Double Bass Help a dummy better understand the general effects of soundpost adjustments

[Chris Fitzgerald]

I've learned a lot about the bass over the years, mostly how to play it more efficiently, but also basic minor setup things like how to straighten a bridge, how to use adjusters correctly, even how to repair a loose seam (although this last is something I tend to farm out because I figure a pro can do it better).

But soundpost adjustment just seems to be a bag of chicken bones and voodoo to me. I think I understand the basic mechanism: The post is a fulcrum of sorts that takes the vibrations the strings produce at the top of the bridge and transmits them to the top, which allows the body of the bass to resonate. There are other complications, like how long/tight the post is and how the bass bar fixed to the top affects this whole process, but that is the gist of it, no?

If this is true, then it would make sense that there would be a set of generally predictable outcomes of moving a post on direction or another. For starters, let's focus on one axis of possible movement : that of moving the post north/south either closer to or further from the bridge foot. Are the effects of this kind of movement generally predictable, and if so, what are the general expected results in terms of tone and feel?

 

[Eric Hochberg]

General SP impressions... Jeff Bollbach Luthier, Inc.: Luthier Rant

Ken Smith says..."the soundpost is usually centered under the G-foot of the bridge about a soundpost thickness below it. Moving the post towards the F-hole slightly will make it deeper but maybe less focused sounding."

I know I've seen tone adjustments by SP placement on TB but they are lost to me in the ether now...

 

[Don Kasper]

a bag of chicken bones and voodoo

By Jove, I think he's got it!
(I'll let the pro Luthiers/Witchdoctors chime in, as soon as the Sacrificial Chicken has been burned...)
Also - Chuck Traeger's book has a few stories (a George Mraz story, in particular), that violate just about all of the accepted "rules"!
Good LUCK!

 

[Don Hergert]

I've mentioned it before here, but if every double bass had a trap door appropriately located for access to the sound post, sound post adjustment would be as well understood as bridge adjustment.

There are a tremendous number of variables to setting up a stringed instrument, and each variable affects all of the others. Sound post adjustments change tone and volume, but so do bridge location and action adjustments, string-type changes and many if not most other setup adjustments.

If a person cannot make these adjustments themselves, how can they ever understand how they change sound?

 

[unbrokenchain]

I've messed and messed with the soundposts on the four plywood basses I've owned. Most of it in effort to get a good sound out of my first (really crappy) bass. Moved it around all over the place, cut it shorter, made a new one, I couldn't for the life of me tell a difference no matter what I did. Most difficult part is getting the post flush to the top and back, stick a little mirror in there to check and keep pulling the post out to shape it, takes forever (for me...).
My '46 Kay has a magnetic access door (here here, dhergert) so moving the sound post is super easy, and since the neck is removable I have to set it up each time I remove the neck. The difference in sound from different sp positions is negligible to my ear, though I haven't tried any extreme positions. I was hoping moving it around would change where the crazy wolf is on the bass, but it doesn't. (Moving the bridge slightly does affect the wolf on that bass fwiw). I just put the soundpost in the "neutral" spot because it seems structurally sound that way, and forget about it.

I'm sure the movement of the post has a much more pronounced effective on a carved bass, but I don't have experience there. Just my $.02, grain of salt n all that.

 

[Yosbank]

I've messed and messed with the soundposts on the four plywood basses I've owned. Most of it in effort to get a good sound out of my first (really crappy) bass. Moved it around all over the place, cut it shorter, made a new one, I couldn't for the life of me tell a difference no matter what I did. Most difficult part is getting the post flush to the top and back, stick a little mirror in there to check and keep pulling the post out to shape it, takes forever (for me...).
My '46 Kay has a magnetic access door (here here, dhergert) so moving the sound post is super easy, and since the neck is removable I have to set it up each time I remove the neck. The difference in sound from different sp positions is negligible to my ear, though I haven't tried any extreme positions. I was hoping moving it around would change where the crazy wolf is on the bass, but it doesn't. (Moving the bridge slightly does affect the wolf on that bass fwiw). I just put the soundpost in the "neutral" spot because it seems structurally sound that way, and forget about it.

I'm sure the movement of the post has a much more pronounced effective on a carved bass, but I don't have experience there. Just my $.02, grain of salt n all that.

