Archive for the ‘DB’s dBs’ Category

Venice to Ljubljana: A Drive-by Noise Barrier Tour

Thursday, June 11th, 2009

by Doug Barrett

Last month I had the opportunity to make the 2.5 hour drive from Venice, Italy to Ljubljana, Slovenia.  First rounding the broad lowlands of the northern Adriatic and then passing through the foothills of the Julian Alps, the Autostrada spans diverse cultures and landscapes.  As one  travels farther east, steeples in small towns change from Italianate towers to onion-shaped domes and the terrain transforms from coastal to mountainous.

While many visitors may focus on the centuries-old villages, ancient ruins, and timeless mountain vistas, the drive also offers quite a sample of modern highway noise barriers.  My fabulous spouse not only tolerated, but (when I was driving) even contributed to a high-speed windshield survey of northern Italian and Slovenian noise barriers.  Several features set these noise barriers apart from those typically found along highways in the United States:

Widespread use of sound-absorptive materials:  Although some U.S. transportation agencies commonly use sound-absorptive materials to minimize the effects of reflected sound (Virginia DOT and New York State DOT come to mind), other state DOTs do not.  Also, while cement-based absorptive materials are popular for highway barriers in the United States, metal panels were the most popular in this region.

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Use of tilted noise barriers: Tilting noise barriers back from the highway is another strategy to reduce multiple reflections of sound between parallel barriers.  This technique, although rare in the U.S., was common here.  Sometimes entire barriers were tilted, other times, each individual panel, or just the upper portion of the barrier was angled.

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Transparent materials: In some locations, scenic views were preserved by using transparent panels.  On many barriers, only the top portion was transparent, typically with the obligatory hawk decal to reduce bird strikes.

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I found the mixing and matching of these features to be especially interesting.  Because the transparent materials are not sound-absorptive, often they were paired with sound-absorptive lower panels.  In some cases, only the transparent upper portion was curved or tilted.  One barrier included a sound-absorptive lower section combined with a transparent upper section, bent towards the road to increase the barrier’s effective height.  The entire structure was capped by a sound-absorptive roll-top, also intended to improve the barrier’s noise reduction benefit.

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Hearing is Believing

Friday, May 8th, 2009

by Doug Barrett

Although “soundscape” may sound jargony compared to more familiar expressions like “landscape” and “viewscape,” the term has been around at least since the 1970s (R. Murray Schafer, The Tuning of the World, 1977).  Soundscape encompasses all aspects of an acoustical environment, and often is used to characterize natural sounds in unspoiled outdoor environments.  Consideration of soundscape, however, also is critical for urban spaces where people live, work, and play.  Within these settings, street traffic, aircraft overflights, and other noise sources can affect the character and even the intended use of an outdoor facility.

On May 19, the Acoustical Society of America and the City of Portland, Oregon Noise Control Office will host a joint symposium on “Urban Design with Soundscape in Mind.”  The symposium’s goal is “to provide planners, architects, engineers and the interested public with tools to use in addressing noise impacts in an urban setting.”  Topics will include “The Political Need to Consider Noise Impacts on Urban Livability” and “The Physical Need to Consider Noise Impacts on Urban Livability.”  A critical, related topic is the development of tools and techniques to help both decision makers and the general public consider future noise impacts, by hearing them with their own ears now.

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After years of struggling to describe the arcane world of noise metrics, logarithmic decibel scales, and frequency-weighting to the public, HMMH decided that there had to be a better way to answer the question “what will it sound like?”  Fortunately, recent advances in digital recording and processing technology provided the foundation for a solution.  Building on these tools, HMMH developed Virtual Soundscapes™ so that listeners can understand what a proposed project will sound like without the distractions of logarithms or even the mention of a decibel.

Using binaural (stereo in-ear microphone) recordings, acoustic models, and specialized sound mixing software, Virtual Soundscapes™ create incredibly realistic stereo simulations by layering together background soundscapes and other noise sources.  These recordings, which may be presented at meetings, installed on a touch-screen kiosk or computer, or posted on a website, allow decision-makers and community members to hear and modify future sound environments through a compelling, interactive experience.  (Use HMMH’s Soundscape Builder™ to create your own Virtual Soundscape™.)

Recently, Central Broward Transit and San Diego International Airport employed customized versions of HMMH’s Virtual Soundscapes™ at large public meetings.  In these venues, listeners experienced interactive simulations using a touch-screen kiosk (touch the screen to select a virtual airplane, train, bus, or barking dog – touch it again to open or close a sound insulated window) and high-quality noise canceling headphones.  Gene Reindel, Manager of HMMH’s Sacramento office, reported back after a successful meeting in San Diego, “I knew that things were going well when listeners reflexively looked skyward when they heard an airplane accompanied by barking dogs!”

