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Noise Barrier Design Handbook

15. Assessing Barrier Effectiveness

Following the construction of a noise barrier system, it may be necessary to evaluate the barrier's acoustical and non-acoustical performance. Such an evaluation may be required for several reasons, including commitments made during the design and/or environmental analysis phases of the project's development or the need to respond to comments related to the barrier's effectiveness made by community residents and motorists.

15.1 Acoustic Effectiveness

By comparing the barrier's insertion loss with the design goal insertion loss, the effectiveness of a barrier after construction can be assessed. The insertion loss (IL) is the sound level at a given receiver before the construction of a barrier minus the sound level at the same receiver after the construction of the barrier. IL may also be expressed as the net effect of barrier diffraction, combined with a partial loss of soft-ground attenuation. The partial loss of soft-ground attenuation is due to the barrier forcing the sound to take a higher path relative to the ground plane. To assess the effectiveness of a barrier system, the following basic steps should be assess the effectiveness of a barrier system, the following basic steps should be followed:

• Select noise sensitive receivers and/or areas for measurement and analysis (see Section 15.1.1);
• Determine barrier insertion loss by measurements and/or modeling (see Section 15.1.2);and
• Compare measured/predicted insertion loss with the design goals of the barrier project.

15.1.1 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, note 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.

15.1.2 Determine Barrier Insertion Loss by Measurements and/or Modeling.

The procedures described in this section are in accordance with the American National Standards Institute ANSI S12.8-1998, "Methods for Determining the Insertion Loss of Outdoor Noise Barriers,"^ref.9 which provides three methods to determine the field insertion loss of noise barriers: (1) the direct measured method; (2) the indirect measured method; and (3) the indirect predicted method. Readers may also refer to the FHWA's "Measurement of Highway-Related Noise"^ref.19 for more detailed discussions on all of the topics contained herein. Also included in this reference are sample field data log sheets.

1. The direct measured method (see Section 15.1.2.1) may be used only if the barrier has not yet been installed or can be removed. Measurements are performed without the barrier to determine "BEFORE" sound levels, and another set of measurements are performed at the same site after construction to determine "AFTER" sound levels.

Note: For a valid determination of barrier insertion loss, BEFORE and AFTER measurements should be made under equivalent source, site, and atmospheric conditions, defined as follows:

• Equivalence in source conditions includes the number and mix of roadway traffic, as well as spectral content, directivity, spatial and temporal patterns, vertical and horizontal positions, and operating conditions of the individual vehicles. To a certain degree, non-equivalence in traffic conditions can be factored out through the use of a reference microphone (See Section 15.1.2.1).
• Equivalence in site geometry entails similar terrain characteristics and ground impedance within an angular sector of 120 degrees from all receivers looking toward the noise source. For research purposes, equivalence in ground impedance may be determined by performing measurements in accordance with the ANSI Standard for measuring ground impedance scheduled for publication in 1999^ref.13. For more empirical studies, or if measurements are not feasible, then the ground for BEFORE and AFTER measurements may be judged equivalent if general ground surface type and conditions, e.g., surface water content, are similar.
• Equivalence in meteorological conditions includes wind, temperature, humidity, and cloud cover. Wind conditions may be judged equivalent for BEFORE and AFTER measurements if the wind class (see Table 7) remains unchanged and the vector components of the average wind velocity from source to receiver do not differ by more than a certain limit, which is defined as follows: (1) for an acoustical error within ±1.0 dB and distances less than 70 m (230 ft), this limit is 1.0 m/s (2 mi/h); (2) for an acoustical error within ±0.5 dB and distances less than 70 m (230 ft), at least four BEFORE and AFTER measurements should be made within the limit of 1.0 m/s (2 mi/h). However, these 1.0 m/s limits are not applicable for a calm wind class when strong winds with a small vector component in the direction of propagation exist. In other words, BEFORE/AFTER measurements in such instances should be avoided.

Average temperatures during BEFORE and AFTER measurements may be judged equivalent if they are within 14 degrees Celsius of each other. Also, in certain conditions, dry air produces substantial changes in the atmospheric absorption of sound at high frequencies. Therefore, for a predominantly high-frequency source (most sound energy over 3000 Hz), the absolute humidity for BEFORE and AFTER measurements should be similar, e.g., within 20 percent.

The BEFORE and AFTER acoustical measurements should be made under the same class of cloud cover (See Table 8).

The advantage of using this method is that it ensures identical site geometric characteristics. However, the disadvantages are that equivalent meteorological and traffic conditions may not be reproducible.

