Tap Tuning

[Craig Bumgarner]

Thanks Bill. I just downloaded and tried SweepGen. Seems to work great. I have amplified PC speakers, they might work, to get started at least. I have Al's DVD. Now that I have a tone generator, thanks again to you, I'll have to give it a try.

CB

[Bob Gramann]

I found when I had the volume loud enough to really make the tea leaves dance that I wanted hearing protection.

[Craig Bumgarner]

Yes, Alan mentions this in his DVD. After watching it a while, I wanted ear protection just from the video

I don't know if my amplified PC speakers will be loud enough, but I think it might be. It is a separated system though w/ a sub woofer and two tweeters, so I've to experiment with which speaker works best, if at all.

[Barry Daniels]

It seems like the quite finger tapping that I do has some unintended benefits. No burst eardrums. <g>

[Bob Gramann]

I worked with the tea leaves a few times. I figured out that I got to exactly the same place as when I tapped and carved. I haven't done the tea leaves in a long time.

[Alan Carruth]

Bill Hicklin wrote:
"4 can be just about any lightweight granular stuff."

Mark Blanchard uses table saw dust, of which he says he has plenty...

You'll need about a 12W amplifier.


Wear hearing protection.

eh, what's that you say....?

WEAR HEARING PROTECTION

Oh, yeah, good idea...

Alan Carruth / Luthier

[Andrew Porter]

Craig, I am trying ot understand how you deflection rig simulates the string loading. Your rig loading is normal to the top plate were most of the string load is at right angles to that, i.e. toward the headstock. I can see there is some torque involved with the string loading. Is the ~20 lbs simulating that torque?

[Craig Bumgarner]

Andrew:

The guitars I build have strings attached at a tailpiece like an archtop or violin, so the bridge sees primarily a downward force compared to a fixed bridge. I know, if one looks closely, even this kind of bridge is rocking back and forth. In slow motion, close up video, it is not strictly up and down, it actually looks more like an earthquake, but for my purposes, I think the downward pressure of the strings and the resulting deflection of the top is the majority of what is going on. Again, this is not exact science, there are lots of subtle things going on besides deflection, but in an otherwise well built guitar, deflection testing helps me be sure I'm in the ball park before I close the box. To me it is a way of accessing the effects of varying stiffness or lack thereof of various pieces in a top or back, but in the assembled stage. It also allows me to adjust while it is still easy to do so.

With a fixed bridge and strings terminated at the bridge, there is a much larger component of load along the string of course and I do not propose my jig simulates this. Roger Siminoff shows a rather complicated jig that does simulate the rocking motion induced by strings on a fixed bridge.

I am not familiar with how others are using deflection testing on fixed bridge guitars, but it appears from this thread that some are. That said, I can imagine that downward deflection would still give you quantified ideas about the stiffness of a top and bracing system and that might give some degree of control even if it does not directly simulate the string load.

CB

[Alan Carruth]

Craig Bumganer wrote:
"In slow motion, close up video, it is not strictly up and down, it actually looks more like an earthquake..."

I'd love to see that video: how did you do it? The motion would be very slight, I'd think. Also, in some tests on an archtop with a tailpiece, I found evidence that the tension change signal of the string is not an effective driver of the top, where it is (marginally) on a flat top.

Alan Carruth / Luthier

[Craig Bumgarner]

Craig Bumganer wrote: "In slow motion, close up video, it is not strictly up and down, it actually looks more like an earthquake..."

I'd love to see that video: how did you do it? The motion would be very slight, I'd think. Also, in some tests on an archtop with a tailpiece, I found evidence that the tension change signal of the string is not an effective driver of the top, where it is (marginally) on a flat top.

I didn't make a video. I was speaking from memory of a Djangobooks forum discussion a couple years ago (search on "earthquake"). I went back and looked at it just now, the actual reference was to slow motion photography, not video, sorry, did not mean to mislead.

There are a number of videos on-line showing plucked strings in slow motion. I like this one http://vimeo.com/4041788 The chaotic nature of the string movement is readily apparent. It is easy to imagine how this chaotic motion would be translated to the bridge and top, both responding in kind.

