Drone noise in my backyard: the challenges for public acceptability Roalt Aalmoes 1 Marta Tojal Castro 2 Naomi Sieben 3 Rui Roosien 4 Royal Netherlands Aerospace Centre NLR Anthony Fokkerweg 2, 1059 CM, Amsterdam, The Netherlands

ABSTRACT The introduction of drones in urban areas for surveillance, parcel delivery services, air taxis, or other services have raised the issue of public acceptability. How can this concept known as Urban Air Mobility (UAM) be successfully introduced in an area without upsetting communities? And how can currently unknown benefits of these services be compared to anticipated drawbacks? Noise impact is already considered to be one of the main concerns for successful introduction of UAM, but focusing on noise levels exclusively may not be enough. Based on recent research on noise annoyance and how it affects individuals and communities, a holistic approach, including noise impact, as well as non-acoustical factors, is promoted to address the annoyance towards these disruptive air vehicles. Other subjective measures should be considered including demographic factors, as well as percep- tional factors, such as the visual environment where these vehicles operate, and emotional factors such as attitude towards drones and air taxis. Using this approach, studies on noise impact of UAM will be able to evaluate the use cases in their intended setting and with the appropriate target com- munities to assess the true impact and define the real challenges to overcome for noise research in the coming decade. 1. INTRODUCTION

The Urban Air Mobility (UAM) concept is considered to be one of the most innovative developments in aerospace industry. According to EASA [1], UAM is “…defined as an air transportation system for passengers and cargo in and around urban environments ”, but Tojal et al. examined that other entities use a comparable or different definition for this concept [2]. In this article, we assume all UAM concepts that may (positively or negatively) affect communities or address public acceptability. UAM operations will take place closer to urban areas than traditional air transport systems, where low-flying operations only take place around airports. Therefore, more communities may be affected by the impact of nearby drones or air taxis. Also, because UAM includes a variety of new types of aircrafts, communities are affected in a different way than conventional aircrafts. The main environ-

1 Roalt.aalmoes@nlr.nl

2 Marta.Tojal.Castro@nlr.nl

3 Naomi.sieben@nlr.nl

4 Rui.roosien@nlr.nl

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mental concerns with respect to drones [3] are noise pollution and third party risk, as well as emis- sions. Although most noise models have electric propulsion, with zero local emissions, the total life cycle analysis costs of UAM, that may introduce other emissions, should not be neglected. Also, secondary concerns should be considered, such as shadow flickering or light pollution at night (visual pollution), privacy concerts, distractions or even effects on flora or fauna. These effects can change the attitude towards these vehicles and may lead to non-acoustical factors that contribute to existing annoyances, resulting in adverse health effects for some [4]. As UAM will introduce a new, formerly unfamiliar noise source to an existing environment, lessons can be learned from introduction of other new noise sources and how this was perceived. A similar case that requires attention is the introduction of wind farms on land: it provides similar chal- lenges such as both visual appearance and a new noise source, although the noise character is different (continuous vs. event-driven). But (negative) attitudes towards wind farms may contribute to re- sistance for wind farm projects, and this may also be true for proposed UAM concepts in cities. Another new technology that may share some resemblance with UAM is the introduction of electric cars: these vehicles are considered positive due to their potential of running on fossil-free, climate- neutral electric energy, and their benefits for local air quality. Also, engine noise is reduced signifi- cantly to the point where slow driving cars are even considered dangerous without additional artificial sounds added to it. Electric drones have similar climate-neutral benefits, although sounds of drone may still be considered noisy because of the particular sound of the spinning rotor blades. But in cases of replacement of traditional aircraft by drones (e.g. for observational purposes), there may still be a net gain. 2. CHALLENGES FOR PUBLIC ACCEPTANCE

Public acceptance is one of the main conditions for successful adoption of UAM. From an engineer- ing point of view, technological difficulties are the main drivers for development of new technolo- gies: control systems should work as foreseen, battery life should be sufficient for the targeted ap- plication, the vehicle should be safe and secure, and costs and revenues should make the solution economic viable. Sometimes, also aircraft noise is a concern from the beginning and designs are op- timized already to support (more) silent operations, or to make sure noise certification levels are met. But if a viable product remains, a proper introduction in society is crucial for the approval of UAM operations. Understanding public concern, informing communities on the need and benefits of operations, and educating about expected annoyance, preferably with participation opportunities in the decision process, helps to gain trust and can shape public’s attitude towards drones.

2.1 Understanding public concern As part of the European project ANIMA (Aviation Noise Impact Management through novel, ex- tensive research has been conducted towards noise annoyance related to aircraft noise [5]. Lessons learned how to engage with communities are valid as well for UAM operations. One of the main influencers mentioned here is media coverage: it does not produce noise responses, but according to Heyes et al it “…can reflect socially shared knowledge, opinions, and perceptions of noise which particularly become relevant in situations of change…” . Another well-known approach is applica- tion of the Balanced Approach [6], where reduction at the source, land-use planning, noise abate- ment procedures, and operating restriction form the four pillars to shape noise management for air- ports. But it is generally recognized that communication should be an integral part of this approach to improve its success rate.

