Abstract: A non-invasive method and apparatus for measuring the volume of an object such as an infant using the principle of the Helmholtz resonator. A container having a cavity therein containing gas and having an opening is subjected to a variable frequency loud speaker. The resonance frequency of the cavity is proportional to the volume of the cavity. By placing an object to be measured in the cavity, the change in the resonance frequency of the cavity before and after the object is placed is an indication of the volume of the object. The loudspeaker is mechanically uncoupled from the opening, the volume of the cavity is large enough compared to the object to be measured so the object will not change the shape of the cavity, and the temperature inside the cavity is maintained substantially constant during the measurements. Various lung measurements may be taken by determining the volume of air inhaled or exhaled from the lungs of an animal.