## 3 Experimental Design and Setup
### Stimuli Preparation
#### Signal Processing and Mixing  
As mentioned above, six recordings of landing helicopters were chosen for the psychoacoustic experiments reported here. Furthermore, four background sound extracts were selected: two sounds with dominant events (i.e., a chiming church bell and singing birds) and two sounds with less dominant events (i.e., water stream and birds). Eventfulness in this context refers to a series of single dominant events with relatively high energy which arise from a rather low level background. Less eventful sounds are, on the contrary, sounds whose momentary sound pressure level do not vary extensively from their A-weighted equivalent continuous sound pressure level, \(L_{\text{Aeq,B}}\)[31]. The categorization of background sounds into eventful or less eventful was carried out by (qualitative) subjective evaluations by the experimenters and another environmental acoustical, as well as by objective differentiation based on signals' intermittency ratio (IR) [31] and \"\(L_{10}-L_{90}\)\". A series of sound temporal dynamics measures is listed in Table 1 to characterize the amount of single events in the signals. Furthermore, Figure 1 shows A-weighted spectrograms of the background sound signals. The signals underwent the following processing steps to prepare them for playback in AuraLab. Most of the processing--beside bass management--was carried out with MATLAB R2016b (MathWorks, Natick, USA). In a first stage of processing, the recordings were calibrated, high-pass filtered (\(f_{c}=20\) Hz), and low-pass filtered (\(f_{c}=10\) kHz) to attenuate irrelevant noise (e.g., microphone inherent noise). The duration of each stimulus was set to 20 s. While background signals were available at a sampling rate of 48 kHz, helicopter signals were upsampled from 44.1 to 48 kHz, which is a requirement of the playback system in AuraLab. The output (mixed) signals were of 16 bit depth. Background and helicopter signals were faded in and out by squared cosine ramps of 0.5 and 4 s, respectively. That is, whereas the background sound was planned to be present during the stimulus duration of 20 s, the helicopter sound was supposed to be arising from and fading out into the background sound. The cuts for the helicopter signals were carried out (level-)symmetrical around their maximum A-weighted noise level \(L_{\text{ASmax,F}}\); i.e., in the range from \(L_{\text{min}}\) to \(L_{\text{ASmax,F}}\), whereby the exact value of \(L_{\text{min}}\) (i.e., a specific minimum level on the \(L_{\text{AS}}\) curve) varied for different flight events, corresponding to a (windowed) signal duration of 20 s. The sound pressure level of the helicopter noise signals and the background signals were modified to reach the target levels. That is, helicopter \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sound** & **Category** & **IR (\%), C = 3 dB** & **IR (\%), C = 5 dB** & \(L_{\text{AE,B,10}}-L_{\text{AE,B,90}}\) **(dB)** \\ \hline Water stream & L & 58.7 & 38.7 & 0.8 \\ Birds (and vegetation) L & L & 62.3 & 48.6 & 5.3 \\ Birds (and vegetation) E & E & 77.3 & 70.9 & 13.5 \\ Chiming church bell & E & 67.9 & 55.2 & 8.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Information about the background sounds at the exemplary \(L_{\text{Aeq,B}}\) of 41 dB(A). For the calculation of intermittency ratio (IR), C is the dB-offset (i.e., the dB difference to \(L_{\text{Aeq,B}}\)) beyond which events were counted [31]. Figure 1: A-weighted spectrograms of the background signals. Hann-windows of 2048 samples (42.7 ms) with an overlap of 50% (1024 samples) were used to generate the spectrograms. For example, whereas the broadband water stream did not contain many dominant events, the eventful birds and vegetation sound (E) contained four clearly audibly distinguishable and dominant events in the following time segments: 1–2, 5–7, 13–15, and 18–20 s.  
signals were attenuated and set to the target \(L_{\text{AE,F}}\). Furthermore, background signals were either attenuated or amplified to scale them to the target \(L_{\text{Aeq,B}}\).