14. BARRIER DESIGN PROCESS

-Anemometer - An anemometer is an instrument used to measure wind speed. Anemometers shall meet the requirements of ANSI S12.18-1994.^ref.10 For general-purpose measurements at distances within 30 m from the source, a hand-held, wind-cup anemometer and an empirically observed estimation of wind direction are sufficient to document wind conditions. For research purposes, or for measurements where the receiver(s) will be positioned at distances greater than 30 m, a high-precision anemometer, capable of measuring wind conditions in three dimensions, integrated into an automated, data-logging weather station, should be used. For all types of measurements, the anemometer should be located at a relatively exposed position and at an elevation of at least 1.8 m (6 ft), preferably at the maximum height reached by the sound during propagation from source to receiver.^ref.9

-Thermometer, Hygrometer, and Psychrometer - A thermometer for measuring ambient temperature and a hygrometer for measuring relative humidity should be used in conjunction with all noise measurement studies. An alternative is to use a psychrometer which is capable of measuring both dry and wet bulb temperature. Dry and wet bulb temperatures can then be used to compute relative humidity. For general purpose measurements, use of a sling psychrometer is recommended. For research purposes, a high-precision system may be needed, such as an automated, fast-response, data-logging weather station.

-Video Camera Recorder - A video camera can be used to record traffic in the field and perform counts off-line at a later time. This approach, however, would require strict time synchronization between the acoustic instrumentation and the camera.

-Counting Board - A counting board is simply a board with three or more incrementing devices, depending on the number of vehicle types. Each device is manually triggered to increment for a given type of vehicle pass-by.

-Pneumatic Line - A pneumatic line may also be used to determine traffic counts. The pressure in the line increases when a vehicle passes over it, causing a mechanical switch to close. The mechanical switch triggers an internal counting mechanism to increment. The disadvantage of using a pneumatic line is that the specific vehicle mix, i.e., automobiles versus trucks, as well as other vehicle types, is not preserved.

-Reference Microphone - The use of a reference microphone is strongly recommended for all existing-noise measurements. Use of a reference microphone allows for the application of adjustments to account for temporal variations in the noise source, e.g., traffic speeds, volumes, and mixes.

Typically, the reference microphone is positioned at a height of 1.5 m (5 ft) above local ground level and located within 30 m (100 ft) of the centerline of the near travel lane at a position which is minimally influenced by ground and atmospheric effects. In addition, site geometry may dictate other reference microphone locations.

-Receiver Positions - In most situations, study objectives will dictate specific microphone locations. Sometimes a single, typical residential area near the existing or proposed highway route can be used to represent other similar areas. If traffic conditions or topography vary greatly from one residential area to the next, receivers at many locations may be required.

Receivers are also typically positioned at a height of 1.5 m (5 ft) above local ground level. However, microphone height(s) should be chosen to represent noise-sensitive receivers, i.e., if multistory structures are of interest, including microphones at heights of 4.5 m and 7.5 m (15 ft and 25 ft) may be helpful. Microphone heights should be chosen to encompass the range of heights associated with all noise-sensitive receivers of interest.

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.}

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.

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 - [(CAL_INITIAL + CAL_FINAL) / 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):

L_adj=10*log₁₀(10^0.1Lc-10 ^0.1La)    (dB)

where:

L_adj 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:
L_adj = 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.

• Roadways: coordinates, including roadway shoulder, vehicle types, traffic counts, vehicle speeds, and interrupted-flow devices, such as stop signs, traffic signals, etc.;

• Receiver: coordinates and height above ground;

• Existing noise barriers or barrier-like objects: barrier type (wall or berm), coordinates, height above ground, and absorptive characteristics;

• Building rows: coordinates, height above ground, and building percentage (the percentage of actual building structure in a row of buildings);

• Ground zones: coordinates and ground zone acoustic characteristics; and

• Terrain lines: coordinates which define substantial changes in ground elevation.

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

• Determine future noise levels for all applicable alternatives - A noise prediction methodology (see Section 14.1.2.2) should be used to predict future noise levels for two cases:

  -No build - The predicted future noise levels without the planned highway project; and

  -Build - The predicted future noise levels after completion of the planned highway project, but without any noise abatement;

• Compare the predicted noise levels for all project alternatives (including the No-Build case) with FHWA noise abatement criteria and existing noise levels to if there are any noise impacts as defined by the FHWA and SHA;

• If noise impacts have been identified, consider noise abatement measures. Noise abatement measures may include, but not limited to:
  1. Avoiding project impact by using design alternatives that result in a reduction in the noise effect, such as altering horizontal and vertical alignments;
  2. Using traffic management measures that are consistent with State statutes regarding the regulation of traffic control devices, vehicle types, time-use restrictions, modified speed limits, etc.;
  3. Acquiring property of interest to serve as a buffer zone to preempt development which would be impacted by traffic noise;
  4. Constructing noise barriers (sound walls or earth berms); and
  5. Insulating and/or air-conditioning public use or nonprofit institutional structures.

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:

• Using the input from the acoustical evaluation, develop the plan, profile, and cross sections of one or several barrier acoustical scenarios, locations, heights, lengths, and estimated costs;

• Document details of the recommended barrier(s) and transmit them to personnel responsible for designing the noise barrier;

• Have the design reviewed by staff who performed acoustical evaluation to assure that any modifications based on engineering requirements did not adversely affect the acoustical performance of the barrier;

• Refine the design, as appropriate;

• Develop accompanying specifications, special provisions, etc.; have appropriate sections reviewed by acoustical personnel; and

• Develop the final Plans, Specifications, and Estimate (PS&E) package.

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 :

• Process options and techniques: formal and informal;
  
• Public meeting/hearing issues; and
  
  - Schedule, number, and progression of meetings and/or field views;
  - Location of meetings: in residences, churches, schools, public meeting places, or in the field;
  - Group size and participants;
  - Display materials: samples, photos, charts, plans, noise panels, etc.; and
  - Acoustical demonstration techniques: FHWA videos and project specific noise tapes.^ref.17, ref.26, ref.77, ref.78, ref.79 and ref.80
  
  
• Barrier system design options;
  
  - How presented;
  - What choices are provided to the public; and
  - Who is the public.