Cutting Through All the Noise
By: Faa Safety Briefing
Tags: Faasteam, Safety, General Aviation, Drones, Noise

Growing up in a neighborhood just over a mile north of JFK Airport’s Runway 22R/L, I know a thing or two about aircraft noise. We’re talking about the mid-1970s too, when 707s, 727s, and DC-9s freely roamed the skies well before the quieter Stage 3 noise requirements were in place. Ah yes, then there was the Concorde. It felt like time stopped for a minute or so as the sleek supersonic airliner passed a mere few hundred feet above my garage on approach. The vibrations from its engines rattled dishes in my mother’s cupboard and good luck if you were at a pivotal point in any TV show. Of course, as a wide-eyed aviation buff, I relished the extra loud engine noise and would always bolt to the backyard to watch the Concorde’s iconic “droop-snoot” in action. However, I’m fairly sure my neighbors did not share my same enthusiasm for these routine aerial encounters. I can’t say that I blame them.

For Flying Out Loud!

Fast forward several decades, and my, how things have changed. I no longer live a half dozen golf swings from a major international airport, but I’m keenly aware of the tremendous strides the FAA has made in terms of reducing and mitigating aircraft noise. Still, the current situation with regard to noise is a dichotomy of sorts. On one hand, the number of people exposed to significant aviation noise in the United States has declined 90% from roughly 7 million in the 1970s, to just over 400,000 today. This also occurred as total enplanements increased by nearly a factor of five, from 200 million in 1975 to more than 850 million today.

Despite this favorable shift over time, a recent noise survey revealed a somewhat curious discovery. Data from the FAA’s Neighborhood Environmental Study (NES), which was released this past January, indicated a substantially higher percentage of people were “highly annoyed” over the entire range of aircraft noise levels, including those at lower levels (below DNL 65 dBA). So how does that happen with so many fewer people exposed to aviation noise? It’s the same question FAA experts are currently tackling in their bid to rethink the way noise is addressed, including figuring out ways to better understand how people perceive and respond to different types of noise. “This is a nuanced story,” explains Dr. James Hileman, the FAA’s Chief Scientific and Technical Advisor for Environment and Energy during a recent webinar on the NES results. “The noise experience today is very different than it was in decades past.”

Indeed it is. What may help explain this growing distaste for aircraft noise are changes in a range of factors including survey methodology, how aircraft operate, population distribution, how people live and work, and societal response to noise. The NES is the first major update the federal government has undertaken to assess annoyance from aircraft noise since a 1992 reassessment of survey data collected in the 1970s was conducted. The NES attempted to capture some of these nuances and provide more recent quantitative data, which the FAA could use going forward. But before we get into those details, a brief background on sound metrics and the agency’s noise efforts might provide helpful context.

Background Noise

Simply stated, noise is unwanted sound. As we mentioned, there is a great deal of subjectivity involved with that assessment, but also a physical, more objective component as well. For example, we use decibels to measure sound intensity on a logarithmic scale. (Fun fact: a decibel is actually 1/10 of a bel (B), a unit originally used to quantify telegraph signal loss and named after Alexander Graham Bell). But to account for the way people respond to sound, we use the A-weighted scale (dBA). This accounts for both intensity and how the human ear responds to sound, focusing on frequencies that would affect us most (e.g., a crying baby vs. the low rumble of thunder).

Because people are exposed to sounds that vary in length and loudness, the FAA must use metrics that take these factors into account. The FAA’s aviation noise webpage (faa.gov/go/aviationnoise) goes into great detail on these metrics, but we’ll focus on one in particular here that helps measure noise exposure in a uniform manner — the day-night average sound level (DNL).

Figure 1

DNL is a metric that reflects a person’s cumulative exposure to sound over a 24-hour period and is the standard metric used for all FAA noise studies. It takes into account both the noise from a particular aircraft, as well as the total number of aircraft operations over a single day (see Figure 1). For example, the DNL value for a single very loud event (like my Concorde experience) could equal that of several hundred aircraft that are less noisy, like near a busy general aviation airport. It also accounts for the complex set of variables that could cause noise to vary, like aircraft weight and configuration, weather, and time of year. To account for increased noise sensitivity at night, DNL is weighted to assess a 10 dBA “penalty” for operations between 10:00 pm and 7:00 am.

The FAA began using a DNL of 65 dBA as a threshold for significant noise exposure in response to the 1976 Aviation Noise Abatement Policy. Together with federal land-use guidelines, this helped the FAA to significantly reduce the number of people living in areas exposed to aviation noise as was pointed out earlier. The agency also established eligibility for noise mitigation funding, like sound insulation for schools or homes.

Peace of Your Mind

To get to the meat of the matter on noise, the FAA also had to find a way to represent the effect of noise exposure on people. Researchers use the term “annoyance” to capture the varied adverse reactions people have to different levels of noise exposure. With the help of social surveys, this relationship can be measured with what’s known as a dose-response curve. If that term sounds like it belongs in a medical textbook, you’re correct. It’s a term also used within the medical field to measure the relationship of an exposure and a reaction. In fact, the FDA used dose-response curves to help pinpoint optimal dosing amounts for COVID-19 vaccines. In the FAA’s case, the dose is aircraft noise and the response is a measure of how it annoys people. Current FAA noise policies, including the DNL 65 dBA threshold, are driven by a dose-response curve developed in the 1970s, known as the Schultz Curve. Although the Schultz Curve was reviewed and revalidated in 1992, its underlying survey data is 40 years old.

That brings us to the FAA’s most recent effort to better understand the impact of aircraft noise exposure, the NES. By measuring responses to a modern fleet using updated data collection and analysis tools, the FAA was able to depict a more contemporary picture of the response to aircraft noise exposure. The NES involved surveying more than 10,000 residents living near 20 representative airports. A range of airport types were selected for the study including larger commercial airports as well as medium-sized airports with a healthy mix of general aviation and rotorcraft operations like Des Moines International and Savannah/Hilton Head International.

As the right side of Figure 2 indicates, the new national curve reflects a much higher level of annoyance and a substantial change in the way people perceive aircraft noise. To help better understand and address these changes, the FAA issued a Federal Register Notice in January (86 FR 2722) to make the public aware of the NES results and to solicit feedback on where the FAA could further direct resources. The agency is now reviewing the more than 4,100 responses received during the comment period with the hope that this feedback will help shed light on the shift in increased annoyance levels and inform future steps.