what the soundpost does

[keyboardclass]

It seems the soundpost supports the right leg of the bridge so the left leg can apply pressure to the bass bar.  I've recently read that it doesn't cause the back to vibrate which is vibrating a plenty anyway.  opinions?  Ta.

[David Burgess]

It does different things at different frequencies.

[keyboardclass]

I know what Savart had to say.  All a bit confusing.  This modern answer makes sense to me. 

[Dwight Brown]

I was under the impression that it caused the back to vibrate in phase with the top?

DLB

[Bodacious Cowboy]

1 hour ago, keyboardclass said:

I've recently read that it doesn't cause the back to vibrate which is vibrating a plenty anyway.  opinions?  Ta.

It absolutely does cause the back to vibrate. But also what David said.

[keyboardclass]

Quote

These vibrations are transmitted to the back through the ribs and blocks, the air inside the violin, and probably also
through the soundpost. Rödig, however, does not share this opinion, because in a violin without a soundpost the
back vibrates surprisingly strongly, often more than the top

From:  https://www.amazon.co.uk/Amadeus-Book-Violin-Construction-History/dp/1574670387/ref=monarch_sidesheet_title

[Shunyata]

As a coupled vibrating system, the violin is EXTREMELY complex.  It isn’t possible to explain  what the soundpost does in a simple statement… it alters the whole instrument.

Wouls be interesting to see a finite element analysis with and without the soundpost.

[FiddleDoug]

Since it directly couples the front to the back, with a more or less rigid member, how can it not transfer vibrations to the back, and cause it to vibrate?

[Don Noon]

18 hours ago, David Burgess said:

It does different things at different frequencies.

17 hours ago, Dwight Brown said:

I was under the impression that it caused the back to vibrate in phase with the top?

56 minutes ago, Shunyata said:

As a coupled vibrating system, the violin is EXTREMELY complex.  It isn’t possible to explain  what the soundpost does in a simple statement… it alters the whole instrument.

Yes, no, yes.

 

 

[keyboardclass]

Quote

As it is so close to the bridge's right foot, the soundpost strongly impedes the bridge's vibrations. This is easily
demonstrated. If, while the violin is being played, the right foot is immobilized with a pair of pliers, the tone is
barely affected; if the same is done to the left foot, volume and timbre are altered a great deal. Thus it is primarily
the bridge's left foot that causes the top to vibrate, which is why the bass bar must lie under the left foot.

..., "When the foot over the bass bar exerts downward pressure on the top, it causes the
back to rise, but when the foot above the soundpost presses on the top, the back is also pressed downward.

More from the book

[Jonathan B]

@keyboardclass, as a non-luthier who last seriously studied physics many decades ago, that is pretty much what I had thought. By anchoring the bridge's treble foot the soundpost means the bridge acts as a lever converting the horizontal vibrations of a string excited by the movement of the bow to vertical movements of the bass foot and the underlying violin top plate.

My thinking is that to first approximation the violin works like a loudspeaker, with the area between the f-holes vibrating and corresponding to the cone of a loudspeaker (the bass bar ensures the area vibrating is more than just the area under the bass-side bridge foot). But the second order effects will be much more complicated and inevitably controversial. A Hi-Fi loudspeaker depends as much on its cabinet as its driving speaker unit, with its ability to deliver sound evenly across the frequencies dependent on the size, shape and volume of the resonant cavity, the position of the vibrating cone and of any apertures, and the acoustic properties of the cabinet materials including any internal baffle.

At least with a violin the geometry and resonant volume is pretty much constant. Judging by discussions on this forum though, even with fairly standard choices for types of wood used the tone can vary considerably with its exact qualities and the precise nature of its graduation. And in ways which seem to depend more on the skills of the maker than the predictions of physics.

My own half-remembered physics makes me think that the resonant cavity will be the main determinant of low to medium frequencies and the nature of the cavity walls for the higher frequencies. WIth of course all actual notes consisting of a whole spectrum of harmonics as well as the fundamental frequency so those less understood effects are involved in the tone of every note.

Is that a proper explanation? If not shoot me down! In the past my curiousity has led me to try to find out more from internet sources, but those seemed to lead to whole PhD theses which were rather too much detail - and still failed to come up with definitive findings.

[David Burgess]

The Strad 3D project has animations of how different parts of a violin move at different frequencies. Things are not as simple as one might wish them to be. At some frequencies, the soundpost is rather stationary. At other frequencies, it is moving quite a bit.

https://strad3d.org/videoTour3.html

[FiddleDoug]

1 hour ago, keyboardclass said:

More from the book

A pair of pliers! You have got to be kidding!!! They didn't even specify what kind of pliers!

" "When the foot over the bass bar exerts downward pressure on the top, it causes the
back to rise"

You're going to have to explain how that happens. Walter Kolneder was not a luthier, violin maker, or scientist!

