## 3 Experimental Analysis of the Acoustic Cloak
### Experiment Results: Wave Guiding Effect of the Cloak Shell  
To avoid interference from reflected sound waves and background noise, we performed the experiments in a semi-anechoic room at the Architecture and Building Research Institute, Taiwan. The environment noise rating was NR-5, the volume of the room 516 m\({}^{3}\) (10 \(\times\) 8.6 \(\times\) 6 m) and the minimum length of the room 6 m, 35 times the wavelength of the sounds being tested (2000 Hz). The low background noise and large testing space created the free-field environment of the acoustic wave. The geometry properties ensured the complete accommodation of the wave period and the free propagation of the wave, thereby ensuring the correctness of the measurement results and the avoidance of unwanted interface noise. To investigate the effects of wave guiding by the cloak shell, we measured the sound pressure field within the zone enclosed by the structure of the cloak shell, as shown in Figure 24. As shown in Figure 24a, we used a B&K Type 4296 omni-directional sound source, which was received using a B&K Type 4190 free-field microphone with a B&K Type 3560E multichannel data acquisition unit. The measurement positions of the microphones were controlled using a 5-axis  
Figure 23: (**a**) 3D diagram of single block structures (multiple elements were produced simultaneously by a 3D printer); (**b**) 3D-printed block element components; (**c**) assembled block elements. testing system, with a computer providing a position accuracy of 0.1 mm. The rectangular area in which the measurements were obtained (152 \(\times\) 342 mm) covered half the cloak shell structure along the _Cx-Cy_ plane, with the microphone positioned just above the cloak shell structure. Measurements were taken in steps of 19 mm in the _Cx_ and _Cy_ directions as shown in Figure 24b. Figure 24c presents the measurement results, indicating the wave propagation behavior inside the half-shell region. The proposed design utilized the transformation for wave propagation around the cloak shell. It is expected that the anisotropic material properties of the cloak shell cause the direction of wave propagation to change from its original direction to a pre-determined path (see Figure 5). Along the  
Figure 24: Experimental setup for measurement in a semi-anechoic room: (**a**) the sound source is excited by B&K Type 4296 omni-directional sound source at 2000 Hz apart from test specimen at 4000 mm; (**b**) the near field measurement inside the cloak shell region was measured using a single free-field microphone scanned in the region between _Cx_ = 0–342 mm and _Cy_ = 0–152 mm, as indicated by the dotted line. (**c**) The results indicate the wave propagation behavior inside the half shell region. horizontal _Cx-Cy_ plane, the cloak shell will concentrate the acoustic waves at its circumference along the propagation direction of the incoming wave, which kept the wave energy from dissipating and allowed it to continue downstream with minimal scattering. As shown in Figure 25, sound field measurements were performed under four conditions: with neither an object nor a shell and only a specimen supporter in space as a reference sound field, with the object (on specimen supporter), with the cloak shell (on specimen supporter), and with the cloak shell and the object (on specimen supporter). Figure 25a-d present the sound field in these cases, respectively. The sound pressure field, as shown in Figure 25a, is for only the measurement facilities and the specimen supporter, with neither an object nor a cloak shell. This field was taken as the reference sound field for further normalized applications. In the field, some minor disturbances were ascribed to the unavoidable local sound source, which was produced by the local sound field in the measurement positions above the specimen supporter. Figure 25b,c plot the results of the individual measurements of the object and the cloak shell. As shown in Figure 25c, although not obvious, higher pressure values were found around the cloak shell.