Black Out the Noise

April 3, 2007

The general public has become more vocal in the demand for DOTs and other highway agencies in the U.S. to reduce the noise generated by highway transportation systems. The Federal Highway Administration (FHWA) has published noise standards for highway projects as 23CFR772. The FHWA Noise Abatement Criteria state that noise mitigation must be considered for residential areas when the A-weighted sound pressure levels approach or exceed 67 dB(A). To put that number in perspective normal conversation is 60 dB(A) and a motorcycle 50 ft away is 80 dB(A).

The general public has become more vocal in the demand for DOTs and other highway agencies in the U.S. to reduce the noise generated by highway transportation systems. The Federal Highway Administration (FHWA) has published noise standards for highway projects as 23CFR772. The FHWA Noise Abatement Criteria state that noise mitigation must be considered for residential areas when the A-weighted sound pressure levels approach or exceed 67 dB(A). To put that number in perspective normal conversation is 60 dB(A) and a motorcycle 50 ft away is 80 dB(A).

To accomplish this reduction in noise level, many agencies in the U.S. are building sound barrier walls at the cost of $1 million to $5 million per roadway mile. Recent research by the California and Arizona DOTs and the National Center for Asphalt Technology (NCAT) at Auburn University, Ala., has been directed at reducing the noise from transportation systems at their source, the tire/pavement interface. This research along with research in Europe has shown that it is possible to build pavement surfaces that will provide low-noise roadways. In January 2005, FHWA announced a Quiet Pavement Pilot Program to demonstrate the effectiveness of quiet pavement strategies and to evaluate any changes in the noise mitigation properties over time.

The noise generated by a vehicle can be classified into three general categories: power-unit noise (engine, fan, exhaust and the transmission, etc.); aerodynamic noise, which is related to the turbulent airflow around the vehicle; and tire/pavement noise. The power unit and the tire/pavement interface are the major sources of roadside noise. At low speeds, power unit noise dominates, while at high speeds-35 mph for automobiles and about 45 mph for trucks-the tire/pavement interaction governs at the roadside.

It has been shown that modification of pavement surface type and/or texture can result in significant tire/pavement noise reductions. Recent studies in the U.S. have verified the results of research done by European highway agencies and have found that the proper selection of pavement surface can be an appropriate noise abatement procedure.

How does it sound?

To be able to study tire/pavement noise it is necessary to have a scientifically reliable method for measuring the acoustical properties of an in-service pavement surface. There are two concepts currently being used for the measurement of roadway noise in the field:

1. Far-field or wayside measurements where the noise level is measured by microphones that are placed along side of the roadway.

2. Near-field or close-proximity (CPX) measurements where the noise level is measured using microphones that are attached to a vehicle or a trailer and are used to measure the noise level at the tire/pavement interface.

There are two approaches for conducting far-field or wayside measurements: statistical by-pass and time-averaged procedures. Both of these procedures consist of placing microphones at a defined distance from the vehicle path at the side of the roadway. See figure 1 for a typical wayside measurement test.

The choice of method depends on the nature of the traffic flow that is being measured. The statistical by-pass methods require the measurement of single vehicles isolated from others in a traffic flow; they can only be applied in situations of low traffic volume. This is the technique generally used for conducting specific research projects. The time-averaged method consists of measuring the noise level over a period of time (six hours or 24 hours or a week) to obtain a thorough understanding of the noise characteristics at a site. This technique is the most commonly used procedure for conducting noise studies associated with highway projects. One difficulty with wayside measurements is that they are time consuming. Another issue is that they determine the noise level at individual points, rather than giving a complete picture of noise in a wider area or neighborhood.

The use of near-field or CPX measurements for conducting noise studies is new in the U.S., although such methods have been used in Europe for years. One approach is a procedure developed by NCAT that mounts microphones inside a trailer that is towed over a pavement surface (Fig. 2). This equipment has been used to test more than 300 pavement surfaces in 23 states. The other approach is one that was originally developed by General Motors and is being used by the California Department of Transportation. It uses microphones that can be mounted on the side of a vehicle. The big advantage of these techniques is the ability to use them to conduct studies of entire highway systems, rather than just a few points.

Near-field measurements have been demonstrated to correlate well to controlled far-field measurements. But it must be realized that they measure only the tire/pavement noise component of traffic related noise. For assessment of the effect of pavement change on traffic noise community wide the wayside methods are needed, as they include both the power train and tire/pavement noise. However, for evaluating tire/pavement noise only the near-field procedures offer many advantages:

1. The ability to determine the noise characteristics of the road surface at almost any arbitrary site;

2. They can be used to check the state of maintenance, i.e. the wear or damage to the surface, as well as clogging and the effect of cleaning porous surfaces;

3. They are much more portable than the pass-by methods, requiring little setup prior to use; and

4. The testing can be done on the run at any selected speed (testing has been done at speeds that range from 48 to 112 kph).

