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Noise Barrier Design Handbook

14. Barrier Design Process

This section describes several general topics involved in the barrier design process. Since each responsible organization has specific guidelines and/or policies related to the type and schedule of the various elements (acoustical, engineering, community involvement) of such a process, some aspects will only be briefly discussed here. State policies related to highway traffic noise are provided in the Noise Policies section of this CD-ROM.

14.1 Acoustical Evaluation

The first step in the barrier design process is an acoustical evaluation. An acoustical evaluation is performed prior to the construction of a new highway or the expansion of an existing one to determine if noise abatement is needed and, if so, to what degree. General steps in performing an acoustical evaluation are as follows:

14.1.1 Select Noise Sensitive Receivers and/or Areas for Measurement and Analysis.

Site selection should be guided by the location of noise-sensitive receivers. Land-use maps and field reconnaissance should be used to identify potential noise-sensitive areas. For obvious reasons, schools, hospitals, and churches are especially sensitive to noise impacts. Noise sensitive residential areas should also be included in a noise-impact assessment. When selecting potential representative sites, keep in mind that the site should exhibit typical conditions (e.g., ambient, roadway infrastructure, and meteorological) for the surrounding area. It is recommended that good engineering judgment be used to select sites, keeping in mind the objectives of the study.ref.19

14.1.2 Determine Existing Noise Levels by Measurements and/or Modeling.

Once the desired noise sensitive sites have been selected, the existing noise levels should be determined for comparison with estimated future noise levels (see Section 14.1.3). The results of this comparison, in concert with FHWA noise abatement criteria and with the responsible organization's policy, should be used to determine the appropriate noise abatement, if any (see Section 14.1.4). Existing noise levels at the desired sites are typically determined from either noise measurements (see Section 14.1.2.1) and/or noise modeling (see Section 14.1.2.2).

14.1.2.1 Noise Measurements.

This section describes briefly the recommended instrumentation, microphone location, sampling period, measurement procedures, and data analysis procedures to be used for performing roadside noise measurements. If measurement of noise levels are desired after a barrier is built (to determine a barrier's effectiveness), refer to Section 15. The procedures described below are in accordance with the FHWA's "Measurement of Highway-Related Noise."ref.19 Readers may refer to this document for more detailed discussions on all of the topics contained herein. Also included in this reference are sample field data log sheets.

Acoustic Instrumentation - Figure 255 presents a generic, acoustic measurement instrumentation setup. All acoustic instrumentation should be calibrated annually by its manufacturer or other certified laboratory to verify accuracy. Where applicable, all calibrations shall be traceable to the National Institute of Standards and Technology (NIST).

Diagram showing a generic noise measurement setup

Figure 255. Generic measurement instrumentation setup

Microphone Location

Table 6. Sampling period.
Temporal nature Greatest anticipated range
10 dB 10-30 dB >30 dB
Steady * 2 minutes N/A N/A
Non-steady fluctuating 5 minutes 15 minutes 30 minutes
Non-steady intermittent For at least 10 events For at least 10 events For at least 10 events
Non-steady, impulsive isolated bursts For at least 10 events For at least 10 events For at least 10 events
Non-steady, impulsive-quasi-steady 3 cycles of on/off 3 cycles of on/off 3 cycles of on/off
* A minimum of three repetitions is recommended, with 6 repetitions being preferred.

Measurement Procedures

  1. Prior to initial data collection, at hourly intervals thereafter, and at the end of the measurement day, the entire acoustic instrumentation system should be calibrated. Meteorological conditions (temperature, relative humidity, wind speed and direction, and cloud cover) should be documented prior to data collection, at a minimum of 15-minute intervals and whenever substantial changes in conditions are observed.
  2. The electronic noise floor of the acoustic instrumentation system should be established daily by substituting the measurement microphone with a dummy microphone. The frequency response characteristics of the system should also be determined on a daily basis by measuring and storing 30 seconds of pink noise from a random-noise generator.
  3. Ambient levels should be measured and/or recorded by sampling the sound level at each receiver and at the reference microphone, with the sound source quieted or removed from the site. A minimum of 10 seconds should be sampled. Note: If the study sound source cannot be quieted or removed, an upper limit to the ambient level using a statistical descriptor, such as L10, may be used. Such upper limit ambient levels should be reported as "assumed." Note: Most sound level meters have the built-in capability to determine this descriptor.
  4. Sound levels should be measured and/or recorded simultaneously with the collection of traffic data, including the logging of vehicle types, vehicle-type volumes, and the average vehicle speed. It is often easier to videotape traffic in the field and perform counts at a later time. This approach, of course, requires strict time synchronization between the acoustic instrumentation and the video camera. The videotape approach can also be used to determine vehicle speed.

