Our resonance principle, according to which each member of the new violin family has been made, can be stated as follows: The main body resonance of each of the instruments tuned in fifths is placed at the frequency of the open third string, and the main cavity resonance at the frequency of the open second string. Another way of stating the principle, and one that includes the instruments tuned in fourths as well as those tuned in fifths, is this: Wood prime is placed two semitones above the lowest tone, and the cavity resonance is a fourth above that. (Wood prime is the strengthened frequency one octave below the main body-"wood"-resonance.) These conditions are exemplified in Heifetz's Guarnerius violin and many other good ones, but they are not found in all good violins.
The loudness curve developed by Saunders is one of our most useful measures for evaluating overall instrument characteristics. We make such a curve by bowing an instrument as loudly as possible at 14 semitones on each string and plotting maximal loudness against frequency. Despite unavoidable variations in any test that requires a musician to bow an instrument, the loudness curve is significant because there is a fairly definite limit to the momentary volume an experienced player can produce with a short rapid bow stroke.
As you will see in figure 4, the loudness ceiling varies for each semitone on a given instrument. The curves of this figure were made by bowing each instrument without vibrato at a constant distance from a sound meter. From them you can see the placement of main body and cavity resonances in eight conventional instruments - two violins, two violas, two cellos and two basses. You can see that in the violins the wood prime adds power to the low range of the G string. In the violas, cellos and basses the two main resonances, which are higher in frequency relative to string tuning, create a condition of somewhat weaker response on the lowest four or five semitones.