Archive for the ‘DB’s dBs’ Category

Interest in Tire Pavement Noise Keeps Rolling at TRB Annual Meeting

Wednesday, February 16th, 2011

by Doug Barrett

The Transportation Research Board (TRB) held its 90th Annual Meeting in Washington DC from January 23-37. Although I’ve attended many of these meetings, the size and breadth of the annual meeting always astonish me. This year’s meeting set a new record (over 10,900) and, as usual, there were workshops and sessions covering everything under the sun related to transportation.

As the primary source of traffic noise, tire-pavement noise continues to be a hot topic in the highway noise world. Although the ADC40 Committee on Transportation-Related Noise and Vibration is just one small part of the annual meeting, in recent years the Committee has broadened its scope by co-sponsoring sessions with other committees. This year, ADC40 co-sponsored three workshops or sessions on tire-pavement topics. Sunday morning’s workshop titled “Implementing Noise-Reducing Pavement Research, an International Perspective: Making Pavement Research Results Work in Practice” was followed by Monday’s session on the “Effect of Asphalt Mixture Composition on Friction and Noise,” and Wednesday’s “Tire-Pavement Noise” session. Co-sponsors of these workshops/sessions included the Committees on Surface Properties-Vehicle Interaction (AFD90), Characteristics of Asphalt-Aggregate Combinations to Meet Surface Requirements (AFK40), and Pavement Maintenance (AHD20).

One topic covered during these sessions was the use of On-Board Sound Intensity (OBSI) measurements. OBSI has gained widespread acceptance throughout the U.S. as a standardized approach for quantifying tire-pavement noise. Recently, HMMH conducted OBSI measurements at locations throughout Virginia, on behalf of the Virginia Department of Transportation (VDOT). The VDOT measurements are part of an ongoing program to document the long-term benefits of pavement overlays. OBSI measurements, however, not only can document the noise reduction benefits of quieter pavements, but also can help to identify louder pavements. As the noise program manager for another DOT said, his state’s ongoing OBSI test program is not about finding the quietest pavements – it’s about identifying the loudest ones!

OBSI Measurement

OBSI Measurement

Scale Modeling for Shut-ins

Thursday, April 1st, 2010

by Doug Barrett

Not so long ago (OK, I’m talking about the 1970s and 1980s, maybe that was a long time ago!) acousticians sometimes had to build real 3-D models. Computer models today can account for shielding provided by terrain and structures, reflections off of and diffraction over and around obstacles, and acoustical impedance of different ground types. But just a couple of decades ago, these calculations were beyond the reach of the best computer models, and acousticians sometimes resorted to constructing actual scale models. These scale models would include buildings and other objects all constructed to scale, much like an architect’s model, but also would include microphones at the receiver locations and artificial noise sources.

Acoustical Scale Model Study (1979)

Acoustical Scale Model Study (1979)

Building a scale model of a house or a street is one thing, but acousticians also had to deal with creating properly-scaled sound waves. For example, one may have been interested in modeling 500-Hz traffic noise which, in the real world, has a wavelength of about two feet. However, in a 1:24 scale model (one inch = two feet), one had to use an artificial noise source with a wavelength of about one inch to model 500-Hz sound. To have a one-inch wavelength, the artificial noise source would need to generate a frequency of about 12,000 Hz.

Acoustical Scale Model Study (1979)

Acoustical Scale Model Study (1979)

Now stick with me, because I promise this all ties-in, but a few weeks ago, I came down with pneumonia and spent a lot of time flat on my back. One day, while recuperating, I was resting on my living room couch. Every now and then I would cough, and each time – almost exactly concurrent with my cough – I heard a faint ringing sound, not unlike a gong sounding in a distant room. At first I attributed this either to my fever or too much cough medicine, but upon reflection, another explanation occurred to me.

To digress for a moment, individuals living close to airports, particularly in locations alongside a runway or behind the start-of-takeoff roll for departing aircraft, sometimes complain of rattling windows or other loose objects, such as pictures on walls or light objects on shelves. The rattling is caused when low-frequency noise (LFN) generated by departing jet aircraft excites, or shakes, the homes. Sometimes the LFN may be difficult to hear directly (our ears do not hear LFN as well as they do higher frequencies), but the wavelengths may be about the same length as major elements of a house. For example, at 50 Hz, the wavelength is approximately 20 feet. When this occurs, LFN can cause elements of the house to resonate, or vibrate, at that particular frequency. This, in turn, causes windows and other loose objects to rattle.

