Measurement of Highway-Related Noise

7. Construction Equipment Noise Measurements for Highway-Related Projects

This section describes recommended procedures for the measurement of highway construction equipment noise. The results of these measurements Can be used to assess the potential noise impact of a construction site associated with a highway-related project.

Highway construction site activity consists of several generic phases, including mobilization, clearing and grading, earthwork, foundations, bridge construction, base preparation, paving, and cleanup. Thus, any noise impact due to a construction site is actually composed of contributions from each of these phases.⁽⁴³⁾

The noise level associated with a particular construction phase is determined by first measuring the levels of individual equipment, then summing the individual contributions over a particular time period. The types and numbers of construction equipment, and the amount of time specific equipment operate in different modes are a direct function of the construction phase.

For the procedures described herein, each type of construction equipment will be characterized by up to four modes of operation as appropriate:

1. the equipment is stationary in a passive operation mode (STATIONARY-PASSIVE, e.g., a bulldozer at idle);
2. the equipment is stationary in an active operation mode (STATIONARY-ACTIVE, e.g., a bulldozer lifting earth, debris, etc.);
3. the equipment is moving to another area within a site but is not actively performing project-related activities (MOBILE-PASSIVE); and
4. the equipment is mobile in an active operation mode (MOBILE-ACTIVE, e.g., a bulldozer moving while pushing earth, debris, etc).

7.1 Site Selection

7.1.1 Site Characteristics

In determining overall noise levels associated with a particular construction site, the first step is to establish reference noise emission levels for each type of construction equipment operating in each of the above four modes. As such, the general site characteristics for determining reference noise emission levels for construction equipment are somewhat similar to those presented in Section 5.1.1 for determining noise emissions for highway vehicles. These characteristics are as follows:

• A flat open space free of large reflecting surfaces, such as parked vehicles, signboards, buildings, or hillsides, located within 30 m (100 ft) of either the construction equipment's path (if measurements of mobile operations are being performed), its stationary position (if appropriate), or the microphone(s).
• The ground surface within the measurement area is free of snow and representative of acoustically hard, e.g., pavement, or acoustically soft, e.g., grass, terrain.
• The line-of-sight from the microphone(s) to the construction equipment being measured unobscured within an arc of 150 degrees.
• A predominant, ambient level at the measurement site low enough to enable the measurement of uncontaminated vehicle pass-by sound levels. Specifically, the difference between the lowest-anticipated, vehicle pass-by, maximum A-weighted sound-pressure level (L_AFmx) and the A-weighted ambient level, as measured at the 15-m (50-ft) microphone, should be at least 10 dB.
• The site to be located away from known noise sources, such as airports, construction sites, rail yards, or heavily traveled roadways, if possible.

7.1.2 Microphone Location

Microphones should be positioned at a height of 1.5 m (5 ft) above ground level (AGL), and placed at a distance of 15 m (50 ft) perpendicular to the equipment's typical operating location (for STATIONARY-PASSIVe and STATIONARY-ACTIVE operating modes), and typical operating path (for MOBILE-PASSIVe and MOBILE-ACTIVE operating modes). For stationary noise sources, measurements should be made at each of 4 positions around each piece of construction equipment, each position representing azimuth angles separated by 90 degrees (See Figure 15).⁽⁴⁴⁾ For mobile noise sources, measurements should be made with each piece of equipment passing by in a left-to-right and a right-to-left direction (See Figure 15).^(44,45) For all measurements, a minimum of three measurement repetitions, and preferably six, should be made.

Figure 15. Microphone positions for construction equipment noise measurements.

7.2 Noise Descriptors

For stationary noise sources, a 30-second L_Aeq should be measured at each of the four azimuth angles. If a 30-second measurement is not possible, shorter durations can be used if the sound level is relatively steady as a function of time. For mobile noise sources, the L_AFmx should be measured. The individual reference levels and the number and type of each piece of construction equipment are then, ultimately, used to compute the total equivalent sound level, L_Aeq,total, for a typical work day during a particular construction phase. Note: Once the L_Aeq descriptor has been established for a typical work day and construction phase, other descriptors Can be computed using the mathematical relationships presented in Section 2. The L_Aeq descriptor may be more useful in assessing potential noise impact due to construction-related activity.

