*Article* **Above and below: Military Aircraft Noise in Air and under Water at Whidbey Island, Washington**

**Lauren M. Kuehne 1,2,\*, Christine Erbe 3, Erin Ashe 4, Laura T. Bogaard 4, Marena Salerno Collins <sup>4</sup> and Rob Williams <sup>4</sup>**


Received: 26 September 2020; Accepted: 4 November 2020; Published: 16 November 2020

**Abstract:** Military operations may result in noise impacts on surrounding communities and wildlife. A recent transition to more powerful military aircraft and a national consolidation of training operations to Whidbey Island, WA, USA, provided a unique opportunity to measure and assess both in-air and underwater noise associated with military aircraft. In-air noise levels (110 ± 4 dB re 20 μPa rms and 107 ± 5 dBA) exceeded known thresholds of behavioral and physiological impacts for humans, as well as terrestrial birds and mammals. Importantly, we demonstrate that the number and cumulative duration of daily overflights exceed those in a majority of studies that have evaluated impacts of noise from military aircraft worldwide. Using a hydrophone deployed near one runway, we also detected sound signatures of aircraft at a depth of 30 m below the sea surface, with noise levels (134 ± 3 dB re 1 μPa rms) exceeding thresholds known to trigger behavioral changes in fish, seabirds, and marine mammals, including Endangered Southern Resident killer whales. Our study highlights challenges and problems in evaluating the implications of increased noise pollution from military operations, and knowledge gaps that should be prioritized with respect to understanding impacts on people and sensitive wildlife.

**Keywords:** military aircraft; noise pollution; ocean noise; Endangered species; human health; animal behavior

## **1. Introduction**

Military aircraft activity in the Salish Sea, Washington State, has been increasing over the past decade due to changes in operations and training for personnel out of the Naval Air Station Whidbey Island (NASWI). Although naval flights have been operating in the area for decades, the recent transition from Northrop Grumman EA-6B Prowler to the more powerful Boeing EA-18G Growler aircraft for electronic warfare has led to increases in the number of complaints about noise, including concern for area wildlife. Consolidation of nationwide training for these aircraft to NASWI increased the fleet size by 44% (from 82 to 118 aircraft) in 2019, with corresponding increases in air carrier practice, electronic warfare training, and overall base operations [1]. The changes at NASWI are reflective of a broader national trend in military base closures and consolidation, which are likely to intensify community noise and air pollution in some areas [2]. The implications of a concurrent change to more powerful aircraft and increased operations for noise pollution have not been

measured, leaving knowledge gaps in the ability to assess vulnerability of both people and wildlife, including Endangered, Threatened or sensitive species.

Worldwide, military transportation and activities are among the least studied sources of noise pollution [3,4]. This is due to a combination of differing regulations in areas surrounding military bases and airfields [2,5], the complexity of conducting research when operation schedules are not publicly accessible, and analytical challenges in measuring and characterizing noise from periodic and intermittent activity [3,6]. For example, the NASWI and other airfields in the United States are not regulated by the Federal Aviation Administration as civilian airports are, resulting in more limited legal bases on which to contest aircraft noise that is disruptive. Military air traffic schedules may be classified or largely unobtainable, making it difficult to conduct monitoring or validate modeled noise data. Lastly, high-intensity but intermittent activities require alternatives to standard community noise metrics, which are geared toward more continuous sources of noise [7]. For these reasons, although studies and reviews exist on the impacts of community noise from civilian airports and highways, independent studies related to military activity are relatively rare [8] and likely to be opportunistic [3,9]. This creates crucial information gaps when the public or agencies try to evaluate the impact of proposed changes in activities [10].

