*3.1. Experiment of Performance Measurement*

A piezoelectric sheet is a material that converts electricity into sound. The tone and intensity of the sounds produced depend on the voltage and the frequency of the electricity supplied to the sheet. In this study, the voltage was fixed at the maximum value of 30 V. The measurement range of the frequency was between 50 and 450 Hz. At the same time, we investigated the relationship between voltage frequency and metal sheet displacements.

#### 3.1.1. Noise

This experiment made use of a handmade soundproof box. The background noise of the testing room was 41.6 dB, while inside the soundproof box it was 31.4 dB. The results were plotted according to the size of the piezoelectric sheet used in the given device. Figure 8a shows the results for three test devices named by case1, case2, and case5, in which the diameter of the piezoelectric sheet was 41 mm. As can be seen in Figure 8, the devices with piezoelectric sheets of the same size had a similar trend in terms of the growth of the noise produced. There was a significant increase in the volume at 150 Hz. With increasing frequency, the results of the noise intensity formed a parabola. The red line indicates the decibel level tested, 50.7 dB when the rotary fan was placed in the soundproof box. This was 1 dB lower compared to the device of piezo actuation jet at 300 Hz. Since the opening length was longer for the case1 and case5 devices, those devices had fewer places on which to fix the piezoelectric sheets than the case2 device, such that they caused more noise due to the free vibrations of the metal foil. The spacing of the piezoelectric sheet was larger in the case5 device. Therefore, the vibration noise produced by the device was about 1–2 dB louder than that produced by the case1 device, which is not very different. Figure 8b shows the results for the device of PAJs in which the diameter of the piezoelectric sheet was 31 mm (e.g., the case3 and case4 devices). As shown in the figure, the noise produced by the case3 device exhibited linear growth, while the slope of the parabola for the case4 device was roughly similar to a straight line. When the frequency was set at 125 to 225 Hz, the volume and the tone for both devices remained similar. Meanwhile, the case3 device produced a higher volume than the case4 device since it had a longer length of opening. The experimental results showed that the three devices of piezo actuation jets produced noise similar to that of the rotary fan at 300 Hz. At frequencies from 60 Hz–150 Hz, however, the noises caused by the three devices of PAJs were lower than that of the rotary fan. In addition, a device of piezo actuation jet has no wind shear effect; therefore, it can reduce its noise effectively by adopting a low operating frequency.

**Figure 8.** *Cont.*

**Figure 8.** The results of noise measurement. (**a**) Case1, case2, case5 (**b**) Case3, case4.

#### 3.1.2. Displacement

When constructing a device of PAJ, the number of adhesives used and the fixed probes will be slightly different due to human error. However, it can be speculated from the findings that the same types of piezoelectric sheets have similar vibration behaviors. Figure 9a shows the displacement results for the test PAJs in which the diameter of the piezoelectric sheet was 41 mm. As shown in Figure 8, the amount of displacement exhibited a stair-step pattern, increasing every 50 to 100 Hz. The displacement divided by the frequency was between about 0.1 to 0.2 μm. Figure 9b shows the displacement results for the test PAJs in which the diameter of the piezoelectric sheet was 31 mm. When the frequency was below 280 Hz, there were no significant changes in the amount of displacement. The amount of displacement increased dramatically, however, when the frequency range was 290 to 300 Hz; the ratio of the displacement and the frequency was between 0.1 and 0.2 μm. The two different types of piezoelectric sheets exhibited completely different vibration behaviors in the same measurement range, yet for both types, the ratio of the displacement and the frequency was between 0.1 and 0.2 μm. The amount of displacement can be estimated using the input voltage frequency and the vibration is related to the type of piezoelectric sheet.

#### 3.1.3. Wind Speed

Since the wind speeds of the rotary fan and the piezoelectric fan are not high, if the distance from the heat source is too great, the air jet cannot exchange heat with the heat source and the effect of forced convection cannot function properly. In this section, the wind speed is used to determine the cooling effect and the size of the wind velocity perturbation can be regarded as the indicator for the strength of the turbulence. Figure 10 shows the wind speed of devices at different distances from the hot-wire anemometer. The results show that the wind speed and placement of the rotary fan and devices are inversely proportioned. In case3, the wind speeds measured at 15 mm and 20 mm are larger than at 10 mm. This is because the wind speed is weak in case3 and generates a pair of vortexes at

15 to 20 mm to disturb the ambient air. Therefore, the wind speed is higher. When placed at 25 mm, it will not be affected. In case4, a larger wind speed is generated at 20 mm, while at 25 mm it is not affected. The devices of PAJ are unstable at low frequencies. The interval time between expansion and compression is longer so that it should be no outward expansion flow field, but only the inward airflow into the cavity. When a higher wind speed is generated during the compression, it causes unstable wind speeds. Therefore, accuracy is lower. When frequencies are above 150 Hz, the time interval is shortened and it can be regarded as a continuous outward airflow and the wind speeds will stabilize.

**Figure 9.** The results of displacement measurement. (**a**) Case1, case2, case5 (**b**) Case3, case4.

**Figure 10.** Each device when the wind speed and placement diagrams under different frequencies. (**a**) Rotary fan (**b**) Case1 (**c**) Case2 (**d**) Case3 (**e**) Case4 (**f**) Case5.
