respectively. The π/ 6 bending refraction beams in the first two cases and the almost totally reflected beam in the third case are clearly observed. The self-collimation effects in square lattice PC or PhC have already been discussed by some researchers [17–21], however, to our knowledge, this effect in triangular lattice structures has not yet been utilized as a means to design an acoustic lens for focusing sound. Here we show its applicability. Unlike most previously designed flat lens using circular EFC, we use a hexagonal EFC to achieve this goal. Usually a size-matched circular EFC having an effective refraction index close to − 1 is preferred, because it has several advantages such as that for any incidence angle a refracted wave can always be excited, and all rays can be brought to the same focal spot. However, usually size-matched EFC can only be realized through sophisticated methods, such as using more than one kinds of host media [14]. Our aim in this paper is to design a simple PC lens without relying on those sophisticated methods, thus the hole-array structure is our choice. In order to reduce the total reflection power, the EFC size of the PC must be large enough and close to that of the host medium. This consideration leads us to choose ωa/ 2 πc$_{t}$ = 0 . 6 as the working frequency. If a slightly higher frequency EFC is chosen, the reflection gets higher, and the image intensity gets dimmer. The anisotropic refraction property of the PC looks like a disadvantage, but we will show that a flat lens of this PC can indeed work very well and it can brought the waves emitted by a point source to a image spot of subwavelength wide. Now we put a point source in front of the lens and study the imaging effect. Simulation results for 9-layer and 13-layer structures are shown in Fig. 3(a)-(c) and (d)-(f), respectively. From (a) to (c) or (d) to (f) the source-lens distance is increased from a to 6 a . According to these simulation results, the shear waves inside the PC indeed propagate along specific directions (the ± π/ 6) as expected. Besides, the thicker the lens, the farther the image locates, consistent with the negative refraction description. Furthermore, when the source moves towards the left, the image also moves towards the left, keeping the source-image distance almost unaltered. The intensities | u | 2 of the displacement field in the focal planes (located at the positions of the image peaks) for the 9-layer and 13-layer lenses are shown in Fig. 4(a) and (b), respectively. In each subplot three source-lens distances ( d = 0 . 8 a , 3 . 3 a , and 5 . 8 a ) are examined, and the image widths w defined by their full-width-at-half-maxima (FWHM) are recorded. The results reveal that the lens can really make a subwavelength image of the 6