Sound Post diameter-How important is it?

panos · #1 06-19-2007, 05:24 AM

I have noticed that the diameter of the sound post differs from bass to bass. Also several doublebass stores sell sound posts with diameters like 15, 16, 17, 18 and 19mm. Mine was 15mm and the one I replaced it (bought from Lemurmusic) is 18mm.

I wonder how the factor of the diameter affects generally the sound and the setup quality of an instrument, and if it is crucial.

Also, how determine the proper sound post diameter that an instrument needs?

Bob Branstetter · #2 06-20-2007, 10:21 AM

Quote:

Originally Posted by panos

I have noticed that the diameter of the sound post differs from bass to bass. Also several doublebass stores sell sound posts with diameters like 15, 16, 17, 18 and 19mm. Mine was 15mm and the one I replaced it (bought from Lemurmusic) is 18mm.

I wonder how the factor of the diameter affects generally the sound and the setup quality of an instrument, and if it is crucial.

Also, how determine the proper sound post diameter that an instrument needs?

While the difference in the total weight and surface contact area might conceivably make a difference in the sound that could be measured with sensitive scientific instruments, for all practical purposes you will never hear any difference. The most important factors are how well the post is fitted to conform to the interior surfaces of the top and back and the how tightly the post fits. The density of the soundpost wood can also affect the sound, but this is not usually an option for you when you buying a soundpost from a retailer.

bonaventura · #3 06-20-2007, 01:22 PM

Quote:

Originally Posted by Bob Branstetter

While the difference in the total weight and surface contact area might conceivably make a difference in the sound that could be measured with sensitive scientific instruments, for all practical purposes you will never hear any difference. The most important factors are how well the post is fitted to conform to the interior surfaces of the top and back and the how tightly the post fits. The density of the soundpost wood can also affect the sound, but this is not usually an option for you when you buying a soundpost from a retailer.

Bob,

Chuck Traeger writes in his book on Setup of DB for Optimum Sound that one of the "genies" is that the edges of the soundpost ends can be sanded away, leaving a smaller overall surface on each end (not rounded, just less flat surface) and that this will produce a better sound. He says it diminishes a twisting action that the top and back produce on the post. Do you have any experiences with that ? Do you have an opinion on that ? Is it worth trying? Would a smaller, e.g., 15 mm, soundpost have any advantage there ?

Bill

1st Bass · #4 06-20-2007, 02:43 PM

Ah, mm!
:-)

Bob Branstetter · #5 06-20-2007, 02:50 PM

Quote:

Originally Posted by bonaventura

Bob,

Chuck Traeger writes in his book on Setup of DB for Optimum Sound that one of the "genies" is that the edges of the soundpost ends can be sanded away, leaving a smaller overall surface on each end (not rounded, just less flat surface) and that this will produce a better sound. He says it diminishes a twisting action that the top and back produce on the post. Do you have any experiences with that ? Do you have an opinion on that ? Is it worth trying? Would a smaller, e.g., 15 cm, soundpost have any advantage there ?

Bill

There's was a lot of discussion about this when the book came out. Do a search and you'll find it. Chuck Traeger likes to experiment and sometimes an experiment works and sometimes it doesn't. I and most luthiers I know are of the opinion that the soundpost should make full contact with the plates to insure the best sound from an instrument. You should note that Chuck also details the accepted traditional method of fitting a soundpost.

**Matthew Tucker** · #6 06-20-2007, 06:14 PM

For my bass, I simply made a soundpost from a piece of cedar with straight grain and a nice ring to it, and planed it down to a dowel shape thin enough to go through the FFs. I fit the post carefully (took quite a long time about this!) and it appears to work just fine. I think I ended up at about 19mm diameter.

I think a narrower soundpost or one with a bevel might be easier to fit since the area of contact is smaller.

Martin Sheridan · #7 06-21-2007, 07:14 AM

For years the suppliers always sent me 19mm posts unless I was getting them for small basses. Lately everyone has been sending 16mm. At this point I can't tell the difference. I think Bob is right in that the fit is very important, not only contact service, but where the post is placed, it's tightness and angle. However, once you get moving the post around to adjust for sound, it is no longer going to fit the same as it did before.

I also like to toss them on a hard floor or workbench and listen and watch them. I like the ones that "dance" around and "sing" the best.

jallenbass · #8 06-21-2007, 07:50 AM

I pulled a pencil thin soundpost out of my 1880s German bass and replaced it with a modern one. No discernible difference to me.

Silversorcerer · #9 06-21-2007, 10:30 AM

One thing we might consider is the distribution of pressure on the plates. If the diameter decreases slightly, the area of the end of the post is decreased by a greater proportion. This increases the pressure on the remaining area where contact is made with the plates. The area of a circle is π (R squared) where R is the radius. Since the radius of the post is squared in arriving at the area of the end of the post, a difference of 3mm in diameter is significant in terms of lbs./ square inch increase on the part of the plate resting on the post. If we take 19mm, we have an area of about, 283.5 square millimeters on which the pressure on the plate is distributed. If we take 16 mm, that goes down to 201 square millimeters. It there is 50lbs. of pressure from the treble bridge foot on the post (hypothetical), a 19 mm diameter post results in .1763 lbs./ square mm. vs. .2487 lbs./ square mm for the 16 mm post. The smaller post diameter may not make a difference in sound, but the larger one is putting less shearing force on the plates at the edge of it's diameter. And it increase by a squared (geometric difference) rather than a simple arithmetic one. So part of the answer on post diameter is in the limit of shearing force the grain structure of the plates will bear. If it makes no difference in the sound, I would think a larger diameter post would be safer.

reedo35 · #10 06-21-2007, 11:43 AM

Quote:

Originally Posted by panos

I wonder how the factor of the diameter affects generally the sound and the setup quality of an instrument, and if it is crucial.