A major question being addressed through research supported by the National Cooperative Highway Research Program is how these techniques relate to each other. It would be of immense value to be able to make a relatively simple near-field measurement and use that value to predict the noise level at a point 100, 200 or 500 ft from the roadway.

Act proper

Research in the U.S. and in Europe has found that the proper selection of the pavement surface can be an appropriate noise abatement procedure and recommend that when designing a low-noise surface, the goals are to:

1. Maximize the sound absorption at 1,000 Hz for high-speed roads and 600 Hz for low speed roads;

2. Minimize the airflow resistance to reduce the aerodynamic influences (or “horn effect” from the tires) by providing an open pavement surface; and

3. Maximize the smoothness of the surface that is in contact with the tire to reduce the impact of the pavement texture on mechanical vibrations of the tire.

Low-noise road surfaces can be built cost-effectively for safety and durability using small top-size aggregate for the surface course, or with a porous surface such as open-graded friction course (OGFC) with a high air void content.

Three types of HMA mixes are used for surfacing of high-volume highways: OGFC, dense graded HMA mixes (DGA), and stone matrix asphalt (SMA) mixes.

OGFCs or porous surfaces typically have lower noise levels than dense-graded surfaces. Studies have shown that an OGFC can reduce noise by 3 to 5 dB(A) when compared to non-porous HMA pavements. This is because the air voids in a pavement allow the air trapped between the tire and the pavement surface to escape (thus reducing the horn effect) and they provide for increased sound absorption. To achieve this improved noise performance, the pores need to be interconnected. The added advantage of these surfaces is that they also provide a reduction of splash and spray and an increase in frictional and hydroplaning resistance of HMA wearing courses.

The noise level from an OGFC surface is dependent on four factors: the interconnected air voids or permeability of the surface; the thickness of the OGFC layer; the gradation (the maximum size of the aggregate) of the OGFC; and the quantity of the binder used in the OGFC mixture (state of the art). Thus, to provide a low-noise OGFC pavement it is desirable to build thick layers with high air voids (15 to 20%) and fine-graded aggregates (3/8 in. maximum size). Most of the OGFC surfaces built in the U.S. have 3.4 in. to 1 1/4 in. thickness. Research is being done by NCAT this fall to evaluate the use of thicker layers. The quiet pavement program in use by the Arizona Department of Transportation has resulted in a reduction of about 10 dB(A) over the transverse tined Portland Cement Concrete pavements that were used to build the Phoenix freeway system.

NCAT has tested 90 dense-graded HMA mixes throughout the U.S. For these mixes, the noise level at the tire/pavement interface has ranged from 93.9 dB(A) to 101.8 dB(A). (Keep in mind that the noise level at the roadside is considerably lower than the level directly under or adjacent to the vehicle.) Generally it was found that the finer the mix (3/8 in.), the lower the noise level. A study done on I-435 in Kansas City showed that it was possible to attain a pavement with a noise level of 95 dB(A) with a dense-graded fine-grade mix.

Stone matrix asphalt mixes are seeing increased use in the U.S. to reduce rutting or permanent deformation. These surfaces have been shown to provide low-noise surfaces. They exhibit the porous nature of the OGFC to a limited extent; and, if a small top-size aggregate is used, they can produce pavements that have a noise level just slightly higher than a fine-graded OGFC surface, 95 dB(A) versus 93 dB(A).

Keep it on low

Research in the U.S. has been directed at two efforts over the last four to five years: the improvement of measurement techniques and the cataloging of the tire/pavement noise characteristics of various pavement types. The result of this work has been new testing capabilities and the identification of actions that can be used now to build low-noise pavements.

Future work is directed toward taking this knowledge and refining techniques for building low-noise pavements, such as the use of thicker OGFC pavements and the use of two-layer OGFC pavements with a fine-graded surface mix over a coarse-graded mix. Another area of interest is the use of even finer SMA mixes. Currently the finest mix is 9.5 mm, and work is now being done to evaluate the use of a 4.75-mm mix.

About The Author: Hanson is senior materials consultant for AMEC Earth and Materials.

Sponsored Recommendations

The Science Behind Sustainable Concrete Sealing Solutions

Extend the lifespan and durability of any concrete. PoreShield is a USDA BioPreferred product and is approved for residential, commercial, and industrial use. It works great above...

Proven Concrete Protection That’s Safe & Sustainable

Real-life DOT field tests and university researchers have found that PoreShieldTM lasts for 10+ years and extends the life of concrete.

Revolutionizing Concrete Protection - A Sustainable Solution for Lasting Durability

The concrete at the Indiana State Fairgrounds & Event Center is subject to several potential sources of damage including livestock biowaste, food/beverage waste, and freeze/thaw...

The Future of Concrete Preservation

PoreShield is a cost-effective, nontoxic alternative to traditional concrete sealers. It works differently, absorbing deep into the concrete pores to block damage from salt ions...