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32 ATTACHMENT Proposed Method of Test for Measurement of Tire-Pavement Noise Using the On-Board Sound Intensity (OBSI) Method Disclaimer 2.2 ANSI Standards The proposed test method is a recommendation of the 2.2.1 ANSI S1.9-1996 (R2006): Instruments for the NCHRP Project 1-44 staff at Illingworth & Rodkin, Inc., and Measurement of Sound Intensity. it includes some modifications to AASHTO Designation Stan- 2.2.2 ANSI S1.40-2006: American National Stan- dard TP076-08, Provisional Standard Test Method for the dard Specifications and Verification Proce- Measurement of Tire/Pavement Noise Using the On-Board dures for Sound Calibrators. Sound Intensity (OBSI) Method. These modifications have 2.2.3 ANSI S1.11 Specification for Octave-Band and not been approved by NCHRP or by any AASHTO Commit- Fractional-Octave-Band Analog and Digital tee or formally accepted for the AASHTO specifications. The Filters. research conducted in this project provided a basis for this 3. Terminology provisional standard. 3.1 Sound intensity--The instantaneous product of acoustic pressure and acoustic particle velocity at a Proposed Standard Method of Test point with direction of propagation defined by the for Measurement of Tire-Pavement particle velocity vector. It corresponds to the acoustic Noise Using The On-Board Sound energy flow through a unit area and the units of Watts Intensity Method (OBSI) per square meter. 3.2 Sound intensity level--Ten times the logarithm of 1. Scope the time averaged sound intensity divided by the ref- 1.1 This document defines the procedures for measur- erence sound intensity (Iref) of 1 10-12 watts per ing tire-pavement noise using the on-board sound square meter [10*Log(I/Iref)]. intensity (OBSI) method. 3.3 Coherence--A measure of the linear dependency of 1.2 OBSI measurements at the source can be used to two signals with a value of 0 being no dependency, characterize the in-service noise performance of and a value of 1 being perfect linear dependence. pavements. Mathematically, it is the magnitude of the cross- 1.3 This procedure is anticipated to change as experience spectrum between two signals squared divided by the increases and additional research allows for the estab- product of the auto-spectrum of both signals. lishment of testing variables over a larger data set. 3.4 SIindex--The sound intensity-to-sound pressure level 1.4 This standard does not purport to address all of the index defined by subtracting the sound intensity safety problems, if any, associated with the its use. It level from the sound pressure level. is the responsibility of the user of this standard to es- 4. Summary of Methods tablish appropriate safety and health practices and A method is described in which a sound intensity determine the applicability of regulatory limitations probe is installed directly on a test vehicle using an prior to use. appropriate fixture and tire-pavement noise from a 2. Referenced Documents standard test tire is measured. Data is acquired over 2.1 ASTM Standards a 440-ft section of pavement at a steady test speed. 2.1.1 F2493 Standard Specification of P225/60R16 Where possible, a test speed of 60 mph is used with Radial Standard Reference Test Tire. alternative speeds of 35 and 45 mph depending on

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33 local conditions and regulations. Sound intensity 6.2.3 Tire inflation pressure shall be measured with levels are measured at the leading and trailing edge a device with an overall accuracy of 1 psi. contact patch of the test tire, either simultaneously or 6.2.4 Vehicle speed shall be measured with a device consecutively, and a minimum of two runs for each with an overall accuracy of 1 mph. Vehicle probe location are made. Data is acquired for 1/3 speedometers may be used if independently octave bands centered at 400 to 5,000 Hz and checked