ASA/CAA '05 Meeting, Vancouver, BC

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Airport Noise Complaints: Talking the Same Language

Nancy Timmerman -
Consultant in Acoustics and Noise Control
25 Upton Street
Boston, MA 02118-1609

Popular version of paper 5aNS6
Presented Friday morning, May 20, 2005
Joint ASA/CAA Meeting, Vancouver, BC

No metropolitan area in the world is free of the noise from commercial aircraft. Citizens complain about the noise, but many feel that the authorities at the airport are not listening, or are speaking a foreign language. There is a disconnect between what the citizens say and the way the airports and their technical consultants describe the noise from aircraft operations.

Why is this so?

Citizens will complain using the following types of phrases. An aircraft "woke me up" (at 4 am). That (enter airline name) flight is always "noisy." The aircraft noise is "too loud." The noise from the aircraft is so loud it "sets off the car alarms." There is "too much" noise. I "cannot hear" the television (or radio or telephone). Why do they always fly on "Sunday (or Saturday) morning?" It's "too late (or early)" for planes to be flying. I "cannot get to sleep" because of the planes. The planes are really "annoying." I cannot use my "backyard" because of the aircraft noise. There are "too many" planes. It has been "(enter number) days without a break". The noise is "continuous." It's been going on "all day." The noise is "constant." The noise "never stops." We get "no break." The planes are "too low." It's "not safe." The plane is "off course." These phrases all refer to what is happening at the time of the call, on that particular day. The airport is operating under a particular set of wind, weather, and runway use conditions.

The airports and technical consultants will use terms like Noise Exposure Forecast (NEF, Canada), Day-Night Average Sound Level (DNL, USA), Community Noise Equivalent Level (CNEL, California), Psophic Index (IP, France), Noise and Number Index (NNI, Ireland), and Weighted Equivalent Continuous Perceived Noise Level (WECPNL, Japan). These terms refer to noise levels measured or computed over a long period of time (usually a year). They are sometimes called "cumulative noise" or "noise exposure" because they can either be viewed as sums or as averages over time. They are based on total (average) numbers of flights at average wind and weather conditions on allocated (average) runway use conditions. There is rarely a real day which is average in all those ways. Because they represent all the varied conditions, the above terms are useful in planning and in regulation.

A typical comment from a citizen might be, "The noise from the planes has to be above 65 dBA." In this case, the citizen is referring to the sound level you could measure at that time as compared with the regulatory DNL of 65 dB. This particular misunderstanding is extremely common in the USA, where residential sound insulation programs (RSIPs) are based on DNL. Neighbors of the airport are interested in having the free home improvements which are the centerpiece of those RSIPs.

For individual aircraft or "noise events", some of the technical terms used are Maximum A-weighted Sound Level (Lmax), the highest sound level measured while the aircraft flew over; Signal-to-Noise Ratio (SNR), how much higher the aircraft noise is than the other sounds; Sound Exposure Level (SEL), a measure of how much sound energy a citizen received while the plane flew over; and Effective Perceived Noise Level (EPNL), a measure of the noisiness of the sound. The first two are measured by an ordinary sound level meter. The others are computed from other data, and so are mere numbers as far as the lay person is concerned. In the above example, the citizen could be understood as meaning Lmax, as opposed to DNL for the airport.

Let's look at the complaint terms in a little more detail.

The complaints can be divided into following groups: 1) those having to do with noise only (too loud, too much, noisy, and annoying), 2) with noise and vibration (car alarms), 3) with noise and sleep interference (woke me up, cannot get to sleep), 4) with noise and other activities (cannot hear, backyard), 5) with noise and time of day (too late/early, Sunday/Saturday morning), 6) with noise and number of flights (too many, constant, continuous), 7) with noise and runway usage or time (no break, never stops, x days without a break, all day), and 8) with something other than noise (too low, off course, not safe). The ones not related to noise were dropped from further consideration.

Most of these terms do not have anything to do with long-term exposure. Groups 1 through 5 are all concerned with the immediate, with aircraft operations which have just occurred. For these, we need to look at the most recent aircraft operation(s). Groups 1 and 2 will be described in terms such as Lmax, which both the airport and citizen can understand. Groups 3, 4, and 5 can also be related to SNR, how much aircraft noise interferes with other things which are going on.

Let's look at the other two groups. Quoting another citizen, "I do not hear long-term average sound levels. I hear the noise from individual airplanes flying over my house."

An airport such as Logan International Airport in Boston, MA, where the author was a noise officer for many years, can have an aircraft arrival or departure more often than once a minute. At those heavily traveled times, the noise from one aircraft may still be heard when the next one comes. In this way, the noise is "constant" or "continuous", and there are "too many" aircraft in a given time. At least two measures try to address this question: the NNI mentioned above, where 4 times as many operations corresponds with 10 dB of noise, and the NA (number above level), which uses Lmax on an "average day", and where 10 times as many operations corresponds with 10 dB of noise. It should be noted that both measures are based on the "average" day.

The FAA operates airports' runway systems in accordance with wind direction, so at any given time, one set of residents is affected by only certain types of aircraft operations. In 2003, Logan had an average of 867 jet operations on any given day, about 433 arrivals and 433 departures. If only one runway pattern in used all day, one set of citizens will receive all of the noise from those arrivals or departures. This is the scenario of Group 7 (no break, never stops, x days without a break, all day). In addition, since the runways are always in the same place, the same set of citizens can receive all the noise from one set of departures and all the noise from a different set of arrivals.

There are two measures which address the noise as well as time (duration) aspect. One is the TA (time above level), which reports how long in the "average" day, the noise is above a given level. The other is the MQI (mean quiet interval), which reports the average time between aircraft events (over a defined time period) that exceed a selected threshold. In the "average" day, it would be the (time in the day - TA)/NA, and is computed this way. Many citizens feel they can understand terms like time above and number above better then DNL.

Finally, the only complaint phrase that might go with cumulative noise or noise exposure is "too much," depending on what exactly the citizen meant. None of the others do. That being said, this author does not believe the citizens average noise levels over a year any more than they do the atmospheric temperature. Their complaints are about that aircraft; last night; today; or maybe this week, but not this year.

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