7.3 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
• Tachometer (optional)

7.4 Sampling Period

For each type of construction equipment, the sampling period will vary depending upon the operating mode (STATIONARY-PASSIVE, STATIONARY-ACTIVE, MOBILE-PASSIVE, and MOBILE-ACTIVE). For each mode, the construction equipment should be operated in a manner which is considered typical for the work period associated with a particular mode. Due to the expected abundance of activity At a construction site, the sampling period may be based entirely on good engineering judgment; and it will be up to the person performing the measurements to ensure that representative high-quality data are obtained.

7.5 Measurement Procedure

1. The instrumentation should be deployed as shown in Figure 15.
2. 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.
3. 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).
4. Ambient levels should be measured and/or recorded by sampling the sound level at each receiver 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.
5. For each mode, the construction equipment should be operated in a manner which is considered typical for the work period and the particular mode.
6. For each equipment type and operating mode, record the L_AFmx or L_Aeq30s, as appropriate.

(Note: Appendix B provides example field-data log sheets.)

7.6 Data Analysis

1. Adjust measured levels for calibration drift (See Section 3.1.4).
2. Adjust measured levels for ambient (See Section 7.6.1).
3. Calculate an energy-averaged level (L_AVG,j) of the L_Aeq30s values obtained for each azimuth angle and each measurement repetition of each equipment type in each stationary mode of operation, j (See Section 7.4).
4. Calculate an energy-averaged level (L_AVG,j) of the L_AFmx values obtained for each measurement repetition of each equipment type in each mobile mode of operation, j (See Section 7.4).
5. Calculate the L_Aeq,i for each equipment type, i (See Section 7.6.2).
6. When all equipment measurements used for a particular phase are complete, compute the L_Aeq,total for a typical workday during that phase (See Section 7.6.3).
7. Perform an assessment of noise impact due to construction equipment activity based on study objectives. In most instances, the L_Aeq,total computed above will be used in Environmental Analyses to compare the potential impact of different construction phases. If a particular noise-sensitive receiver is a primary concern in the study, it is suggested that long-term existing-noise measurements be made at that location, in accordance with the recommendations in Section 4.

7.6.1 Ambient Adjustments

If measured levels do not exceed ambient levels by 4 dB or more, i.e., they are masked, then those data should be omitted from data analysis.

If measured levels exceed the ambient levels by between 4 and 10 dB, 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:

• L_adj is the ambient-adjusted measured level;
• L_c is the measured level with source and ambient combined; and
• 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)

7.6.2 Determination of the Equivalent Sound Level for Each Type of Construction Equipment

The equivalent sound level for a particular type, i, of construction equipment is computed as follows:

L_Aeq,i = 10 x log₁₀⁴Σ_j=1 [ 10^{(L_AVG,j ⁄ 10 x T_j)} x (T_j ⁄ T_total) x N_j]           (dB)

where:

• L_Aeq,i is the equivalent sound level for equipment type i;
• j is the operating mode, where up to four modes are applicable for each type of equipment;
• L_AVG,j is energy-averaged level obtained in operating mode j;
• T is the operating mode duration, in seconds; and
• N is the number of pieces of equipment type i operating in mode j.

For example:

• L_AVG,1 = 65.5 dB for T1 = 600 seconds and N = 3 pieces
• L_AVG,2 = 86.7 dB for T2 = 5500 seconds and N = 2 pieces
• L_AVG,3 = 71.0 dB for T3 = 350 seconds and N = 2 pieces
• L_AVG,4 = Not applicable

Therefore:

L_Aeq,1 = 10log₁₀ [(10^65.5/10 x (600/6450) x 3) + (10^86.7/10 x (5500/6450) x 2) + (10^71.0/10 x (350/6450) x 2) = 89.0 dB

7.6.3 Determination of the Total Equivalent Sound Level

The total equivalent sound level for a typical work day during a particular construction phase is computed as follows:

L_{Aeq, total} = 10 x log₁₀^kΣ_{i = 1} [10^{(L_{Aeq, i} ⁄ 10)}]           (dB)

where:

• L_Aeq,total is the total equivalent sound level for a typical work day during a particular construction period;
• k is the number of different types of equipment; and
• L_Aeq,i is the equivalent sound level for equipment type i.

For example:

• L_Aeq,1 = 89.0 db
• L_Aeq,2 = 81.7 db
• L_Aeq,3 = 79.0 db
• L_Aeq,4 = 80.5 db

Therefore:

L_Aeq,total = 10log₁₀ [10^89.0/10 + 10^81.7/10 + 10^79.0/10 + 10^80.5/10 ] = 90.6 dB