On the mechanism of accordion pitch bending

This paper presents a minimal model explaining pitch bending (a decrease in pitch) that occurs in an accordion when the bellows is operated forcefully while a key is pressed partially. The phenomenon is attributed to the pressure loss at the vent hole of the reed chamber opened partially by the pallet. This pressure loss causes the internal pressure of the reed chamber to fluctuate in a way that exerts a stronger force to close the reed near its closed position and a weaker force when it is open. Consequently, the restoring force of the reed is partially offset, leading to a decrease in the effective spring constant and thus causing the pitch to bend. To elucidate this mechanism, the qualitative behavior is first explained using an equivalent circuit, followed by the presentation of a physical model of accordion sound production. Through numerical simulations and theoretical nonlinear oscillation analysis, the model, incorporating adjustable pressure loss coefficients whose calibrated values are hydrodynamically realistic, yields results in good agreement with those from the accordion pitch-bending experiment of Okada, Nishimiya, and Cho [J. Acoust. Soc. Japan 81(11), 739-742 (2025)], which was conducted under controlled airflow and pallet-opening conditions.