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Resources -How to Set a Sound Post
Introduction: Basic theory of the sound postPart 1:About the soundpost setter, and choosing the wood
Part 2:Determining the height and fitting the post
Part 3:Positioning and adjusting the post
This basic guide will be expanded with time.Emailme your questions about this topic, and I will
answer in the text of the document. In this way all will benefit.
Introduction: Basic Theory of the Sound Post
This introduction is meant to a guide to those who have little or no
knowledge of violin construction, particularly concerning the sound
post. Hence it will be largely explained in simple, practical terms.
Below is a diagram of a violin cut laterally throught the center thatillustrates the position of the sound post in a completed violin.
The sound post is a small dowel of Spruce that is held by friction
between the top and back plates of a violin (viola, cello, or string bass),situated under the treble side of the bridge. It has both structural and
tonal importance.
Structurally, it supports the top plate, acting like a pillar under the
bridge. Without it, a violin would "cave in" on the treble side. A good
violin, however, may not be harmed if it is without a sound post for ashort period of time, but an instrument should not be left in this
situation for long.
Acoustically, the sound post transfers vibrations from the top plate tothe back plate of the instrument. It also alters the vibration of the top
plate. Its placement, length, thickness, grain orientation, and wood
selection influence the tone of an instrument. An instrument without asound post will sound weak, thin and hollow.
On the bass side of an instrument is a "bass bar", a cross section ofwhich can be seen in the illustration above. It has both acoustic and
View of a sound post through
the end pin hole at the bottomof the violin. Also seen are
some of the cleats whichreinforce the back seam.
The sound post can easily be
seen through the f-hole on the
treble side of the instrument.
http://www.violins.on.ca/luthier/soundpost1.htmlhttp://www.violins.on.ca/luthier/soundpost1.htmlhttp://www.violins.on.ca/luthier/soundpost2.htmlhttp://www.violins.on.ca/luthier/soundpost2.htmlhttp://www.violins.on.ca/luthier/soundpost3.htmlhttp://www.violins.on.ca/luthier/soundpost3.htmlmailto:[email protected]:[email protected]:[email protected]:[email protected]://www.violins.on.ca/luthier/soundpost3.htmlhttp://www.violins.on.ca/luthier/soundpost2.htmlhttp://www.violins.on.ca/luthier/soundpost1.html -
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structural roles as well.
Part 1: About the Sound Post Setter, and Choosing the Wood
A: The Sound Post Setter
The sound post setter is a small metal tool about 23cm/9" long. The sharp point at one end is pressed
into the post to hold it for inserting into the instrument. The other end, which looks like a hand, is for
moving the post once it has been placed in the instrument. (Sound post setters available here).
B: Selecting the Wood
The post is made of a dowel of spruce, usually about 5.5-6mm in diameter for a full size violin. Thepost must be small enough to fit through the f-hole at the notches, which is usually the widest point.
This is true for all instruments (from the smallest violin to the largest bass). The grain should be fine,
straight and even, preferably matching the wood that was used in the top of the instrument. So if the toplate instrument is coarsely grained, use a post with wider grain; if it is filely grained, use a post that
has closer growth rings. (Sound post material available here).
Part 2: Determining the Height and Fitting the Post
A: Determining the Height of the Post
Once a sound post setter has been obtained, and an appropriate piece of material selected, the next step
will be to cut and fit the sound post. There is a tool which is a great help to determining the height of
the post, called a sound post gauge, which greatly simplifies this process.Soundpost gauges can befound here.
I will also explain here explain how to do it without a guage. A small needle file can be used to
ascertain the approximate height for the post.
Insert the file (or other appropriate tool) in the upper 'eye' of the f-hole until it touches the backof the instrument, making sure that the file is perpendicular to the plane of the instrument(parralell with the ribs, or sides).
Place your thumb nail on the file to mark the length needed. Mark this length on the sound post. The end grain direction of the sound post must run
perpendicular to the grain in the top. This is important to the performance of the instrument.
Cut the dowel somewhat longer than what is required for the post With a single-cut file, file the ends at angles to match the arch of the top and the back.
B: Inserting and Fitting the Post
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To place the post inside the instrument, insert the sharp end
of the setter into the post, about 2/3 of the way to the top(picture A)
Gently lower the sound post into the instrument (picture B).
Once inside the instrument, place the bottom of thesoundpost onto the back in the approximate place where it
will be. It may be easier to start by placing the soundpostcloser to the centre of the instrument, and then pulling it
toward the f-hole once it has been placed.Picture A
Picture B
Part 3: Positioning and Adjusting the Post
The correct position of the sound post is behind the bridge foot (toward the tailpiece), as seen in theabove diagram. Generally, if the sound post is too short, the upper wing of the f hole will sink with the
strings tensioned and the post in the above position. If it is to tall, the wing will protrude. Adjust as
necessary.