calibrated by a device with an overall accuracy to ensure that data quality criteria are met. The re- of 1 mph. sults from the leading and trailing edge positions for 6.3 Test Tire each run are averaged together and then the tire 6.3.1 Measurements shall be conducted using the averages for individual runs are averaged, resulting in ASTM F 2493 P225/60R16 (16 inch) Standard the overall A-weighted OBSI level and 1/3 octave band Reference Test Tire (SRTT). Test tires shall be levels that are reported for each pavement section. operated in only one rotational direction for 5. Significance and Use the test life of the tire. The test tire shall be 5.1 This test method defines procedures to quantify tire- mounted on the right side of the test vehicle pavement noise very near the source in isolation on a non-driven axle. from other vehicle noises. 6.3.2 Unless a specific inflation is required, the test 5.2 Using the method and the specified standard test tire shall be inflated to a pressure of 302 psi tire, measurements can be compared across different cold. pavements and among different users of the method. 6.3.3 The test tire shall be loaded with the existing, 5.3 The method can also be used to compare the tire- unloaded weight of vehicle plus the personnel pavement noise generation of different tires, includ- and equipment to perform the testing unless ing truck tires, if the intent of the measurements is to specified otherwise in the test plan. Loading of compare tire noise generation on some defined set of the test tire shall be 850100 lbs. pavements. 6.4 Test Vehicle 6. Equipment 6.4.1 The test vehicle shall provide a non-driven, 6.1 Acoustic Instrumentation non-steering tire/wheel mounting location. 6.1.1 The sound intensity level shall be measured 6.4.2 The tire and wheel at the test position shall using a sound intensity meter or equivalent rotate freely without extraneous noise of any measurement system meeting the require- kind. ments of ANSI S1.9-1996 (R 2006) and re- 7. Measuring Procedure quirements of ANSI S1.11. 7.1 Probe Location 6.1.2 The sound intensity probe shall consist of two 1 /2 phased matched condenser microphones 7.1.1 Sound intensity shall be measured at two installed on two 1/2 microphone preampli- points, one opposite the leading edge of the fiers. These shall be attached to a plastic probe contact patch and one opposite the trailing holder that provides a 16-mm center-to-center edge (Figure 1) spacing of the microphones as measured from 7.1.2 The leading and trailing edges of the contact the center of the microphone diaphragms re- patch shall be defined as the points where the sulting in a "side-by-side" SI probe configura- tire tread is 0.1 inch above the pavement sur- tion. The midpoint between these microphones face (Figure 2). shall be used in positioning the probe. The mi- 7.1.3 The measurement points shall be 31/4 inches crophones shall be protected from airflow above the ground. using a spherical foam windscreen approxi- 7.1.4 Measurements shall be made in a plane surface mately 31/2 in diameter. parallel to the sidewall of the tire with the 6.1.3 Acoustic calibration of the entire data acquisi- measurement plane 41/2 inches from the tire tion system shall be performed with a sound sidewall at the measurement location. calibrator that fulfils the requirements of 7.1.5 The probe shall be supported by a fixture capa- ANSI S1.40 Class 0 or Class 1. ble of maintaining it in the specified position 6.2 Non-Acoustic Instrumentation for the duration of the test. The fixture shall 6.2.1 Air and surface temperatures shall be measured be designed to minimize extraneous noise and with a device with an overall accuracy of 1.8F. wind turbulence. Measurements of the leading 6.2.2 Wind speed shall be measured with a device and trailing edge may be made concurrently with an overall accuracy of 5%. using a two-probe fixture configuration.

