Phil Howson (Ph.D.) has recently had his paper "Rhotics and Palatalization: An Acoustic Examination of Upper and Lower Sorbian" published in the journal Phonetica. The work is from his first generals paper. He traveled to Germany, with the help of the Germany/Europe Fund, and collected data at the Max Planck Institute for Evolutionary Anthropology and the Sorbian Institute.
In the paper he examines the acoustics of rhotics in Upper and Lower Sorbian (related Slavic languages spoken in eastern Germany) to better understand the resistance of rhotics to palatalization, and the acoustic cues of rhotics as a class. You can find the paper here.