Ultrasonic Attenuation in Thin Specimens Driven through Buffer Rods

A method is presented for measuring ultrasonic attenuation in thin solid specimens. The sample is bonded to one end of a buffer rod, and a transducer is bonded to the other end. All three elements must have plane parallel faces. The system is excited by an rf burst shorter than the round-trip travel time of the elastic wave in the buffer but longer than three times the round-trip travel time in the specimen. The specimen and its interface with the buffer act as a multiple-path elastic wave interferometer. The envelope of the returning echo in the buffer rod shows a stepwise amplitude modulation, one step per round trip in the specimen. The amplitudes of the first three steps are measured at rf frequencies at which the phase delay in the specimen is an integer multiple of π. From the amplitudes, the attenuation in the specimen and the reflection coefficient at the buffer-specimen interface are computed. Measurements can be made over a broad frequency band with a single transducer, so the frequency dependence of the attenuation can be determined. The method has been applied to thin brass samples. The results are presented and interpreted.