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A Waterfall in Brooklyn (part 2 of 2)

Monday, April 20th, 2009

by Doug Barrett

Famous for its rush-hour traffic, the Brooklyn Queens Expressway (BQE or I-278) runs from southern Brooklyn to the Grand Central Parkway in Queens, carrying a daily traffic volume of approximately 140,000 vehicles.  The most notable bridge along the project corridor, the triple cantilever, is a reinforced concrete, multi-level structure built in 1948.  It carries six lanes of the BQE on two cantilevers, with the three eastbound lanes located above the three westbound lanes.  The third cantilever features the Brooklyn Heights Promenade, a pedestrian walkway with views of the East River and the Manhattan skyline.  Now over 60 years old, this structure is in need of rehabilitation (excerpted from the ACTT BQE Project workshop). 

Brooklyn Queens Expressway

Brooklyn Queens Expressway

In addition to being an ingenious way to squeeze a major highway through a narrow corridor adjacent to the historic Brooklyn Heights neighborhood, the triple cantilever is notable for being one of the most elegant and effective highway noise barriers in the U.S.  Much like the parapet along the edge of the Empire State Plaza, the Promenade itself acts as a noise barrier by blocking the direct sound-propagation path from traffic below.  As a result, pedestrians enjoying the Promenade may be completely unaware that a steady stream of cars and trucks is passing just below their feet.  HMMH currently is providing noise consulting services for the project scoping phase of the triple cantilever’s rehabilitation.  Last summer, we conducted a noise measurement program to document existing conditions and also assist in forecasting future noise levels.  In addition to the usual noise sources that often complicate urban traffic noise measurements (aircraft overflights, helicopters, sirens, barking dogs, curious passersby, etc.), we encountered something new during BQE measurements.

By chance, our measurement program coincided with an entirely unrelated public art project featuring the temporary installation of artificial waterfalls at various locations around New York City.  The closest waterfall to the project site was hundreds of yards away across an abandoned pier area, destined for future use as the new Brooklyn Bridge Park.  One day we conducted a set of measurements to document the fall-off in noise levels as one moved away from the edge of the Promenade.  Much to my delight, as I stepped back from the railing above the streaming traffic that afternoon, the roadway noise melted away and I was greeted by the sound of a cascading waterfall along the Brooklyn waterfront.

Noise Measurement Site

Noise Measurement Site

Waterfalls

Waterfalls in New York City

A Waterfall in Brooklyn (part 1 of 2)

Wednesday, April 15th, 2009

by Doug Barrett

Being a noise geek has its pluses and minuses.  On the upside, entertainment, or at least distraction, due to acoustical phenomena seldom is far away.  Whether it’s my fascination with the reflected “phipp…phipp…phipp” heard while driving windows-down past a row of utility poles or the tonal qualities of different pavement types, my fabulous spouse has long tolerated this quirk.  Every now and then however, we come across a “noise moment” that even she admits is really cool.

A few weeks ago, while in Albany, New York, attending a college hockey tournament, we took a late-afternoon stroll around the Empire State Plaza just prior to the finals.  This 98-acre government plaza, conceived by Governor Nelson Rockefeller to be “the most electrifying capital in the world,” was constructed between 1965 and 1978.  One of the Plaza’s more attention-grabbing features is the futuristic performance venue known as “the Egg.”  Like something from the Jetsons, this unique, cantilevered structure appears to hover overhead, but in reality is anchored by a central core that descends underground six stories.

The Egg

The Egg

The Egg

The Egg

At one point we stood at the edge of the Plaza looking down on a roadway passing below.  As we turned and walked away from the parapet, the traffic noise from below first decreased (due to the parapet acting as a noise barrier), but then increased again in volume.  Oddly, the traffic noise seemed to come not from the roadway, but from above us near the center of the Plaza.  Both of us were fascinated by the clarity of the auditory “image” that we heard as each individual vehicle passed from one side to the other – just as if we were standing by the side of the road as the vehicles zipped by!

Traffic noise reflecting off the Egg’s underbelly produced this surprising effect.  By blocking the direct sound-propagation path from the roadway, the 40-foot high retaining wall acted as a noise barrier.  At the same time, like a giant inside-out parabolic mirror, the smooth, convex Egg reflected traffic noise to the plaza below as the virtual image of each vehicle traced its way across the Egg’s surface.

After taking this in, we had to hustle to make it to the hockey game on time, and all thoughts of acoustical phenomena were lost to the action on the ice.  Well, mostly.  Did you ever notice how the “clank” of a hockey puck striking the goal post is audible even in an arena with thousands of screaming spectators?  But later, after the disappointment of “my” team’s loss began to wear off, I reflected some more on what we had heard beneath the Egg, and another historic New York State public works project came to mind (to be continued).