## 4 Noise Level Analysis Results  
The level of noise generated by rotating gears was measured at the top cover of the medical compression machine.  
The main cover is in contact with the body, so it does not propagate noise. The results of an analysis of the eight cases are summarized in Figure 13. In Figure 13, the noise patterns were represented with STFT plots. In the STFT plots, the time and frequency were shown on the horizontal and vertical axis, respectively, and the color contour indicates the amplitude (dB) of the frequency component.  
The plots were generated by sequentially overlapping frequency transformations for a specific time interval (0.05 s). Figure 12a shows the analysis results of the initial model. In  
Figure 11: Gears rotation by the motor.  
Figure 12: Torque according to motor’s rotational velocity: (**a**) Input rotational velocity; (**b**) Output torque.  
all cases, the noise level peaks at 950 Hz, which is the GMF component of the gear pairs calculated using Equation (11).  
This result suggests that the GMF component generated from the pair of gears has the most significant impact on the system.  
The STFT results in Figure 12 show the typical white noise pattern caused by the meshing of the toothed gears. White noise is excited by the transmission error of mating gears and can be described as a resonance due to gear mesh frequency.  
The highest noise level at the top cover and the weight of each case are summarized in Tables 5 and 6.  
The initial tolerance applied in Cases 1 and 2 increased the noise level to 1.3% and 2.7%, respectively. Poor bearing assembly was found to have a more significant effect.  
Figure 13: Noise level analysis results by case: (**a**) Initial model; (**b**) Case 1; (**c**) Case 2; (**d**) Case 3; (**e**) Case 4; (**f**) Case 5; (**g**) Case 6; (**h**) Case 7; (**i**) Case 8.  
Combining Tables 5 and 6, the following results can be obtained. In Case 3, where the cover material was changed from polycarbonate to aluminum, a material with a better sound absorption property, the weight increased by 8.4% while the noise level was reduced by 8.2%. For Case 4, where the driven gear material was changed from S45C to polyacetal, the noise level and weight were reduced by 7.5% and 8.5%, respectively. Both the weight and noise level could be reduced by changing the material for the driven gear. The fatigue strength of S45C and polyacetal are 307.7 [21] and 40-45 MPa [22], respectively. The polyacetal gears are vulnerable to fatigue because gears are exposed to repeated loads. For Cases 5-7, structural reinforcement increased the weight, but the noise levels were lowered. For Case 5, the gear cover was enclosed with the solid frame, the weight increased by 21.5% (600 g), and the most significant level of noise reduction was achieved--11.8%.  
This result could be attributed to the sealed gear cover, through which significant noise is propagated. The arch-shaped reinforcement also increased the weight of Case 6 by 1.9%, but the noise level was reduced by 7.2%. Increased cover thickness in Case 7 also increased the weight by 2.1%, with a noise reduction of 7.2%. Based on these results, structural reinforcement (Case 6) and increased cover thickness (Case 7) are preferred for noise reduction while keeping the weight increase minimal. Reduced gear thickness and a consequent decrease in weight and contact area of the gears led to noise reduction (Case 8). However, as described earlier, the pressure required for therapeutic body compression is 40 kPa.  
The load required to achieve this pressure level is applied to the gears in contact, and therefore, increased stress due to a smaller contact area may cause fatigue problems.