| Official Luthiers Forum! http://luthiersforum.com/forum/ |
|
| steel string vibration envelopes not equal? http://luthiersforum.com/forum/viewtopic.php?f=10101&t=26683 |
Page 1 of 1 |
| Author: | Doug Powdrell [ Wed Mar 24, 2010 4:25 pm ] |
| Post subject: | steel string vibration envelopes not equal? |
Is it common for one string to not vibrate with the same general consistency as the others? Could this be an indication of poor contact at the nut or saddle??? Typically, D strings do not appear to vibrate with same intensity as others, using Martin style bridge with pins not parallel to saddle.. Sound is fine. Could it be improved if all strings vibrate consistently??? Logic says yes....Working to modify bridges so that each string exerts equal torque on saddle....i.e. variable string slots...??? thanks. |
|
| Author: | Alan Carruth [ Thu Mar 25, 2010 12:25 pm ] |
| Post subject: | Re: steel string vibration envelopes not equal? |
Doug Powdrell wrote: "Typically, D strings do not appear to vibrate with same intensity as others, using Martin style bridge with pins not parallel to saddle.. Sound is fine. " Well, there's a good clue that maybe there's nothing wrong. "Could it be improved if all strings vibrate consistently??? Logic says yes...." 'Improved' how? If the sound is fine now, even and so on, then making one string 'more effective' that the others would tend to make it less even, I'd think. "Working to modify bridges so that each string exerts equal torque on saddle....i.e. variable string slots...??? " I know a lot of folks don't think so, but the main force of the string to produce sound comes from the up-and-down motion of the string relative to the top. The bridge can rock, but the tension change signal is typically about 1/7th as strong as the transverse (up-and-down or side-to-side)signal, and since the top is built to resist allowing the bridge to torque forward, it's the transverse force that provides most of the power to the top. I can say this with a _lot_ of confidence, since I've made the measurements. You can get all the gory details on my web site, on the 'Acoustics' page, as a .pdf file. This is not to say that the torque of the bridge makes no contribution to the sound, but from what I can tell (I'm still working on that particular experiment) the height of the strings off the top is the most important variable there. That's close enough to 'the same' on most guitars so that small changes won't have much effect, IMO. Break angle by itself seems to have only a very small effect, if any. The really important things in the way the transverse force works are the string tension and the 'characteristic impedance' of the string. The latter is basically a measure of how hard the string can push on the top. Back in the old days (Leopold Mozart) they used to think that you wanted all of the strings to have the same tension. It turns out that if you do that, they will have different impedances, and will tend to sound a little uneven. Modern string sets vary a bit in tension from string to string and have impedances that are nearly matched, as far as I can tell. In other words, they compromise a little so that things like the 'feel' that depend on tension are not too uneven, and neither is the sound. What that might do is give you strings that 'sound' and 'feel' more or less the same, but vibrate with slightly different amplitudes. There are some other things that could be getting into the act as well. For example, the top vibrates in response to string forces, and that, in turn, shakes the strings. It's possible that the D string gets less of this than other strings: maybe the top isn't moving as much at that frequency, or it's not moving in that place. There's also the question of how you're measuring the string motion. Strings can follow a pretty devious path, moving both vertically and horizontally, with any point on the string making some pretty weird gyrations sometimes. It could add up to a lot of energy in the string for not a lot of apparent motion in one or another plane. At any rate, I go back to my observation that, so long as things sound OK, what are you worried about? It's nice, and even useful, to understand this stuff, and I've spent 'way too much time and energy doing just that. But, in the end, if it ain't broke... |
|
| Author: | Doug Powdrell [ Thu Mar 25, 2010 7:24 pm ] |
| Post subject: | Re: steel string vibration envelopes not equal? |
Alan-Thanks so much for your thoughts on this......I have read your articles in the Red Books/GAL magazines and look forward to meeting you someday....it's great to have such a person on the OLF. I adhere to the concept that the transverse motion of the bridge pumps the soundboard. You indicate that 'the height of the strings off the top is the most important variable here'.....but when I visualize a floating bridge with tailpiece (low angle) on a steel string, there's less vertical 'down' force to move the soundboard....and then my mind says increase this, via string break angle. I'll admit I try to design so that the saddle is as tall as possible (3/16" or so) without excessive loading of the bridge. Heck, they may not last as long, but seem to put out the sound. I think Cumpiano indicates the saddle height is the volume knob on the acoustic....(but at a price, if exceeded). In looking at Siminoff's table of string tensions (.012-.053"), ADG are near 30#, while E, B,E are around 22#. I follow your concept of 'unequal tensions' and 'characteristic impedance'....thanks. Very interesting concept that 'maybe the top isn't moving as much at that frequency, or it's not moving in that place'....great comment ,thanks....does one need to design so that these vibrations appear equal. No. Does a professional guitar player care about this? No experience here. How am I measuring string motion.....Yes, I agree string vibration is a complex affair...starts off horizontal, oscillates toward vertical and various combinations thereof..... I am simply visually comparing horizontal....my bad. String sound is not dull or muted, just seems to stop vibrating before all the others, even the ones with less mass and less tension.....soundboard is absorbing this frequency. In summary, I build to let the right brain play (I have some left brain stuff-mech/civil engr.) that creeps out every now and then...need to suppress.....if it sounds good, what's the problem???Just trying to use statics analysis to optimize the bridge/saddle/string break angle for 'even' sound (and look?) I need to strum other guitars besides my own and see how they vibrate/sound. Thanks again, Alan. I'll just keep on building and experiment along the way..... |
|
| Author: | Alan Carruth [ Fri Mar 26, 2010 1:45 pm ] |
| Post subject: | Re: steel string vibration envelopes not equal? |
"...but when I visualize a floating bridge with tailpiece (low angle) on a steel string, there's less vertical 'down' force to move the soundboard....and then my mind says increase this, via string break angle..." My own experinece with archtops is that reducing the break angle over the bridge almost always seems to help the sound. Far from becoming louder with more break, often they are simply 'choked'. Break angle on a flat top, with the strings fixed to the bridge, works differently, of course, and is a bit more complex. Look at it this way: when the string is 'down', below the line it follows when it's at rest, it's pulling downward on the soundboard, and when it's up, it's pulling upward. The down and up forces are simply a function of the angle and the tension of the string. So long as the string is in contact with the top of the saddle, it's going to be transmitting as much signal as it can: if the force oscillates from 0 to 10, that's the same as if it oscillates from 2 to 12, except that the latter has a DC component, if you follow me. Thus, in terms of transmitting signal, you only need enough break angle over the saddle to keep the string in contact when it's all the way up. As I say, I'm in the middle of a rather involved (aren't they all?) experiment looking at the relative influences of saddle height and brake angle on a classical guitar. From what I can see so far, a fairly large change in break angle makes only a minimal change in the sound of the guitar, if any. Altering the string height off the top changes the static torque on the bridge, which seems to effect the way the top works a bit, and also gives more prominance to the tension change signal. With the strings further off the top there is more of the second partial in the mix, if my measuremebnts are to be believed. We've got some other things to look at, including doing some listening tests to see if people can hear things we can measure, or maybe things we can't. That one is going to take a while. |
|
| Page 1 of 1 | All times are UTC - 5 hours |
| Powered by phpBB® Forum Software © phpBB Group http://www.phpbb.com/ |
|