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34 Side View 7.3.2 Temperature--Air and pavement tempera- ture shall be measured at the beginning of the OBSI measurement set and monitored such that changes of 10F are detected. Testing shall be restricted to a temperature range from 40 to 105F unless the purpose of the testing is intended to evaluate the effects of tempera- Tire Contact Patch: ture. During the OBSI measurements, tire sur- 3 in. Leading Edge Trailing Edge face temperature shall be measured every half- hour or sooner if environmental conditions are rapidly changing. 7.3.3 Wind speed and direction--Average wind SI Probe 4 in. speed and direction shall be monitored and Locations Top View noted throughout the testing. No restrictions Figure 1. OBSI probe locations relative to on wind speed or direction shall be applied. test tire and pavement surface. However, data validity checks shall be used to identify when wind conditions have adverse effects on the OBSI measurement. 7.2 Acoustic Calibration 7.4 Test Section 7.2.1 Prior to each set of measurements, the sound The test section shall have the same nominal intensity probes and measurement system shall material and surfacing for its length. The test be calibrated with the acoustic calibrator. At the section shall be free of debris to the extent pos- end of each set of measurements or after 4 hours sible. The test section shall be nominally (whichever is shorter), the calibration shall be straight and free of dips and swells. repeated. If the second calibration differs from 7.5 Acoustic Data Acquisition the first by more than 0.2 dB, the set shall be 7.5.1 Sound intensity shall be measured using a "lin- repeated. ear average" (energy average) over a specific 7.2.2 Local values for air temperature and baromet- time interval. Generally, an averaging time of ric pressure shall be entered into the analyzer 5 seconds shall be used for a test speed of for proper calculation of sound intensity prior 60 mph. For 45 mph, the averaging time is to OBSI measurement. 6.7 seconds. For 35 mph, it is 8.6 seconds. Pave- 7.3 Environmental Conditions ment sections that are too short to allow this 7.3.1 Pavement dampness--The pavement shall be averaging time or are suspected of not being dry. For known non-porous pavements, this consistent throughout the specified shorter criterion shall be followed from visual inspec- averaging period are acceptable as long as all tion. For porous pavements, testing shall not Data Quality Criteria are met. be conducted on the pavement if it is known 7.5.2 The mean sound pressure level of the probe mi- that rain has occurred in the vicinity of the test crophone pair and coherence of the sound site within 48 hours. pressure signals between the microphone pair shall be measured. Microphone signals shall also be recorded for additional post-processing if required. 7.5.3 OBSI and other acoustic data shall be acquired at minimum for the 1/3 octave bands centered at 400 to 5,000 Hz. 7.5.4 Microphone signals shall be filtered by the 0.1" Thick A-weighting spectrum shape at the input to the analyzer. 7.5.5 For each 440-ft section of pavement tested, a minimum of two measurements for the lead- ing and trailing edge probe locations shall Figure 2. Orientation of the OBSI probe to the edge made. It is recommended that three or more of the tire contact patch--trailing edge position. measurements of each section be performed. If

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35 data quality criteria are not met for at least two and trailing edge of the contact patch. If a single of the runs, the measurements shall be re- probe is used, multiple runs shall be averaged to- peated until they are. gether arithmetically for the leading and trailing 7.6 Data Quality Criteria edges separately. The leading and trailing averages 7.6.1 Audio monitoring--The sound pressure sig- shall then be averaged on an energy basis. If dual nals shall be acoustically or visually monitored probes are used, the level of the two probes shall be av- as they are acquired. Any unusual noises such eraged on an energy basis for each run. The energy as rattles, excessive wind noise, stones embed- averages for individual runs shall then be averaged ded in the tire tread, etc., shall be observed and together arithmetically. the cause of such noises shall be identified and 9. Data Reporting remedied. 9.1 The specific acoustic data reported shall depend on 7.6.2 The direction of the sound intensity shall be the specific needs of the test as defined in the test positive for all data reported as valid. Positive plan and final report. As a minimum, the following direction is defined to be sound propagating tire-pavement average data shall be reported: overall away from the test tire. A-weighted OBSI level summed over the frequency 7.6.3 Mean sound pressure level minus sound in- range of valid data with the range noted; 1/3 octave tensity level (SIindex) shall be less than 5 dB and band levels over the range of valid data. greater than -1 dB in all 1/3 octave bands for all 9.2 Any exceptions to this stated OBSI procedure must data reported as valid. be reported. 7.6.4 Coherence--The ordinary coherence between 9.3 Other information that shall be reported include air the two microphones constituting the probe and pavement temperature range during testing, lo- shall be greater than 0.8 for all frequencies cation and description of the test pavement, the date below 4,000 Hz. of the measurement, period of the performance of 7.6.5 Overall A-weighted sound intensity levels for the measurements, and test speed. measurements made of the same pavement 9.4 Additional information to be made available on re- section shall be within 1 dBA. The range in quest shall include wind conditions during the mea- sound intensity level between runs shall be less surements, barometric pressure used for air density than 2 dB in all 1/3 octave bands for all data re- calculation, coherence, SIindex, probe configuration, ported as valid. and test vehicle make and model. 8. Data Processing 10. Precision and Bias OBSI data shall be processed into levels representing 10.1 Precision is expected to be within 0.5 dB. the combination of the noise sources at the leading 10.2 Bias is expected to be within 1.5 dB.