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2.2 Dealing with change Research to reduce noise annoyance is traditionally based on reducing overall annoyance for a con- sidered group of people. Daily equivalent noise level maps are used to determine the number of peo- ple exposed, and based upon exposure-response doses, the number of highly annoyed people can be determined. Measures to reduce this number, that can be any measure part of the Balanced Approach, are considered an improvement. But these numbers do not consider local shifts in noise exposure. Such shifts mean a different group of people is exposed to noise, which can lead to more complaints as they are not used to these levels of noise. A good example is the introduction of the 5 th runway at Schiphol: a measure to shift aircraft annoyance away from the more urban areas around Schiphol towards a more rural area. This new runway became the most preferential runways for operations, and according to the calculations, less people should be noise affected. However, the number of com- plaints increased significantly from 200.000 before the opening of the 5 th runway towards 700.000 after the opening for the same amount of air traffic [7]. This case shows that new or more noise for affected populations can generate proportionally higher number of complaints than expected. 2.3 Other personal and social factors It’s not easy for everyone to cope with increased noise. Scientific research [8] shows that noise sen- sitive people have a stronger negative reaction to noise than others. Research on drone noise should therefore consider noise sensitive people and they can be identified them using a Weinstein [9] ques- tionnaire. There is also literature [10] showing that multiple social and personal factors can influence the relationship between the noise source and the experienced annoyance. Especially the social fac- tors are interesting, as they are linked to situations, and thus variable, and (partially) shared between individuals. Examples of these social factors include trust in authorities, history of noise exposure and expectations of residents. A recent study shows that expectations of residents can influence whether aircraft noise is more annoying in an urban environment or in a park [11]. This shows that the history of noise exposure and expectations mediate the relation between noise and the experienced annoyance. On the one hand, the urban environment may match the noise to expect, but on the other hand, the green environment may be more bearable for other participants of this study. Again, we would expect similar results for drones instead of aircraft. Later in this paper, some other studies that touch this research field will be described, but more research is needed to explore this relation. A personal factor that moderated the relation between noise and annoyance is age. There is liter- ature showing [12] that noise annoyance increases in age for adolescents and peaks at 18. Interest- ingly, the response to noise of adults between 18-30 years old is moderate and declines steadily after the age of 30. Even though personal factors influence annoyance, Miedema & Vos [13] show in their study that noise sensitivity and fear of the noise source have a much larger impact. Nonetheless, this relation is unclear for drone noise and annoyance and also needs to be explored further. 3. OPPORTUNITIES FOR IMPROVING PUBLIC ACCEPTANCE

3.1 Useful applications Negative impacts for communities can be mitigated by showing positive factors like usefulness for the same community. Benefits of the application can help reduce or downplay the perceived (nega- tive) impact. This subjective behaviour can be seen in our current world: annoyance of road noise is generally accepted as most people also own a car and make use of the same infrastructure that creates the annoyance. A study by Peterson [14] on the annoyance of nearby wind turbines also asked if people received economic benefits from the turbines. Those that did receive economic benefits evenly

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reported on noticeability of the wind turbines as others, but were much less annoyed than those not receiving these benefits.

3.2 Information and education Informing and educating noise-affected communities are essential when introducing drone services. There are indications [11] that the recognition of a visual appearance of the associated drone noise helps to reduce annoyance: although there might be one group of people that subjectively feel their annoyance feeling strengthened by seeing a drone, another group might feel “relieved” that the un- known sound source can now be explained. That would explain why results whether the visual ap- pearance of a drone may have less influence on the annoyance than expected. When approaching communities, lessons can be learned from prior research, in particular in re- lation to airport noise management. Asensio [15] describes four participation techniques with in- creased level of community participation: information, consultation, participation, and empower. De- pending on the problem and the state of maturity of the operations, the appropriate level of participa- tion can help to improve community and airport relations. If we replace “airport” by “drone service provider”, similar community participation can be applied to improve relationship with communities and increasing opportunities for public acceptance. 4. ACTIVITIES RELATED TO PUBLIC ACCEPTABILITY AND DRONE NOISE

To demonstrate which activities take place to address public acceptability for UAM, a non-exhaustive number of examples is presented here as an example. There will be other initiatives as well, and with the raising maturity of UAM concepts, new initiatives to address public acceptability will increase as well.

