...When we tap on a piece of wood the sound generated can tell us important information useful to making a successful instrument.
...First, the example of testing the tap tone frequency of a relatively thick piece of tone wood:
..In this example, a piece of violin making wood was tapped with the end of a wooden pencil. At the peak of the response curve is the frequency of that piece of wood. Notice that it is a specific frequency and that no other tones are generated but that one. (the spike line at the far left has to do with the sound analyzer circuitry).
...However, by the time the wood is thinned to the shape it will have in the completed instrument, a very different sound response is returned by the same pencil tap (or knuckle tap):
...Here is the violin plate being tapped with the eraser end of a low toned wooden pencil. In this example, the plate frequency is shown as 324 HZ, (my current goal frequency for the back plate is 198HZ and 176 for top plate). But now notice how broad the response curve is. Strong sound is generated from about 200HZ all the way up to about 800 HZ (the horizontal scale is 0-1,000 HZ on the screen.
...Now here is the crux of the problem: Our ears/brain hear all this data at once and must make sense out of it in a split second. The sound we are interested in the lowest major frequency that our brain decides it the bass or fundamental frequency. In this case that frequency is 324 HZ. However there is much sound generated at higher frequencies and our ears/brain are more sensitive to hearing this higher frequency.
...Another important concept to understand is that as a violin plate is thinned and the lower frequency (fundamental frequency or tap tone) decreases, the higher sound generated goes UP at the same time. In other words, as the plate of other violin wood parts are lightened and thinned, the sound spectrum generated by the wood part, broadens and becomes more complicated. Said a different way, as the tap tone is lowered by thinning the wood plate, the high frequency of the same tap tone goes up at the same time.
...In order to use tap tones as a useful tool in adjusting violin wood parts, we must develop a skill in hearing either the upper tap tone or lower tap tone at will. More normally, we want to ignore the upper tap tone range and only hear the lowest proment (fundamental) tap tone and adjust based on that. We can however, use the change in the upper tap tone as a check, since it is easier to hear.
...You can practice to develope this skill by listening to orchestra music and begin to hear specifically the highest musical parts only and then to listen again for only the lowest toned part being played. Once you begin to have some skill in this, begin to listen to the tap tones of the plates of violins. When you have this skill, you can begin to use tap tones effectively in shaping (finally) the parts of the violin family instruments to your advantage. This is necessarry because of the great variability of different wood pieces, sometimes even in the same tree. The range of tap tones for seemingly identical pieces (say fingerboards or tailpieces or tuning pegs) can easily be an octave range (that is one piece has twice the tap frequency as the other identically shaped piece of the same wood variety).