In reviewing the experiments we should summarize the arguments indicating that the observed phonon signals are due to non thermal phonon radiation emitted in the decay and recombination of injected quasiparticles. It appears very difficult to explain the observed structure in the dependence of the signal amplitude as a function of the generator voltage in terms of heat pulses, e.g., phonons with Planck's distribution. An estimate of the possible heat effect arrives at much smaller signal amplitudes than observed. In addition, only little temperature dependence of the signal amplitude is to be expected. In contrast to this, quantum processes provide a straightforward understanding of the experimental findings.
