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].
Bolin et al. [
13] and Nilsson et al. [
14] reported that auditory masking effects [
16,
17] of the natural background sounds on the noise could be exploited to improve the quality of the urban soundscape. For this, however, the level of a background sound must be comparable or higher than the level of the noise [
16,
17,
18,
19], in the case of which it might be incorrect to label them as “background” sounds. Nevertheless, because of its temporal variation, a background sound of singing birds was found to be a less effective masker than broadband natural sounds, such as a water stream (i.e., brook), deciduous vegetation, and coastal breaking waves [
13]. On the other hand, bird sounds were found to improve pleasantness more strongly than water features [
15]. Furthermore, Jeon et al. [
12] reported that among nine different background sounds, “water stream” and “waves of lake” were found to be more preferable than other sounds to be used as mixed background sounds for road traffic noise as well as for construction noise.
De Coensel et al. [
15] argued that not only the auditory masking, but also other perceptual processes, such as auditory attention, might affect the perception of noise in a mixture of noise and background sounds. Thereby it is assumed that, e.g., differences in the temporal structure of sounds are associated with different degrees of attention to them.
Yang et al. [
20] suggested that values of psychoacoustic parameters fluctuation strength [
16,
21], sharpness [
16,
22], and loudness [
16,
23] were the three key indicators for characterization of sounds in soundscape. It can be hence summarized that not only different background sources (water, birds, vegetation, etc.) might influence the perception of the noise differently, but also the temporal structure [
13,
24], spectral structure [
13,
18,
19], and the level [
12,
13,
14,
25] of the background sound could be important parameters for the altered perception of the noise. In accordance with that, Aletta et al. [
26] suggested a holistic approach when categorizing the soundscape, considering the physical spectro-temporal characteristics, as well as the type and the meaning (i.e., the semantic content) of sounds.
With that background, this paper investigates possible effects of background sounds of different types and levels on the perceived annoyance of foreground noise. In particular, sounds of landing helicopters were used as an exemplary noise source. Unlike studies which investigated relatively high-level background sounds—partially masking the noise—in order to improve the urban soundscape [
12,
13,
14], the study presented here focused on the usage of background sounds in psychoacoustic experiments and in virtual acoustic applications.
Section 2 of this paper introduces briefly the experimental concept. Experimental design and setup follow in
Section 3. Experiments 1 and 2 and their results are presented in
Section 4 and
Section 5, respectively. After a discussion of the outcomes in
Section 6,
Section 7 will conclude the paper.
2. Experimental Concept
Acute annoyance reactions to helicopter noise were investigated under laboratory conditions. The observed annoyance ratings correspond to “short-term annoyance” [
27] or “psychoacoustic annoyance” [
16], similar to the reported laboratory experiments by Schäffer et al. [
28] and Taghipour et al. [
8]. The term “short-term” refers to the time period during and after an acoustic stimulus’ playback and before the next stimulus is presented [
9].
To investigate differences in short-term annoyance, stimuli were generated from field recordings (see Taghipour et al. [
8]) of helicopter landings and from background ambient sounds. In both experiments presented here, three design parameters, i.e., two continuous variables, flight event’s A-weighted sound exposure level,
, and background sound’s A-weighted equivalent continuous sound pressure level,
, as well as the categorical variable
background sound type (i.e., eventful vs. less eventful) were systematically varied to study their individual and combined associations with short-term annoyance ratings.
To avoid confusions between the foreground sound (i.e., the helicopter noise) and the ambient background sound, in this article, “noise” and “sound” are repeatedly used for these two categories of sounds, respectively. However, it should be noted that, in general, sounds are not to be labeled or prejudged as “noise” per se. It is rather for each individual observer to judge subjectively whether they perceive a sound as noise. Pleasantness or unpleasantness (including declaring a sound as “noise”) is not a definitive, but a (subjectively) perceived quality of sound, which can only be interpreted in the context under investigation [
9,
29].