I'm sure you are right about bridge & tailpiece vs. a fixed bridge. By terminating the strings right on the top, there has to be more load transmitted to the top directly and that has to count for something. In your test, how did you define "effective"? My completely unscientific perception is acoustic archtops (not the electric ones that look like archtops) and Selmer style guitars with their tailpieces are considerably louder than most fixed bridge steel string guitars, at least in a performance environment. Of course, volume may not be the only definition of effective and it takes more than a bridge to be loud. The lack of "effectiveness" may actually be part of the Selmer tone by not driving certain frequencies well.

I wonder what a violin with a fixed bridge would sound like?

While we're at it, here's another question..... How much of the sound that the guitar body projects comes from the string vibrations being transmitted through the bridge. 100% or nearly so? Or does the vibrating sting alone produce enough energy to get the top going on its own, at least to some substantial degree? I guess that would be easy enough to test, just set up a taut sting over a guitar body, but attached to something other than the body and pluck it. If there was no physical contact, the only way to transmit the energy would be through sound waves and my guess is there just isn't enough energy that way to do it. I suppose there is a well known answer to this, but I don't know it.

CB

[Clay Schaeffer]

Hi Craig,
The bridge acts as a "bridge" to carry the vibrations across from the string to the top. The material it is made of creates an impedance mismatch which can be helpful to do this. Different materials create different impedances which is why some materials make better bridges than others. This would merit a whole discussion of it's own.

[Barry Daniels]

It has not been my experience that tailpiece guitars (archtops) are louder than fixed bridge guitars. Just the opposite in fact.

Some archtops can project their sound quite well in one direction (forward), but that is a whole other thing than just total volume.

[Alan Carruth]

Thanks for clarifying, Craig.

That's a cool video, but it's hard to interpret without some more information. In particular, you'd need to know the pitches of the notes being played, and the frame rate of the camera. Maybe that information is in the thread. I don't think most video cameras have a high enough frame rate to 'freeze' the vibrations of even very low pitched strings, so what you're probably seeing is some sort of of 'aliasing', similar to the illusion of wheels on moving wagons turning backwards that we used to see on TV westerns. It looks as though the camera was catching the string somewhere near once every ten cycles or so, and you see the slight change in position since the last frame because it's not exactly synchronized. What lines up best in a case like that are the waves for the tenth (or whateverth) partial, so that's what you see. You can see similar stuff if you hold a vibrating string up to your computer monitor and get the frequencies synchronized with the refresh rate.

There's a cool java app you can download that shows how a plucked string vibrates, although I don't have the URL handy. I talk about it in the 'String Theory' paper on my web site, on the 'Acoustics' page, and I may have put the URL in the notes at the end. I'll say that I Iearned a lot on that project; not least that, although strings are the 'simple' part of the system, they're far from simple in themselves!

"In your test, how did you define "effective"? "

A big impetus behind that string project was to settle the question of whether it's the 'transverse' string signal or the 'tension change' that is the big driver of the top. Basically, as the string vibrates up and down with respect to the top, it pushes the top up and down, in a loudspeaker like motion, so that's one force on the bridge. When the string is displaced from a straight line the tension is higher,and it rocks the top of the bridge toward the nut. Since it's off-center twice per cycle ('up' and then 'down') the tension change signal is frequency doubled with respect to the transverse signal. The usual model (see Fletcher and Rossing: 'The Physics of Musical Instruments' for example) says that the transverse signal is stronger then the tension change. Also, it's easier to move the top 'up and down' than it is to rock the bridge, and the loudspeaker movement should produce sound more efficiently, due to lack of 'phase cancellation'. Thus one might expect the transverse vibration of the string to produce more sound. There are, however, people who dispute this. Since most of the argument I've seen is based on calculations of the string forces on the one side (and my math chops are weak), and experiments that look questionable to me, and are not well documented on the other, I figured I'd just look up the experiment where somebody acutally measured the forces. When I couldn't find it, I measured them myself. It took a lot longer than I thought it would.