While often considered an acoustic barrier, the air-water interface may effectively transmit sound in certain situations (e.g., in calm conditions and for vertical incidence; [11,12]). This opens up the need to more critically examine underwater impacts of civil and military aviation noise, which have typically been considered negligible [13]. Of paramount concern in the Salish Sea are the Southern Resident killer whales (*Orcinus orca*, SRKW), which were listed as Endangered under Canada's Species at Risk Act in 2001 and the U.S. Endangered Species Act (ESA) in 2005. Endangered by chemical pollution, food shortages, and vessel traffic, additional anthropogenic stressors should be examined for the potential to put recovery of SRKW out of reach. Protecting foraging areas is important because SRKWs are food-limited, and because they are more vulnerable to disturbance while feeding than during any other activity [14]. Another species of particular concern in this region is Threatened marbled murrelet *(Brachyramphus marmoratus)*, a non-migratory seabird that makes use of protected and shallow coastal areas for foraging. In short, we see a number of compelling and timely reasons to measure in-air sound levels from Growlers to assess impacts on humans and terrestrial wildlife, and explore whether Growler noise is audible under water in areas used by SRKWs, marbled murrelet, and other wildlife.

In this study, we evaluated the potential bioacoustic impacts of noise from Growlers and implications for the Puget Sound and Salish Sea region. Noise pollution is usually studied as a public health issue for people [3], or, less commonly, as an anthropogenic impact on wildlife [8]. Terrestrial and aquatic impacts and species are usually considered and studied separately. Though understandable, this compartmentalizing does not acknowledge the cascading changes that can occur in ecosystems as a result of new noise sources, or that anthropogenic noise impacts species and taxonomic groups broadly [15,16]. In this study, we therefore adopted an integrative approach by measuring both in-air and underwater noise, and then interpreting those levels against established impacts for humans as well as sensitive terrestrial and aquatic wildlife. Our study seeks to answer two questions: (1) Does noise from military aircraft have the potential to impact aquatic as well as terrestrial habitats? and (2) How do measured levels in air and under water compare with thresholds known from previous studies to impact humans, terrestrial, and marine wildlife? We then use these results to critically examine the processes by which noise impacts are assessed and mitigated, helping to bridge the gap between monitoring and management of noise pollution.

## **2. Methods**

#### *2.1. Study Area*

Whidbey Island, where NASWI is located, is near the border of Canada and the United States of America, and forms the northern border of Puget Sound. The island is approximately 88 km long, and 2.4–19 km wide; it is the largest island in Washington State. Whidbey Island was historically inhabited by people of multiple Native American tribes that maintain reservations in the surrounding area today, including the Lower Skagit, Swinomish, Suquamish, and Snohomish. The island is currently home to 70,000 residents living in multiple medium- and small-sized communities. The majority of the island's economic activity is directly or indirectly related to the Navy's presence, but other economically important activities include farming, fishing, tourism, and real estate/vacation home purchases. Public concerns about impacts from NASWI are not limited to Whidbey Island, but are present throughout Island County, which relies on a reputation for remote and peaceful tourist opportunities (e.g., the San Juan Islands).

The NASWI is the largest single employer on Whidbey Island, with a base population of approximately 10,000 soldiers, civilians, and contractors. NASWI was first commissioned and constructed in the early 1940s and has undergone various eras of expansion and contraction. Currently, NASWI consists of two airfields (Ault Field and OLF Coupeville) with four runways (Figure 1a). Aircraft are housed at Ault Field, but both airfields are used for field carrier landing practice (FCLP) by Growlers. FCLPs are intended to replicate conditions for carrier-based takeoffs and landings and feature repeated "touch-and-go" flights; a certain number of these must be conducted at night to adequately prepare pilots. Although FCLP is the dominant type of aircraft training at NASWI, other base aircraft activities include electronic warfare and air-to-air combat training in nearby military operations areas [10], submarine detection, and cargo aircraft training [17].

Schedules for base activities are not published, with the exception of a courtesy (i.e., non-official) notification of the airfield and approximate time frame for FCLPs for that week [18]. Information on base operations, aircraft activity, and corresponding noise impacts is otherwise available only in the form of general estimates (e.g., annual operations, modeled maximum loudness in selected areas) conducted as part of the Environmental Impact Statement (EIS) process [17] and corresponding Biological Opinions for ESA-listed species [19]. The U.S. Department of Defense policy is to model rather than monitor noise from military operations, and no noise monitoring has been done by the U.S. Navy to date.

