Measuring Resonant Frequencies of an Acoustic Guitar

by Mark French

Originally published in American Lutherie #143, 2021



Many luthiers track resonant frequencies of their acoustic guitars, often with the goal of meeting some target values. It’s common for the first two resonant frequencies of a full-sized acoustic guitar to be near 100Hz and 200Hz respectively. Some builders use detailed frequency measurements during the build process. For example, Greg Byers described his methods in his 2017 GAL Convention workshop (AL#134). Another good discussion is in Volume 1 of Contemporary Acoustic Guitar Design and Build by Trevor Gore and Gerard Gilet.

Resonant frequencies are a function of the ratio of stiffness over mass. Specifically, frequency is proportional to √(stiffness/mass). This simple relationship is harder to apply at higher modes, but it describes the effect of the structure on the lowest modes well enough for us. In practice, it’s easier to reduce resonant frequencies than to increase them. Selectively thinning parts of the top or shaving braces reduces stiffness more than mass, and so tends to reduce resonant frequencies. It’s very uncommon for luthiers to add anything, either mass or stiffness, to their guitars in order to modify dynamic response, though it is certainly a valid thing to do. Trevor Gore is the only one who comes to mind, though there may be others I’ve not heard of.