Basics of Air Resonances

by W.D. Allen

Originally published in American Lutherie #1, 1985 and Big Red Book of American Lutherie Volume One, 2000



Stringed musical instruments with soundboxes typical of the guitar and violin families have many internal air resonances. The resonance with the lowest frequency is called the Helmholtz resonance, and its importance to the quality of the instrument is appreciated. The resonances with higher frequencies have been referred to by different names: higher Helmholtz, cavity modes, or standing wave modes. These resonances have been measured and documented for several different instruments, but there seems to be little information on their controlling parameters.

The intent of this article is to give the instrument builder some understanding of the air resonances, what parameters establish the frequencies, and some insight into the potential for using this information to make better instruments. A minimum-math, pictorial approach with approximation and rounded-off numbers will be used. Showing the effects of the controlling parameters is the objective, not the absolute value of a number.