the tap tone frequencies/Chladni patterns themselves and the decay rate of the modes are certainly two of them. The 'frequency domain' stuff turned out to be pretty easy to do with analog equipment, and got tried first. At some point we're going to have to correlate things in the 'time domain' as well.

One question I have with looking at the relative strengths of modes is whether you're taking into account the changes in mode shapes as you work. When you hold and tap a top the tap tone contains a mix of sounds from different modes: those that are not active at the holding point and active at the tapping point, with upper and lower frequency limits set by the nature of the 'hammer' and the tap. Since we know that the mode shapes change it's possible that at one stage in the processs you're hitting close to t a node line of a given mode, while later you could be tapping nearer an antinode, and getting much more sound from that same mode. This could have little to do with how 'active' the mode itself is. It's possible to measure things like the effective mass and loss factor of any given mode, of course, so long as it's reasonably well isolated in frequency, but those things take time.

In the end, of course, all of the theoretical quibbles will have to give way if the technique works. I have observed that the theorists are _always_ able to come up with some way to explain data, even if they have to resort to eleven dimensions to do it!

The idea is to use the same tapping technique that is often shown when people give tap tuning demonsrations. Hold the top and thump it. If you hear something that doesn't sound like it's being muffled by the holding hand then that's a good thump. Try thumping in a different place. Try holding in a new place. etc. For this experiment I recorded three thumps. Two of the thumps yielded very similiar frequencies. The third thump was close but had a couple of different modes. I imagine you would do this as you work at shaving braces. Like the tap tuners. I thumped with my knuckles, again like the tap tuners. Perhaps there is some room to refine my technique.

In my case I started with a top with braces pretty much where I wanted them-- using my pull it out of my hat technique. Now I may do a bit more shaving.

After I got my charts it was hard to tell what I had so a sprinkled some glitter. The frequencies I found on my charts were near the modes. If nothing else, this is a good way to hunt for modes.
Here's some shots of some of the modes. The top is 14 fret 00 size, mostly Martin lower bout, a bit of Carruth around the neck block.

Al's statement about the nature of the 'hammer' and a post I read in another forum where Tim McKnight describes how he uses the fleshy part of a finger to do his tapping made me think it would be a good experiment to try to measure how a tap done McKnight style would differ from the knuckle tapping I was doing.

Tapping at the bridge location it was easy to hear that the knuckle tap had higher frequency components. Here's a plot of the modes I found using both taps. Red circles are from the knuckle tap. Blue diamonds from the fleshy finger. The taps were normalized to the mode which was at 87 Hz. It's hard to see but there's a diamond and a circle there.

I decided to try constraining the edge of my top plate to see how that changed things. I just used some spool clamps to hold the plate to my outside mold. I gave the plate a knuckle tap and recorded the result. Constraining the plates seems to have coralled the lower modes into a single mode where the lower bout is pumping as a unit - that's what I saw with glitter on the constrained plate. At higher frequencies the plate behaves in a fairly similiar way when constrained or free.

It seems that the raw spectrum data from a constrained tap and a free tap is the best way to show what happens.