bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurement of throat impedance and far-field directivity for acoustic horns
Authors: Post John T. , Hixson Elmer L.
Published: October 2005

Experiments suitable for the measurement of throat impedance and directivity of acoustic horns were designed and implemented using an axisymmetric exponential and tractrix horn. The horns have equal length (55.9 cm), throat radius (2.54 cm), and mouth radius (27.1 cm). A modern implementation of the ‘‘reaction on the source’’ method was compared with a two-microphone method for the measurement of acoustic impedance. The indirect reaction on the source method was found to be less accurate but has the capability of insitu-type impedance measurements. The far-field pressure was derived by measuring the near-field pressure and mathematically extrapolating to the far field, a technique which has been used in the area of antennas since the early 1970s.
Complex image theory for sound propagation above ground
Authors: White Michael J. , Li Y. L.
Published: October 2005

A very simple and accurate method is introduced for the computation of the sound field above an impedance ground or a porous half-space. The procedure is based on complex image theory. Using Prony’s method, the slowly varying part of the integrand in the Sommerfeld integral is approximated by a short series in terms of complex exponential functions. Combining the series of exponential functions and the remaining integrand, the resulting integral can be exactly evaluated by the Sommerfeld identity. A new closed-form expression of the sound field is represented as the original source, its quasistatic image, and three ‘‘images’’ with complex amplitudes and positions. Avoiding numerical evaluation of the Sommerfeld integral makes these closed-form expressions computationally very efficient. Numerical results show that these closed-form Green’s functions are very accurate for the computation of the near and intermediate fields.
Determining the acoustic parameters of snow by impulsive waveform inversion
Authors: Albert Donald G.
Published: October 2005

The amplitude and waveform shape of atmospheric acoustic pulses propagating horizontally over a seasonal snow cover are profoundly changed by the air forced into the snow pores as the pulses move over the surface. Attenborough’s four-parameter, rigid-ice-frame porous treatment of snow is applied to the inverse problem, where the observed waveform is used to determine the acoustic parameters of the snow. Horizontally propagating acoustic pulses were generated using blank pistol shots fired 1 m above various seasonal snow covers and measured by surface microphones 60 m away. Various inversion strategies, both frequency and time domain, were investigated to determine automatically the acoustic parameters that accurately predict the observed waveforms. Preliminary results show that the effective flow resistivity and depth of the snow are the controlling parameters. [Work supported by the Directorate of Research and Development, U.S. Army Corps of Engineers.]
New fast field programs for a multiple-source system
Authors: Li Y. L. , White Michael J.
Published: October 2005

A new fast field program (FFP) has been developed for numerical computation of sound pressure from a phased multiple-source or multiple-receiver system in a layered medium. The new FFP is based on a far-field approximation. Using the stationary phase method, the twofold integral for sound pressure in the far field is reduced to a onefold integral. The onefold integral makes the new FFP very efficient and more practical than computation performed individually for each source. A criterion for validity of the far-field approximation is also given. The sound pressure of several phased-array systems in a layered medium has been computed. Numerical results from the new FFP are compared with some well-known results. The agreement is excellent.
A nanosecond acoustic source: Experimental investigation of pulsed dielectric breakdown of water and its dependence on pressure and conductivity
Authors: Jones Hugh M. , Kunhardt Erich E.
Published: October 2005

Spark gaps are finding use as underwater acoustic sources in sonar and prospecting applications. It is thus important to determine the influence of pressure and conductivity on the dielectric breakdown of water. Well defined rectangular pulses (80-kV, 3-ns rise time, 100-ns duration) have been applied to a gap (0.04–0.21 cm), between Rogowski profile electrodes, containing de-ionized, nondistilled water, de-ionized, distilled water, or sodium chloride solutions (0.001–1.0 M). Breakdown in these liquids has been studied at pressures up to 400 atm. Calibrated voltage dividers situated on the source and load sides of the test gap permitted measurement of the interelectrode potential and the current response. From these measurements, the time lag to breakdown, breakdown voltage, power input to the liquid, and temporal characteristics of the breakdown process have been determined. The breakdown time lag increases with increasing pressure and gap width, and decreases with increasing field. Moreover, it is weakly dependent on the conductivity of the liquid. An Ohmic current-voltage relation has been observed during the prebreakdown stage. A dynamical model has been developed to explain these results and is also presented at this meeting. [Work supported by ONR.]
Processes leading to cavity formation and dielectric breakdown in liquids subjected to high electric fields
Authors: Jones Hugh M. , Kunhardt Erich E.
Published: October 2005

