Speakers can achieve a high degree of synchrony when reading a prepared text together. Under these constraints, there is necessarily a very tight coupling of production and perception. In a first experiment, we demonstrate that speakers can successfully synchronize with selected recordings of others obtained in a synchronous speaking condition. We then have speakers attempt to synchronize with modified recordings, in which the original recording is replaced with altered speech. The goal is to find out the physical properties of the speech signal which permit the coupling required for synchronization. It is demonstrated that the energy envelope itself is not sufficient to support coupling, while pitch information is essentially unimportant.