Body-volume changes drive the Helmholtz air mode in stringed instruments

Evidence

Why would air go in and out of the f-holes? There are two reasons: First, some of the body deformation modes may be changing the interior volume of the violin, which will force air in and out at the period of the mode. Second, even for a rigid box, there is the Helmholtz mode to consider, wherein the air at the f-holes acts as a mass, and the air inside the violin acts as a spring. In fact, these two mechanisms interact, since the changing body volume is a drive on the Helmholtz mode. For example, squeezing a slightly deformable bottle with an open neck at its resonant Helmholtz frequency will cause it to speak relatively loudly at that frequency. — Eric J. Heller, Why You Hear What You Hear- An…, 1794

Categorization

• DEFINE: Explains the two mechanisms forcing air through f-holes: body volume change and Helmholtz mode, and how they couple.
• SHARE: The squeeze-the-deformable-bottle-at-its-Helmholtz-resonance demo is perfect for a physicist friend who owns a water bottle.
• XREF: connects to: Trapped air in a drum bowl alters the head’s mode frequencies from ideal values — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1302; Body resonances aligned to open-string fundamentals give each string instrument its timbre — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1126