In the example in Figure 1, the background (or residual) sound level in the absence of any identifiable noise sources is about 45 dB. During roughly three-quarters of the time, the sound level is 50 dB or less. The highest sound level, caused by a nearby motorcycle, is 73 dB, while an aircraft generates a maximum sound level of about 68 dB. The question then becomes: how do we "measure" this variable community noise?

MAXIMUM SOUND LEVEL

One way of describing a sound environment is to measure the maximum sound level - in this case, the nearby motorcycle at 73 dB. However, the aircraft sound, although not as loud as the motorcycle, lasts longer. Studies have shown that human response to noise involves both the maximum sound level and its duration.

SOUND EXPOSURE LEVEL

A second way of describing this sound environment is to measure the sound exposure level (SEL), which is the total sound "energy" of a single sound event and takes into account both the intensity and duration. One way to understand SEL is to think of it as the sound level you would experience if all the sound energy of an event occurred in one second. In the sample time history in Figure 1, the motorcycle generates a SEL of about 77 dB, while the aircraft generates an SEL of about 81 dB.

EQUIVALENT SOUND LEVEL

The maximum sound levels and the SEL measure individual events that may occur only once, or may occur several times during the day. The number of times these occur is also important in measuring the noise environment. One way this could be done would be to count the number of times per day that the SEL exceeds 80 dB, plus the number of times it exceeds 75 dB, and so on.

A more efficient way to describe both the number of events and the SEL of each is the time-average of the total sound energy over a specified period. This is referred to as the equivalent sound level (Leq). In our example shown in Figure 1, the Leq is roughly 56 dB.

DAY-NIGHT AVERAGE SOUND LEVEL

Finally, one additional factor, which is also important in "measuring" sound, is the occurrence of sound during nighttime hours. People are normally more sensitive to intrusive sound events at night, and the background sound levels are normally lower at night because of decreased human activity. Therefore, a "penalty" may be added to sound levels that occur during night hours (between 10:00 p.m. and 7:00 a.m.). The 24-hour average sound level, including a 10-dB penalty, is known as the day-night average sound level (DNL). The 10-dB penalty means that one nighttime sound event is equivalent to 10 daytime events of the same level.

COMMUNITY ANNOYANCE

Airport noise exposure is more likely to have a behavioral effect on people, rather than cause them physical injury. Because it is difficult to predict an individual's response to noise, a community response index has been developed to evaluate aircraft noise impact potential.

Annoyance is a summary measure of the general, adverse reactions of people to noises that disrupt their daily activities - telephone conversations, TV/radio listening, sleep, or simple tranquility.

Extensive research has found that day-night average sound levels (DNL) correlate very well with community annoyance from most environmental noise sources. Some studies found that communities report themselves slightly more annoyed by aircraft noise than by surface transportation noises.

NOISE EXPOSURE AND LAND-USE COMPATIBILITY GUIDELINES

Federal agencies have adopted guidelines for compatible land uses with environmental sound levels. Land-use is normally determined by property zoning, such as residential, industrial, or commercial. Noise levels that are unacceptable for homes may be quite acceptable for stores, warehouses, or other businesses.

While the federal government has no decision-making role in land use planning, the FAA reviews and approves, as required by statute, airport noise compatibility programs.

FAR PART 150

The FAA has implemented an Airport Noise Compatibility Planning Program (also known as "FAR Part 150"), which assists airport operators in managing sound. The FAR Part 150 Noise Exposure and Land Use Compatibility Program is aimed at balancing an airport's operational needs and its impact on the surrounding community. Its purpose is to reduce noise impacts on existing non-compatible land use and to prevent the introduction of new non-compatible land uses in the areas impacted by aircraft noise.

In general, guidelines state that most land uses are considered to be compatible with DNLs that do not exceed 65 dB, although Part 150 declares that "acceptable" sound levels should be subject to local conditions and community decisions. Nevertheless, a DNL of 65 dB is generally identified as the threshold level of aviation noise and other sources of community noise, which are "significant." However, a DNL below 65 dB does not mean that that level of noise annoys no one. A small percentage of people are simply more sensitive to noise when most other people, and a small percentage are barely annoyed even at high noise levels. A combination of factors can cause different people to interpret sounds as "unwanted" noise. A measure of noise impact, such as day-night average sound level, provides a reliable indicator of overall community response, but does not predict how any single individual will respond.

As a result, there is probably no minimum level of transportation noise at which no one is annoyed. General guidelines for noise compatibility identify day-night average sound levels between 55 and 65 dB as "moderate exposure" and as generally acceptable for residential use. Above a DNL of 65 dB, guidelines identify the noise impact as "significant," and this designation is currently a factor in decisions to provide federal funds for mitigation projects.

COMMUNITY INVOLVEMENT

The FAR Part 150 Noise Study has an extensive public participation process with committees of residents, airport users, public officials, government agencies, and technical consultants (a Community Advisory Team and a Technical Advisory Group). Public meetings will be scheduled to coincide with various milestones of the study. A variety of public information materials will be made available to interested residents or businesses.

FREQUENTLY ASKED QUESTIONS

What does a Part 150 Noise Study consist of?

The FAR Part 150 Noise Study requires two types of documents that must be submitted to the Federal Aviation Administration (FAA) for approval of the Airport's program - a Noise Exposure Map (NEM) and the Noise Compatibility Program (NCP).

What is a Noise Exposure Map (NEM)?

Noise Exposure Maps are developed using a specialized computer model developed by the FAA known as the Integrated Noise Model (INM). Often called "contour" maps, because they define areas of equal day-night average sound levels (DNL). Noise exposure maps illustrate where aircraft noise occurs in and around the airport and at what sound level. The FAA requires the consideration of "significant" noise levels, specifically aircraft noise levels of DNL 65 to 75 in the environs of airports.

What is a Noise Compatibility Program (NCP)?

The Noise Compatibility Program evaluates and recommends various noise abatement and land use mitigation measures to reduce noise impacts and ensure land use compatibility within the vicinity of the airport.

How do I participate in the FAR Part 150 Noise Study?

The FAR Part 150 Noise Study has an extensive public participation process with committees, meetings, and a public hearing. Anyone is invited to join the Community Advisory Team, or attend Technical Advisory Group meetings and the public hearing. Comments can also be emailed to noise@pcrguam.com or called in to our hotline at (671) 642-4500.

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