## 3 Noise Analysis  
### Aircraft Noise Assessment Methods  
The aircraft noise assessment units vary, and the resulting assessment criteria are all different. Currently, South Korea uses Weighted Equivalent Continuous Perceived Noise Level (_WECPNL_), as shown in Equation (4). _WECPNL_ is a unit proposed by the International Civil Aviation Organization (ICAO) as a measure of long-term, continuous exposure by aircraft, accounting for the metallic noise characteristics of aircraft. In Korea, noise is calculated close to the actual noise felt by residents by the average of the highest noise of aircrafts weighted by the number of flights per day, right, and in the midnight hours.  
\[WECPNL=L_{max}+10\log(N_{2}+3N_{3}+10(N_{1}+N_{4}))-27 \tag{4}\]  
\(L_{max}\) represents the average of the maximum noise measured for a day over 10 dB above background noise when the planes pass through. \(N_{1}\) represents the number of aircrafts passed between 24:00 and 07:00, \(N_{2}\) between 07:00 and 19:00, \(N_{3}\) between 19:00 and 22:00, and \(N_{4}\) between 22:00 and 24:00.  
However, _WECPNL_ tends to be over-approximated, and other environmental noise in the country applies the equivalent noise method (\(L_{den}\)), making it difficult to compare with aircraft noise, which is limited to reflecting the sum of continuous noise exposure. In addition, advanced aviation countries, such as the U.S. and Europe, have adopted the equivalent noise method. Therefore, the Ministry of Environment of Korea will change the aircraft noise unit from _WECPNL_ to the equivalent noise method \(L_{den}\) in order to unify it into an international standard, which is scheduled to take effect in 2023.  
The equivalent noise assessment method complements the shortcomings of the _WECPNL_ and is linearly related to _WECPNL_. It is also appropriate to use as an aircraft noise assessment indicator considering continuity with the current standard because it is easy to automatically measure with integration without simplification due to the development of noise-measurement technology.  
Figure 9: (**a**) Result of clustering when the number of vertiports is 10; (**b**) result of clustering when the number of vertiports is 40.  
The equivalent noise assessment method is the average noise divided by 24 h, after measuring the equivalent noise level every hour for a day, and weighted hourly. In addition, the two divisions of the time zone are \(L_{dn}\) (day-night average sound level), and the three divisions of the time zone are \(L_{den}\) (day-evening-night average sound level). In the case of the two divisions, the daytime \(L_{d}\) is between 07:00 and 22:00, and the nighttime \(L_{n}\) is between 22:00 and 07:00, and only the night time zone is weighted by 10 dB, as shown in Equation (5). In the case of the three divisions, the daytime \(L_{d}\) is between 07:00 and 19:00, the evening \(L_{e}\) is between 19:00 and 22:00, and the nighttime \(L_{n}\) is between 22:00 and 07:00. This case is weighted by 5 dB in the evening and 10 dB in the nighttime.  
Therefore, in this paper, noise analysis is performed on the basis of \(L_{den}\), as shown in Equation (6), as the equivalent noise assessment easily measures noise and distinguishes day, evening, and night time zones and will be changed from _WECPNL_ in 2023.  
\[L_{dn}=10\log\left[\frac{1}{24}\left(15\cdot 10^{\frac{L_{d}}{10}}+9\cdot 10^{ \frac{(L_{n}+10)}{10}}\right)\right] \tag{5}\]  
\[L_{den}=10\log\left[\frac{1}{24}\left(12\cdot 10^{\frac{L_{d}}{10}}+3\cdot 10^{ \frac{L_{d}+5}{10}}+9\cdot 10^{\frac{L_{n}+10}{10}}\right)\right] \tag{6}\]