## 4 Results and Discussion  
### Effects of Acoustic Wave Field on Particle Number Concentration  
When the temperature was adjusted to 160 \({}^{\circ}\)C, the DEH particle size was 0.5 \(\upmu\)m. The effect of SPLs on acoustic agglomeration under different acoustic frequencies is shown in Figure 5, where f is the acoustic frequency. The number concentration of DEH particles in the initial state without the acoustic field is indicated in the original curve. As the SPL increased considerably, the concentration number decreased. It can be concluded that large SPLs are conducive to acoustic agglomeration. The trend of decrease, however, was not evident, and a significant decrease was only observed at the 1600-Hz frequency. The optimal condition occurred when the SPL was 143 dB and the  
The effect of acoustic agglomeration is reflected by the reduction ratio of the PNC. This ratio was calculated using Equation (10). \[\eta=1-\frac{N}{N_{0}}, \tag{10}\]  
where \(\eta\) is the reduction ratio, \(N_{0}\) is the original PNC, and \(N\) is the PNC after agglomeration effects. Figure 5: Effect of sound pressure levels (SPLs) on 0.5-\(\upmu\)m particles at different frequencies in acoustic agglomeration. Figure 8 illustrates the reduction ratio of 0.5-\(\upmu\)m particles at different acoustic frequencies and SPLs. A larger SPL resulted in a greater reduction in the number concentration of particles. The increase in the SPL mainly increased the amplitude of the air medium and particle vibrations. As the amplitude increased, the range of motion of each particle also increased. The relative motion among particles also increased, thus increasing the probability of collision and agglomeration. The optimal frequency for the acoustic agglomeration of the 0.5-\(\upmu\)m particles in the selected frequency band was 1600 Hz. Figure 6: Effect of SPLs on 2-\(\upmu\)m particles at different frequencies in acoustic agglomeration. The reduction ratios of the 2-\(\upmu\)m particles at different acoustic frequencies and SPLs are shown in Figure 9.  
It can be observed that the acoustic agglomeration effect on 2-\(\upmu\)m particles at 800 Hz improved, and there was a downward trend at 1200 Hz. The effect when the SPL was 143 dB and the frequencies were 1600 and 2000 Hz was evident, i.e., the reduction ratio reached approximately 45%. The reduction ratio curve of the 4-\(\upmu\)m particles at different SPLs and frequencies is shown in Figure 10. This curve was similar to that of the 2-\(\upmu\)m particles, and the optimal frequency remained at 2000 Hz; however, the reduction ratio of the 4-\(\upmu\)m particles was considerably larger than that of the 2- and 0.5-\(\upmu\)m particles. Figure 7: Effect of SPLs on 4-\(\upmu\)m particles at different frequencies in acoustic agglomeration. By comparing the acoustic agglomeration results of the three particle sizes, it can be observed that, within the selected 800-2200-Hz frequency band, the agglomeration effect on the 4-\(\upmu\)m particles was more evident under the same acoustic condition. Some scholars [21] found that a larger particle number concentration led to a better acoustic agglomeration effect. The concentration number of the 4-\(\upmu\)m particles was low and it was only one-third of that of the 0.5-\(\upmu\)m particles. This is because the evaporation of DEHs in the monodisperse aerosol generator was constant. The acoustic agglomeration effect on the 4-\(\upmu\)m particles, however, was considerably better than that on the 2- and 0.5-\(\upmu\)m particles. Figure 8: Reduction ratio of 0.5-\(\upmu\)m particles under different acoustic conditions. Figure 9: Reduction ratios of 2-\(\upmu\)m particles under different acoustic conditions. When SPL = 143 dB and f = 2000 Hz, \(\eta_{4}\) = 0.814, whereas \(\eta_{0.5}\) = 0.254 when SPL = 143 dB and f = 1600 Hz. It can be observed that the particle size distribution compared with the number concentration had a more significant effect on acoustic agglomeration.