After 36 years and a couple false starts, Honolulu is approaching another opportunity to build a mass-transit system.

Like the weather, noise is a subject on which many people have an opinion. How much noise the new transit system will produce has generated enough controversy that the City Council voted unanimously on Wednesday to go back to the drawing board and reconsider their decision to endorse steel-on-steel transit vehicle technology. To address citizens' concerns, the council decided that three vehicle technologies be re-evaluated: rubber tires, magnetic levitation (maglev) and steel wheels on steel rail.

The ultimate technology selected would have vehicles running on a dedicated guideway structure commonly referred to as an aerial structure. In the world today, steel-wheeled vehicles are by and large the most common technology in use. Rubber-tired vehicles have a much more limited use and are most often seen in people-mover systems at airports. A monorail, such as in Seattle or Disneyland, is a form of rubber-tired system. Today, aside from technology test tracks in Germany, Japan and the U.S., there is only one revenue-producing line (in Shanghai) using a maglev train, although a federal demonstration project is still under consideration for the U.S. Mainland.

For all three technologies, the noise emission characteristics have been measured extensively and are well known. In spite of this, some widespread misconceptions or lack of understanding of transit noise persist and to a large degree are colored by an individual's prior exposure.

All three vehicle technologies being considered for Honolulu generate some noise. The generic vehicle noise emission characteristics published by the Federal Transit Administration indicate that noise exposure from a single maglev vehicle is the least of the three, whereas a steel-wheeled intermediate-capacity vehicle is not quite twice as loud (about 8 decibels) as maglev. A rubber-tired vehicle is slightly, but not much, quieter (2 decibels) than a steel-wheeled vehicle. A meaningful comparison of noise levels for different transit technologies requires that the distance from the vehicle to the measurement point be the same and speed of the vehicles being compared be the same. Otherwise, adjustments to account for any differences must be made for it to be an apples-to-apples comparison.

One commonly held belief is that steel-wheeled vehicles could sound like the transit systems in New York and Chicago. The El in Chicago and the New York City subway have been around for a long time, 100 years in the case of New York. Consequently, even though the vehicles are modern, much of the infrastructure is old. In Chicago, the oldest aerial structures are of light-weight steel-plate construction. In more recent times, quieter, reinforced concrete guideway construction is used instead of this older type construction.

Transit vehicle technology and the means of attenuating noise have resulted in much quieter transit systems today than were in existence 100 years ago. Furthermore, environmental laws and regulations, which did not exist then, impose limits on the amount of noise that is acceptable in the community through which a new transit line will pass.

One of the big differences in steel-wheeled transit technology today is the use of continuously welded rail. The joints in the rails of yesteryear have been eliminated, resulting in a smoother and quieter ride. Another major change is that transit systems today with steel-wheeled vehicles employ regular maintenance programs geared to keep trains functioning optimally where it matters most, at the wheel and rail interface. A byproduct of this maintenance also helps minimize the main source of noise at the wheel and rail.

As rails wear, they require regular grinding to keep them smooth to achieve optimum traction. The steel wheels also wear and to keep them round and the treads within tolerance, they are machined and ultimately replaced with new ones when they can no longer be ground. It is in the interest of the transit agency to maintain smooth rails and round and smooth wheels.

Another source of noise associated with older steel-wheel transit systems is the squeal produced when trains traverse short-radius curves. Modern transit systems can be designed to avoid short-radius curves unless the tracks are in the street. A concrete aerial structure guideway inherently has large sweeping curves, thus avoiding this problem.

One of the most important measures that can be used to reduce the airborne component of wayside noise from trains is a sound wall. On aerial structures, relatively short walls (30 inches high) close to the track provide a substantial reduction in wayside noise. Additional noise reduction can be obtained by installing sound-absorbing material to the inside faces of the sound walls. A well-designed and constructed sound wall on an aerial structure can attenuate wayside noise so that it will sound as if it is less than half as loud as the noise without the wall (i.e., about 10 to 12 decibels less). In Vancouver, the transit system sound walls are transparent, which has the benefit of lessening the visual impact as well as reducing noise.

Hong Kong has gone to the extreme of covering sections of the aerial structure where tall high-rise condominiums are close to the tracks. The appropriate noise-control mitigation to use is dependent on site-specific circumstances. Where the ambient conditions are quiet and/or where residences, or hospitals are close to the transit guideway, more substantial mitigation may be necessary compared with noisier conditions (e.g., middle of a busy boulevard) or where residences are far away. For all three vehicle technologies, it is possible to achieve wayside noise levels that are about 70 decibels A-weighted (dBA) at 50 feet from the guideway. For comparison, a diesel-powered city bus typically produces a level of about 80 dBA at the same distance, which subjectively would sound twice as loud.

In summary, transit vehicles with rubber tires are only marginally quieter than steel wheel vehicles in general. But most importantly, with a well-designed and properly mitigated system it is possible to obtain a relatively quiet environment adjacent to a transit line with either steel wheel or rubber tire vehicles. The downside of maglev is its cost, but these are all decisions the City Council must wrestle with in making its final decision.