#### *2.2. In-Air Acoustic Data Collection*

Growlers were recorded in air at Moran County Beach Park (48.3693, −122.6662), the nearest public location from the underwater recording site (see below) on September 13 and 16, 2019, and located under FCLP flight track 14 for Ault Field (Figure 1b). On both days, FCLPs were scheduled from "Morning to Late Afternoon", and were done on track 14, with jets circling south to north; as a result, the recorder was capturing sound associated with landings. An observer logged the type and number of all visible and (in the case of takeoffs) audible aircraft events and noted the direction of travel and flight activity as landing, pass, or takeoff. A Songmeter SM4 autonomous recorder (Wildlife Acoustics, Maynard, MA, USA) collected audio data from aircraft landings and flyover events. Sound was sampled at 48 kHz and with zero gain added. The Songmeter was deployed between 0930 and 1530 on September 13 and between 1100 and 1500 on September 16. A sound level data logger (Extech 407760; Nashua, NH, USA) was deployed at the same time recording A-weighted sound levels (dBA) at 1-s intervals; however, the data logger failed to record on the 13th, so simultaneous sound pressure levels were collected with audio data on September 16 only.

**Figure 1.** Map of the study area, location of underwater and in-air sound recordings, and schematic of hydrophone mooring. (**a**) Whidbey Island and surrounding areas, showing the location of Naval Air Station Whidbey Island's two airfields (Ault Field and OLF), largest cities and towns, and state parks. Critical habitat areas (Areas 1–3) for Endangered Southern Resident killer whales are shown. (**b**) Location of SoundTrap (orange circle) and in-air recordings (yellow circle) relative to Ault Field and field carrier landing practice (FCLP) flight tracks 32 and 14 (blue lines). Flight tracks were georeferenced from the Environmental Impact Statement [17]; two other flight tracks around Ault Field (tracks 7 and 25, oriented east-west) are not shown. (**c**) Schematic of the SoundTrap mooring, which was deployed from 15 August–11 September 2019. The SoundTrap was housed in a coated wire cage attached to a concrete block anchor with a 2 m line; a 30 m line attached to buoys kept the SoundTrap suspended above the sea floor.

The number of aircraft events during the FCLPs on September 13 and 16 was summarized from the visual observations for the period of 1100–1500 (when FCLP activity occurred) on both days. To provide a visual representation of the timing and duration of FCLPs, long-term spectral averages (LTSAs) were generated for the same periods each day in MATLAB, using 1-s and 1-Hz resolutions.

#### *2.3. Underwater Acoustic Data Collection*

Growlers were recorded under water with a SoundTrap 300 STD autonomous recorder (Ocean Instruments, Auckland, New Zealand) that was factory-calibrated and programmed to record continuously at 96 kHz sampling frequency (fs) prior to deployment. The SoundTrap was deployed off the northwest coast of Whidbey Island, approximately 1400 m from the end of the east-west runway and 1000 m from the shoreline (Figure 1b). This location is below the path of aircraft taking off to the west, and FCLP flight tracks 7 and 32.

The SoundTrap was suspended in a metal cage 2 m above the sandy mud sea floor, and was moored using a system of concrete blocks, sinking line, and two floats (Figure 1c). The SoundTrap was deployed twice for two weeks, totaling 28 days of data collection. In between deployments, the SoundTrap was retrieved for charging and downloading data. It was first deployed on 15 August

2019, at 13:54 PDT at 48.3492, −122.6917, at a depth of 33.2 m, and then on August 29, 2019, at 12:12 PDT, in a similar location (48.3494, −122.6907), at a depth of 34.7 m. The weather throughout this month was variable, consisting of rain, wind, sun, and clouds. Growlers taking off to the west flew over the SoundTrap at an altitude of 120–190 m above sea level.

