CMB Distortions from Damping of Acoustic Waves Produced by Cosmic   Strings
  We study diffusion damping of acoustic waves in the photon-baryon fluid due to cosmic strings, and calculate the induced $\mu$- and $y$-type spectral distortions of the cosmic microwave background. For cosmic strings with tension within current bounds, their contribution to the spectral distortions is subdominant compared to the distortions from primordial density perturbations. 
Hiroyuki Tashiro, Eray Sabancilar, Tanmay Vachaspati
astro-ph.CO hep-ph hep-th