4.1 Drone perception studies Even though noise annoyance of conventional aircrafts and helicopters has been studied extensively, relatively little research has been conducted on the noise emission of UAMs. What has been shown however, are greater annoyance rates towards UAM vehicles compared to aircraft noise [16] and also towards road traffic, specifically the drive-by of a car or a van [17]. When solely focussed on noise emission of drones, Christian and Cabell, as well as Gwak et al. have shown a relatively high level of annoyance. It is however interesting that in the study of Gwak et al., they found a significantly higher level of annoyance towards medium and large drones when compared to aircrafts, but smaller drones showed an actual decrease in annoyance compared to aircrafts at the same level of loudness. Not only are drones considered more annoying than aircrafts and road traffic, a study conducted within the NLR [18] also shows higher annoying scores for drones than helicopters. Here, it is inter- esting to investigate if this result could be explained due to the function helicopters often have, where they are perceived as more useful, e.g. as a trauma helicopter. Pre-existing attitudes towards drones already influence the annoyance someone feels towards a drone, but the perceived usefulness of a drone could alter this attitude. Further research is currently conducted to measure the influence of the perceived usefulness of a drone on annoyance scores. This NLR study [18] also investigated the non-acoustic factors in drone annoyance. Results show that a drone is seen as more annoying when it hovers compared to when it flies by. This could prove to be an issue for the implementation process, mainly at a busier central point where drones fly to and from. Interestingly however, this study showed no significant difference in a drone that visibly flew over or was only audible.

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Another study from Torija, Li and Self [19] was conducted with a similar approach, where seven either urban or rural environments were simulated using virtual reality. In these environments a sta- tionary drone was presented with a visual model, or with audio only. Results show that the drone in an urban environment was considered more annoying compared to the drone in the rural park area. The drone was especially considered as less annoying in locations closest to road traffic. In this study however, they found an increase in annoyance when the drone was only auditory, compared to a drone that was also visible. Even though research has been conducted on the perception of drones, there is a need for more subjective non-acoustic measures, as to why drones are perceived as more annoying and how this annoyance could be explained. To achieve this, personal and social non-acoustic factors, such as expectations of drones, history of noise in residents, demographic factors, etc. should be explored to create an integrated image of the perceived annoyance of UAM in multiple settings for different groups of people. 4.2 Dutch Drone Delta The Dutch Drone Delta (DDD) is an interest group that consists of future stakeholders of the Dutch UAM ecosystem. The DDD aims to accelerate the safe and effective deployment of UAM in the Netherlands with studies, questionnaires and pilots. It is organised into five separate tracks address- ing social acceptance, long distance operations of drones, integration of unmanned systems with manned aviation, drone delivery, and finally UAM (with an emphasis on the Urban factor). Among others, NLR assessed current obstacles for Dutch UAM deployment with a stakeholder question- naire, including social acceptance and noise. A number of use cases have been tried out, including a drone package delivery to a ship near the Port of Rotterdam. 4.3 AMU-LED In the SESAR JU project AMU-LED, research is being performed to prove the viability of UAM. Through real flight demonstrations, several aspects of UAM will be showcased, being public ac- ceptance one of them. The demonstrations will take place in 5 different locations: Cranfield (UK), Amsterdam (NL), Enschede (NL), Rotterdam (NL), and Santiago de Compostela (SP); making use of multiple vehicles to perform different missions (e.g. passenger transportation, medical delivery, air ambulance, surveillance, etc). These demonstrations allow the project to get real data on the level of acceptability of UAM. To this end, three focus groups have been set up – one per country – to analyse their level of acceptability on different aspects of UAM. The focus groups are composed of 20 people from different backgrounds and age groups, who will have to respond to the same sur- vey in two phases: a first phase, without seeing the drones flying in real life, and a second, after see- ing them fly. The idea behind the two-phase interview is to prove how perception (and acceptabil- ity) changes once the public can understand fully what drone flights entail. With the results of the surveys, recommendations will be made to the authorities on how to introduce UAM in cities in an acceptable way to the public. 5. CONCLUSION

The research on public acceptance of drones, and the noise impact in particular, is still making its maiden voyage. First, a clear definition of what is meant by UAM or by the intended drone applica- tions is needed to assess its impact on communities. Then, traditional research on noise impact, as it has been done with other modes of transport and is still going on, is valuable and can certainly de- termine a relation between drone traffic and expected annoyance. But psycho-acoustic and non-

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acoustical factors may play an even larger role than with transport noise that is accepted by society, especially in this exploratory and incubation phase. Fortunately, there are sufficient researchers and interest groups that see the challenges in this field, and help to do laboratory studies or field studies to enhance the understanding, and mitigate certain annoyance to allow successful introduction. But developers of drone applications should take note that a holistic approach, including monitoring public response in relation to acoustical and non-acoustical factors, is needed to ensure public ac- ceptability that is comparable with, and preferably higher than, other modes of transport. 6. REFERENCES

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