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Brewerpaul
Jun-21-2005, 9:21pm
I just had some work done on my fiddle including having a new sound post fitted. It got me thinking, why isn't there a sound post in a mandolin? It's sort of similar to a violin in some ways. Wouldn't a post help transmit the sound to the back so it resonates too?

Bill Snyder
Jun-21-2005, 9:53pm
Soundposts don't work on plucked string instruments. Pluck the fiddle. Do you want your mandolin to sound like that? http://www.mandolincafe.net/iB_html/non-cgi/emoticons/rock.gif

Dave Cohen
Jun-22-2005, 10:36am
True enough; soundposts don't work well on plucked stringed instruments. But that short answer doesn't give any hows or whys. I haven't looked, but I hope that some of the old discussions on the subject are sufficiently accessible for newcomers. Maybe a note or two on this should be included in a FAQ or two.

A soundpost does several things in violin family instruments. First, it introduces some asymmetry into the plate modes (both top and back plates). Second, it does provide some coupling between top and back plates, but only for certain modes. The reason for that is that the soundpost, when properly located, is quite near to the nodes for some of the plate modes. for those particular modes, there will be either no coupling, or very weak coupling between the plates. Are these things "good"? Well, apparently for violins, they are, though they are not completely understood from a physical standpoint. Third, the soundpost tends to dissipate some of the energy supplied to the violin body through the strings via the bridge. For a violin family instrument, that is a good thing. In those instruments, the periodic (read "rapidly repeated") driving force supplied by the bow supplies excess energy, and some of it contains high frequency components which would result in harsh sound radiation from the violin. The dissipation of those high frequency components is thus a good thing for bowed instruments, but would greatly attenuate the response of plucked stringed instruments. A single pluck or pick stroke, no matter how forceful, just doesn't contain anywhere near the total amount of energy in a sustained bow stroke, nor does it contain the undesireable high frequency components of the bow stroke.

http://www.erols.com/judcohen

blammo
Jun-22-2005, 10:38am
Soundposts don't work on plucked string instruments. Pluck the fiddle. Do you want your mandolin to sound like that? http://www.mandolincafe.net/iB_html/non-cgi/emoticons/rock.gif
hmmÖmy bass has a soundpost. and it sounds waay better plucked than bowed. (probably due to my awful bowing technique http://www.mandolincafe.net/iB_html/non-cgi/emoticons/tounge.gif )

Lee
Jun-22-2005, 10:39am
Dave, not to hijack the thread, but could you post your insights concerning violin type tailpieces and the mandolin.

Dave Cohen
Jun-22-2005, 9:38pm
I forgot to add, in response to Brewerpaul, that the backs of plucked stringed instruments do indeed vibrate. Mechanical vibration is imparted from the top plate to the back plate via the sides ("ribs") and also by the air in the instrument body cavity.

Re tailpieces; I also am concerned about hijacking this thread. Probably best to start another thread on tailpieces. Iirc, tailpieces have also been discussed in another thread in the builders' section, though I can't remember exactly when.

jmkatcher
Jun-22-2005, 11:19pm
I seem to remember reading that some L&H instruments had soundposts. Can anyone verify that?

Paul Hostetter
Jun-23-2005, 2:28am
Lyon and Healy imported lots and lots of violins and so on, all of which had soundposts. On other non-typical string instruments? Not that I ever heard of. I have tried soundposts for people on a number of occasions, just to satisfy their curiosity about what happens to guitars and mandolins and so on. They tend not to want them to remain in place for very long.

Maybe I can offer a simpler version of why soundposts go in violins and their ilk. Iíve been in violin repair for more than 30 years, and 65% of my work is with violin family.

1) Violins donít have frets, and without such a clear point of contact for the vibrating string, its signal decays pretty fast. But since you use a bow, that decay-prone note can be sustained as long as the bow is in motion and in contact. So the simple physics of the string is very different.

2) All the plate mode considerations aside, the real point of the post is to carry some of the string energy to the back. (Iím not so sure about characterizing this as the dissipation of "high frequency components" though.) Violin family instruments, and viols for that matter, which include double basses, are generally held away from the playerís body so the back is not dampened. It thus has the potential to do something audible, and the soundpost facilitates that. Take a soundpost out of a violin and play it and it sounds rather thin, as though itís running on half its cylinders. You donít hear undesirable high frequencies - you hear diminished frequencies.

Part of the greater energy of a violin string in motion comes from the bow, but another part is the considerably greater top pressure engendered by the tall bridge. Set a cello up with a neck angle that couples with a bridge like youíd see on the fairly analogous archtop guitar, and the note, whther plucked or bowed, will be rather pathetic. Same thing when you simply bow a guitar: not much happening. Itís not just the difference between plucking and bowing.

Comparing plucked and bowed instruments is difficult because so many things about each of them are so different as to be almost unrelated.

What can always be done, and I have certainly done it enough, is to try putting a soundpost in a mandolin and seeing what happens. It wonít hurt it, so try it. Have someone who really knows how to fit a post do it for you, and try it in different locations. Youíll learn a lot. Or try taking one out of a violin for the same reason. It wonít hurt it, so try it. I have even made fretted fingerboards for violins (deadly), and fretless guitars (interesting). The more you do hands-on experimenting like this, the more you learn, and the more you realize why things have evolved to where they are.

http://www.lutherie.net/harp.bass.sm.jpg

Jim Hilburn
Jun-23-2005, 8:03am
I've made the case to several bass guys that the upright bass is being used improperly by being plucked and that someone needs to design an upright just for this purpose, but I usually just get blank stares.
But that has nothing to do with mandolins, does it.

Dave Cohen
Jun-23-2005, 9:50am
Paul the "real point" of the soundpost is *not* to carry some of the string energy to the back. The sound post does do some of that, but in a very frequency dependent way. At some frequencies, it does just the opposite. For the main body mode which couples with the main air mode, there appears to be a node running through almost exactly where the soundpost is located. So for that combination, the soundpost is *not* carrying much of the string eneergy to the back plate. Also, carrying the string energy to the back plate is not at all the same as dissipating the unwanted high frequency components. Please note that I did *not* characterize carrying the string energy to the back plate as being the same as dissipating unwanted high frequency components.

AS I said above, back plates do vibrate without the aid of a soundpost. Motion of the top plate imparts motion to the ribs and to the air inside the instrument body cavity. The lowest body mode consists of both plates moving in a trampoline-like motion (except for the node apparently imposed by the soundpost!), and occurs at two (occasionally three) frequencies. In one case, the plates are moving in opposite directions, and the ribs are alternately bulging out and caving in. In the other case, the plates are moving in the same direction, and the ribs are moving up and down without bending. In both cases, the ribs are a coupling link between the top and back plates. The moving air in the cavity also couples the top and back plate motions, though the coupling may be weaker.

This stuff is not a matter of my hypothesis or opinion; it has been in the primary peer-reviewed literature for decades. You can certainly disagree with it. There are lots of people who believe that the Earth was literally formed in six days. Far be it from me to change their minds. But if you disagree with what is in the literature, you are bucking an extremely large body of empirical evidence.

http://www.erols.com/judcohen