The choice of landing helicopters as foreground noise was rather exemplary. A number of takeoffs and landings of civil helicopters and propeller-driven aircraft was available, which were calibrated, analyzed and already tested with regards to the perceived annoyance [
8]. In order to avoid possible variations due to the type of aircraft (helicopter vs. propeller-driven aircraft) and procedure (takeoff vs. landing), only landing helicopters were chosen for this study. Whereas the ecological validity or plausibility of this particular noise situation is supported by landing civil helicopters in everyday life for medical, rescue, special transport, or sport (hobby) purposes, it should be noted that usage of other types of foreground noise samples (e.g., road traffic) might exhibit higher ecological validity [
30]. Similar experiments are planned with other noise sources.
6. Discussion
Beside the differences in the number of observations and the range of
, experiments 1 and 2 were designed and conducted similarly. It seems therefore plausible to compare the results of the two experiments and to try to summarize the results in a combined analysis. Annoyance rating histograms for the two experiments are shown in
Figure 9. Median annoyance ratings in experiments 1 and 2 were 5 and 6, respectively. Furthermore, mean annoyance ratings in experiments 1 and 2 were 5.54 and 5.47, respectively. Considering that the
(or
) ranges of the two experiments were comparable, this outcome is not surprising. In conjunction with the similarity of the linear mixed effects models—i.e., Equations (
2) and (
3)—this could be interpreted as a quasi-indicator for reliability and reproducibility of the experiment. Since experiments 1 and 2 were conducted by the second author and the first author, respectively, this is additionally indicative of the objectivity of the experiment conduction.
The effect of
on perceived annoyance was very similar in both experiments: annoyance increased with increasing
. The value of its regression coefficient,
, was equal to 0.413 and 0.360 in Equations (
2) and (
3), respectively. In both experiments, stimuli containing eventful background sounds were rated systematically higher (i.e., more annoying) than those containing less eventful background sounds. In the mid range of
, generally, with increasing
, annoyance tended to increase and decrease for stimuli with eventful and with less eventful background sounds, respectively (see
Figure 5 and
Figure 8). The differences between the annoyance ratings for these two categories of background sounds saturated (i.e., vanished) in the lower and higher end of the
range, which is similar to saturation effects in general psychometric functions [
37,
38]. That is, lower and higher than certain
thresholds (for these laboratory experiments 34 and 54 dB(A), respectively), it would not make a difference whether the background sound is eventful or less eventful. In the case of the lower threshold, the background sound might be generally irrelevant in presence of a much louder and partially masking flight event. In the case of the higher threshold, the background is probably clearly present and audible in foreground regardless of its eventfulness.
While the differences between annoyance ratings for the eventful and the less eventful background sounds were robust and alike for the two experiments, their differences to the baseline stimuli containing only helicopter noise (i.e., no background sound) were not similar in the two experiments.
Table 5 shows mean and median annoyance for these three categories. In experiment 1, on average, annoyance from the stimuli with eventful background tended to be higher than annoyance from the helicopter stimuli (with no added background sound). In experiment 2, on average, annoyance from the stimuli with less eventful background tended to be lower than annoyance from the helicopter stimuli (with no added background sound). Further investigation are needed in order to explain this difference and/or to provide more clarity in this regard.
Annoyance increased slightly with increasing playback number in both experiments. The value of its regression coefficient,
, was equal to 0.017 and 0.060 in Equations (
2) and (
3), respectively. This is in accord with other laboratory studies [
8,
28,
39]: generally, annoyance increases with increasing number of stimuli listened to in an experimental session. This emphasizes the importance of a randomized playback order of the stimuli, as done for the two experiments presented here.
For both experiments, including subjects’ random intercept in the linear mixed effects models improved the models significantly. This confirms findings of other psychoacoustic laboratory experiments [
8,
28,
40,
41]. Furthermore, this shows the general interindividual differences in the preferences and sensibilities of human observers to background sounds [
42]. It should be noted that including subjects’ age and gender in the models, i.e., Equations (
2) and (
3), did not improve the models significantly.