+1
Link Removed

 

[bassedsouth]

For starters, let's focus on one axis of possible movement : that of moving the post north/south either closer to or further from the bridge foot. Are the effects of this kind of movement generally predictable, and if so, what are the general expected results in terms of tone and feel?

To try and answer your question from my personal experience... I don’t think much is totally predictable when moving the post a little more North or South .
I’m no expert luthier by any means but have learned to cut and fit posts correctly in all my basses over the years. It’s my personal belief that it’s the diffence in pressure to the top plate that makes more of a difference to the sound than anything else when moving a post around , the actual pressure applied to the top/back increases or decreases slightly due to variations in the distance between the back and top plates at any given sound post location.
Anyway , that’s just my take on it - if I’m wrong or not is open to debate

 

[Chris Fitzgerald]

My general interest in this subject has to do with the fact that my experience has largely been like @unbrokenchain 's experience - IOW, that moving the post seems to make a difference, but that I adjust my technique quickly to get the sound I'm looking for and can't really tell an appreciable difference in the long run. The posts on both of my regular basses have not been adjusted in over 10 years, and I have never felt the need to try.

My more immediate interest is that I have a strange "wolf" for lack of a better word on my travel bass that basically only shows up when amplified; the E-F-F# area just below the octave is louder than the rest or the amplified signal, and resonates strangely on the few gigs I've played it on. Granted, these were loud gigs where I am sitting right next to a loud drummer, but I have been able to replicate the phenomenon here at home by watching the levels of the pickup running through a meter that shows amplitude, and those notes jump out when I play them. Oddly, I can feel the back of the bass vibrate in a sort of frantic way against my left leg when playing in that area, and it doesn't do that on any other notes in any other register.

So I was wondering if moving the post is likely to have any affect on this phenomenon. It seems especially important for me to better understand the mechanics because the post is spring loaded on this bass, making moving it seem even more awkward for an unskilled person like myself, and because the nearest luthier who is reputed to be good with post adjustments is 2 hours away, works by appointment, and my schedule is crazy. Plus, it's hard to replicate a gig environment in a bass shop, even if you bring your amplifier!

 

[unbrokenchain]

CF - does the phenomenon happen acoustically when you bow those notes? If not, have you tried a different pickup, or moving the pickup? The wolf on my Kay is between the 2nd octave Ab and A mainly on the A string, and manifests as a heavy warbling between two pitches even with the other strings muted, one pitch being the one I'm stopping and the other being the one the bass wants to sing. With pizz it more feels like a "dead spot." Both apparent acoustically.

It’s my personal belief that it’s the diffence in pressure to the top plate that makes more of a difference to the sound than anything else when moving a post around , the actual pressure applied to the top/back increases or decreases slightly due to variations in the distance between the back and top plates at any given sound post location.

I tend to set the soundpost so that there's enough tension that it will stay up if I stand the bass up carefully without strings on it, but just barely - if I bump it any way it will fall right over. But moving the post east/west would seem to affect the pressure the most depending on the arch of the top and back. How that affects tone, wolfs, and relative amplitude of different notes is more than I can surmise...

 

[bassedsouth]

The posts function is structural support.Yes it does have an influence on sound and feel , but maybe if it had been called a ‘support rod’ instead , it would have less of an impact on how we intellectualize it

 

[Don Kasper]

I am not a Luthier...or a Witchdoctor.
I've never completely understood the voodoo behind "moving the soundpost" -North/South or East/West - is it the actual placement of the post in relation to the G foot that changes the sound, OR, is it the subsequent change in "tightness" of the post? (OR, both?)
There is an adjustable soundpost (Anima Nova?) that would be fascinating to experiment with ($$$) in terms of placement/fit/tightness.

