## 1 Introduction  
There are a variety of applications in the context of laboratory noise research or virtual acoustics for which background sounds are useful or, in many cases, necessary. The following is a non-exhaustive list in this regard. It is possible to generate noise synthetically by means of emission synthesizers using physical and/or parametric approaches [1; 2; 3; 4; 5; 6; 7]. Adding recorded or synthesized (natural) background sounds to the noise samples helps to increase the degree of realism of the auralization. It is further possible to use propagation filters to simulate farther distances [4; 8], in the case of which the recorded background sounds would also be sent to the target distance. Whereas in this case, only the source is aimed to be sent to the target distance, the background sounds are actually supposed to remain close to the observer. This problem can be solved with adding similar background sounds in the observer position [8]. When noise is recorded by a calibrated single microphone, as long as the exact momentary positions of the source and the observer are recorded as well, it would be possible to auralize the movement of the source on a multi-loudspeaker system. In this case, however, background sounds contained in the original recording (e.g., birds) would incorrectly move with the foreground noise from one loudspeaker to the other. Since the background sound should commonly be relatively static (with respect to its location), its movement with the noise could be extremely unrealistic and irritating. This might be solved by adding static background sounds to the moving noise which could partially mask the (moving) inherent background sound [8]. When virtual urban scenes are designed or virtual sound walks are simulated, they typically contain one or more background sound sources [9; 10]. It is assumed that sounds of water features, birds, or natural vegetation would improve the virtual living (or urban) environment [11; 12]. Generally, in a psychoacoustic experiment on acoustic comfort, sound pleasantness, or noise annoyance, stimuli are typically played back successively (i.e., one after the other). While the subjects are sitting in a lab, it is asked to imagine they would be sitting in an office, in the garden, in a wind park, at a street corner, or in other scenarios. Thereby, typically no sound of the imaginary environment--but rather silence--exists in-between the stimuli. It would be much more realistic to have a background sound present in the room for the whole duration of an experiment, from which each stimulus then arises into the foreground. Furthermore, it should be mentioned that, although noise recordings are ideally carried out carefully such that they are free of dominant background sounds, still some sort of background sounds are present in any natural or everyday environment. Hence, the recorded foreground sound samples also contain some background sounds. In psychoacoustic experiments on acoustic comfort, pleasantness, or noise annoyance this introduces a possible source of error or bias, which should be controlled for. The examples given above show the relevance and importance of background sounds in virtual audio and psychoacoustic applications. Whereas these examples justify the vast usage of background sounds in the respective contexts and for particular reasons, only a few studies investigated the effects they could have on the perception of the respective scenes. A series of studies reported that background sounds in presence of noise sources might improve the acoustic quality of the urban soundscape [12; 13; 14; 15]. For example, natural water features in urban environments were reported to reduce the loudness of road traffic noise [14; 15] and of wind turbines [13]. They were further reported to reduce annoyance from the road traffic or wind turbines [12; 13]. [PERSON] et al. [13] and [PERSON] et al.