Mobile

Mobile

Getting back to my coughs and the mystery ringing noise — we happen to have a vaguely Calderesque mobile hanging in our living room directly above the couch where I was resting. The entire mobile spans a few feet and consists of nylon fabric panels stretched across wire frames. The largest panels, which are about six to eight inches across, would resonate at roughly 1,500 to 2,000 Hz. One possible explanation is that this also was the frequency range of my coughing. Now imagine that these six- to eight-inch panels represent walls of a 12- to 16-foot wide room built for a 1:24 scale model study. Keeping with the 1:24 scaling comparison, my coughs would scale down to about 60 to 80 Hz, not unlike LFN generated by a jet aircraft. So maybe it wasn’t too much cough medicine after all. Perhaps my coughing had created a low-budget scale model for LFN rattle effects!

That Crackly Sound

Friday, December 18th, 2009

by Doug Barrett

Four more days to go until the longest night of the year and it’s starting to get cold here in New England!  Last night’s low was around 14 degrees F. in Boston, and last weekend, up in New Hampshire, it was colder still.  My fabulous spouse and I live about five miles from Boston’s Logan International Airport, and depending on which way the wind is blowing, sometimes we are treated to a steady stream of departing aircraft.  The other day, after a particularly cold morning, she asked why the aircraft sound different in cold weather – both louder and “crackly” as she put it.

The short answer is that it’s complicated.  As for the long answer, I suspect that she’s noticing the dependence of atmospheric absorption of sound on temperature and relative humidity.  Leo Beranek said it better than I could: “energy is extracted from a sound wave by rotational and vibration relaxation of the oxygen molecules in the air.  The vapor content of the air determines the time constant of the vibration relaxation… In addition, this ‘molecular absorption’ depends in a major way on temperature.” (Leo L. Beranek, Noise and Vibration Control, McGraw Hill, 1971, P.170)

It’s especially interesting that at any particular relative humidity, molecular absorption peaks at a given temperature.  In other words, one can’t simply make a generalization that sound “travels” better when it’s cold outside.  Adding to the complexity, the effects of temperature and humidity vary dramatically depending upon the frequency of the sound.  In general, wider variations exist at higher frequencies.  Notice on the first figure below (reproduced from Beranek) that there is relatively little sensitivity to temperature or humidity at 500 Hz – only about 3 dB per 1,000 feet across a temperature range of 100 degrees F.  In contrast, at 8 kHZ, the second figure shows a whopping variation of up to 50 dB per 1,000 feet over the same temperature range.

Atmospheric Absorption for aircraft-to-ground propagation (500 Hz, 1,000 Hz, 2,000 Hz)

Atmospheric Absorption for aircraft-to-ground propagation (500 Hz, 1,000 Hz, 2,000 Hz)

Atmospheric Absorption for aircraft-to-ground propagation (8,000 Hz)

Atmospheric Absorption for aircraft-to-ground propagation (8,000 Hz)

Regarding the “crackly” sound, here are some molecular absorption values that I pulled from the figures: At 500 Hz to 1 kHZ (approximately speech frequencies), one expects molecular absorption at a rate of about 1 to 2 dB per 1,000 feet of propagation on either a pleasant summer morning (60 degrees F. and 50% RH) or on a nippy winter morning (10 degrees F. and 20% RH).  That is to say, at lower to mid-range frequencies, there’s not a big seasonal difference.  At 8 kHZ, however, one expects absorption at a rate of about 20 dB per 1,000 feet with the summer conditions, but only about 6 dB per 1,000 feet with the winter conditions.  Less high-frequency absorption with cold, dry weather equals more “crackly” sounding!

FHWA Proposes Revisions to Procedures for Traffic Noise Abatement

Tuesday, October 13th, 2009

by Doug Barrett

On September 17, the Federal Highway Administration (FHWA)issued a notice of proposed rulemaking (NPRM) and request for comments related to 23 CFR Part 772.  For those involved with traffic noise analysis, 23 CFR Part 772 is the source from which all other state or local guidance flows.  While maintaining much of the existing regulation, the proposed changes would provide many clarifications, as well as some new additions and other changes that may influence traffic noise studies for many years to come.

Highway Traffic

Highway Traffic

First, a little background information for those not familiar with 23 CFR Part 772.  This portion of the Code of Federal Regulations addresses “Procedures for Abatement of Highway Traffic Noise and Construction Noise” and provides the basis for State transportation agency noise abatement standards in use throughout the U.S.  The regulation, originally developed as a requirement of the Federal-Aid Highway Act of 1970, applies to highway construction projects where a State department of transportation has requested federal funding for the project.  The FHWA provided further clarification and guidance in the 1995 document “Highway Traffic Noise Analysis and Abatement Policy and Guidance.”   While updating the 1995 document, the FHWA determined that certain changes to 23 CFR Part 772 were necessary.

The current NPRM proposes numerous changes to the existing regulation.  While soliciting comments on all the proposed changes, the FHWA is specifically seeking comments on three additions to the regulation:

  • The first would require highway agencies to determine a baseline cost-reasonableness value.  This value, which would require FHWA approval, may include the actual construction cost of noise abatement, cost per square foot of abatement, and either the cost per benefited receiver or cost per benefited receiver per dB(A) noise reduction.  In addition, highway agencies would be required to reevaluate these costs at least every five years.
  • The second addition would provide a change from past FHWA guidance regarding when third parties may contribute additional funds to a noise abatement measure or aesthetic treatment.  The NPRM would allow third party contributions only after the highway agency has determined that the noise abatement measure is feasible and reasonable.
  • The third addition would require each highway agency to maintain an inventory of all constructed noise abatement measures.