The use of liquid-filled gaps as acoustic sources requires an understanding of the contributions from various microscopic processes to the dynamics of current growth, cavity formation, and dielectric breakdown. A model is presented for the development of cavities and dielectric breakdown in liquids subjected to uniform high amplitude electric fields with durations of microseconds or less. In this model, breakdown proceeds in four stages: formation of a nucleation site in the liquid near an electrode, growth and expansion of this site until the local density is reduced sufficiently for electron-impact ionization to occur, growth of an electron avalanche and its transformation into an ionizing front, and propagation of the front via a sequence of processes occurring in the region ahead of the front, namely, heating by electron injection, lowering of the liquid density, and avalanche growth and retardation. Expressions for the duration of each of these stages are derived and their behavior with pressure and field strength analyzed. From this, a critical energy criterion for breakdown is obtained. Comparison is made with experimental results for water, salt solutions, and the liquid noble gases, and for pulse durations in the microsecond and submicrosecond time scales. [Work supported by ONR.]
Radiation force, equilibrium position, and shape for acoustically levitated bubbles driven above resonance
Authors: Asaki Thomas J. , Marston Philip L.
Published: October 2005

The equilibrium positions and shapes of stably levitated bubbles on the Earth in an ultrasonic standing wave have been measured and calculated for bubbles larger than resonance size (radius≊140 μm). The method of levitating bubbles typically having radii ≳400 μm was previously described [T. J. Asaki, P. L. Marston, and E. H. Trinh, J. Acoust. Soc. Am. 93, 706–713 (1993)]. The equilibrium position gives a measure of the radiation force on the bubble. Trapped bubbles exhibit an oblate shape of primarily quadrupole distortion. They are positioned near to and above the axial pressure minimum in the levitation chamber. These results are in good quantitative agreement with predictions based upon the multipole projections of the acoustic radiation pressure on the bubble, taking into account the monopole acoustic response of the bubble. The monopole response is limited by inertia and radiation damping for the size range of bubbles considered. A simplified theory for the shape (based on the deformation resulting from the Bernoulli pressure of incompressible potential flow around a sphere) is generally consistent with the measurements for the smaller bubbles observed. [Research supported by ONR.]
Evolution of surface properties of acoustically levitated bubbles in aqueous solutions and seawater and sources of anomalous damping
Authors: Asaki Thomas J. , Marston Philip L.
Published: October 2005

The complex free-decay frequency was measured for quadrupole oscillations of acoustically levitated bubbles. Single bubbles were stably levitated for periods of up to 3 h. During this time the natural frequency and damping were measured at approximately 2-min intervals. Bubble oscillations were detected by means of an optical extinction technique. Each free-decay record was taken by abruptly terminating the amplitude modulation used to induce shape oscillations. The method is remarkably sensitive to the presence of impurities which adsorb onto the surface from solution. Damping associated with ‘‘clean’’ small bubbles agrees with predictions associated with viscous and boundary layer considerations for oscillations about a spherical shape. The complex frequency obtained for controlled bulk concentrations of surfactant (Triton X-100) agrees qualitatively with planar surface capillary-wave studies. Salt water solutions and Puget Sound seawater are also examined. Anomalous additional damping was found in association with larger bubbles (radius ≳750 μm) and possible sources will be considered. [Research supported by ONR.]
Acoustic microstreaming within a gas-filled bubble
Authors: Nyborg Wesley L.
Published: October 2005

When surface waves occur on the boundary of a gas-filled space surrounded by liquid, the resulting motions of the gas include velocity components parallel to the gas–liquid interface. Because these satisfy a nearly nonslip condition, boundary layers are established and small-scale acoustic streaming is generated in the gas phase. Approximate theory suggests that significant convection can occur, and may play a role in sonic enhancement of rate processes which occur at gas–liquid interfaces. For example, intrabubble streaming may contribute to the enhancement of rectified diffusion observed by Gould [J. Acoust. Soc. Am. 56, 1740–1746 (1974)].
A model for cavitation damage during ultrasonic cleaning
Authors: Qi Quan , Harris John G. , Johnson Robert E.
Published: October 2005

A model is proposed in this report to describe the potential growth of an existing crack in the structural component caused by collapsing bubbles during ultrasonic cleaning. A local pressure rise is generated by the collapse of a single bubble and the momentum of the collapse is described by the Kelvin impulse I(t). By assuming a uniform pressure distribution and using dynamic fracture mechanics, we obtain a simple relation that expresses the dynamic stress intensity factor K(t) as a function of the Kelvin impulse. Further simplification is achieved by assuming spherical bubble deformation in the description of the Kelvin impulse, as suggested by Blake and Gibson [Ann. Rev. Fluid Mech. 19, 99–123 (1987)], and this leads to a relation between the dynamics of a single bubble and the stress intensity factor K(t). The time history of K(t) is discussed for a completely collapsed bubble obeying the Rayleigh equation. Once the dynamical stress intensity factor exceeds the fracture toughness KIC crack will start to grow. Repeated action of collapsing bubbles eventually leads to the failure of the component. [Work supported by the Manufacturing Research Center of the University of Illinois.]
A computational model of auditory localization
Authors: Martin Keith D.
Published: October 2005

