Measurement of Highway-Related Noise

4. Existing-Noise Measurements in the Vicinity of Highways

This section describes recommended procedures for performing existing-noise measurements in the vicinity of highways. Existing-noise measurements include measurements made either prior to a highway project, including the construction of a new highway or the expansion of an existing one (BEFORE), measurements made subsequent to project completion (AFTER), or measurements of both the BEFORE-project and AFTER-project condition. This section does not address the assessment of highway noise barrier performance, which is covered separately in Section 6. The difference in sound levels BEFORE a highway project is started and AFTEr it is completed, combined with the overall level associated with the completed project, gives an indication of the expected noise impact.⁽³⁵⁾

4.1 Site Selection

Site selection should be guided by the location of noise-sensitive receivers.

4.1.1 Site Characteristics

Site characteristics depend on the purpose of the existing-noise measurements: (1) establishing an overall sound level for the purpose of assessing noise impact of a nearby highway; and (2) establishing a change in sound level prior to a highway project relative to the sound level upon project completion.

4.1.1.1 Overall Sound Level Measurements

Land-use maps and field reconnaissance should be used to identify potential noise-sensitive areas. Schools, hospitals, and churches are especially sensitive to noise impacts since they require very low levels to facilitate activity. Noise-sensitive residential areas should also be included in a noise-impact assessment. When selecting potential representative sites for overall sound level measurements, keep in mind, that the site should exhibit typical conditions (e.g., ambient, roadway, and meteorological) for the entire community. It is recommended that good engineering judgment be used to select sites, keeping in mind the objectives of the study.

4.1.1.2 Change in Sound Level Measurements

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 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.⁽²⁵⁾

Average temperatures during BEFORE and AFTER measurements may be judged equivalent if they are within 14 °C of each other. 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, as determined from Table 4.

Equivalence in traffic conditions includes the volume 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 4.1.2.1).

4.1.2 Microphone Location

When performing measurements to establish the change in sound level, 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.

4.1.2.1 Reference Microphone

The use of a reference microphone is strongly recommended for all existing-noise 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.

Typically, the reference microphone is positioned at a height of 1.5 m (5 ft), and located within 30 m (100 ft) of the centerline of the near travel lane at a position which is minimally influenced by ground attenuation and atmospheric effects (See Section 3.2). However, the specific location of the reference microphone may be defined by the location(s) of any noise-sensitive receiver(s) (See Section 4.1.2.2).

4.1.2.2 Receiver

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

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.

In terms of microphone height, 1.5 m (5 ft) is the preferred position. However, microphone height(s) should be chosen to represent all noise-sensitive receivers of interest, 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.

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 meteorological conditions (See Section 3.2).

4.2 Noise Descriptors

The equivalent sound level (L_Aeq) should be used to describe continuous sounds, such as relatively dense highway traffic. The sound exposure level (L_AE), or the maximum A-weighted Sound level with fast time response characteristics (L_AFmx) 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 (L_den) may be used to describe long-term noise environments (typically greater than 24 hours), particularly for land-use planning. Note: Once the L_Aeq and L_AE noise descriptors are established, other descriptors can be computed using the mathematical relationships presented in Section 2.

4.3 Instrumentation (See Section 3)

• Microphone system (microphone and preamplifier)
• Graphic level recorder (optional)
• Measurement/recording instrumentation
• Calibrator
• Microphone simulator
• Pink noise generator
• Windscreen
• Tripod
• Cabling
• Meteorological instrumentation
• Vehicle-speed detection unit
• Traffic-counting device

4.4 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 nature is expected to vary substantially, 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 5 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)

* A minimum of three repetitions is recommended, with 6 repetitions being preferred.

4.5 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 (wind speed and direction, temperature, humidity, and cloud cover) should be documented prior to data collection, at a minimum of 15-minute intervals, and whenever substantial changes in conditions are noted.
2. The electronic noise floor of the acoustic instrumentation system should be established daily by substituting the measurement microphone with a dummy microphone (See Section 3.1.5). 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 (See Section 3.1.6).
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 L₉₀, 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, as defined in Section 5.1.3, 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.

4.6 Data Analysis

4.6.1 Overall Sound Level Measurement Analysis

1. Adjust measured levels for calibration drift (See Section 3.1.4).
2. Adjust measured levels for ambient (See Section 4.6.3).
3. Compute the mean sound level for each receiver by arithmetically Averaging the levels from individual sampling periods.
4. Perform an assessment of the averaged sound levels based on study objectives.

4.6.2 Change in Sound Level Measurement Analysis

1. Adjust measured levels for calibration drift (See Section 3.1.4).
2. Adjust measured levels for ambient (See Section 4.6.3).

3. For each measurement repetition of each BEFORE-AFTER receiver pair, the noise level difference should be determined by subtracting the difference in adjusted reference and receiver levels for the BEFORE case from the difference in adjusted reference and receiver levels for the AFTER case:

   Difference_i = (L_Aref - L_Arec) - (L_Bref - L_Brec)           (dB)

   where:

   • Difference_i is the noise level difference at the ith receiver;
   • L_Brec and L_Arec are, respectively, the BEFORE and AFTER adjusted source levels at the ithe receiver; and
   • L_Bref and L_Aref are, respectively, the BEFORE and aFTER adjusted reference levels.
4. Compute the mean sound level for each receiver by arithmetically Averaging the levels from individual sampling periods.
5. Perform an assessment of the averaged sound levels based on study objectives.

4.6.3 Ambient Adjustments

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 data 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, ambient contribution becomes essentially negligible and no correction is necessary):

L_adj = 10 x log₁₀ (10^{{ 0.1L_c}} - 10^{0.1L_a})           (dB)

where:

1. L_adj is the ambient-adjusted measured level;
2. L_c is the measured level with source and ambient combined; and
3. L_a is the ambient level alone.

For example:

• Lc  = 55.0 db
• La  = 47.0 db

Therefore:

L_adj = 10 x log₁₀(10^{(0.1 x 55.0)}-10^{(0.1 x 47.0)})= 54.3 dB