Other portions of the NPRM provide definitions or clarification of terms included either in the current regulation or in the 1995 policy document.  For example:

  • “Residence” would be defined to include all dwelling units, e.g., owner-occupied, rental units, and mobile homes.  In addition, “multi-family dwelling” would be defined.
  • “Special land use facilities” would be defined and would include picnic areas, recreation areas, playgrounds, active sport areas, parks, motels, hotels, schools, places of worship, libraries, hospitals, cemeteries, campgrounds, trails, and trail crossings.
  • The terms “planned, designed, and programmed” and “date of public knowledge” would be defined to clarify the cut-off date for considering new residential construction for noise abatement.
  • “Severe noise impact” would be defined.  Areas meeting this definition may be eligible for (1) exterior noise abatement that exceeds the allowable cost for feasible and reasonable abatement measures, or for (2) consideration of interior noise insulation for privately owned structures.
  • States would be required to define a “Substantial increase” criterion that is between 10 and 15 dBA over existing noise levels.
  • “Benefited” receptors must recieve “at least 5 dBA reduction” and benefited recievers must be included in determining cost reasonableness, even if not impacted.
  • The noise analysis study area would be required to extend at least 500 feet from all termini of the project build alternatives.

The NPRM also would require highway agencies choosing to participate in a Type II (retrofit) noise barrier program to develop an FHWA-approved priority system, based on a variety of factors, and rank the projects.  While some state agencies currently employ system-wide prioritization lists (HMMH has assisted the Massachusetts Turnpike Authority, the District of Columbia, and the New York State Thruway Authority in the development of their Type II prioritization lists), others currently construct Type II barriers based on ad hoc funding made available on a project by project basis.

This is a summary of only some of the proposed changes.  The full NPRM is available at http://edocket.access.gpo.gov/2009/pdf/E9-22386.pdf.  Comments on the NPRM must be received by November 16, 2009 and may be submitted via the Federal eRulemaking Portal.

TRB Noise and Vibration Committee Visits Home of the Pop-top

Wednesday, August 12th, 2009

by Doug Barrett

During the last week in July, transportation noise enthusiasts convened in Dayton, Ohio for the Transportation Research Board’s Committee on Transportation-related Noise and Vibration (TRB Committee ADC40) annual summer meeting.

2009_0307_MiniConceptA

The mission of the Transportation Research Board, one of six branches of the National Research Council, is to “provide leadership in transportation innovation and progress through research and information exchange, conducted within a setting that is objective, interdisciplinary, and multi-modal.”  With 31 technical presentations, a tour of the U.S. Air Force Research Laboratory at Wright-Patterson Air Force Base, vendor/sponsor displays, a chance to meet state DOT representatives one-on-one during a 90-minute “Speed Stating” session, and the official Committee meeting itself, this summer’s gathering was right on target with TRB’s mission statement.

ADC40 has long been an active committee, and I’m always impressed by how much valuable information is exchanged at these summer meetings.  With a total attendance of somewhere around  100 individuals, a large percentage of the attendees also are presenters, sponsors, exhibitors, or organizers.   The technical sessions are uniformly well attended, and the follow-up questions show that this crowd knows its audience well and presents topics that are pertinent to many in the group.  HMMH gave four presentations covering the range from a new ANSI standard for estimating noise-induced awakenings, to issues regarding highway noise barrier studies, to a noise-barrier design contest at Amsterdam’s Schiphol Airport.  As in years past, other hot topics included research on tire/pavement noise, news on FHWA’s  updated version of the Traffic Noise Model (TNM 3.0), and policy updates from FHWA and state DOTs.

There was time for some fun too, including a chance to see the Dayton Dragons take on the Peoria Chiefs (Class “A” baseball) on Tuesday evening.  For aviation buffs though, a visit to the National Museum of the U.S. Air Forcewas a stand-out treat.  The museum covers the history of aviation, from the Wright Flyer to moon landings and beyond, with as impressive an assortment of vintage aircraft as you’re going to find under one roof – ok, actually three enormous roofs.  On the bus ride back from the museum, the driver treated us to a bonus monologue of Dayton history and trivia.  One got the feeling that the driver was readily capable of expanding our 15-minute version to a much more complete history, with the level of detail dictated by the length of the ride.  At any rate, if not for this, I may never have known that Dayton gave birth not only to the Wright Flyer, but also to the step ladder and pop-top can.

A final note, many thanks to the Organizing Committee for all their efforts, and congratulations to the successful meeting!