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It has been shown recently that a low-energy collective excitation mode that can be found on bare metal surfaces has acoustic(linear) dispersion and arises due to the coexistence of a partially occupied quazi-2D surface state band with the underlying 3D bulk continuum. Since acoustic dispersion allows the confinement of light on small surface areas in a broad frequency range, it becomes relevant for variety of photonics applications. We demonstrate that collective acoustic excitations can be employed in unidirectional waveguide formed by the interface of a metal and 2D photonic crystal where only a forward propagating surface acoustic plasmon mode is allowed to propagate. To describe acoustic plasmons we employed a simplified model based on the effective 2D dielectric function in which 3D bulk electron continuum is taken into account in terms of 2D Fourier transform of the screened interaction term. To investigate transport properties of the structures within this frequency range we used finite-difference time-domain (FDTD) method that allows calculating the propagation of EM waves through media with full tensorial magnetooptic permittivity. By using numerical simulation we have shown that implementing surface acoustic plasmons offers a possibility to operate such a one-way waveguide structure in the terahertz frequency range.