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SOUND POST ADJUSTMENT FOR BEST PERFORMANCE
...The general guide for sound post placement for full sized violins is that the near side of the sound
post should be 1/8" behind the back side of the bridge and centered on the right foot of the bridge, or
perhaps slightly outboard from this position, but inside the outside border of the right hand bridge foot.
...With the model violin I make I find that a measurement of 0.13" is best (0.13" - 0.125" = 0.005"difference from normal).
...I believe that the entire range of adjustment is in that +/- 0.005" range from the 1/8" standard. See test
chart below to see actual test results showing how loudness and tone changes with slight movements ofthe sound post.
... Along with the relative position to the bridge, the post should be vertical and both ends should
closely match the angle of the plate at the contact point.
...Before you move the sound post I recommend that you use a fixture to hold the violin horizontal and
which firmly clamps the instrument down so both of your hands are free. It is a very good idea tolessen the string tension to say only 1/4 of normal tension to lessen the pressure on the sound post so
that it can be moved easier and to prevent damage to the wood at the contact point, especially to thesoft wood of the spruce top plate.
...I use the special flat sound post pliers to move the top of the sound post and a sound post tool tomove the bottom end of the 'post. Normally I move the top end of the sound post first until it is in the
position I want, and then increase the string tension to about 1/2 tension and then move the bottom end
of the post until it is exactly perpendicular (to the plane of the back plate's perimeter edges).
... When I make a sound post, I draw on an arrow in ink showing 'up' and facing the right side of the
instrument; that way, at a glance I can see which end of the sound post should be up and be sure thatthe post has not turned.
...The grain lines of the sound post should be oriented from side to side, that is perpendicular to thegrain of the top plate.
...The sound post needs to be a certain length within +/-0.002" of the ideal length. When in position and
with no string tension, the sound post should stand in position without falling when the violin is gently
tipped. It is the string tension that really clamps the sound post into position. You may want to rub a bit
of white blackboard chalk onto the ends of the sound post to help it keep it's position, when installing anew sound post.
...As a practical length, I suggest that you add 0.010" to the ideal centerline length when making yournew sound post. This will add a slight preload and make it less likely to shift or fall out of position.
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...Here I am measuring the bottom end angle for a
new sound post. The angle protractor
measurements are written down for futurereference. The center-line length of the sound
post is also measured at this time.
... Notice that this tool has small wood sound postmaterial epoxied to the ends of a commercially
available tool. These fit looselyto begin with andthe epoxy mixtured holds them on while inserting
through the sound holes and raising into position;
then the epoxy sets up with the ends in position,at the correct end angles, in the violin being
measured. No guessing anymore!
... Once the sound post top end is installed and put into
position, this tool allows measuring to see if the sound post is
vertical. The bottom has a cup shape that nestles against thesound post, at which time the top mounted bubble balance is
checked for level. You can then see which way the bottom of
the sound post needs to be moved in order for the post to
become vertical.
...It must be remembered that in all operations involving
moving the sound post, no tool must ever come in contactwith the wood of the violin around the sound holes. The
spruce is very fragile all around the sound hole opening and
because cross grain wood this thin can be easily broken off, itmust not be touched... thus the importantance of having both
hands free to manilupulate the special tools and a fixture that
firmly holds the violin in place.
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...Sound meter is held in position ( by the pictured metal rod) 14" from the violin's bridge while
maximum loudness of the violin's bowed open strings is recorded on chart (as pictured in photo to theleft).
...The bottom chart unitsare in inches behind the
bridge back to the back
side of the sound post (inhundredths of an inch).
The right side chart unitsare DB of loudness
recorded from the sound meter; each 10DB is double the sound loudness. This
chart is typical for a full sized violin.
Notice that the maximum loudnessincreases about 30% when the ideal soundpost position is used (and all strings reach
a peak at the same point as the tone is
maximized (tone impression line is beingpointed to with the pen). Once this point is
determined and recorded, it becomes the permanent ideal sound post location and need not be
concerned about in the future.
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...Also note that a change in positon of only +/- 0.020" will be a loss of between 20 and 30% of the
total possible power and tone quality. The sound post position is not an arbitrary setting but one that the
instrument insists on for proper playing.
...When doing the above test, the sound post must be moved (after first re-installing the holding fixtureand losing the string tension) , then the sound post made exactly vertical again in its new position, and
then the string tension brought up to concert pitch, bridge straightened up and the maximum loudnessbowing test repeated for the next data point. Thus this test can easily consume an hour of time to do
properly, but the results are valuable!
EXACTLY MEASURING FOR, CUTTING, AND FITTING A SOUND POST...
