Official Luthiers Forum!

My friend plays the fiddle and swears this is critical

This from wikipedia

"In a string instrument, the sound post is a small dowel inside the instrument under the treble end of the bridge, spanning the space between the top and back plates and held in place by friction. It serves as a structural support for an archtop instrument, transfers sound from the top plate to the back plate and alters the tone of the instrument by changing the vibrational modes of the plates."

so any relevance to guitars? anyone tried one?

No. It's for bowed instruments, not plucked.

Adding to the previous comment. The sound post as used in the violin family would kill the vibrations on a plucked instrument. On the bowed instruments, the bowing action continually adds energy to sustain the vibrations.

Think about what the sound post does. When you bow a string on a fiddle, you're pushing it sideways, and the top has to move in and out in order to push any air. The soundpost 'nails' the treble foot of the bridge down, so that it can't move, turning the bridge into a bellcrank. The top of the bridge moves sideways, causes the whole thing to rotate around the treble foot, and the bass foot pushes down on the top. The bassbar underneath carries that force along the top.

This comes at a cost: Carleen Hutchins once told me that the sound post cuts the overall efficiency of the fiddle in half. The trick is that it mostly cuts the output in a high frequency range that nobody wants to listen to anyway, and it also enhances the output in the low range, where you need it. Pre-soundpost violins had three strings: they lacked the low G.

On a guitar you normally pluck the strings in a way that causes them to move 'vertically' with respect to the soundboard, so thay can push it effectively. Thus you don't need the post. Besides, on the guitar you need all the efficiency you can get; the violin gets plenty of power from the bow, but there's not much energy in a pluck.

I _have_ put a post in a guitar a few times: archtops that feed back too much when played with enough gain to be heard over the horns, usually. It cuts out the feedback nicely, but kills the acoustic sound.

Vibrating tops, like strings, have harmonic nodes. If the soundpost is placed correctly, it rests in a harmonic node and "voices" the instrument. however, it also reduces the overall output of the instrument the way touching a harmonic node on a string reduces the output of the guitar, but is neccesary to keep the top from collapsing, as violins are not braced in quite the same way as a guitar is.

If placed incorrectly, it simply stops the instrument from resonating, like if you touch a guitar string in the wrong spot.

The string pressure at the bridge squeezes the top down onto the post, holding it in place.

In an acoustic guitar, the string tension torsions the top away from the back.

_________________
Old growth, shmold growth!

Think of how the violin instruments sound plucked. As others have said the top just isn't free to freely vibrate as in a guitar, it's continuosly driven by the bow.

_________________
Jim Watts
http://jameswattsguitars.com

When installing sound posts into acoustic guitars, the secret is that you need to make sure you use enough of them.

I've found that 6 to 8 does the trick, all cut about 1/4" inch longer than you would think they should be.

I make 3 or 4 and throw them in the trash can, for good luck. I don't put any in the guitar!

_________________
Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars