AN ACOUSTIC METHOD FOR MEASURING INFANT BODY VOLUME

Abstract

An acoustic plethysmograph that measures body volume has been developed for body composition studies on premature infants. It is based on the principle that the resonance frequency of a Helmholtz resonator depends, in part, on the volume of the resonator chamber. Thus, when an object is placed inside the chamber, the resonance frequency changes in proportion to the volume of the object. The prototype system consists of a 20 × 20 × 45 cm plexiglass chamber with a hinged front for access. A loudspeaker, driven by a variable frequency tone generator, is aimed at a 7.5 cm diameter opening in the top surface of the chamber. The frequency of this tone is monitored by a frequency counter. The resonance frequency corresponds to the frequency at which the sound level inside the chamber, as detected by a microphone in the chamber, reaches a maximum. Chamber temperature was maintained at 35°C and the sound pressure level below 75 dBa. The device was calibrated by measurement of change in resonance frequency when known volumes were placed in the chamber. The volumes of a number of inanimate objects of unknown volume were measured, first by the acoustic technique, then by water displacement. The two methods differed by less than 2% for volumes between 250 and 1500 ml. Eight anesthetized minipigs were measured acoustically, and after sacrifice, measured by water displacement. The two measurements differed by less than 5%. When the sacrificed pigs were measured acoustically, the acoustic and water displacement measures differed by less than 1%.