This section describes recommended procedures for the measurement of highway noise barrier insertion loss. Insertion loss is defined as the difference in sound level at a receiver location with and without the presence of a noise barrier, assuming no change in the sound level of the source.
The procedures described in this section are in accordance with ANSI S12.8-1987,(6) which provides three methods to determine the field insertion loss of noise barriers: (1) "direct" BEFORE/AFTER measurement; (2) "indirect" BEFORE measurement at an equivalent site; and (3) "indirect" predictions of BEFORE levels.
The "direct" BEFORE/AFTER method requires performing measurements at a site before the barrier has been constructed to determine "BEFORE" levels, and another set of measurements at the same site after construction to determine "AFTER" levels. The advantage of using this method is that it insures identical site geometric characteristics. However, the disadvantages are that equivalent meteorological and traffic conditions may not be reproducible.
The "indirect" BEFORE method requires performing measurements at a site with a barrier to determine "AFTER" levels, and another set of measurements at an "equivalent" site without a barrier to determine equivalent "BEFORE" levels.
A site may be judged equivalent if geometric, atmospheric, and traffic conditions are determined to be essentially identical for the BEFORE case as compared with the AFTER case. Geometric equivalence refers to the terrain characteristics and ground impedance at the site. Atmospheric equivalence refers to temperature, humidity, and wind speed and direction (See Section 6.1.1). Traffic equivalence refers to vehicle type and mix.
The BEFORE and AFTER cases for the "indirect" BEFORe method should be studied simultaneously, if possible. In other words, the ideal situation is to make BEFORE and AFTER measurements simultaneously at adjacent locations. The primary advantage to 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.
The "indirect" prediction method requires performing measurements at a site with a barrier to determine AFTER levels, and using a highway-traffic, noise-prediction model, such as the Federal Highway Administration's Traffic Noise Model (FHWA TNM®), to predict sound levels at an equivalent site without a barrier. This method is inherently the least accurate of the three methods presented herein.
Site selection for all three measurement methods is guided by site geometry, and the location of noise-sensitive receivers.
For valid comparison of BEFORE and AFTER sound levels, equivalence in site geometry, meteorological, and traffic conditions must be established.
Equivalence in site geometry entails similar terrain characteristics and ground impedance within an angular sector of 120 degrees from all receivers looking towards 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 the second half of 1996.(37) 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 3 in Section 3.2.1) 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 is 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.(25)
Average temperatures during BEFORE and AFTER measurements may be judged equivalent if they are within 14 ° C of each other. Also, in certain conditions, dry air produces substantial changes in sound attenuation 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.
The BEFORE and AFTER acoustical measurements should be made under the same class of cloud cover (See Table 4 in Section 22.214.171.124).
Equivalence in traffic conditions includes the number and mix of roadway traffic, as well as spectral content, directivity, and spatial and temporal patterns 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 126.96.36.199).
The use of a reference microphone is strongly recommended for all barrier insertion loss measurements. 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 1.5 m (5 ft) directly above the barrier (See Figure 12), 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 (See Figure 13). This location should remain the same for all measurements, including measurements at the equivalent site, where the barrier is not present.
Figure 12. Reference microphone-position 1.
Figure 13. Reference microphone-position 2
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 useful to position microphones at offset distances from the barrier which corresponds 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 3 m and 6 m (10 ft and 20 ft) may be helpful. Microphone heights should be chosen to encompass all noise-sensitive receivers of interest (See Figure 14).
Figure 14. 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.
Note: For receiver distances greater than 100 m (300 ft) from the source, atmospheric effects have a much greater influence on measured Sound levels.(8,38) In such instances, precise meteorological data will be needed to ensure BEFORE and AFTER equivalence of the meteorological conditions (See Section 3.2).
The equivalent sound level (LAeq) should be used to describe continuous sounds, such as relatively dense highway traffic. The sound exposure level (LAE), or the maximum A-weighted sound level with fast time response characteristics (LAFmx), should be used to describe the sound of single events, such as individual vehicle pass-bys. The day-night average sound level (Ldn) and the community-noise exposure level (Lden) may be used to describe long-term noise environments (typically greater than 24 hours), particularly for land-use planning. Note: Once the LAeq and LAE noise descriptors are established, other descriptors can be computed using the mathematical relationships presented in Section 2.
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 are 15 minutes, 1 hr and 24 hr. Measurement repetitions at all receiver positions are required to ensure statistical reliability of measurement results. A minimum of three repetitions for like conditions is recommended, with six repetitions being preferred. Table 5 in Section 4.4 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 S1.13-1971 and ANSI S12.9-1988.(16,47)
The following steps apply for all methods except the BEFORE predictions for the "indirect predicted" method, which is discussed separately in Section 6.5.1.
(Note: Appendix B provides example field-data log sheets.)
If measured levels do not exceed ambient levels by 4 dB or more, or if the levels at the reference microphone do not exceed those at the receivers, then the barrier insertion loss cannot be determined.
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 measured levels must be corrected for ambient as follows (Note: For sound levels which exceed ambient levels by greater than 10 dB, ambient contribution becomes essentially negligible and no correction is necessary):
Ladj = 10 x log10 (100.1Lc - 100.1La) (dB)
Ladj = 10 x log10(10(0.1 x 55.0)-10(0.1 x 47.0)) = 54.3 dB
Due to multiple reflections between source and barrier and/or edge diffraction at the top of a barrier, a 0.5 dB correction factor to reference microphone sound levels in the AFTER case may be applied. Good engineering judgment, based on repeatability through measurements, should be used to determine the magnitude and necessity of this correction. For example, if for several runs (i.e., greater than six), a consistent repeatable difference at the reference microphone position in the BEFORE and AFTER case occurs, and it can be proven that the traffic during both cases were equivalent, then the difference can be attributed to edge diffraction effects. The edge diffraction correction factor will be a negative value which is added directly to the sound level measured at the reference microphone in the AFTER case (See Section 6.6.3).(22,31) Note: Larger corrections due to parallel barriers may be necessary.