### *2.4. Data Analysis*

Visual observations confirmed the occurrence of 23 single Growler flights over the Songmeter on September 16, during an FCLP session with only one aircraft operating. Opportunistic observations on four dates (August 15, 28, 29, and September 12) visually confirmed the occurrence of ten Growler flights over the SoundTrap. These overflights were manually identified in the recordings using Audacity© (Version 2.3.2; retrieved 20 September 2019 from https://audacityteam.org/) and a 15-s audio file was saved for each overflight. The audio files were analyzed using a custom script in MATLAB (version 2018b; The MathWorks Inc., Natick, MA, USA). Each 15-s file was calibrated and then Fourier-transformed in 1-s Hann windows (i.e., the number of Fast Fourier Transform components NFFT equaled the number of samples per second) with 67% overlap. A time series of band levels was computed between 20 Hz and 20 kHz, corresponding to the frequency band occupied by Growler overflights. The peak band level was assumed to correspond to the time when the plane was directly overhead. The mean-square sound pressure spectral density (in short, power spectral density, PSD; [20]) from the corresponding 1-s window was saved. A 140-min sample of underwater ambient noise (i.e., sound received at this location from all sources but the signal of interest: airplane noise) was collated from before, in between, and after the overflights, and also calibrated and Fourier-transformed in 1-s Hann windows with 67% overlap.

Over all overflights, median and quartile PSD levels, one-third octave band levels, and weighted levels were computed. One-third octave band levels were obtained by integrating the PSD into bands that are 1/3 of an octave wide, then applying 10×log10 [20]. One-third octave band levels were compared to published audiograms to estimate which parts of the in-air and underwater noise spectra might be audible to the two ESA-listed species (SRKW and marbled murrelet), and at what levels. Audiogram data were extracted from publications using the software program WebPlotDigitizer (Version 4.2; A. Rohatgi, Pacifica, CA, USA) if data tables were not published. The killer whale underwater audiogram followed the model proposed by Branstetter et al. (2017) [21]. In the absence of killer whale critical ratio data across the frequency band of Growler noise, one-third octave bands were used as a conservative estimate (see Figure 4A in Erbe et al. 2016) [22]. There is no audiogram available for marbled murrelet, so audiograms of other seabirds were used as surrogates. In-air and underwater audiograms for cormorant (*Phalacrocorax carbo sinensis*) were measured by Johansen et al. (2016) [23], the in-air audiogram for the lesser scaup (*Aythya a*ffi*nis*) duck by Crowell et al. (2016) [24], and the in-air audiograms for common murre (*Uria aalge*) and Atlantic puffin (*Fratercula arctica*) by Mooney et al. (2019) [25]. One-third octave bands were also used for the birds in air and under water [24,26]. We report A-weighted levels for humans as well as audiogram-weighted levels for the animals in air and under water. Audiogram-weighting involved filtering the sound spectrum by the animal audiogram prior to integration over frequency. In praxis, the audiogram was interpolated to 1-Hz resolution for comparison to the noise spectrum, also in 1-Hz resolution. Over the range of frequencies where the noise PSD levels exceeded the audiogram levels, the audiogram levels were subtracted from the noise PSD levels at each frequency, yielding differences in dB at each frequency. Differences were converted to linear quantities (by applying 10ˆ(level/10)), which were then integrated over frequency, and the result was converted to a level-quantity (by taking 10×log10), yielding the audiogram-weighted level in dBth.