Regarding Chris's travel bass @ chris fitzgerald
Are these notes on the G string just below the G harmonic? This area (just below or above the 1st partial harmonic) seems to be a fertile wolftone area, more commonly on the A string, though.(IME)
If you attempt to bow those notes, Fortissimo, you should be able to determine if it is "wolftone" related or not - a wolftone will collapse (sound-wise) when pushed to loud volumes. It will not vibrate normally, but will "choke" and fight the bow, and there may be abnormal oscillations of the string as it vibrates, both audibly and visually. (IME).
I would try to have the soundpost moved to see if the problem gets better or worse (time consuming and not easily done, esp. on a travel bass - as there may be special tools or procedures that only the maker would know of...It would be worth a call to Kolstein, IMO.)
I had a wolftone that DISAPPEARED after some (DIY!) trial & error soundpost adjustments - I made sure to record and mark the Old and New Positions, just in case things got worse.
Finally, if it is not a wolftone, it may be just the inherent resonance of the instrument, but it may still benefit from soundpost experimentation - I would even try radical placement just to see if the problem responded to a change.
Good Luck.

 

[Don Kasper]

The posts function is structural support.Yes it does have an influence on sound and feel , but maybe if it had been called a ‘support rod’ instead , it would have less of an impact on how we intellectualize it

Does it also acoustically couple the Top and Back Plates? (I read this somewhere...)
Thank You.

 

[bassedsouth]

Does it also acoustically couple the Top and Back Plates? (I read this somewhere...)
Thank You.

There seems to be varying consensus on that - I’m really not sure if it has any real positive interaction with the back...

 

[Chris Fitzgerald]

@Don Kasper @unbrokenchain I just tried bowing up there, and it does not appear to be that sort of wolf acoustically. It's more that it's a bit louder and resonant in that area, enough to drive the pickup a little out of control. Yesterday I hooked up a compressor and a sweepable mid EQ, and found that it tends to be tamed when I cut between 170 and 200 hz; of course, there are other "good" frequencies in this range. The other way to tame it was to apply the compressor to limit the points where these notes came out louder, which seems to mostly work. It happens with various pickups, and seems to be a vibrational thing that I don't hear acoustically so much as feel kinesthetically...and it appears the pickups are doing the latter. in practice it feels/sounds like "on the verge of feedback, but not quite there".

My guess is that moving the post would likely move the vibration to another pitch, but who knows? The highly regarded luthier I asked about this said something to the effect that moving the post closer tends to make the bass feel "tighter", which for me would be a good thing because it feels a little looser than my regular basses, probably because of the subtracted 1/2" of string length. But it could also be a simple string thing, or maybe the bass doesn't like the spirocore G... god knows it wouldn't be the first bass to have that issue! If I had an Evah medium lying around, I'd throw that on there and try it.

 

[Inky13]

Half the battle is just finding a decent metal coat hanger!

 

[fu22ba55]

I don't yet understand the secret black magic of HOW to adjust a soundpost, but this video helped explain the why to me a bit.

 

[unbrokenchain]

Sounds like you've got a workable solution with the EQ/compressor combo

My guess is that moving the post would likely move the vibration to another pitch, but who knows?

That was my hope in moving the post around on my Kay since my wolf was too close to my Ab for comfort. Post placement didn't affect it, but moving the bridge south (and the post, the same amount) made the wolf less intense and also seemingly lower in pitch. I found this out accidentally, the neck Condino made for my Kay is huge and he had the bass setup with the bridge above the f-hole notches so the string length was a little closer to normal. I decided I wanted that little bit of extra clearance on the bouts for the bow so I moved the bridge/post for that reason, and it inadvertently helped the wolf. Now my string length is 43" but I can deal

Sounds like its a different issue than the one on your bass, but easier to move the bridge than the post, you might experiment in that way.

 

[Povl Carstensen]

Half the battle is just finding a decent metal coat hanger!

The cleaners have it!

 

[Povl Carstensen]

As I am for a deep, soft feel, I have always had the post about 2 cms south of the bridge. I am however looking foreward to having my post shortened a bit. It is presently standing under the inside of the bridge leg. And I am hoping that shortening it, and getting it more directly under the bridge leg (but still about 2 cms south) will give me even more "sound". But who knows..

 

[Chris Fitzgerald]

Years ago I used to make tonal adjustments on an old Czech carved bass by moving the bridge around a bit. i figured I was in effect sort of making a soundpost adjustment by changing the orientation of the bridge foot and post. Many posts here on TB dissuaded me from doing that, although no one would actually say what the structural risk I would be taking by doing so would be. In any case, that wouldn't be a good option for the travel bass because currently the adjusters are out as far as I am comfortable with them being out, and moving the bridge back even a little would lower the strings, so...