Data Analysis Procedures

1. Adjust measured levels for calibration drift as follows:

If the final calibration of the acoustic instrumentation differs from the initial calibration by greater than 1 dB, all data measured with that system during the time between calibrations should be discarded and repeated; and the instrumentation should be thoroughly checked.

If the final calibration of the acoustic instrumentation differs from the initial calibration by 1 dB or less, all data measured with that system during the time between calibrations should be adjusted by arithmetically adding to the data the following CAL adjustment:

CAL adjustment = reference level - [(CALINITIAL + CALFINAL) / 2] (dB)

For example:
reference level = 114.0 dB
initial calibration level = 114.1 dB
final calibration level = 114.3 dB
Therefore:
CAL adjustment = 114.0-[(114.1+114.3)/2] = -0.2 dB

2. Adjust measured levels for ambient as follows:

If measured levels do not exceed ambient levels by 4 dB or more, i.e., they are masked, or if the levels at the reference microphone do not exceed those at the receivers, then those data should be omitted from analysis.

If measured levels exceed the ambient levels by between 4 and 10 dB, and if the levels at the reference microphone exceed those at the receivers, then correct the measured levels for ambient as follows (Note: For source levels which exceed ambient levels by greater than 10 dB, the ambient contribution becomes essentially negligible and no correction is necessary):

Ladj=10*log10(100.1Lc-10 0.1La)    (dB)

where:

Ladj is the ambient-adjusted measured level;
Lc is the measured level with source and ambient combined; and
La is the ambient level alone.

For example:
Lc = 55.0 dB
La = 47.0 dB
Therefore:
Ladj = 10*log10(10(0.1*55.0)-10(0.1*47.0)) = 54.3 dB

3. Compute the mean sound level for each receiver by arithmetically averaging the levels from individual sampling periods.

14.1.2.2 Noise Modeling.

There are many noise prediction methodologies being used by the highway noise community.ref.29, ref.73, ref.74, and ref.75 The current state-of-the-art in highway noise prediction is the FHWA Traffic Noise Model, Version 1.0 (FHWA TNM®). Readers are directed to TNM's Trainer CD-ROM, which provides a detailed tutorial on using TNM and three companion reports (TNM's User's Guide, Technical Manual, and data base report).ref.4, ref.5, ref.6 and ref.7

Following is a list of site characteristics to be included in the modeled analysis. These site characteristics can be determined from site visits, photos, aerial plans, etc.

14.1.3 Determine If There Are Any Future Noise Impacts.

Existing noise levels are compared to future noise levels to establish if there will be any future noise impacts on the surrounding area. A noise impact associated with highway traffic noise occurs when: (1) predicted future noise levels exceed existing levels by a State Highway Agency (SHA) determined amount (i.e., "substantial increase"); and/or (2) predicted future noise levels approach or exceed the SHA's impact criterion level. This level is typically at least 1 dB(A) less than FHWA's Noise Abatement Criterion.

General steps in determining the need for noise abatement are as follows:ref.76

14.1.4 Determine Feasibility and Reasonableness of Noise Abatement.

The feasibility and reasonableness of noise abatement measures are based on the responsible organization's established criteria. A noise abatement measure is primarily considered feasible if the minimum noise reduction goal (as defined by the SHA) can be achieved at impacted receivers. However, keep in mind that even if an acoustically effective barrier may be feasible in theory (from computer modeling of the site), there may not be a physical way to actually build it making it unfeasible.

The determination of reasonableness is more subjective. Some factors typically considered in the determination of reasonableness include, but are not limited to:

  1. Cost of abatement;
  2. Amount of noise impact;
  3. Noise abatement benefits;
  4. Life cycle of abatement measures;
  5. Environmental impacts of abatement measure;
  6. Views of impacted residents; and
  7. Input from public and local organizations.

14.2 Develop Barrier Design

If the acoustical evaluation has determined that a noise barrier is appropriate and both feasible and reasonable, the barrier design process begins. General steps in developing the barrier design are as follows:

It is important to note that the steps listed above should be considered part of an iterative process where both the acoustical and civil engineers work together to resolve conflicts which arise during the design of a noise barrier. A balance between conflicting issues must be the ultimate goal, while preserving the primary function of the barrier system, which is effective and substantial noise reduction.

14.2.1 Community Participation.

Most agencies have specific guidelines and/or regulations which dictate the process of involving the public in the design of a noise barrier. The type, manner, and timing of community involvement activities vary widely between agencies. Many State policies are included on the CD-ROM which accompanies this Handbook. In general, community involvement programs normally include consideration of the following :

Updated: 07/13/2011
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