Noise  Primer
&
Commuter  Rail  Basics

Burnt Fork Creek

Watershed Alliance Meeting

November 13, 2001

 Defining Sound

Sound is created when objects vibrate, resulting in a minute variation in surrounding atmospheric pressure called sound pressure.  The human response to sound depends on the magnitude of a sound as a function of its frequency and time pattern (EPA, 1974).  Magnitude measures the physical sound energy in the air.  The range of magnitude from the faintest to the loudest sound the ear can hear is so large that sound pressure is expressed on a logarithmic scale in units called decibels (dB). 

A-Weighted Noise-Monitoring Equipment 

A-weighted noise-monitoring equipment is most commonly used when conducting a community noise study.  The readings obtained from this type of equipment are similar to how humans perceive sounds of low to moderate magnitude.   The letter “A” indicates that the sound has been filtered to reduce the strength of the very low and very high-frequency sounds, much as the human ear does.  If the noise readings were taken without the A-weighting noise-monitoring equipment, the results would include the noises that are out of human hearing range and produce higher readings.  Noise levels that are from an A-weighted noise-monitoring equipment are indicated by (dBA).

 

Figure 1-1 illustrates the A-weighted levels of some common sounds.  The range of human hearing extends from about 0 dBA for young healthy ears (that have not been exposed to loud noise sources) to about 140 dBA.  When sounds exceed 110 dBA, however, there is a potential for hearing damage, even with relatively short exposures.

Data Obtained from Noise Readings 

In measuring community noise, noise readings are taken every second and from that an L_eq (dBA) is obtained.  A L_eq is  defined as a receiver’s cumulative noise exposure from all events over a one-hour period.  Because humans are more sensitive to noise disturbance during nighttime hours, a day-night equivalent sound level (L_dn) is frequently used to estimate the overall disturbance people experience from environmental noise.  The L_dn is measured over a full 24 hour time period, with events between 10 pm to 7 am increased by 10 dBA to account for greater nighttime noise sensitivity.  The effect of this penalty is that any event occurring during the nighttime hours is equivalent to ten events during the daytime hours. This emphasizes L_dn toward nighttime noise is to reflect that most people are easily annoyed by noise during the nighttime hours when background noise is lower and most people are sleeping.

Typical Community Noise Levels 

The FTA manual Transit Noise and Vibration Impact Assessment (April 1995) uses L_dn levels to characterize community noise in residential areas. Figure 1-2 defines typical community noise levels in terms of L_dn.  Most urban and suburban neighborhoods are in the range of L_dn 50 dBA to 70 dBA.  Residential neighborhoods that are not near major sound sources are usually within the range of L_dn 55 dBA to 60 dBA. However, if there is a freeway or moderately busy arterial nearby, or any nighttime noise, then the L_dn is usually in the range of 60 dBA to 65 dBA.  A noise level of 70 dBA reflects a relatively noisy environment.  This reading could be associated with an area that has buildings located on or close to a busy surface street, freeway, or airport.  This noise level would usually be considered unacceptable for residential land use without taking special measures in order to enhance outdoor-indoor sound insulation.

Land Use Categories 

Three land use categories have been developed for the various land use types within a project area.  Table 1-1 defines the various land use categories.  The land use category is a factor used in determining if there are any impacts from the change in noise exposure due to a proposed project. 

Table 1-1   Land use Categories and Metrics for Transit Noise Impact Criteria

 Amount of Noise Impact from a Proposed Project

 

Figure 1-3 and Table 1-2 illustrate the amount of impact a proposed project has on an area based on the land use category, existing noise exposure (dBA) and the proposed noise exposure (dBA).  The noise impact criteria in Table 1-2 and Figure 1-3 are based on comparing the existing outdoors noise levels and the future outdoors noise levels that would result from the proposed project. 

 Figure 1-3 Noise Impact Criteria for Transit Projects

Table 1-2   Noise Levels Defining Impact for Transit Projects 

The noise levels incorporate both the absolute criteria, which considers activity interference caused by the proposed project itself, and the relative criteria, which considers the annoyance created due to the change in the noise environment caused by the proposed project.  These criteria were developed so that they could be applied to various transit modes, to recognize the heightened community annoyance caused by late-night or early-morning transit service, and to respond to the varying sensitivity of communities to projects under different background noise conditions.

Table 3 describes the perceived effect a person has from an increase or decrease of noise levels by various amounts.

Table 3  Perceived Impacts from Noise Level Changes

Adding Noise Readings 

Because noise is measured on a decibel scale, combining two noise levels is not achieved by simple addition. For example, combining two 60 dBA noise levels does not equal a noise level of 120 dBA (which is near the threshold of pain), but yields 63 dBA, which is lower than the volume at which most people listen to their televisions.  Table 1-4 contains criteria used when adding two or more noise reading together.  An example of adding noise readings follow Table 1-4.

Table 1-4   Criteria for Adding Two Noise Readings

Figure 5   Train Noise Sources

Emory Campus to Tucker Noise Segment Preliminary Noise Impact Results

Noise receptors for the segment between the Emory campus and Tucker station were divided into two categories:  those near grade crossings where trains must blow horns to warn autos and those outside those areas.  The closest residences in the segment inside and outside the horn zone are estimated to be at a distance of 80 feet and 95 feet from the project corridor respectively and have estimated existing L_dns of 70 dBA (without horn) and 79 dBA (with horn). 

 

Utilizing FTA methodology and equations the project noise contribution was calculated and the potential for train noise impact was assessed.  Applying the FTA criteria for defining train noise impacts the results of the preliminary impact analysis indicate that there would be no impact at residential receptors located beyond 34 feet in the No Horn Zones and beyond 266 feet in the Horn Zones.

 

Table 5 summarizes the results of the Emory campus – Tucker stations segment noise impact analysis. 

Typical Locomotive, Double-deck Coach Configuration

Double-deck Coach Interior (upper level)