The sharp change in behavior at periods above and below approximately 1 day in the solar wind spectra, reported by Goldstein and Siscoe, is discussed in terms of a damping-growth transition for ion acoustic waves in a density gradient recently observed in laboratory experiments. The region immediately surrounding the sun (~10 RS radial extent) is envisaged as a low-pass filter for compressional perturbations, with a sharply defined upper frequency of ~10-5 s-1.