  We explore what can be said on the effective temperature and sound speed of a
statistical ensemble of fluid phonons present at the onset of a conventional
inflationary phase. The phonons are the actual normal modes of the gravitating
and irrotational fluid that dominates the protoinflationary dynamics. The
bounds on the tensor to scalar ratio result in a class of novel constraints
involving the slow roll parameter, the sound speed of the phonons and the
temperature of the plasma prior to the onset of inflation. If the current size
of the Hubble radius coincides with the inflationary event horizon redshifted
down to the present epoch, the sound speed of the phonons can be assessed from
independent measurements of the tensor to scalar ratio and of the tensor
spectral index.
