A uniformly planted spruce forest functions acoustically like a sonic crystal.

Evidence

Site 2 (planted spruce) was the most uniform and regular forest site. It consisted of spruce trees that had been planted at the same time on a square grid with trunks between 15 and 30 cm in diameter. Photographs of this site appear in Fig. 1(b). The ground was flat and free of vegetation and other debris; only needles were present (see Fig. 10). The trees had been pruned to remove the lower branches. This site is similar to a “sonic crystal” (Sanchez-Perez et al., 1998) in appearance, with the trees located on a square grid with a spacing of about 2 m between trunks. — Donald G. Albert, Michelle E. Swearingen, Frank E. Perron, and David L. Carbee, Low Frequency Acoustic Pulse…, 38

Categorization

• DEFINE: ‘Sonic crystal’ is a technical term here applied by analogy: regularly spaced trees on a square grid act as a periodic acoustic structure.
• XREF: Connects to phononic/photonic crystal physics and to periodic metamaterials — same geometry, different wave medium.