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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:

• Select noise sensitive receivers and/or areas for measurement and analysis (see Section 14.1.1);
• Determine existing noise levels by measurements and/or modeling (see Section 14.1.2);,
• Determine if there will be any future noise impacts (see Section 14.1.3); and
• Determine the feasibility and reasonableness of noise abatement (see Section 14.1.4).

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).

Figure 255. Generic measurement instrumentation setup

• Calibrator - An acoustic calibrator provides a means of checking the entire acoustic instrumentation system's (i.e., microphone, cables, and recording instrumentation) sensitivity by producing a known sound pressure level (referred to as the calibrator's reference level) at a known frequency, typically 94 or 114 dB at 1 kHz or 124 dB at 250 Hz. The calibrator used for measurements described herein should meet the Type 1L performance requirements of ANSI S1.40-1984(R1997) or IEC 60942.^ref.68 and ^ref.69
• Microphone Simulator - The electronic noise floor of the entire acoustic instrumentation system should be established on a daily basis by substituting the measurement microphone with a passive microphone simulator (dummy microphone) and recording the noise floor for a period of at least 30 seconds. A dummy microphone electrically simulates the actual microphone by providing a known fixed (i.e., passive) capacitance which is equivalent to the minimum capacitance the microphone is capable of providing. This allows for valid measurement of the system's electronic noise floor.^ref.70
• Pink Noise Generator - The frequency response characteristics of the entire acoustic instrumentation system should be established on a daily basis by measuring and storing 30 seconds of pink noise. Pink noise is a random signal for which the spectrum density, i.e., narrow-band signal, varies as the inverse of frequency. In other words, one-third octave-band spectral analysis of pink noise yields a flat response across all frequency bands.
• Windscreen - A windscreen should be placed atop all microphones used in outdoor measurements. A windscreen is a porous sphere placed atop a microphone to reduce the effects of wind-generated noise on the microphone diaphragm. The windscreen should be clean, dry, and in good condition (a new windscreen is preferred). Typically, the effect on the measured sound level due to the insertion of a windscreen into an acoustic instrumentation system can be neglected.
• Microphone System (Microphone and Preamplifier) - A microphone transforms sound pressure variations into electrical signals that are, in turn, measured by instrumentation such as a sound level meter, a one-third octave-band spectrum analyzer, or a graphic level recorder. These electrical signals are also often recorded on tape for later off-line analysis. Microphone characteristics are further addressed in ANSI S1.4-1983(R1997).^ref.12 A compatible preamplifier, if not engineered as part of the microphone system, should also always be used. A preamplifier provides high-input impedance and constant, low-noise amplification over a wide frequency range. Also, depending upon the type of microphone being used (refer to Reference 18), a preamplifier may also provide a polarization voltage to the microphone.
  
  The microphone system (microphone and preamplifier) should be supported using a tripod or similar device, such as an anchored conduit. Care should be taken to isolate the microphone system from the support, especially if the support is made up of a metal composite. In certain environments, the support can act as an antenna, picking up errant radio frequency interference which can potentially contaminate data. Common isolation methods include encapsulating the microphone system in nonconductive material (e.g., nylon) prior to fastening it to the support.
• Graphic Level Recorder - A graphic level recorder (GLR) connected to the analog output of the measuring or recording instrumentation is typically used in the field to provide a visual, real-time history of the measured noise level. A GLR plot varies in level at a known, constant pen-speed rate and response time that may be adjusted to approximate both slow and fast exponential time-averaging. It is valuable in visually judging ambient levels and verifying the acoustic integrity of individual events in real-time during measurements.
• Recording Instrumentation - There are two basic types of tape recorders: analog and digital. Analog recorders store signals as continuous variations in the magnetic state of the particles on the tape. Digital recorders store signals as a combination of binary "1s" and "0s." Not all field measurement systems will include a tape recorder. A recorder offers the unique capability of repeated playback of the measured noise source, thus allowing for more detailed analyses. The electrical characteristics of a tape recorder shall conform to the guidelines set in IEC 1265 and ANSI S1.13-1995 for frequency response and signal-to-noise ratio ^{ref.71 and ref.72}
• One-Third Octave-Band Analyzer - When the frequency characteristics of the sound source being measured are of concern, a one-third octave-band analyzer should be employed. In most cases, such a unit would not be employed directly in the field but would be used subsequent to field measurements in tandem with tape-recorded data. Such units can be employed to determine noise spectra, as well as compute various noise descriptors (see Section 3.2). Use of octave-band analyzers is not precluded; however, one-third octave-band analyzers are preferred.
• Sound Level Meter - For the purposes of all measurements discussed herein, sound level meters (SLMs) should perform true numeric integration and averaging in accordance with ANSI S1.4-1983(R1997) ^ref.12. Selection of a specific model of sound level meter should be based upon cost and the level of measurement accuracy desired.
• Meteorological Instrumentation - It is recommended that meteorological data, including temperature, relative humidity, and wind speed and direction be measured simultaneously with all acoustic data. For microphone distances within 30 m (100 ft) of the noise source, atmospheric effects, especially air turbulence, can affect measured sound levels (see Section 3.3.3). These effects typically increase with increasing distance from the noise source. Meteorological equipment shall include:
• 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.
• Traffic Instrumentation - For many transportation-related measurements, the collection of traffic data, including the logging of vehicle types, vehicle-type volumes, and average vehicle speed may be required for: (1) determination of site equivalence (see Section 15.1.2); or (2) input into a highway traffic noise prediction model. This section discusses various instruments for the counting and classification of roadway traffic, including the use of a video camera, counting board, or pneumatic line. If none of these instruments is available, meticulous pencil/paper tabulation should be used.
• 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.

Microphone Location

• 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.

• Sampling Period - Different sound sources require different sampling periods. Depending upon the characteristics of the sound source, a longer sampling period is needed to obtain a representative sample, averaged over all conditions. Typical sampling periods range from 2 to 30 minutes. In special instances where the temporal variation is expected to be substantial, longer sampling periods, such as 1 hr or 24 hr, may be necessary. Measurement repetitions at all receiver positions are required to ensure statistical reliability of measurement results. A minimum of 3 repetitions for like conditions is recommended, with 6 repetitions being preferred. Table 6 presents suggested measurement sampling periods based on the temporal nature and the range in sound level fluctuations for a particular sound source. Guidance on judgment of the temporal nature of the sound source may also be found in ANSI S1.13-1995 and ANSI S12.9-1988.^{ref.70 and ref.72}

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:

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:

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.