DETERMINATION OF THE VISCOUS AND THERMAL-CHARACTERISTIC LENGTHS OF PLASTIC FOAMS BY ULTRASONIC MEASUREMENTS IN HELIUM AND AIR

The acoustic wave most commonly transmitted and detected in the high-p orosity absorbent materials used in noise control is generally the air -borne slow compressional wave. In a new experiment, the air saturatin g the sample is replaced by helium and the transmission is studied at ultrasonic frequencies (70-600 kHz). The experiment is quite easily pe rformed using standard ultrasonics and vacuum equipment. The main purp ose of this work is to propose a method to determine simultaneously bo th the viscous and thermal characteristic lengths with the same precis ion. These two parameters characterize the viscous and the thermal int eractions between the frame and the fluid at high frequencies. The cha racteristic lengths are deduced from the high-frequency asymptotic beh avior of either the velocity or the attenuation curves obtained in the sample saturated by air and by helium. It also appears that due to th e properties of helium, the discrepancy previously observed between pr edictions and measurements is shifted toward higher frequencies. (C) 1 996 American Institute of Physics.