The sound post is part of the internal structure of the double bass. It is a wooden dowel that sits between the back and the top of the bass. The sound post contributes to the tone and resonance of the instrument. It is held in place by the tension of the strings, rather than being glued on. This makes it possible to make fine adjustments to the sound post by moving it slightly. The sound post is typically set just below the foot of the bridge on the G-string side of the bridge. Setting and adjusting a sound post is a difficult procedure that takes finesse, the proper tools and patience.
Purchase a sound post setting and adjustment kit. You can find these kits online or at music stores that specialize in acoustic instruments. The kit should include a sound post position index tool or gauge, a sound post setting tool, a mirror and a mallet.
Loosen the strings on the double bass. Turn the tuning pegs on the headstock until the strings are loose. This relieves the tension on the double bass, making it possible to install the sound post.
Insert the sound post position index or gauge through the top of the F-hole. Hold the gauge until it touches the back of the bass. The gauge needs to be in the same position where the sound post will be installed. Write down the measurement.
Make a mark on the sound post a fraction longer than the gauge measurement. Cut the sound post with the single cut file.
Connect the sound post positioning tool to the sound post. The tool should be connected to the sound post about one-third of the length from the top. Insert the sound post through the F-hole and align it against the back of the bass, 1/2 to 3/4 inch below the foot of the bridge on the G-string side of the bridge.
Insert the mirror through the opposite F-hole and use the mirror to align the sound post. Push the sound post upright so that it sits vertically between the back and the top of the bass.
Tighten and tune the strings to concert pitch. The double bass is tuned E-A-D-G. Play the bass and carefully listen to the resonance and tone of the instrument. The sound post contributes significantly to the tone of the bass.
Loosen the strings slightly. Insert the mallet into the F-hole and lightly tap the sound post. Tune the strings and listen to the difference in tone. Continue the process until you are satisfied with the tone of your double bass.