...Fitting the sound post accurately is made more difficult because both the top and bottom plates of the
violin are curved at the contact with the ends of the sound post; besides this, the angle of the end is
usually different at the two ends.
...The sound post is approximately 1/4" in diameter and is made of spruce wood. The grain of the soundpost is oriented at 90 degrees to the grain of the wood for the top plate... that is, the grain of the one
goes across the grain of the other. Normally there should be between five and seven growth rings
across the 1/4" width of the sound post. Less grains for a warmer tone and more for a colder tone, is the
general rule.
...In this article I will outline my method for carefully measuring the exact length of sound post neededfor a particular instrument and how to measure the end angles and accurately cut them, all at the correct
overall length (length of the centerline of the sound post).
First, I will be helpful if we have some basic sound post tools:
Sound post retrieval tool (for when the post is loose inside instrument.
Sound post pliers, to manipulate the top end of the sound post
Sound post setter, adjustment tool; useful to adjust the
bottom position of 'post.
Vertical alignment measuring tool
Handmade sound post installing tool; or just use the
sound post setter's pointed end
Twin-layer thin plastic gage for measuring position of
sound post through the "ff" hole
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...This is the tool that will allow you to
tell if the sound post is exactly vertical
when inside the violin. The bottom has ashape that 'cups' around the sound post
and is parallel to the vertical shaft of the
tool.
...The soldered-on brass rod is to allow
the tool to be easily turned but is notabsolutely necessary.
...On top (out of the picture) is a 'bulls-
eye' level mounted on a wood block that
slides over the vertical shaft. (I remove
for storage convenience when not inuse).
...This tool is commercially available and the inner metal shaft slides up inside
the brass tube so that it can measure the centerline length of the sound post thatis needed. The thumb screw locks it into the correct length; the tool is then
removed from the violin "ff" hole opening and measured. You will notice in
this photo that I have added two wooden ends which are tipped to match theinside plate contours...more on this later.
...The plastic post location tool. The two
pieces are identical and the end is cut as
shown so it can hook around the sound
post when it is in position inside theinstrument. The end are stapled together
for that the two pieces stay in 'registrar'.
...Thus by looking at the outside piece
you can tell where the inside piece is compared to the bridge.
...A vernier caliper measures the center-line length of
the sound post to the nearest 0.001". be sure to
measure in the center of the tipped ends of the wood
tips (90 degrees to the tool...not shown)
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...This tool measures the end angles of the sound post tool
so we know what angles to 'cut' on the sound post ends to
accurately fit the instrument. Usually the end angles aredifferent so it is a good idea to put a mark on the sound
post as to which way is 'up'...I draw an arrow on the side
facing the "ff" hole so it is clearly visible through thesound hole.
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...(this is the bottom of the holder fixture) Notice that
the top section lifts off the four support feet. This
allows you to move the instrument so that you canrecapture the sound post after it falls over and is
rolling around inside; without having to unclamp theinstrument from this holder (saves time and
frustration).
...This is the final tool you need to make which allows
for the accurate hand sanding of the end angles and
which also allows you to exactly control the length ofthe sound post.
...It is bolted to the mitre gage of my table saw andslides in the slot. The end angle of the post is set on
the mitre gage. The sanding disc is flat and the cut is
made with by sliding the mitre gage in and out byhand while feeding the motionless sanding disc out
(my 'Shop Smith' table saw allows the saw spindle to
slide in and out with a lever...perfect for thisapplication.
...These three photos show more details of the sound post tool that allows you to measure how verticalthe sound post is once it is installed in the instrument. It fits in through the right hand "ff" hole by
turning it sideways.
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...Now I will show you the whole process of fitting a new sound post for and instrument from the
beginning...
...The first step is to mount two cut off pieces of sound post stock with oversize holes drilled in theircenter about 70% of the length deep (just larger enough to allow the wood ends to pivot and conform to
the shape of the inside of the instrument, when put inside and raised into position). Then some 5 minute
epoxy is mixed up and put on the steel shaft ends of the sound post length finding tool. The ends arecovered with the two wood 'post ends and then they are gently held in position while they are passed
through the right hand "ff" hole. Once inside, the tool is maneuvered so that it is in the position of the
sound post and then the tool is lengthened to firmly contact the top and bottom plates and then thethumb screw is firmly locked down. Do this with the tool vertical as shown by the bubble balance. The
wood ends tip and conform to the inside plate shape, then the glue sets up.
...It is best to fit the sound post with the instrument lightly strung up with the bridge in properposition and just enough tension in the strings to make them straight. Once the sound post is in
position, then the string tension can be brought up to proper tension and that string pressure will hold
the sound post firmly in position.
...Here the end angles of the sound post are being
measured with the protractor tool (and recorded (ofcourse) for future reference....
...The needed length of the sound post may change
over time but the needed end angles should remain
constant.