Why the open strings are not the same loudness as fingered notes (often)?
...One of the characteristics of a fine violin is that all the notes are 'even'. By this is meant that open strings are the same loudness as fingered strings; that all the strings produce notes of the same loudness (with the same bowing effort, or course); and that the notes all up and down each string are equally as strong. [ It might be worth mentioning that "even and strong and vibrant" is better than "even and faint and uninteresting".]
...For the sake of discussion; let us suppose that the on the 'e' string the open note does not match the power of the fingered notes. For the sake of argument, let us say that in this imaginary instance, the open note is strong and powerful but the fingered notes on this 'e' string are much more subdued.
...For this 'test' we need a 'tapper': We can use our nuckle as this produces an accurate tone, or we can use an ordinary wooden pencil (preferably on with a low or dull tap tone when struck against a hard surface), unsharpened. It is a good idea to put a piece of tape around the metal cylinder that holds on the eraser so it does not come in contact accidentally with the violin or its varnish.
(you may quiet the strings with a soft cloth inserted betwean the bottom of strings and top of fingerboard.)
...Now tap on the side of the 'nut' (the small ebony block that the strings rest upon at the peg box) along side the 'e' string. Now make a comparison tap on the side of the tailpiece where the 'e' string goes over the bar. Notice how similar in tone they are (the overtones may well be different; but listen for the root or base tone). Another way to test this relatioship is to tap down on the string, both at the nut and at the contact bar of the tailpiece. These tap tones must be identical for the optimum power and clarity.
...Now tap again on the side of the nut and then a comparision tap on the side of the fingerboard in the area where notes are not as pleasing. Are these two tap tones different?
...I believe you will discover that when the open strings sound very strong and focused, the tap tone at the side of the ( on on top of the string at the) nut will closely (or perfectly) match the side tap (or downward tap on the string at the string bar) tone of the tail piece. Where the notes are weak, the tap tone on the side of the adjacent fingerboard will be different when compared to the side of the tailpiece.
...So, in this case the reason the open string and fingered notes are different is because the frequencies do not match. However, they can be made to match for excellent performance.
...The ideal situation is when the nut, fingerboard and tailpiece all have the same side (and down ward) tap-tones. (both sides are tuned seperately). Then the instrument will be performing to its potential.
...All these can be adjusted to matching tones. Your may enjoy the following detailed articles: