End Pin Adjustment for best tone and performance...
...This acoustical agreement is important for the tone of the instrument to be clear. When these tap tones are 'off', the tone may sound 'garbled', like radio static when a station is not coming in clearly.
...Make this adjustment after the nut has been adjusted to agree with the fingerboard (which balances open and fingered string notes).
...The adjustment is very simple, if a bit of a bother. If the end pin tap tone is too low (compared to the nut) then the end pin is removed and the inside end is sanded or filed to shorten it, which will raise its tone. Remember that the frequency changes as the square of the change in length, so constantly check it against the a downward tap on the center of the nut, as you adjust the end pin frequency.
...You may either tap the end pin sideways against a hard wood table top or similar surface or tap the end pin downward in its inner end to hear it's tone (frequency); both methods should give the same indication. I use my special tapping (low-toned) pencil and tap down on the nut with my left hand and immediately follwoing, then tap the end pin down with my right hand so I can hear the two sounds closely one after the other... lets the ear/brain have a good indication to judge from.
... If the end pin is too high (compared to the nut) then you need not remove the end pin from the instrument, simply cover the varnished surfaces on the back of the violin with lightly applied masking tape, to protect the varnish, and then file or sand the head, probably with a small sanding stick, (on its outside surface) to bring the tone down to a perfect match with the nut. You can polish the end of the nut with a piece of leather or canvas or what ever works best for you to bring the wood back to a shine.
...I imagine that you will be surprised how much difference this adjustment can make!
Tap downward against the center of the nut...
...If the end pin tap is too low then sand the inner end until the frequency comes up to the nut tap frequency. Constantly turn the end pin while you sand to keep it a uniform shape..not 'lop' sided on its inside shape.
...Another way to raise the frequency without removing the end pin is to sand or scrape wood evenly from the outside diameter of the raised turning along side of the tailgut. Be sure to protect the varnish from accidents. This works well and only small amounts of wood are usually removed to effect adjustment.
...This is the downward tap against the side of the end pin when the end pin is in the instrument. Here it is useful to have two (identical) tapping pencils so the two tones can be heard one right after the other.
...This is the sound specturm from a tap on one plate of a recent violin, and will serve as an example of what a tap tone is composed of and what we are to listen for:
...On the left is sound level in Db; on the bottom you see the frequencies generated by a tap in the center of the top plate of my Opus #22 violin. The first prominent sound peak is the 'fundamental tap tone' that we are to listening for. However, our wonderfully designed ears (brain, nerves, etc.) hear all these generated sound frequencies at once. The highest pitches (in this case 4,000 hertz, vibrations per second) is the easiest for your ears to hear, as our hearing is more sensitive to high frequency sound. However, the 'fundament' tap tone is the first major peak (in this case about 322HZ) in the sound envelope; this is the tap tone frequency, that we want to hear and compare.
...For most of the fittings on a violin, the back plate fundamental tap tone seems to be the best goal frequency for a fine sounding instrument. {I try to use 353HZ for the back plate when I can.}