Anyway - One of the measurements I made was designed to see if the tension change force produced any noticable amount of sound on an archtop guitar. To do this I took advantage of the fact that the overtone content of a plucked string depends on where you pluck it. If you pluck it at a point 1/n of the length, where n is a whole number, every nth partial will be missing from the signal. This is basically because you're plucking the string in a place where those partials would not be moving. It's sort of the opposite of playing 'harmonics' on the guitar, where you pluck the string (starting a bunch of partials in motion) and then touch it at, say, the 12th fret, half way along the string, and suppress all of the odd-numbered partials that have to move there. Plucking the string just in the middle does the opposite, and only activates the odd partials.

Remember, though, that the tension change signal is frequency-doubled with respect to the transverse frequencies of the string: if the string plucked in the middle has only odd-numbered partials in the transverse signal, the tension change signal will have only even numbered ones. Plucking the A string in the middle sounds the 1st, 3d, 5th and so on, partials in it's transverse signal: 110 Hz, 330 Hz, 550 Hz, and so on. The tension change signal has the 'even' multiple frequencie: 220, 440, etc. Plucking a string in the center, using a 'wire break' technique that 'plucks' the string at a known point and starts it moving in a known direction, can thus tell you whether the tension change signal can produce any sound from that configuration: if you record the sound, and it contains even-order partials, the tension change signal is producing them.

I tried this on a flat-top guitar, and saw some energy (not much) in the even partials. A harp, where the string tension pulls upward on the soundboard directly, showed much more, and the 'odd' and 'even' partials decayed at different rates, suggesting that the two signals were coupled diferently into the soundboard. Trying the same thing on an archtop guitar gave no significant energy in the even partials: they can't drive the top by 'rocking' the bridge, and the back angle of the neck is not sufficient to couple the tension directly to the vertical top motion to any significant degree (as my Indian calculus prof used to say (rapidly): "As the djangle goes to djero, the sine of the djangle also goes to djero: what I mean I am now explaining").

Obviously, this was one set of experiments on particular instruments: your milage may vary. It's an easy thing to replicate, though.

"How much of the sound that the guitar body projects comes from the string vibrations being transmitted through the bridge. 100% or nearly so? Or does the vibrating sting alone produce enough energy to get the top going on its own, at least to some substantial degree?"

How much sound does a Les Paul guitar make when you pluck the string if it's not plugged in, and not resting on a table?

For the most part, strings are too small to move much air at the frequencies they vibrate at. As far as the string is concerned, it's more on the lines of trying to run in knee-deep water: you don't so much start currents moving as just create eddies that steal energy. To make any amount of sound, you have to attach the string to something that's big enough and stiff enough to move some air, and light enough that the string can move it. Hence, soundboards.

The soundboard is not the only part of the gutiar that the string can move, of course, but its the only part that's big enough to produce much sound. Neck vibrations, for example, can take some energy from the string, but they don't make much sound because the headstock is just too small to be a good soundboard. To prove this, you can hold the guitar up by pinching the neck around the nut or first fret between your thumb and finger so that the guitar hangs freely, and tapping on the back of the headstock. This will activate the first 'xylophone bar' resonant mode of the guitar, usually (on steel strings) at something like 60-70 Hz. Although the headstock will vibrate a lot, you have to put your ear right up to it to actualy hear the sound. If, however (as can happen, particularly on classical guitars) this resonant mode is high enough in pitch to couple with the top at the 'main air' frequency, it can actually move some air in and out of the soundhole, and alter the timbre of the guitar in the low range. That stored energy doesn't do much good unless it's tied into a soundboard or cavity resonance that actually move some air.

I'll note in passing that this seems to be the only 'neck' mode that matters on acoustic guitars. It's different on solid bodies, where the neck is the most flexible part of the system, and can steal enough energy to reduce what's in the string for the pickup to hear. Basses are particularly prone to this, since the bridge is 'way down on the end, where the bar' mode moves the most, and that's why they have more dead notes.

This post has gone on long enough.....

Alan Carruth / Luthier

[Jason Rodgers]

This post has gone on long enough.....

But is why we keep coming back for more!