Barzegar, Mohammad AminDavies, CliveGrafton, Miles2026-06-082026-06-082026-05-29Acoustics Australia, ISSN: 0814-6039 (Print); 1839-2571 (Online), Springer, 1-19. doi: 10.1007/s40857-026-00385-30814-60391839-2571http://hdl.handle.net/10292/21336Helmholtz resonance provides a well-established acoustic basis for determining volume via the resonance-frequency–volume relationship. However, frequency-tracking methods are typically too slow for dynamic measurements. We present an alternative physical model, the sound-pressure quality-factor (SPQF) model, which estimates volume in real time from cavity sound-pressure amplitude, avoiding frequency hunting. The model follows from the equations governing the driven, underdamped vibration of the port-air mass. The resonator is excited at its empty-cavity natural frequency with a single-tone drive; inserting a sample reduces the steady-state pressure amplitude, from which displaced volume is inferred. We validate the method with liquid and solid samples in 1-, 2-, and 3-L cavities and in a mechanically adjustable chamber under dynamic conditions. The approach achieved millilitre-level accuracy for solids and relative expanded uncertainty U, k = 2 < 0.1% of cavity capacity in static tests, and it tracked liquid discharge at ~ 15–20 Hz. On the mechanically variable resonator, SPQF tracked piston-driven volume changes for speeds up to 75 mm·s⁻1, delivering ~ 20 measurements in 1.5 s.Open Access. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.51 Physical Sciences40 Engineering5103 Classical PhysicsBioengineering02 Physical Sciences09 EngineeringAcousticsHelmholtz resonanceCavity volumetrySound pressure quality factor (SPQF)Journal ArticleOpenAccess10.1007/s40857-026-00385-3