2. The indirect measured method (see Section 15.1.2.1) may be used when the barrier has been installed prior to any direct BEFORE measurements and cannot be removed to permit such measurements. In this case, the BEFORE condition is simulated at an equivalent site without the barrier. In this case, BEFORE and AFTER measurements should be performed simultaneously at adjacent locations, if possible. The primary advantage of using this method is that it insures essentially the same meteorological and traffic conditions. The difficulty is that an adjacent equivalent site may not always be available. If an adjacent equivalent site is available, then this method is preferred over the direct measured method.

Note: For a valid determination of barrier insertion loss, BEFORE and AFTER measurements should be made under equivalent source, site, and atmospheric conditions as discussed for the direct measured method.

3. The indirect predicted method may be used if neither direct BEFORE measurements or indirect BEFORE measurements at an equivalent site are possible. In this case, BEFORE levels are predicted using a highway-traffic, noise-prediction model, such as the Federal Highway Administration's Traffic Noise Model (FHWA TNM^®). ^{ref.4, ref.5, ref.6 and ref.7} The resulting insertion loss should be referred to as "partially measured." This method is inherently the least accurate of the three methods presented herein (see Section 15.1.2.2).

15.1.2.1 Noise Measurements.

This section describes briefly the recommended instrumentation, microphone location sampling period, measurement procedures, and data analysis procedures to be used for performing BEFORE and/or AFTER measurements using any of the three methods described above.

• Instrumentation - Figure 255 in Section 14.1.2.1 presents and describes 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).
• Microphone Location - When performing measurements to determine barrier insertion loss, it is important to remember that microphone locations relative to the sound source in the BEFORE and AFTER cases should be as close to identical as possible.

  -Reference Microphone - The use of a reference microphone is strongly recommended. Use of a reference microphone allows for a calibration of measured levels, which accounts for variations in the characteristics of the noise source, e.g., traffic speeds, volumes, and mixes.

  In most cases, a reference microphone is placed between the noise source and other measurement microphones at a height of at least 1.5 m (5 ft) directly above the top edge of the barrier (see Figure 256), and at a distance from the sound source sufficient to minimize near-field effects. Typically, a minimum, standard distance of 15 m (50 ft) from the noise source is used. If the barrier is located less than 15 m from the source, the reference microphone should be placed at a distance of 15 m from the noise source, but at a height such that the line of sight between the microphone and the ground plane beneath the source is at least 10 degrees (see Figure 257). This location should remain the same for all measurements, including measurements at the equivalent site, where the barrier is not present.^{note 3}

  

  Figure 256. Reference microphone - position 1.

  

  Figure 257. Reference microphone - position 2.

  -Receiver Positions - In most situations, study objectives will dictate specific microphone locations. As such, this section presents a very generic discussion of microphone locations and assumes no specific study objectives have been identified.

  Generally, it is convenient to position microphones at offset distances from the barrier which correspond to incremental doublings of distances (e.g., 15, 30, and 60 m [50, 100, and 200 ft]). Often times measurement sites are characterized by drop-off rates as a function of distance doubling. In terms of microphone height, 1.5 m (5 ft) is the preferred position. If multi-story 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 all noise-sensitive receivers of interest (see Figure 258).

  

  Figure 258. Receiver positions.

  For the purpose of determining barrier insertion loss, it is important to remember that microphone locations relative to the sound source in the BEFORE and AFTER cases must be identical. There may be instances when receivers are placed on the lawns of homes within the community adjacent to a noise barrier.

• Sampling Period - Different sound sources require different sampling periods. For multiple-source conditions, 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 variations are 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 in Section 14.1.2.1 presents suggested measurement sampling periods based on the temporal nature and the range in sound level fluctuations of the noise source. Guidance on judgment of the temporal nature of the 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. For valid comparisons of BEFORE and AFTER measured levels, the equivalence of meteorological conditions, i.e., wind, temperature, humidity, and cloud cover, should be established. It is assumed that equivalence of site parameters, such as terrain characteristics and ground impedance, were established prior to performing measurements. Sampling periods in which equivalence cannot be established should be excluded from subsequent analysis.
  2. 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

3. 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

4. If appropriate, determine the reflection and/or edge-diffraction bias adjustment.^{note 4}

5. Compute the barrier insertion loss or lower-bound to insertion loss for each source-receiver pair as follows:

where:

• IL_i is the insertion loss at the i_th receiver;
• L_Bref and L_Aref are, respectively, the BEFORE and AFTER adjusted reference levels;
• L_edge is the reflection and/or edge-diffraction bias adjustment; and
• L_Brec and L_Arec are, respectively, the BEFORE and AFTER adjusted source levels at the i^th receiver.

