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RESEARCH
Sound is caused by air pressure fluctuations, generated by vibrations, and received by a person (or microphone). Its measurement is commonly expressed in terms of decibels (dB) that allow the quietest possible sounds to be easily scaled against the loudest sounds.
To define noise, one must think in terms of how sound is perceived. This is a little more complex, since we are all human, and thus have different opinions on what sounds are objectionable. For example, think of music… do you enjoy classical? Rap? Country? Heavy metal? While we may all hear the same sound, some will consider these as noise, and others will not. And while these definitions are important, it should be noted that in PSC, we sometimes use noise in place of sound.
We can detect sounds that range from 20 cycles per second (Hertz or Hz) to 20,000 Hz (20 kilohertz or kHz). However, the ear is most sensitive to frequencies between 250 Hz and 10 kHz.
Pavement noise, or more correctly tire-pavement noise, is the result of the mechanical interaction between the tire and the pavement surface. While it may seem simple at first, closer inspection reveals that this interaction is extremely complex. This is due in large part to the unique geometry of the tire tread along with irregularities in the pavement surface (for example, the presence of “chips” or possibly “grooves”). As a vehicle rolls, sound (noise) is generated, which then propagates away from the tire, partially reflecting off of the road, other cars, walls, and other obstacles.
Sound pressure involves microphone measurements that indirectly measure the air pressure changes as sound waves pass by. The second, sound intensity (sometimes called SI) is a little more involved but arguably more valuable. SI uses an array of specialized microphones to take measurements. Interpretation of the results involves sophisticated mathematics, but the results are more meaningful. SI not only reports “dB”, but also identifies directionality of the measurement. In other words, this technique allows for the effects of “background noise” to be significantly reduced.
The PB methods measure noise from the roadside, while CPX methods measure noise close to the tire-pavement contact zone. The SI method is a CPX method that recently has gained significant popularity. This method allows for a continuous measure of tire-pavement noise as one travels down a road. “Noisy” and “quiet” areas of pavement can be readily identified and quantified as a result. Pass-by methods are important too, however, since they allow for a sampling of different types of vehicles (and tires). The disadvantages are that they are only representative of a small area of pavement, and the test setup can often be difficult due to the need for a “clear zone” around the microphone (no walls or other obstacles that can reflect sound).
Vehicle engine and exhaust noise, among other sources, can sometimes be significant – particularly for heavier vehicles (trucks) or while driving at slow speeds. This is another reason why PB testing will remain an important test, since it measures a combined noise value, not just that generated due to tire-pavement interaction.
This will involve both better tire design and better pavement surfaces. Numerous “solutions” have been tried worldwide, and many advancements have been noted. For example, porous pavements (asphalt and concrete) have been used previously and have often demonstrated significant noise reduction. For asphalt pavements, “rubber” pavements have also shown promise, as have more “open graded” mixtures. For concrete pavements, some surface textures have proven beneficial, including burlap dragged textures, exposed aggregate surfaces, and longitudinal tining or grinding. Numerous surfacing techniques continue to be experimented with around the world. However, one must remember that in addition to their noise abatement capabilities, other considerations must also be made including: safety, smoothness, cost, and durability. How the noise-reducing ability of a given pavement surface changes over time should also not be overlooked.
Traffic noise measurements commonly fall into one of two categories: Wayside and Source. The Wayside methods measure noise at the “side of the road”, and are the most common traffic noise measurement technique in use today. While this technique provides a good indicator of the exposure that an abutter may have, its application is limited due to strict test requirements, not to mention that it only represents the noise level for a very short segment of pavement.
Meanwhile, source methods measure noise close to the tire-pavement contact patch. The two most common Source measurements include On-Board Sound intensity (OBSI) and Close Proximity (CPX). Both are relatively advanced techniques that are becoming increasingly popular since they provide for a continuous measure of tire-pavement noise as one travels down a road. “Noisy” and “quiet” areas of pavement can be readily identified as a result. Currently, FHWA and AASHTO are reviewing a draft standard for OBSI, and similar guidelines for CPX testing is in the works, which will likely be modeled after an existing ISO draft specification.
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