Archive for April, 2009

Trains and Cracks

Monday, April 27th, 2009

by Lance Meister

Will the train damage my home?  This is a question we get all the time in our business.  The short answer is “probably not.”  Here’s the long answer:

Trains do create vibration, and if you live close enough to the tracks, you can probably feel it when the train goes by.  However, there is a very large difference between the point at which a human feels vibration and the point at which vibration can cause damage to even the most fragile structures.  Human sensitivity to vibration starts at around 65-70 VdB.  This is the point where you start to feel that very low frequency rumble and are maybe a little annoyed by it.  Damage to structure starts to occur around 95-100 VdB, and this is only for minor cosmetic damage, such as small cracks in plaster.

Because vibration, like noise, is measured on a decibel scale, every 10dB increase in the vibration level is a ten-fold increase in the energy of the vibration!  So the difference between 65 VdB and 95 VdB is not a 50% increase in the vibration, but a 1000 time increase in vibration.  That’s a pretty big difference between being able to feel vibration and having vibration cause damage to a building.

Another way to put this into perspective is to compare the vibration from a train to other sources of vibration you might experience in your home.  The vibration generated by a person running up the stairs, or slamming a door is equivalent to, or greater than, the vibration caused by a train in the nearby walls and floor.  Of course, you can always yell at your teenage son or daughter to stop running up the stairs and slamming the door to their room!  But the point is this: your house doesn’t suffer catastrophic damage when this occurs.  Houses are built to withstand some vibration.

A common occurence and something you may have experienced yourself, especially near new transit or rail lines, is that a person feels a train go by, senses the vibration, and then happens to look at a wall or the ceiling and sees a crack.  The immediate (and perfectly reasonable) response is to associate the two things.  The reality is that the crack may have been there for a long time and was just never noticed.  Buildings settle over time, and this can create small cracks in plaster.  In addition, seasonal variations in temperature and even the lowering of the water table under your house can put stresses on the foundation and walls and create cracks.  An older home may already have all these issues sorted out, but a new home may take some time to settle, and small cracks may appear over time, whether or not you live near train tracks.

This is not to say that it is impossible for the vibration from a train to create superficial cracks.  If the train tracks are located very close and the trains are traveling at high speeds, there is a small potential for some minimal damage.  However, if you can just feel the vibration, or are only slightly annoyed by it, chances are it is well below the threshold for damaging your home.

Earth Day +1

Monday, April 27th, 2009

by Mary Ellen Eagan

I had good intentions of posting yesterday, but the day got away from me.  Perhaps it was meant to be, since we environmentalists can now bask in the warmth of spring (finally here in Boston!) and review the Earth Day announcements that are meaningful for the first time in my life (well, since Earth Day began).

Other signs that Washington (dare I say the US?) cares:

  • – The House Energy and Commerce Committee is holding hearings on the Waxman-Markey climate bill.  Yesterday, Ray LaHood testified and gave DOT’s support.
  • – Also yesterday, Senators Carper, Inhofe, Boxer and Kerry introduced legislation directing the EPA to study the environmental impact of black carbon and the most cost-effective ways to reduce its emissions (especially interesting since Inhofe opposes the EPA’s regulatory efforts on climate).
  • – And now even WalMart is on the Green bandwagon.  I heard a radio spot this morning on the environmental benefits of buying a double package of Cheerios – I find it interesting that they’ve found a way to make consumption virtuous!
  • – My five-year old spent last week learning about “Reduce-Reuse-Recycle” at pre-school.  She took it to heart and wanted to take home the paper towel from the public restroom.  Next challenge is her big sister, who claims to be an environmentalist, but takes 20-minute showers.

Here’s to hoping next year’s Earth Day will celebrate the passage of climate legislation and other real policy changes.

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).

OMEGA Community Noise Study: Indices to enhance understanding and management of community responses to aircraft noise exposure

Wednesday, April 15th, 2009

by Mary Ellen Eagan

A recent study conducted in the U.K. demonstrates what we have known anecdotally for years – that DNL is not an effective tool for communicating the impacts of aviation noise.  The study asked focus group members to evaluate a range of traditional and “supplemental” noise information.  The goal of the study was not to undermine existing contour-based noise metrics but rather to establish whether these could be enhanced if other explanatory information was added.

The study concluded the following:

  • -There was considerable variation among focus group members in their interpretation of different metrics used to illustrate the same noise environment.
  • -There was general dissatisfaction among members of the public with the aggregated indictors such as Leq and Lden.
  • -All the aggregated indicators required considerable explanation before participants understood the illustrations.
  • -A preference for metrics that disaggregate key elements of aircraft noise: namely, time, frequency of events and individual sound levels.
  • -A desire for a wider range of noise exposure illustrations, especially among members of the public living close to airports.
  • -Universal acknowledgement that bar charts, for specific locations illustrating the numbers of events within ranges of maximum sound levels for given periods of the day, were the most informative and easiest to interpret of all the metrics viewed. 
  • -Consensus that the flight path densities maps were the most visually attractive despite the lack of specific noise data contained therein.  To combat this, a number of participants suggested that this image could be overlaid on aggregated noise footprints such as N70 or Leq contours.
  • -That the public is more interested in site specific information that is easy to interpret in relation to their own personal exposure, rather than more complex images that may provide a comprehensive overview of the whole noise environment around an airport, as conventionally used by planners and decision-makers.

See figures below for examples of flight path density maps HMMH has prepared at Fort Lauderdale International Airport for their Quieter Skies Report.

Airline Jet Arrivals

Airline Jet Arrivals

Airline Jet Departures

Airline Jet Departures