For example:

L_Aref = 78.2 dB
L_edge = - 0.5 dB
L_Arec at receiver 1 = 56.3 dB
L_Bref = 77.7 dB
L_Brec at receiver 1 = 65.0 dB

Therefore:

IL₁ = (78.2 - 0.5 - 56.2) - (77.7 - 65.0) = 21.4 - 12.7 = 8.7 dB

Note: The lower bound to barrier insertion loss is the value reported when ambient levels are not directly measured without the sound source in operation, i.e., "assumed" ambient.

15.1.2.2 Noise Modeling.

As stated earlier, the indirect predicted method requires performing measurements at a site with a barrier to determine AFTER levels, and using a highway-traffic noise-prediction model to predict sound levels without a barrier. This method is inherently the least accurate of the three methods presented. 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 to three companion reports (TNM's User's Guide, Technical Manual, and data base report). ^{ref.4, ref.5, ref.6 and ref.7} This section describes briefly the procedures associated with the modeling approach.

1. Determine sound levels for the AFTER case according to Section 15.1.2.1.
2. Using the measured traffic data and the observed site data, input the necessary information into a highway-noise prediction model, such as the FHWA TNM to compute BEFORE levels at the reference position and at each receiver position. It is possible that modeled levels at the reference position may differ substantially in the BEFORE case, as compared with the measured AFTER case. In such instances, the difference observed at the reference microphone shall be used as a calibration factor for all other measurement positions.
   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.
3. Compute the insertion loss according to Section 15.1.2.1. The resulting insertion loss should be referred to as "partially measured."

15.2 Non-acoustic Effectiveness

An evaluation of a barrier system's non-acoustic effectiveness can include objective factors such as structural durability of the barrier and the appearance of the barrier over time. These objective factors generally require that a considerable time period elapse following barrier construction before any evaluation can be made. On the other hand, subjective factors such as community acceptance of the noise barrier system and public perceptions can be evaluated in the time period shortly after the construction of the barrier, as long as the traffic volumes have stabilized at the facility. A wide range of sampling and interviewing techniques are available to obtain input from adjacent landowners and the motoring public.

15.2.1 Community Acceptance.

Opinions expressed by landowners immediately adjacent to a noise barrier typically reflect their perceptions of the noise levels actually heard versus those levels which they had expected to hear. For example, a barrier may be providing a full 10-dB reduction, but if the final with-barrier levels are still near 66 or 67 dBA, it still may be judged as "too loud" or "ineffective" or "no better than before." In addition, perceptions related to the barrier can relate to the intrusiveness of noise and/or the barrier from either an acoustical or non-acoustical standpoint. Landowners living farther from a noise barrier sometimes complain about noise generated by a new installation. In such situations, while noise levels are typically not loud enough to require consideration of noise abatement, a different or new noise exists as the result of a new or upgraded installation. Closer to the installation, residents adjacent to a highway along which a new barrier was constructed on the opposite side may complain about increased noise.

While the potential for opinions and perceptions such as those noted above is inevitable with the construction of any new barrier system, dealing with such factors is primarily a policy-related issue, as compared to a noise barrier design issue. Similar to the specific noise barrier design and analysis policies unique to each state, each state also has a specific and unique policy (whether formalized or not) related to community involvement, public participation, and post-construction evaluation factors. The reader is referred to the specific state noise policies (contained on the companion CD-ROM) for more information.

15.2.2 Cost.

All states have procedures for evaluating the cost and effectiveness of noise barrier systems. For the most part, a barrier's cost and acoustic effectiveness are factors considered in determining barrier feasibility and reasonableness. The reader is referred to the specific state noise policies (contained on the companion CD-ROM) for details related to the particular items evaluated in such determinations.

Since barrier feasibility and reasonableness evaluations are typically performed during the design phase, they are normally based on modeled noise levels and historical barrier costs. As such, they may not necessarily reflect the final and true cost or effectiveness of a specific as-constructed barrier. If a more refined cost-effectiveness value is desired, a post-construction evaluation may be performed to take into account the following post-construction conditions:

• Measured noise levels;
• Revised insertion loss calculations;
• Costs of modifications made during construction;
• Actual construction bid costs; and
• Actual material costs.

Evaluating costs in this detail may still not provide the true costs related to a specific barrier system, particularly on large projects for which the noise barriers comprise just one relatively small component. Front end loading of specific construction items is often found in construction bid documents for the purpose of establishing contract cash flow. On projects for which noise barrier construction is the major or only item, a determination must be made as to whether or not to include items such as insurance, maintenance, protection of traffic, mobilization, etc. in the barrier cost. More discussion on such cost implications is found in Section 13.