William Bowlby

A model for insertion loss degradation for parallel highway noise barriers

An algorithm is developed and validated for the prediction of the reduction, or degradation, of traffic noise barrier insertion loss when a second barrier is placed on the opposite side of the highway. The algorithm combines the basic emission, propagation, and barrier attenuation features of the Federal Highway Administration traffic noise prediction model with a geometrical acoustics approach for multiple reflections. The resultant model can accommodate any number of source lanes or receivers, three vehicle categories, and independently variable barrier heights and absorption coefficients. The model was validated against mathematical, scale model, and full scale field data received from other researchers, and has proved to be a good predictor of insertion loss degradation.

Seasonal Measurements of Aircraft Noise in a National Park

Jackson Hole Airport is the only air carrier airport located entirely in a National Park, that being Grand Teton. Concerns over noise impacts led to an Airport/Park agreement setting allowable levels, restricting flights and flight paths, and calling for compliance monitoring in the noise sensitive areas. This study documented seven days of continuous noise level measurements made at four sites in the Park during four consecutive seasons in 1985-86. Individual sound exposure level measurements at B-737/200 jets, Swearingen Metro turboprops, and general aviation aircraft permitted computation of “aircraft-only” day/night levels (DNL) at each site for each day. Measurement of half-hourly average sound levels permitted computation of “total” DNL for comparisons. Results showed general compliance with the DNL limits, except when winds necessitated northerly departures, in which case a single B- 737 could cause an exceedance. There were no statistically significant differences in mean aircraft DNL for each sit...

INTEGRATED DECISION SUPPORT TOOL FOR EVALUATING TRANSIT AND LAND-USE COORDINATION POLICIES

Policy makers in many urban areas have begun to embrace the principles of transit-oriented development as a means to create more livable communities and of light rail transit as a means to address congestion and air-quality problems. A policy-oriented screening tool for applying rigorous technical analyses to transit-oriented development policies to give decision makers meaningful information about a wide range of potential land-use, transit service and financing, and parking management policies is described. The decision support tool (DST) uses a simplified version of the regional travel forecasting model and includes an air-quality analysis module. The DST was validated to the Memphis regional travel forecasting model, and enhancements were incorporated for studying the effects of specific land-use, transit, and parking-policy assumptions. DST provides policy-related output data such as percentage of development in the corridor; transit ridership; annual transit operating cost, revenue, deficit, and cost recovery ratio; annual transit capital cost; and air-quality benefits. A sample application of the model for the Poplar Corridor in Memphis, Tennessee, is presented.

Determination of traffic noise barrier effectiveness.

This study evaluated the benefits of a noise control program involving construction of over

Field Evaluation of Reflected Noise from a Single Noise Barrier—Phase 1

13 million of traffic noise barriers along Interstate 440 in Nashville, TN. In addition, much of I‐440 was depressed in a rock cut with an objective of reducing the traffic noise levels. There were four basic objectives to the study: (1) determine barrier insertion losses at various sites for various geometric configurations; (2) determine change in community noise levels (from before the highway project); (3) study effectiveness of sound‐absorbing blocks used on one section to minimize multiple reflections between parallel barriers; and (4) investigate reasons for any differences between predicted and measured insertion losses. Measurements of A‐weighted average sound levels and noise reduction were made at over 40 sites. Calibrated predictions of levels without the barriers were also made. Differences in design geometries, terrain, and construction techniques made it impossible to generalize results from one mea...

ASSESSING NOISE REFLECTIONS OFF THE UNDERSIDE OF ELEVATED BRIDGE STRUCTURES: PROCEDURES USING THE FHWA TRAFFIC NOISE MODEL

This research studied the change in sound levels and characteristics caused by sound reflections off a reflective (non-absorptive) noise barrier on the opposite side of a highway. The analysis was done using measurements at five barrier sites and adjacent sites without a barrier under equivalent site, source, and meteorological conditions. Changes in broadband and 1/3 octave band equivalent sound levels (Leq) were studied, along with statistical descriptors, particularly the L₉₀ and L₉₉ metrics. Evidence is seen of increased Leq both across the road from the barrier and at positions between the road and the barrier. In some cases, even greater increases in L₉₀ and L₉₉ suggest a sustaining of the received sound due to the reflections. Spectrograms, visualizations of the spectral time history, reveal that the presence of the barrier causes sound levels to increase over a broad range of frequencies and causes higher sound levels to be sustained for a longer period of time. Psychoacoustic metrics were calculated and combined into three measures of annoyance. Annoyance metrics opposite the barrier tended to increase relative to the No Barrier site in lighter traffic where individual vehicle pass-bys are distinct.

KNOWLEDGE ACQUISITION FOR DOMAIN EXPERTS

Noise reflections off the underside of elevated bridge structures have the potential to increase traffic noise levels and to degrade the effectiveness of noise barriers at noise-sensitive land uses near proposed highway projects. Assessing the extent to which these reflections will occur requires identifying those roadway segments from which traffic noise will reflect off the underside of an elevated bridge structure and reach a receiver. The geometry involved in assessing noise reflections can often be complex, involving numerous receivers, roadways, and barriers. Because of this complex geometry, manual analysis is beyond the scope of most noise analyses. As a result, assessing noise reflections off the underside of elevated bridge structures has traditionally been difficult and time-intensive. A procedure can be used to assess noise reflections off the underside of elevated bridge structures using features of FHWA’s Traffic Noise Model. The procedure is intended to provide a screening level of analysis to determine whether or not noise reflections off the underside of elevated bridge structures will increase traffic noise levels or degrade noise barrier performance in noise-sensitive areas. If the results of the noise reflections analysis using this screening level procedure are found to increase traffic noise levels or degrade barrier effectiveness, more detailed analysis may be warranted.