# Methods for Noise Event Detection and Assessment of the Sonic Environment by the Harmonic Index
## 2 Materials and Methods
### Data Processing  
#### 2.2.1 Detection of Noise Events  
Individual events in the sound pressure level (SPL) time history of the environmental noise are usually identified by an exceedance-based detection algorithm. As detailed in the extensive literature review reported in [15], the onset of a noise event is detected when the instantaneous SPL exceeds a threshold level L\({}_{\text{B}}\) for a duration \(\tau\) in s, and with an emergence of at least E dB. Noise events are only retained when the time gap (or noise free interval) since the previous detected event is larger than \(\tau_{\text{g}}\)[16]. A script running in the \"R\" environment, version 3.6.3 [25], was developed to import each of the 1 s short L\({}_{\text{Aeq}}\) time series as input in a text file format, formed by four columns; that is date, time, short L\({}_{\text{Aeq,1s}}\) in dB(A) at 1 s interval, and a label indicating the corresponding source, namely road traffic noise (RTN) or something else (ANE; anomalous noise event), as recognized by the ANED procedure [19]. Among the several criteria proposed to detect noise events, the following were considered (Figure 3), consider the outcome of the extensive studies carried out in [15,16]:  
1. Noise levels exceeding the threshold L\(\beta\) = LAeqT + C dB, according to the formulation of the intermittency ratio (IR) [17], where LAeqT is the continuous equivalent level  
Figure 2: Sites in Milan selected for noise monitoring (satellite images from Google Earth Pro 7.3.4.8248) [22]: (**a**) Site “Ml139” Via Leonardo Bruni; (**b**) Site “Mm129” Via Benigno Crespi; (**c**) Site “Mh117” Viale Enrico Fermi. referred to the measurement time T and 3 dB is the value chosen for the constant term C [17] (Figure 3a). This algorithm is denoted as \"IR\" hereinafter. 2. Noise levels exceeding the threshold L\(\beta\) = LAeq\({}_{\text{r}}\)run + 3 dB, where LAeq\({}_{\text{r}}\)run is the running LAeq (Figure 3b). This algorithm is denoted as \"Lr\" hereinafter. 3. Noise levels exceeding the threshold L\(\beta\) = LA50 + 10 dB, namely the NAL50E10 metric [16], where LA50 is the 50 th noise level percentile; that is the noise level exceeded for 50% of the measurement time T (Figure 3c). This algorithm is denoted as \"L50\" hereinafter. 4. SPL onset, as sum of progressive positive SPL increments, greater than 10 dB from the event start time \(\tau_{\text{s}}\) (Figure 3d). This algorithm is denoted as \"O10\" hereinafter. Each of the above algorithms were applied to detect noise events in the 1 s short L\({}_{\text{Aeq},\text{Is}}\) time series with the following options:  
* Without any condition on the event duration \(\tau\) and time gap \(\tau_{\text{g}}\) (or noise free interval) between adjacent events, hereinafter denoted as NC. * Event duration \(\tau>\) 2 s and time gap \(\tau_{\text{g}}\) equal to 5 (T5) and 10 (T10) s, hereinafter denoted as C.  
Figure 3: Parameters of the sound event detection for each algorithm: (a) algorithm “IR” based on IR; (b) algorithm “Lr” based on L\({}_{\text{Aeq},\text{run}}\); (c) algorithm “L50” based on L\({}_{\text{A50}}\); (d) algorithm “O10” based on the sum of progressive increments of SPL. \(\tau\) is the duration of the sound event, starting at time \(\tau_{\text{s}}\) and ending at time \(\tau_{\text{e}}\), and \(\tau_{\text{g}}\) is the time gap between sound events. When a sound event was detected, in order to assign the relevant source, the labels provided by the ANED procedure at each second along the duration of the sound event itself were considered. The source was assigned according the following rules:  
* Unique source labels, assigned source corresponding to that indicated by all the labels when they were always the same (either road traffic noise (RTN), or something else, i.e., anomalous noise event (ANE). * Mixed source labels, assigned source corresponding to that indicated by the majority of the labels when they differed. * Equal number of RTN and ANE labels, no source assignment.