Heavy vehicles (trucks and buses) are significant contributors to overall traffic noise levels; noise from one heavy truck is equivalent to noise from about 10 passenger cars. Therefore, a thorough understanding of how heavy vehicles emit noise is crucial to the prediction and mitigation of traffic noise. Noise from heavy vehicles comes from a variety of sources including exhaust stacks, muffler shells, exhaust pipes, drivetrain, air intake, cooling fan, tires, and aerodynamic characteristics. The relative contributions of these sources vary with vehicle type, operating conditions, and (for tire noise) the type of pavement. Noise wall heights are determined through modeling and depend on the distribution of noise source heights. Since truck and bus exhaust stacks are often the tallest noise sources, highway noise walls are typically designed so the top of the exhaust stack is obscured from the receiver sight. If, however, the top of the exhaust stack is not the major noise source, then shorter height noise walls could be built to achieve the same acoustical objective. The current treatment of heavy vehicle noise for highway conditions in the current FHWA Traffic Noise Model (TNM v.2.5) puts about 50% of heavy vehicle noise at the top of the exhaust stack, about 12 ft high, and the other 50% at ground level, regardless of vehicle speed or pavement type. A number of recent studies have, however, indicated that in modern trucks the majority of noise is generated lower and most may be tire/pavement noise. Others studies have also shown a strong dependence of overall heavy vehicle noise on tire type and pavement type. Therefore, it is essential to have more information about noise sources than can be observed in the standard pass-by measurements currently used to generate data for the FHWA TNM. A recent NCHRP Project 08-56 (NCHRP Report 635: Acoustic Beamforming: Mapping Sources of Truck Noise) developed and experimentally validated a practical acoustic beamforming measurement technique to localize and quantify the noise sources on typical trucks operating under actual roadway conditions. Sound maps and continuous distributions of the source heights were obtained for multiple truck pass-bys on a highway using a microphone array designed and fabricated for the study. With this advanced methodology now available, it provides the opportunity to conduct nationwide roadside truck and bus noise measurements to obtain statistical height distribution data on noise sources that can be used for traffic noise models.
The objectives of this research were: (1) to determine height distributions and spectral content for heavy vehicle noise sources, and (2) to establish and begin populating a new heavy vehicle (truck and bus) noise source database for incorporation into traffic noise models, including future versions of the FHWA TNM acoustical code. The research should collect data using a sample of commonly used heavy vehicles that reflect the current national fleet across the country and use data collection and analysis methods that reflect the current state of technology in acoustic beamforming. Data collection should be done on in-service roads or test track(s), on a range of commonly used pavement types (i.e., quiet and loud, flexible and rigid) with known On-Board Sound Intensity (OBSI) levels. OBSI measures should conform to AASHTO TP 76, Standard Method of Test for Measurement of Tire/Pavement Noise Using the On-Board Sound Intensity (OBSI) Method.