Measurement of Highway-Related Noise

Appendix D Sample Report Documentation

The objective of this appendix is to exemplify the types of information to be documented in a field measurement report. For the purposes of this appendix, assume existing-noise measurements were performed (See Section 4).

D.1 Site Sketches

The measurement site was located on Route 95 (a 2-lane highway) 0.8 km past Exit 21. A reference microphone was attached to a mast, placed at a height of 1.5 m above the roadway pavement, and located at a 15 m offset position from the centerline of the near travel lane. Another portable mast was fitted with three microphones, placed at heights of 1.5 m (low), 4.5 m (middle), and 7.5 m (high), and located at a 30-m offset position. When referring to microphone heights, the high, middle, and low convention will be used for the remainder of this report. Figures D1 and D2 present the plan and elevation views, respectively.

Figure D1. Measurement site plan view.

Figure D2. Measurement site elevation view.

D.2 Source Description

The source was constant free-flowing traffic traveling on Route 95. Traffic volume and mix were recorded on video cassette and used to obtain vehicle counts. Vehicles were counted and classified in three categories: automobiles (A); medium trucks (MT); and heavy trucks (HT). Vehicles were further grouped by direction (eastbound and westbound). Vehicle counts and average speed for each test run are presented in Table D1.

( - ) Denotes test run was removed from the population of events to be analyzed (See Section D.7 for an explanation).

D.3 Instrumentation Description

Note: A list of instrumentation is presented in Table D2. Each noise measurement system consisted of a General Radio Model 1962-9610 random-incidence electret microphone, connected to a Larson Davis Model 827-0V preamplifier. The microphone/ preamplifier system was mounted in an insulated nylon holder and connected via cable to a Larson Davis Model 820 Type 1 Precision integrating Sound Level Meter/Environmental Noise Analyzer (LD820). The microphone/preamplifier combination was positioned 0.3 m from the mast and placed in its shadow as viewed from the roadway. This position insured minimum errors due to reflections from the mast structure.(11) Brüel & Kjær Model UA0237 windscreens were placed atop each microphone to reduce the effects of wind-generated noise on the microphone diaphragm.

Pre-processing and storage of the measured noise level data were accomplished by the LD820. Each unit was programmed to continually measure, energy average, and store A-weighted noise levels with fast-exponential response characteristics at a rate of two data records each second (1/2-second averages).

A passive microphone simulator was used to establish the electronic noise floor of each system. In addition, the frequency response of each system was tested using pink noise generated by a Cetec Ivie Model IE-20b random noise generator.

Traffic speed was obtained with a CMI Doppler radar gun set up 6 m off the edge of the near travel lane, approximately 100 m west of the microphone centerline (See Figure D1). The Doppler radar was directed at the departing westbound traffic, thus minimizing the possibility of individual vehicles slowing down after detecting the radar signal. Readings were observed visually from the radar's digital display, and recorded continuously during each measurement period at a rate of approximately one reading every 10 seconds.

A Panasonic Model AG170 video camera was set up on a nearby overpass to record pass-by traffic at the measurement site. The camera was time-synchronized with the LD820's, so that the noise data could be correlated with the traffic data.

D.4 Meteorological Data

A Climatronics Model EWS weather station continually recorded temperature, humidity, wind speed, and wind direction data on a continuous strip-chart recorder with a paper speed of four inches per hour. Wind speed and direction were measured at a height of 7.5 m above the ground (height equivalent to the highest microphone position); temperature and humidity were measured at a height of 1.5 m above the ground. In addition, cloud cover was documented periodically, as well as significant changes in weather conditions.

Using the known recorder paper speed and the time marks produced on the strip-chart, a time scale was transposed on each chart and the 5-minute measurement period for each test was identified.

The average wind speed and average wind direction re magnetic north (degrees) were computed for each 5-minute test run. The 5-minute averaged wind speed (WS) and direction (WD) were then used to compute the vector component of wind speed in the x-y plane from the source to receiver (VWS) for each test run.

Meteorological data are presented in Table D3. Note: Cloud cover class 2 was observed for the duration of the measurement day.

^* Wind Direction re Magnetic North
( - ) Denotes test run was removed from the population of events to be analyzed (See Section D.7 for an explanation).

D.5 Ground Surface Characterization

The roadway surface was composed of dense-graded asphaltic concrete. The roadside terrain between the road and the receivers was relatively flat and composed of packed clay with low-cut grass.

D.6 Measurement Procedures

At the beginning of the measurement day, a complete system check was performed on the entire measurement system. To establish the electronic noise floor of each system, a passive microphone simulator was substituted for each microphone. The frequency response of each system was tested by recording a 30-second sample of pink noise. In addition, 30 seconds of calibration data were recorded at the beginning and end of the measurement day.

Data were collected at a rate of two samples per second. After collecting data for ten consecutive 5-minute test runs (5-minute spacing between each run), approximately 30 seconds of calibration data were measured and stored for all microphones. Data collection then calibration were repeated until a total of thirty 5-minute test runs were measured and stored.

At the end of the measurement day, the 1/2-second noise data stored in each LD820 were downloaded to an AST Premium Exec Model 386SX/20 notebook computer and stored on floppy disk for later off-line processing.

D.7 Acoustical Data

Processing of the noise data files stored on floppy disk was accomplished off-line, using the LD820 support software in tandem with the Acoustics Facility-developed computer program, RFILE. The LD820 software was used to obtain a graphical history plot (noise level versus time) for the test runs identified in the field as potentially contaminated. These plots were examined and all questionable test runs were removed from the population of events to be processed.

The RFILE program, using the 1/2-second data stored in each file, was used to compute the equivalent A-weighted sound levels for each 5-minute test run (L_Aeq,5min). The L_Aeq,5min values were adjusted for calibration drift. No ambient adjustments were necessary. The final L_Aeq,5min values are presented in Table D4. Computation of experimental error is shown below.

Experimental Data Error Calculation

1. Compute Variance* for:

   • Background (Not computed if measured level background by 10 dB):

     Reference Microphone Position   0.0
     High Microphone Position   0.0

   • Difference (Corrected source levels at reference microphone position minus calibration corrected source levels at the

     High Microphone Position   0.012

   • Bias:

     Type	Amount	Amount/2	(Amount/2)2	Variance
     Calibrator	0.25	0.125	0.016	0.016
     Cal. Drift	0.23	0.115	0.013	0.013

2. Sum of Variances (Sum of above items)   0.041
3. Standard Error (Square root of Sum of Variances)   0.202

* Note:   Variance = ( Σ )² = [n ∑ (X_i)²- (∑ X_i)²] / [n (n-1)]; where n is number of levels and X_i is value of i^th level.

( - ) Denotes test run was removed from the population of events to be analyzed (See Section D.7 for an explanation).