To evaluate the scope of potential impact at the ecosystem level, we compared the distribution of recorded (i.e., received) levels in air and under water with thresholds of behavioral and physiological stress responses for humans and a suite of representative terrestrial and marine species. Selection of representative species, responses, and thresholds from the literature was guided by two criteria: if the species occurred in or was a reasonable surrogate for species in the Salish Sea area, and if the study used noise stimuli that was a sensible proxy (i.e., low-mid frequency, broadband) for aircraft noise. Whenever possible, studies that established or modeled a noise–dose relationship were used; in the case of modeled probability, the 50% likelihood of response was used as the threshold. Despite recognition that human-weighting of sound pressure levels is understood to be potentially unsuitable for wildlife [16,27], we found that most terrestrial studies nonetheless evaluated responses to A-weighted sound pressure levels. In addition to thresholds for people [28,29], the final suite of terrestrial species (or genus) contrasted against in-air received levels were: marbled murrelet [30], owls [31–33], harlequin duck (*Histrionicus histriónicas*) [34], and caribou (*Rangifer tarandus*) [35]. Marine species selected for contrast with underwater received levels were: killer whales [36], common murre [37], harbor porpoise (*Phocoena phocoena*) [38], herring (*Clupea harengus*) [39], and California sea lion (*Zalophus californianus*) [40].

Although our study design did not allow for comparison of underwater sound from Growlers with other surface-confirmed anthropogenic sources (in this area, primarily boats), we used LTSAs to visually represent and contrast underwater sound from Growlers and vessels. We used the weekly notifications of FCLPs (Table S2) to focus on dates and time periods (e.g., "Midmorning", "Late Afternoon") when training was scheduled for Ault Field, and created LTSAs for these periods (1-s and 1-Hz resolutions) to identify periods when both Growler noise and vessel noise were present. Three 1-h LTSAs were generated to visualize the underwater soundscape under varying flight and vessel activity.

#### *2.5. Comparison of Sound Levels and Flight Activity with Prior Studies*

To place sound levels and flight activity at NASWI in the context of those documented in other studies, we conducted a literature review to identify studies of impacts of military low-altitude flights (MLAF) on people and wildlife. We restricted our search to these studies because the noise strength, onset rate, and intermittent nature of MLAF are distinct from commercial or general aviation aircraft [7,41]. In particular, comparable environmental noise levels (>100 dBA) are encountered only rarely in other contexts [3]. Our initial search resulted in 26 primary research articles that evaluated impacts of MLAF on people or communities (i.e., annoyance, hearing damage or loss, and effects on mental and physical health), and 34 articles that examined impacts on wildlife (Data S1). A subset was removed before extracting noise data; reasons for exclusion included inability to obtain full articles, reporting of events only (vs. noise), or non-relevant context (e.g., air shows) (Data S1); some studies also had multiple publications related to the same dataset (Table S1). The final number of studies from which noise metrics were extracted was 12 (people) and 18 (wildlife) (Table S1). The number of studies that have measured or modeled impacts of underwater noise from aircraft was too low for meaningful analysis, and included studies therefore only reflect in-air conditions.

From each study, three metrics were extracted or estimated: (1) maximum received sound level, (2) typical or average number of daily events > 100 dBA, and (3) total daily duration in seconds > 100 dBA (Table S1). If a typical number of daily MLAF events was not reported, we calculated the average number of daily events as the total reported events divided by the number of days when recording took place; since military activity usually occurs almost exclusively on weekdays, weekend days were excluded from this formulation. A threshold of 100 dBA was used because it was relevant to the current study and is frequently used as a reporting threshold, facilitating the extraction of metrics across disparate studies that could include events both below and above that threshold.

The region of study, year in which the study was conducted, and (for wildlife) the focal taxonomic group were also extracted. If a study included multiple noise treatments or examined geographic areas with different received levels, metrics were extracted for each treatment or geographic area. If the duration of individual flight events was not reported, a conservative estimate of 4 s per overflight event (based on mean reported event duration across all field studies) was used (Table S1). The same metrics were then calculated for the current study using the sound pressure level data and observed flight events from September 13 and 16. These two dates do not represent maximum daily periods with FCLPs, which is up to 8 time periods per day, but typical and moderate activity on training days (Table S2). The relative positions of different studies with respect to the three metrics were contrasted separately for people and wildlife.
