Helmholtz resonance is the resonant oscillation of the gas in a cavity having a constricted neck or port that is open to the atmosphere, under the influence of an external driving frequency. If the driving frequency matches the natural frequency of the cavity and port, the system will resonate with a gain determined by its physical dimensions. The combination of cavity and port can be modelled by a mass and spring, giving the relationship between resonator dimensions, gas properties and resonant frequency; the spring force is caused by the air in the cavity, and the mass by the oscillating air in the port. Thus, the free space in the cavity and hence the volume of any object inside it, can be determined by measuring the resonant frequency.

A 3L resonator was calibrated using precise quantities of water and the resonant frequency measured to within 0.005Hz. Regular and irregular solid objects were then tested along with particulates of varying size and density. Additional results have been obtained with resonators of 1L and 2L capacity.

For particle sizes larger than 3mm and for liquid and single large solid test objects, accuracy in volumetric measurement was approximately +/-0.1% of full-scale fill. For particles smaller than 3mm the measurement accuracy was highly dependant on particle size, and measurements of volume shifted from hydrodynamic volume to bulk volume, as sound penetration was restricted by the decreasing interstitial pore size.