title: Ultrasonic properties of a suspension of microspheres supporting negative group velocities.
abstract: The ultrasonic attenuation coefficient, phase velocity, and group velocity spectra are reported for a suspension that supports negative group velocities. The suspension consists of plastic microspheres with an average radius of 80 microm in an aqueous medium at a volume fraction of 3%. The spectra are measured using a broadband method covering a range from 2 to 20 MHz. The suspension exhibits negative group delays over a band near 4.5 MHz, with the group velocity magnitude exceeding 4.3 x 10(8) m/s at one point. The causal consistency of these results is confirmed using Kramers-Kronig relations.
authors.affiliation: Department of Physics and Astronomy, and National Center for Physical Acoustics, University of Mississippi, University, Mississippi 38677, USA. jmobley@olemiss.edu
authors.forename: Joel
authors.initials: J
authors.lastname: Mobley
authors.forename: Robert Evans
authors.initials: RE
authors.lastname: Heithaus