I seem to remember asking this same question several months ago...
Sound Post Diameter

**Matthew Tucker** · #11 06-21-2007, 03:14 PM

Quote:

Originally Posted by Silversorcerer

19 mm diameter post results in .1763 lbs./ square mm. vs. .2487 lbs./ square mm for the 16 mm post

Thats one way of putting it.

Or you could say simply that at one extreme a needlepoint post may provide a better fulcrum for vibration, but will spike through the plates, while at the other extreme a telegraph pole will have no chance of going through the plates but won't do much for vibration. Find your compromise!

You do love your maths, don't you Silver!

Silversorcerer · #12 06-21-2007, 03:57 PM

Quote:

Originally Posted by Matthew Tucker

Thats one way of putting it.

Or you could say simply that at one extreme a needlepoint post may provide a better fulcrum for vibration, but will spike through the plates, while at the other extreme a telegraph pole will have no chance of going through the plates but won't do much for vibration. Find your compromise!

You do love your maths, don't you Silver!

Mostly what I was trying to illustrate was that a 16% reduction in diameter produces a 41% increase in pressure. The relationship is not linear. So with the smaller diameter post, the bridge can take much less of an accidental knock without cracking the top. Knowing that, I'd be inclined to err on the side of caution and use the larger post, particularly if I was going to knock off the hard edge a bit.

Jake deVilliers · #13 06-21-2007, 04:30 PM

Then how would you see Traeger's 'hourglass' soundpost concept.

It could have the same footprint on top and bottom but be lighter and (he says) more flexible through the middle. I'm not sure I see the post as a vibrating member but I'm open to the concept.

Silversorcerer · #14 06-21-2007, 05:45 PM

Quote:

Originally Posted by Jake deVilliers

Then how would you see Traeger's 'hourglass' soundpost concept.

It could have the same footprint on top and bottom but be lighter and (he says) more flexible through the middle. I'm not sure I see the post as a vibrating member but I'm open to the concept.

Well, certainly if it is narrower in the middle it is more flexible, but why that would be a good thing escapes me. If the post is mostly a fulcrum, it seems like it would do it's job better if it is stiff. Reducing the friction at the edges makes intuitive sense and it would result in a gradual increase in pressure at the edge not unlike a tapered caul used to clamp wood. The tapered edge of the caul keeps it from digging in at the edge. But it seems if one narrows the center of the post that one shifts the fulcrum point or flex point away from the end of the post to the middle. I can't see how that would be an improvement in the function of the post, but who knows. Experimentation would be required and a great deal of it.

**Matthew Tucker** · #15 06-21-2007, 07:06 PM

As far as I remember the hourglass idea is to allow the post some twisting motion, allowing freer vibration of the corpus. But it's only going to be a tiny effect compared with the construction of the rest of the bass. And its only Traegers theory, and there are all sorts of other variables subject to experimentation too.

A thought on experimentation... the theoretical opposite of an hourglass would be a sphere, and whilst it would be devilishly easy to fit and move around a spherical post, it would be a devil to get between the FF holes!

Jake deVilliers · #16 06-21-2007, 07:49 PM

And the sphere would have an even smaller footprint on the top and back. I'll let the wizard do the math.

Unless, of course, you inflated it in place! Fully adjustable.

Silversorcerer · #17 06-22-2007, 06:59 AM

A side note:

The original sound post in my 4/4 5-string DB was a fat 21 mm diameter post. This was supplied by the maker of course. I fitted it in the spring when the weather was still cool. When I fitted a new post 19 mm diameter (could not locate a 21 mm blank despite several inquiries) in the maximum heat and humidity of the summer, it came out about a 1/16 of an inch longer. I didn't notice a change in sound with the new post. Certainly it's possible to custom make a post, but I wondered is if anyone has seem another 21 mm diameter sound post or if this is rare, possibly for 4/4 size instruments only?

**Matthew Tucker** · #18 06-22-2007, 07:24 AM

I just measured my workshop broomhandle with a micrometer.

Exactly 21mm

Bob Branstetter · #19 06-22-2007, 08:13 AM

Quote:

Originally Posted by Silversorcerer

Certainly it's possible to custom make a post, but I wondered is if anyone has seem another 21 mm diameter sound post or if this is rare, possibly for 4/4 size instruments only?

I ran across one that was nearly 25mm in a true 4/4 size (not what they are calling 4/4 today). I suspect that it was actually 1 inch since this was back in the 1960's before the metric system became so universal. This monster had the biggest and ugliest ff holes I've ever seen to go along with it's 45" string length.

Making good soundpost blanks in virtually any practical size isn't all that difficult if you have the right tools and some good, well aged, straight grained spruce. Here's (page down) a tool like the one I've been using to make my soundpost blanks for over 40 years. All that is required is a little fine sanding after the blank is completed and that's just for cosmetics.

1st Bass · #20 06-22-2007, 08:43 AM

Here's another way to make soundposts-- this one was for violins, but it will work for any size--the two plates were set as close together as possible, and the holes are concentric-- the entering hole has to accommodate the square stock, and the exit hole takes the finished size. The router bit was just a straight cutter bit. The whole gizmo was clamped down on a router table. It worked extremely well...as I recall it did not even require sanding. And it was very, very fast. All made out of scraps of plywood.

The apparatus is positioned so that the router bit is exactly lined up on the inside edge of the finish-side hole. I think there was a "U"-shaped hole in the bottom plate to allow positioning of the router bit. Build it for free, make all the soundposts you want. I think you'd have to make a different jig for each size of soundpost stock, but still...