I recommend that two tapping tools, one held in each hand, be used so you can tap on a second area immediately after the first thus allowing the ear can more easily hear differences
Two low toned wooden pencils can work well. I will now describe a technique to do tap tone adjustments using two pencils. Try and pick out two with the same tap sound as there is considerable variety even among seemingly identical pencils. Sharpen the end in the usual pencil manner. We will be tapping directly downward with the pencil shaft held vertically, touching the wood with the lead point. This will allow us to tap in tight situations and close along side the strings.
When doing tap tone testing, work in a quiet room with good acoustics. We want to compare two wood violin parts. The technique is to tap one part and immediately after to tap the second part. I start by tapping the reference part, that is the part that I am listening to and not changing, with my left hand and immediately after tap the second part (the part I will change if necessary) with the pencil in my right hand. In this technique we will use both the highest and lowest parts of the sound spectrum of the tap tone thus providing a check on our hearing accuracy. When the violin parts are in ideal acoustical adjustment, both the lowest (fundamental) tap tones and the highest overtones of the two parts will match each other.
First to generate the lowest part of the tap sound, use only the lightest possible taps so that the taps are just barely audible; this will minimize overtones. Note if the second tap is higher or lower than the first. Next, tap the same two parts using a brisk downward stroke of the pencils listening exclusively for the highest overtones, again noting if the second tap is higher or lower. As the fundamental tap tone goes lower, the highest overtone goes higher. In other words, the range of the sound spectrum widens as the fundamental tap tone is lowered. This is the principle that we are now going to use to our advantage.
If the second fundament tap is lower than the first (using only the lightest possible taps); then the second high overtone tap will be higher than the first (using a brisk downward tap to emphasize the high overtones). This means that the wood part under investigation (the second part we tapped) needs to have its fundamental tap tone raised (and its high overtone tap lowered).
If the second fundamental tap is higher than the first (using the lightest possible taps); then the second high overtone tap will be lower than the first (using brisk downward taps) This means that the wood part under investigation ( the second part we tapped) needs to have its fundamental tap tone lowered (and its high overtone tap raised).
By using both fundamental and high overtone taps to verify the tap tone match of the two parts I imagine you will make meaningful and accurate adjustments with just a little practice.
Tuning Violin Family Bridges: