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The synchronization of a four-dimensional hyperchaotic system is analyzed. Two state observers are designed with different methods to realize rapid synchronization of the response and drive systems respectively. When the system parameters are known, we analyze the design processes of two observers and verify their effectiveness. Analysis shows that the synchronization schemes are simple and effective and they do not need to construct Lyapunov functions. Furthermore, they have certain universality. Finally, the synchronization systems are applied to voice secure communications. The encryption and restoration of the speech signal are simulated through experiments. Theoretical analysis and simulation results show the feasibility and effectiveness of the scheme.