A radiation force technique for determining ultrasonic attenuation

The NPL primary standard radiation pressure balance has been modified to enable phase-insensitive through-transmission measurements of attenuation to be made. The accuracy of the basic technique has been assessed by careful measurements on Dow Corning 710 silicone fluid contained in NPL liquid reference cells of various thickness. Several important sources of measurement uncertainty have been identified and studies to assess their significance are described. Interfacial reflections occurring at the two cell membranes result in transmission loss, multiple reflections and, most importantly, standing waves set up between the sample and transducer. Water currents generated in the reference ultrasonic beam, caused by convection and acoustic streaming, may lead to overestimates of the attenuation coefficient of up to 4%. The most important source of uncertainty is a temperature rise which occurs during the measurement and is a consequence of the absorbed power. This effect, which may lead to underestimates in the attenuation coefficient of up to 20%, has been verified by a combination of thermocouple measurements and calculations performed using available theory, both of which are presented. The implications for attenuation measurements in general are discussed.