For each measurement repetition and BEFORE/AFTER pair, the insertion loss, or its lower bound, should be determined by subtracting difference in adjusted reference receiver levels for case from case:
ILi = (LAref + Ledge - LArec) - (LBref - LBrec) (dB)
IL1 = (78.2 - 0.5 - 56.2) - (77.7 - 65.0) = 21.5 - (12.7) = 8.8 dB
The lower bound to barrier insertion loss is the value reported when ambient levels are not directly measured without the sound source, i.e., "assumed" ambient.
*Note: There are several useful rules-of-thumb for estimating noise barrier insertion loss. If the line-of-sight is broken by the barrier between the source and the receiver, barrier insertion loss is typically 5 dB. For each additional 1 m (3 ft) of barrier height beyond the line-of-sight blockage, an increase in barrier insertion loss of 1.5 dB can be considered typical. Noise barriers are usually designed with an insertion loss goal of 10 dB in mind. Actual barrier insertion losses of between 6 and 8 dB are quite common.
*In addition, insertion loss due to buildings is dependent on the amount of gap, or opening, between buildings in the same row. Typically, 4.5 dB attenuation is attainable for the first row of buildings, and an additional 1.5 dB for each subsequent row, up to a maximum of about 10 dB.
Also, to achieve any substantial amount of attenuation due to foliage, such as trees and bushes, foliage must be at least 30 m (100 ft) deep and dense enough to block the line-of-sight. Typically, as much as 5 dB attenuation is attainable.(26,39)
One of the consequences of noise barrier construction on one side of a roadway, is the possibility of noise reflecting to the opposite side of the roadway. Increases in sound level due to a single reflection can practically range from 0.5 to 1.5 dB, with a theoretical increase of 3 dB when 100 percent of the sound energy is reflected. A 3 dB increase is generally just slightly perceptible to the human ear.
Although the overall sound level increase due to reflections off a single barrier may not be readily perceptible, the frequency of the reflected Sound may alter the signature of the source as perceived by residents on the opposite side of the road. This change in the general character of the sound may be perceptible, although no conclusive research has been done in this area.
However, construction of barriers on both sides of the highway may not solve this potential problem. Sound reflected between both barriers may cause degradations in each barrier's performance anywhere from 2 to as much as 6 dB, i.e., a single reflective barrier with an insertion loss of 10 dB may only realize an effective reduction of 4 to 8 dB if another reflective barrier is placed parallel to it on the opposite side of the highway.
There are several methods used to minimize the reflections from single barriers and reflections between parallel barriers:
For parallel barriers, ensure that the distance (width) between the two barriers is at least 10 times their average height relative to the roadway elevation (width-to-height ratio or w/h ratio).
In recent studies,(22,25) it was determined that as the w/h ratio increases, the insertion loss degradation tends to decrease. This decrease was attributed to: (1) the decrease in the number of reflections between the barriers; and (2) the weakening of the reflections due to geometrical spreading and atmospheric absorption. Table 7 provides a guideline of three, general w/h ratio ranges and the corresponding barrier insertion-loss degradation ( IL) that can be expected.
|w/h Ratio||Maximum ΔIL in dB (A)||Recommendation|
|Less than 0.1||3 or greater||Action required to minimize degradation|
|10:1 to 20:1||0 to 3||Degradation acceptable in most instances|
|Greater than 20:1||No measurable degradation||No action required|
Apply acoustically absorptive material on either one or both barrier facades. Absorptive treatment may be categorized by the amount of incident sound that a barrier absorbs. Currently, the Noise Reduction Coefficient (NRC) is the measure of choice. NRC is defined as the arithmetic average of the Sabine absorption coefficients, Α Sab, at 250 Hz, 500 Hz, 1000 Hz, and 2000 Hz. Measurements to determine the Sab of a facade should be made in accordance with the American Society of Testing and Materials (ASTM) Recommended Practice C 423-90a (Reverberation Room Method).(40) An alternative method for computing the NRC is to determine the absorption coefficients using ASTM Recommended Practice C384-95a (Impedance Tube Method).(41) The Reverberation Room method provides a measure of material absorption for randomly incident sound while the Impedance Tube method provides a measure of absorption for normal incident sound. Typically, the reverberation room method is used for determining NRC.
NRC values theoretically range from 0 to 1, where 0 indicates that the barrier will reflect all the incident sound, and 1 indicates that the barrier will absorb all the incident sound. However, very often when a material is tested in a reverberation room (ASTM C423-90a), NRC values higher than 1 may be computed. This is the result of an anomaly in the test procedure. To correct for this anomaly, and, in turn, obtain a meaningful NRC, the four absorption coefficients should first be normalized such that the highest one is equivalent to 1.0, and the factor that was applied to the highest one should then, in turn, be applied to the remaining three coefficients. Typical NRC values for an absorptive barrier range from 0.6 to 0.9.
A barrier may be described by the amount of noise it transmits, i.e., its Sound Transmission Class (STC). Measurements to determine the STC of a section of a barrier should be made in accordance with ASTM Recommended Practice E 413-87.(42)
Usually it is assumed that the sound transmitted through a barrier is negligible relative to that which is diffracted over the top, i.e., the sound transmitted is at least 20 dB below that diffracted. Most state transportation agencies specify a minimum STC for barriers constructed within their state.