Judith L. Rochat

Two-Dimensional Focusing of Sonic Boom Noise Penetrating an Air-Water Interface

In the late 1960s and early 1970s research on supersonic aircraft determined that sonic boom noise would penetrate the surface of the ocean. It was assumed that the surface of the air-water interface was perfectly flat. Today, sonic boom noise is once again a topic of interest. The present work concentrates on two-dimensional focusing and defocusing, caused by ocean surface swell, of the penetrating sonic boom waveform. A finite difference algorithm is used to calculate tbe pressure levels underwater due to a rounded sonic boom waveform interacting with the water interface. Numerical results are consistent with the predictions of known theories involving 1) the existence of pressure disturbances underwater and 2) their penetration depth being a function of the aircrafts speed. These calculations also indicate that 1) the swell of the ocean surface focuses and defocuses the waveform with increasing effect as the ocean wave height increases and 2) the percent change (from a flat ocean interface to a wavy ocean interface) in pressure values due to the swell increases with increasing Mach number.

Focusing of sonic boom noise penetration into a homogeneous wavy ocean: Complex surfaces and wavelength comparisons

The last decade has seen a revived interest in the study of sonic booms; this is due to an upcoming new breed of supersonic passenger aircraft along with a heightened awareness of marine environmental issues. Sonic boom noise penetrating into a flat, homogeneous ocean is a topic several researchers have already addressed. The primary goal of the authors’ research is to study the effects of realistic ocean features on noise penetration. These features include a wavy ocean surface and bubbles near the surface. This presentation will focus on a wavy ocean surface, specifically on somewhat complex surface profiles and on sonic boom effective wavelength/ocean wavelength comparisons. Results using finite difference simulations indicate that a somewhat complex wavy ocean surface profile slightly augments the underwater pressure field (as compared to a flat surface); this result was also found when studying simple surface profiles. Some of the trends observed, however, were not as obvious as seen when studying th...

THE USE OF COLOR IN THE SCIENTIFIC VISUALIZATION OF ACOUSTICAL PHENOMENA

Color is commonly used in representing scientific data, especially as color output devices have become widely available. The misuse of color can lead to confusing or even misleading representations of data. The purpose of this paper is to provide a review of guidelines for using color effectively, specifically aimed at the computational acoustics community. The question of whether or not to use color at all is initially raised, and gray-scale is suggested for appropriate cases. Human color deficiencies in technical audiences are then described, and the consequential choices of equally-accessible color schemes are discussed. The topics of human color perception models, hypsometric tints, contoured plots, and histogram equalization are introduced. Color palettes specifically suited for acoustical phenomena, such as linear and dB magnitudes, scales for positive and negative data, circular palettes for phase, and methods for illustrating different classes of phenomena (e.g., wave types) on a single plot, are discussed in detail. Color examples and Mathematica code are provided, and final recommendations are made for choosing effective color schemes.

The effects of fuzzy attachments on compressional and shear waves in a plate

Using the theory established by A. D. Pierce et al. [ASME Paper 93‐WA/NCA‐17 (1993)], regarding fundamental structural‐acoustic idealizations for structures with fuzzy internals, the effects of fuzzy structures on different wave types are examined. In this problem, the structure that undergoes vibrations is a rectangular elastic plate mounted in an infinite baffle. On one side of the plate is a fuzzy structure, represented as a random array of point‐attached spring‐mass systems. The known theory explains the effect of these attachments on bending waves in the plate. In this presentation, the theory is extended to isolated compressional and shear waves and predicts that, in either case, the fuzzy structure can be modeled in the system solely by (1) an added frequency‐dependent mass and (2) an added frequency‐dependent damping. These results are similar to those for bending waves. [Work supported by ONR.]

Effective flow resistivity of highway pavements.

In the case of highway traffic noise, propagating sound is influenced by the ground over which it travels, whether it is the pavement itself or the ground between the highway and nearby communities. Properly accounting for ground type in modeling can increase accuracy in noise impact determinations and noise abatement design. Pavement-specific effective flow resistivity values are being investigated for inclusion in the Federal Highway Administration Traffic Noise Model, which uses these values in the sound propagation algorithms and currently applies a single effective flow resistivity value to all pavement. Pavement-specific effective flow resistivity values were obtained by applying a modified version of the American National Standards Institute S1.18 standard. The data analysis process was tailored to allow for increased sensitivity and extraction of effective flow resistivity values for a broad range of pavements (sound absorptive to reflective). For porous pavements (sound absorptive), it was determined that examination of the measured data can reveal influence from an underlying structure. Use of such techniques can aid in the design of quieter pavements.

