Symmetric Pair Plasmas with Impurities and Acoustic Wave in it
  A motivation for the study of symmetric pair plasma dynamics lies in the insight they provide into the dynamics of more general plasmas. In the symmetric pair plasma, when temperature both species is the same, acoustic wave are absent. However in presence other species (impurities) in fullerene pair plasmas. for example two-temperature electrons, acoustic waves are possible. 
Barbara Atamaniuk, Krzysztof Zuchowski