[Don Noon]

1 hour ago, Jonathan B said:

My thinking is that to first approximation the violin works like a loudspeaker, with the area between the f-holes vibrating and corresponding to the cone of a loudspeaker

38 minutes ago, David Burgess said:

The Strad 3D project has animations of how different parts of a violin move at different frequencies. Things are not as simple as one might wish them to be.

The loudspeaker concept only holds (somewhat) at the lowest few modes, up to 600 Hz at most.  Above that (and that's where you find the differences between great and horrid violins), you get modes of increasing complexity and detail where if you optimize only for the loudspeaker concept, you'll miss a lot of the good stuff and get a horrid result (from experience).

 

[Jonathan B]

@Don Noon I did admit that the higher frequencies (including the harmonics that make up the tone of any note) were beyond my simplistic physics. It isn't obvious (to me) that the soundpost necessarily forms part of that though, it is only a small part of the resonant surfaces whose precise nature will define those high frequency tones.

[sospiri]

Surely the pupose of both the bass bar and the sound post is to allow the belly to be a very light construction thus improving tone and power?

[Don Noon]

The higher you go in frequency, the smaller the vibrating patches (antinodes) become and the closer together the nodal lines get.  Simplified, the presence and precise placement of the post can make a difference in these higher mode patterns as well as define the coupling between the modes and the trebel bridge foot... and thus affect tone.  A few of the midrange modes might be common and analyzable, but most of them are different depending on the individual instrument.

[Deo Lawson]

Record a short piece on your violin. Then, if you feel equipped for such a task, take the soundpost out of a violin, and record yourself playing again. No damage to the top will occur in such a short time.

Then you will understand what the soundpost does. All the fancy physics explanations are basically brain candy.

[Marty Kasprzyk]

One of the things a soundpost often does to old instrments is to crack their top plates.  Another thing it does is to deform their arch shapes if they don't crack first.

[Marty Kasprzyk]

14 minutes ago, Marty Kasprzyk said:

One of the things a soundpost often does to old instrments is to crack their top plates.  Another thing it does is to deform their arch shapes if they don't crack first.

Oops!  I'm wrong. Probably the arch shapes are distorted first and then they crack.

[LCF]

1 hour ago, Deo Lawson said:

Record a short piece on your violin. Then, if you feel equipped for such a task, take the soundpost out of a violin, and record yourself playing again. No damage to the top will occur in such a short time.

Then you will understand what the soundpost does. All the fancy physics explanations are basically brain candy.

A series of experiments you can perform:

Take a cheap and worthless fiddle. Note how it plays and sounds.  Carefully remove the back then glue on a piece of 9/16" mdf in its place.  Prepare a new sound post. How does it sound now???

Next cut a hole through the top under the treble foot of the bridge.  Prepare a sound post which supports the treble foot against the back. How does it sound now?

 

Does this prove anything?

Hell I don't know.

[keyboardclass]

I play on countless soundpostless violins at auctions so I'm well aware of the soundpost's tonal effect.  The question is what change in the instrument's response brings that about?

[Jonathan B]

9 hours ago, Don Noon said:

The higher you go in frequency, the smaller the vibrating patches (antinodes) become and the closer together the nodal lines get.  Simplified, the presence and precise placement of the post can make a difference in these higher mode patterns as well as define the coupling between the modes and the trebel bridge foot... and thus affect tone.  A few of the midrange modes might be common and analyzable, but most of them are different depending on the individual instrument.

Thanks, that is a helpful description which explains a role of the soundpost in tone quality (upper frequency components) as well as tone generation.

Is that effect mainly on the thinner more flexible upper plate? Or in different words, what is the result ot @LCF's thought experiment using MDF?

(To be fair, that experiment really needs MDF which is arched like the original back as well as thick, so that there is no change in either the volume or the shape of the resonant cavity just in its ability to vibrate).

[Don Noon]

12 hours ago, LCF said:

A series of experiments you can perform:

Take a cheap and worthless fiddle. Note how it plays and sounds.  Carefully remove the back then glue on a piece of 9/16" mdf in its place.  Prepare a new sound post. How does it sound now???

6 hours ago, Jonathan B said:

To be fair, that experiment really needs MDF which is arched like the original back as well as thick...

I did that experiment a few years ago...   

 

 

[Jonathan B]

Fascinating @Don Noon. Being unfamiliar with your experimental set-up - and not finding it in the link - are those the sound spectra for a single excitation note with each back, or the acoustic power out for similar excitations at each frequency, or something else? (Looks to me a bit like the sound spectra from white noise excitations, but that doesn't quite make sense in terms of how a violin is played).

Clearly though the different back material affects the spectrum, with the lower line at higher frequencies possibly meaning less colour in the tone and probably less projection. Or have I interpreted that wrong?