Phase-shifted air motion through f-holes reinforces sound radiation at high frequencies

Evidence

If the body is contracting and expanding, changing the volume inside the cavity of the violin, air will be forced in and out. But what is the phase of the air motion relative to the drive? This phase is important to the sound radiation. If the drive is slow, it is clear that air will go out as the body contracts; this scenario is below resonance. Well above the Helmholtz resonance frequency (and all the body modes are well above it), we have a phase shift of 180 degrees, meaning that air will go in the f-holes just as the body is contracting! A little thought reveals that this is a good thing; it adds a further rarefaction near the violin just when it is contracting, and thus it adds to the sound power that will be radiated. Any modes that change the volume of the violin have a monopole component to their radiation, gaining loudness as a result. This is the happy result of asymmetry of many of the modes, so that collapsing regions of the body are not fully compensated by expanding ones, and vice versa a half-period later. — Eric J. Heller, Why You Hear What You Hear- An…, 1795

Categorization

• XREF: Directly parallels the ported loudspeaker’s above-resonance phase reversal described in section 13.6 of this same book.