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Sound post tension...

Discussion in 'Setup & Repair [DB]' started by dhergert, Nov 20, 2018.

  1. dhergert

    dhergert Gold Supporting Member

    Jan 17, 2018
    Blue Zone, California
    I've recently had an eye-opening experience with the sound post in my Alcoa and wanted to hear if this is at all useful or practical for "normal" wooden double basses...

    First, there are some points about my Alcoa that should be mentioned:

    1) A typical stock Alcoa body is welded together and the top and back are not likely to separate from the ribs.
    2) My Alcoa has had a custom welded-patch repair to the top under the bridge area.
    3) My Alcoa has had a hinged trap-door installed in the driver side C to facilitate repairs and other customization. This trap door has allowed me to do some experimental and prototype sound post work.
    4) I've created and successfully installed a platform-topped sound post to provide more distributed support for the top repairs under both feet of the bridge.
    5) This platform-topped sound post is adjustable for length in 6 discrete locations and in one general location.
    6) Up to this point I have been using low tension strings on this bass (Innovation SilverSlaps E and A under SBW Deluxe G and D)

    A couple of weeks ago I increased the tension of this adjustable sound post by increasing its overall length by probably about 1.5mm. The goal was to reduce some subtle rattles that I was hearing while playing pizz at what was then the instrument's top volume. While 1.5mm isn't much, it did make the sound post significantly tighter.

    This has resulted in the desired elimination of the rattles. It has also noticeably increased volume and note clarity and it has reduced overtones, enough so that I'm no longer concerned about changing to higher tension strings in order to produce greater volume. Since making this change, I've played this bass in groups where I previously couldn't hear my bass acoustically, and now I can hear it very well, so I'm very pleased with this sound post change...

    I'd really like to hear thoughts about this:

    For a "normal" wooden double bass, is it (1) safe and (2) helpful to increase sound post tension when trying to achieve higher volume with low tension strings?
    Last edited: Nov 20, 2018
    Povl Carstensen likes this.
  2. wathaet


    May 27, 2007
    I think a general rule is hard to define, but I can tell you what my Pellacani and Hill do.
    The Hill actually sounds bigger with a looser post, but response and clarity is better with 1-2mm more length. It is louder where it matters in orchestra with a loose post, but maybe a bit more high end with a longer post.
    My Pellacani had too short a post by far and seemed only to like super low tension like genssler or tuned down solo. Once it was normalized it got bigger sounding, lost the scratch on the D string and sounds great with heavy strings all of a sudden. To be a bit unhelpful, it also seems different with each type of string. The variables drive me nuts!

    As for safe, I did it on a rather expensive 250 year old bass, but I am a bit nuts.
    Povl Carstensen and dhergert like this.
  3. dhergert

    dhergert Gold Supporting Member

    Jan 17, 2018
    Blue Zone, California
    Thank you Wathaet...

    Regarding that difference with each set of strings, my guess from seeing this sudden change with my bass is that maybe each bass has an optimal total tension of the strings combined with the tension of the sound post... And that perhaps the effect of these variables is something that is only becoming known as people start using adjustable sound posts more and more. Or maybe it's only visible to people like yourself and others here, who have been around a whole lot of double basses in the process of hearing, playing and maintaining them.

    And of course there is the weather changing now too -- while my aluminum bass isn't prone to humidity changes, it is prone to metal contraction and expansion due to temperature changes, so there might also be something going on there.

    These are very interesting beasts, double basses.
    wathaet likes this.
  4. BobKay

    BobKay Supporting Member

    Nov 5, 2012
    Estero, Florida; USA
    My recent experience is just the opposite of yours. Within the last two months I had sound post adjustments on both my hybrid and full ply. In both cases the soundpost was too long, probably because it was fitted in a drier climate and I live in SW Florida. On the hybrid I also changed from Spiro mittels to EP weichs. More volume, bigger sound. Maybe the top was being choked by the too-long posts. So I can’t confirm your results.
    dhergert likes this.
  5. Of all the possible “fiddlin” that can be done in a proper set up and maintenance on a doghouse, sound post placement and adjustment seems to me the most alchemical in both tone improvement and the actual process!
    dhergert likes this.
  6. dhergert

    dhergert Gold Supporting Member

    Jan 17, 2018
    Blue Zone, California
    I agree, different results, however your experiences may support my theory... That each bass has an ideal combination tension made up of both the string tension and the sound post tension -- and of course that is assuming the sound post is in the right location.

    Alchemy, yes. With multiple variables that are going to be different with every bass, it has to be very much like magic. And while adjustments may be excellent, perfection may never be verifiable.

    Wouldn't life be boring if there wasn't any mystery?
  7. james condino

    james condino Spruce dork Commercial User

    Sep 30, 2007
    asheville, nc
    Trap doors / access panels are wonderful in that someone like yourself can make these kinds of experiments to change both the position and tension ten different times within about 20 minutes, removing the alchemy, and then make a rational decision on their own. All basses should have access panels of some kind instead of having to play the #$%&*ING ship in a bottle game every time you need or are curious about a 30 second sounpost adjustment or a 15 minute repair instead of taking the whole top off!!!!!!!!!
  8. dhergert

    dhergert Gold Supporting Member

    Jan 17, 2018
    Blue Zone, California
    James, thank you for chiming in here. I totally agree!

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