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Double Bass DIY soundpost and tail gut adjustment

Povl Carstensen

Povl Carstensen

Jul 19, 2014
1,096
825
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Copenhagen
www.povlerikcarstensen.dk
#1
A couple of years ago my bass went through a major repair with a neck block reset. The top was off and was serviced. Got a new bridge and sound post in the process. My main impression was that the bass sounded quicker, but perhaps a bit smaller, less deep.
Then I changed the tail gut to Efrano gut. In the proces, the sound post fell. And when I reset it, it ended up a bit towards the inside of the G side bridge foot (1,5/2 cms?). Perhaps the bass had settled a bit, perhaps my luthier pulled it tighter than I preferred.
A couple of weeks ago, I went for it. Loosened the strings. Pulled out the post, shortened it about 2 mms (ofcourse the "back" end). Reset it, and it now ended more in line with the bridge foot, perhaps some mms towards the outside. And the result was as hoped. A more full, deeper sound, a bit softer feel. Actually, perhaps the G string speaks a bit softer, but it is okay, since the overall balance is better. Yesterday was the first gig, and the bass definitely had more presence.
While I was at it, I actually also shortened the tail gut. In principle, always do one adjustment at a time, but well, any way... It had stretched, so I shortened from about 3,5 cms "swinging" length, to about 2 cms. The object of this is to get a more focused attack (the tail piece can "wobble" less). In the process the afterlengths tuning went from 2 octaves and a minor 7th, to 2 octaves and a 5th. Except for the E, which due to different curvature of tail piece and bridge, or a sligthly asymetrical bass overall, is "off".
It has been a bit nerve wrecking, but overall I am liking the change. I am about at the point, where I can't think about more things to "do" to my bass.
 
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#3
Here is a bit of basic soundpost instruction. This guy has some interesting videos, and some not so much.
One thing, I would never knock my soundpost around without holding it with something. I use my wire coat hanger thing.
IMG_20190202_123108.jpg
 
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#5
Povl Carstensen said:
... A couple of weeks ago, I went for it. Loosened the strings. Pulled out the post, shortened it about 2 mms (ofcourse the "back" end). Reset it, and it now ended more in line with the bridge foot, perhaps some mms towards the outside. And the result was as hoped. A more full, deeper sound, a bit softer feel. Actually, perhaps the G string speaks a bit softer, but it is okay, since the overall balance is better. Yesterday was the first gig, and the bass definitely had more presence. ...
Click to expand...

To be fair, I'll tell you why I'm asking this question before I ask it: I have a theory that for optimum volume and tone, with higher tension strings a double bass needs less compression on a sound post and that with lower tension strings a double bass needs more compression on the sound post. Of course in order to implement that, less compression is accomplished with a shorter sound post, higher compression is accomplished with a longer sound post.

With that in mind, what kinds of strings are/were you using before and after your sound post length change?
 
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#6
Don Hergert said:
To be fair, I'll tell you why I'm asking this question before I ask it: I have a theory that for optimum volume and tone, with higher tension strings a double bass needs less compression on a sound post and that with lower tension strings a double bass needs more compression on the sound post. Of course in order to implement that, less compression is accomplished with a shorter sound post, higher compression is accomplished with a longer sound post.
Click to expand...
I heard that somewhere else too.

The reason might be that the amount of clamping the top and back by the pressure of the soundpost might have an optimum to define the vibrational knot at the soundpost position.

With too much pressure the clamping might be so high that the top cannot wiggle at the soundpost position because of stiffness and pressure.

With too less pressure the knot might not be defined well enough when the top moves up and down with modes of vibration that don‘t have a knot there. (With more pressure these modes would get damped.)

But this is only my theory.
 
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#7
I have a different, perhaps simpler, theory on string tension and SP tightness. This is all based on my current string search, and my own recent/current study of SP tension, on my new bass.

I think the primary job of the SP is to keep the top plate in a "neutral static loading", or "optimal" position. Some call this "pre loading". If the top plate is in a "neutral static loading" position, then when we play, the entire vibrational energy of the strings (heavy or light gauge strings) will have full potential of top plate excursion. We can also feel a nice arco response when the top plate is loaded just right. Does that make sense?

Notice how when we put on heavy string they tend to choke the bass? That's because the top plate is "pre loaded" downward near the downward limit of its vibrational excursion, so there's no room for the top to move downward anymore. The arco response tells us this. Therefore to counteract the loading of heavy strings you need a tighter SP to transfer that string load to the back plate, thus keeping the top plate centered in its neutral range of excursion. I think this is the reason many violin makers "spring" the bass bar. But I am in the no-spring bass bar camp, because we can keep the top plate neutral by way of SP tightness.

You have the opposite effect with light gauge strings: With light strings, and if your SP is in tight, it pushes the top plate upward (or anti pre loading) thus also choking the sound by restricting the full upward excursion of the top plate. Does this make sense?

So now you can extrapolate my "neutral static loading" theory to seasonal effects of SP tightness due to changes in humidity and temperature. Some folks are really anal with multiple SP for every season, when perhaps one will do, tapped in or out as necessary, to maintain optimal neutral static loading of the top plate.

With all of this experimenting, I am now in the camp that SP tightness is more important than actual SP placement. So why don't I "upgrade" to adjustable SP? Well, first I'm going to see how hard it is for me to maintain optimal SP tightness by hand with a traditional wooden SP over the next year.

What do the experts think about all of this? Am I crazy?
 
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#8
You can also look at the top as on a membrane of a drum. If you press on the membrane the pitch of the drum rises but also the vibrational patterns and their frequencies change and damping rises.
Anyway a high pressure on bridge for and soundpost effectively clamp the part of the top from bridge foot to soundpost, the top cannot vibrate as freely as with less pressure.
 
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#9
Interesting comments. I play with Efrano plain guts in the "weich" version. So, low tension. Intuitively I have always gone for a "soft" feel, and "deep" tone. So I would not pull the sound post very hard. One thing is that my bass is a rather longish 3/4, about 106,5 cm. However, I am pretty certain that before long, I will try and move the post about half the distance towards the bridge. To see if that gives a hint of attack, mid frequenzy presence. Think Mingus versus Paul Chambers, or something like that. Will certainly report.
 
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#10
I moved the soundpost a bit north towards the bridge. Say from 1½ SP diameter to about 1 SP diameter away. I just loosened the D and G to do it. So now I am inside the "allowed" distance. I did not sound like Paul Chambers before, and it did not make me sound like Charlie Mingus. The living room result is it seems a bit easier to play in tune, to adjust intonation. Perhaps because there are a bit more overtones, treble information. The feel is still soft etc. Should be, with my string set up. As I am getting more comfortable doing this, I guess I will try at some point to move even closer. Overdo it, to understand how it works.
About the same time, I moved the tail gut. For some strange reason, my end pin assembly sits a bit to one side of the bass (anyone else experienced that?). Forcing the tail gut a bit, so it comes over the saddle more centered, seemed to give a livelier feel. Its an easy experiment to do, and I did not have to loosen the strings.
Heres a video on that detail:
 
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#13
stefaniw80401 said:
Yes, exploring the extremes will help you notice the more subtle changes. I've always wanted to try placing the post north of the bridge foot. Seems like it should be safe to do -- no?
Click to expand...
From a structural point of view it should not harm the box if you keep I in the same distance area as below the bridge foot. But since the c-bouts are a lot closer there, there is less vibrational width of the top which leads to less good vibrational coupling to the back, specially for the low frequencies.
I guess that the soundpost there gets you a hard sound with less bass resonance. But it is always best to try yourself, since theories might not cover all aspects, probably leading to wrong results.
 
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