30 August 1999

The Silent Highway

by Kate Melville

A whole university department dedicated to studying highway noise? Of course, but only in America.

Purdue University' Institute of Safe, Quiet and Durable Highways which starts in September is dedicated to understanding the physics of highway noise. According to Bob Bernhard, the institute's director, "Traffic noise is a persistent long-term environmental effect of highways…. Residents rarely complain about being able to see a highway, but they often complain about highway noise".

With the cost of sound barriers running at up to US$1 million per mile (sorry but you will have to do the translations for local currency and kilometers) this new institute may help both those affected and eventually the hip pocket of taxpayers (in theory).

What may be of real interest is that most acoustic pollution from vehicles does not come from engine noise but rather from the interface of tires and road surfaces. For many years engineers suspect several mechanisms maybe to blame, including:

Compressed air trapped between a tire's tread and the road surface bursts from the confining spaces, causing pops and whistling sounds

Block-like shapes in the tread smacking against the road surface, like so many small hammers.

Vibration from tread blocks and the underlying belts that radiates energy outward, producing sound (just like stereo speakers).

What the engineers at Purdue propose to do is to study both road surfaces and tires and to change their designs to produce less noise.

Research will include the use of lasers and sound waves to analyze noise-producing mechanisms in rotating tires. Interestingly the engineers will also study the porous pavements that are used in Europe to build quieter roads. This may sound pretty obvious but Bob Bernhard says, " European researchers have hypothesized the mechanisms of noise generation and built quiet, porous road surfaces. These hypotheses haven't been proved, and that's what we are going to go after".

Eventually the research work will expand to include research dealing with other transportation-related noise, like truck engines, as well as issues involving highway safety and durability.