Federal Highway Administration (FHWA) Roadway Construction Noise Model (RCNM)

Roadway construction is often conducted in close proximity to residences and businesses and should be controlled and monitored in order to avoid excessive noise impacts. To aid in this process, the Volpe Center Acoustics Facility, in support of the Federal Highway Administration (FHWA), has developed a construction noise screening tool. The FHWA Roadway Construction Noise Model (RCNM) is a newly developed national model for the prediction of construction noise. The model is based on the construction noise prediction spreadsheet developed for the Central Artery/Tunnel Project in Boston, MA (CA/T Project or ‘‘Big Dig’’) by Erich Thalheimer of Parsons Brinckerhoff Quade & Douglas, Inc. The CA/T Project is the largest urban construction project ever conducted in the United States and has the most comprehensive noise control specification ever developed in the United States. RCNM incorporates the CA/T Project’s noise limit criteria and extensive construction equipment noise database, where these parameters can...

Variability of pavement noise benefit by vehicle type

The Volpe Center Acoustics Facility, in support of the California Department of Transportation (Caltrans), is participating in a long‐term study to assess several types of pavement for the purpose of noise abatement. On a four‐mile stretch of a two‐lane highway in Southern California, several asphalt pavement overlays are being examined. Acoustical, meteorological, and traffic data are collected in each pavement overlay section, where microphones are deployed at multiple distances and heights. Single vehicle pass‐by events are recorded primarily for three vehicle types: automobiles, medium trucks, and heavy trucks. Data are analyzed to determine the noise benefit of each pavement as compared to the reference dense‐graded asphaltic concrete (DGAC); this includes a modified Statistical Pass‐By Index as well as average Lmax values for each vehicle type. In addition, 1/3‐octave band data are examined. Automobiles and heavy trucks are the focus of this paper, where benefits due to pavement will be presented fo...

Highway traffic noise measurements compared to predictions from FHWA’s Traffic Noise Model

In 1998, the United States Federal Highway Administration (FHWA) released a new tool for highway traffic noise prediction and noise barrier design, the Traffic Noise Model (TNM). In order to assess the accuracy and make recommendations on the use of TNM for the FHWA, the Volpe Center Acoustics Facility performed numerous roadside measurements, obtaining over 100 h of traffic noise data from highways around the country. For each site, acoustical, meteorological, and traffic data were collected simultaneously throughout the measurement period. Spectrum analyzers were used to collect 1/3‐octave band A‐weighted equivalent sound levels, and the microphones were deployed at distances from 50 to 1300 ft from the roadway and at two heights off the ground, the number of microphones used being site dependent. In comparing the measured and TNM‐predicted sound levels, results indicate that TNM is, on average, within 1 dB of the measured data for all types of sites combined. Grouping the data by site types shows how w...

Effects of wind‐wave generated bubbles on sonic boom noise penetration into the ocean

Supersonic aircraft, particularly the projected high speed civil transport (HSCT), fly over the ocean to minimize the sonic boom noise impact on land. This noise interacts with the ocean surface, causing an underwater pressure disturbance. In steady, level supersonic flight at less than Mach 4.4 over a homogeneous ocean, the underwater disturbance is a decaying evanescent wave persisting in depth to 100 m or more. Realistically, however, the ocean is not homogeneous and contains wind‐wave generated bubble clouds near the surface. In the current research, a two‐dimensional finite difference simulation is used to predict underwater sound levels due to a sonic boom impinging on an ocean with bubble layers near the surface. Each of the bubble layers has an approximate sound speed appropriate for its corresponding depth; bubbles in the ocean can cause the sound speed to drop as low as the speed of sound in air. In this presentation the effects of bubble layers on the sonic boom noise penetration will be descri...

Sonic‐boom noise penetration into the ocean: 1995 update

This talk is an update to a previously given presentation [ 1850–1851 (A) (1993)] in which a first‐pass model for the penetration of sonic boom noise into the ocean was described. Since that presentation was given, a number of refinements to the theory have been identified, the primary of which is the effect of aircraft Mach number on the duration of the sonic boom. The general trend for aircraft flying at a fixed altitude is that the evanescent acoustic wave underwater due to a sonic boom penetrates more deeply for increased aircraft speeds [ 159–162 (1995)]. Other refinements currently under development include appropriate initial models for sound level decay as a function of depth and for focusing and defocusing of the penetrating sonic boom noise by ‘‘frozen’’ sinusoidal and trochoidal ocean surface waves. [Work supported by NASA Research Grant NAG‐1‐1638.]