Similar further analyses were carried out with
,
, or
instead of
in the linear mixed effects models. This was done to investigate whether—beside the differences caused by the background type—the subjects were annoyed mainly by the flight event levels (F) or by the level of the mixture of flight and background sound (S). All the linear mixed effects models with
,
, or
led to higher AIC and BIC than the same models with
, which indicates that
was a better predictor of short-term annoyance.
Table 6 shows Pearson correlation coefficients for bivariate correlations between annoyance and these level variables.
Table 6 confirms the above analysis: annoyance correlated more strongly with the flight event sound exposure level (
) than with the stimulus level variables.
The outcome of this study strengthens the theories of urban sound design. Enabling the presence of less eventful background sounds (such as water stream, birds, and vegetation) of mid
range should be effective in reducing perceived annoyance from noise sources in living areas which are affected by them. The highest annoyance rating in experiment 1 was given for the stimuli with the eventful church bell in background. Similarly, Kang and Zhang [
43] reported that natural and culture-related sounds (e.g., music) were preferred compared to artificial sounds. More importantly, Hong and Jeon [
44] reported that, in particular, water and bird sounds have been usually evaluated as the most effective and most favorable sounds to improve urban sound environments [
11,
12]. The results of the present study indicate that, for background sounds to be effective in reducing perceived annoyance and improving the soundscape, not only it is important that they are natural, e.g., from water and birds, but also they should exhibit a low degree of eventfulness. Furthermore, their sound pressure levels should not be too low or too high.
On the other hand, if the goal is, for example in a psychoacoustic experiment or in virtual acoustic demonstrations, not to affect the perception of the foreground sound (i.e., noise)—and hence to avoid a weakening in the validity of the experiment—the data suggest to use relatively low-level natural background sounds with a low degree of eventfulness. For the data collected from the two presented experiments, this
range was about 34 to 41 dB(A). Consistent with this outcome, Taghipour et al. [
8] reported that two mixed “birds and vegetation” sound samples exhibiting low degrees of eventfulness and sound pressure levels (
) of around 37.5 dB(A) were found to be optimal for a 3D auralization of aircraft noise.
De Coensel et al. [
15] and Hong and Jeon [
44] reported that, in similar contexts, (mixed) birds sounds were more pleasant or more preferable than sound of water features. In the present study (i.e., in experiment 1), no significant difference was found between annoyance from stimuli containing less eventful water stream and those containing less eventful birds.
7. Conclusions
Two laboratory psychoacoustic experiments were reported in this paper, in which short-term perceived annoyance from (mixed) sounds were collected. Stimuli consisted of foreground helicopter noise and background ambient sounds. The main predictor of annoyance was helicopter’s . The stimuli containing eventful background sounds were associated with higher annoyance ratings than the stimuli containing less eventful background sounds. Generally, increasing accentuated this difference, however, with saturation effect at the lowest and highest (at which no significant difference was observed between these two categories of background sounds). Furthermore, the statistical analysis did not lead to a unique correction factor compared to the baseline stimuli containing only helicopter flight events (i.e., no background sound). The observed differences were within one point on the ICBEN 11-point scale, however, different for the two experiments. Further studies are needed in this regard, as the quantification of such a correction factor is helpful (and partially essential) for applications in psychoacoustic experiments on annoyance and virtual acoustic demonstrations.
With respect to applications in urban sound design and acoustic comfort improvement, the outcomes suggest that enabling less eventful background sounds of water stream, birds, and vegetation could decrease the perceived annoyance by the residents. For this purpose, a mid range seems to be appropriate. With respect to applications in psychoacoustic experiments and in order to ensure the experimental validity (with respect to the foreground aircraft noise), the outcomes suggest to use low-level water stream, birds, and vegetation sounds which exhibit a low degree of eventfulness.