Light Rail has arrived in Seattle. The first link, a 14-mile electric railway system, opened in July 2009, making access from South Seattle to the downtown core, easier and more efficient. The ability of a light rail system to run along streets and share space with road traffic makes it an ideal choice for an urban setting. Seattle’s new light rail operates on embedded tracks at street level, along elevated structures and through tunnels. This study focused on the street level segment and the potential increase in noise levels for residents along the alignment.
In order to accommodate the light rail, an arterial through Seattle’s Rainier Valley neighborhood was widened to allow the trains to travel down the center of the roadway. Not only did this introduce a new noise source (the light rail train) into the community, it also slightly increased existing traffic noise as the widening of the street relocated traffic nearer to the homes. Figure 1 shows the configuration of rail and traffic and the proximity to residences.
Figure 1. The light rail travels down the center of the road
Studies were conducted to predict the level of increased noise for homes along the alignment. The metric used to define the light rail noise was the Day-Night Level (Ldn). This metric is based upon a 24-hour average of sound with a penalty added to any sounds occurring between the hours of 10:00 PM and 7:00 AM. The penalty is intended to reflect a higher sensitivity to noise during typical sleeping hours. The traffic noise metric was based upon an hourly Equivalent Level (Leq), which is the average sound in a one-hour period. The loudest hour is used to assess impact.
The initial program included 137 residences, with predicted exterior sound levels due to the project, that were high enough to be considered “impacted”. Sound Transit, the local Transit Authority, received a grant from the Federal Transit Administration (FTA) to launch a Residential Sound Insulation Program (RSIP), designed to reduce transportation noise intrusion into homes. Each home received an acoustical audit to measure the level of sound insulation provided by the existing construction. Figures 2 and 3 demonstrate two approaches to measuring the sound insulation properties of window elements.
Masurement approach #1
Masurement approach #2