A novel computational model of localization has been implemented. Using a simplified model of the cochlea filter bank, maximum-likelihood estimates of azimuth and elevation are formed from interaural intensity and phase differences at energy onsets in multiple critical bands. Preliminary results show that the model is capable of resolving front/back confusions and other elevation ambiguities resulting from the ‘‘cones of confusion.’’ The onset sampling strategy is intended to model psychoacoustic phenomena such as the precedence effect and allows the model to deal with multiple sound sources and reverberant environments. Examples of the model’s ability to resolve ambiguous signals will be presented and compared with human performance. [Work supported by NSF.]
Spontaneous remediation of sensorineural hearing loss in an infant girl
Authors: Meier Richard P. , Zakia Renée A. E. , Champlin Craig A.
Published: October 2005

An infant girl (JD) was assessed as having a mild-to-moderate sensorineural hearing loss in her left ear and a moderate-to-severe sensorineural loss in her right ear, based on results of auditory brain-stem response (ABR) at age 3 months and soundfield testing at 4, 5, and 9 months (Better ear PTA=55 dB HL). JD was fitted with hearing aids binaurally. The one risk factor in her birth history is prenatal polydrug exposure, including cocaine. She had recurrent otitis media, but tympanometry and other evidence weigh against middle ear problems as the cause of her hearing loss. A follow-up ABR at 13 months revealed JD to have normal hearing, a result confirmed by audiometric testing of her unaided hearing thresholds (PTA=10 dB HL). The implication is that, between ages 9 and 13 months, JD’s hearing improved to normal levels, possibly as a result of delayed maturation of the peripheral auditory tracts. Early detection of hearing loss may reveal more children like JD who have sensorineural losses that spontaneously remediate with development. Such cases would highlight the need for repeated testing of unaided thresholds in hearing-impaired infants, especially if aggressive intervention is contemplated. [Work supported by NIH and the Texas Advanced Research Program.]
Acoustic evaluation of pharmacodynamics
Authors: Daly D. B. , Daly D. M.
Published: October 2005

Ability to classify sets of certain synthetic sounds fluctuates with vigilance. In patients with genetic hypersomnia, performance can fluctuate from well-defined classes with narrow transitions to intervals with wide transitions where only stimuli at extremes are identified consistently. LRCs were used to measure whether performance differed from chance and if so, whether it differed from a composite of normals [Daly etal., J. Neurophysiol. 44, 200–222 (1980)]. In a group of subjects independently treated for genetic hypersomnia (11–13 yr old: IQ≳120; meds: 15–20 mg methylphenidate, τ=2 h), rested untreated performance over 4-min periods in 24-min intervals changed ∼100-fold (from normal to periods with wide transitions). At 10–12 min following dissolution of single, sublingual 10-mg doses, transitions narrowed and performance began to improve (re: normal composite). Performance continued to improve for 10–15 min, then remained relatively consistent (in some cases within normal limits for 40–50 min). Performance returned to untreated levels in an orderly fashion: Well-defined transition areas gradually widened, then spread toward extremes. Under similar conditions subsequent dose-response functions for an individual differed little. Name brand and generic preparations apparently dissolve at different rates; some differences among individuals may reflect variations in excipients or compounding.
The effect of a diver’s glove on vibrotactile thresholds
Authors: Travis Jeffrey D. , Schoppe Joan
Published: October 2005

An experiment measuring vibrotactile thresholds in human subjects was performed. Vibration thresholds at frequencies between 100 and 400 Hz under normal conditions were compared with other researchers’ published results. To examine how diving gear would affect a sea diver’s vibratory sensitivity, vibrotactile thresholds were determined on the middle phalanx of subjects with and without diver’s gloves. Two kinds of diver’s gloves were tested, one designed for cold water and the other for warm water. It was established that diver’s gloves do have an effect on vibrotactile thresholds, and that the specific type of glove (cold water or warm water) alters the normal shape of the threshold curves in different ways.
Effect of voice quality on the tense/lax distinction for English vowels
Authors: Holt Lori L. , Lotto Andrew J. , Kluender Keith R.
Published: October 2005

For a variety of East and West African languages, voice quality covaries with tongue root advancement. In these vowel systems, an advanced tongue root vowel is produced with breathy phonation, whereas, a modal voice quality is used with nonadvanced tongue root. A potential explanation for this regularity, based on the interaction between acoustic effects of vocal-tract shape and of voice quality, was evaluated. Because it has been suggested that the advanced tongue root contrast is similar to the tense/lax contrast in English, male and female series varying perceptually from tense to lax were synthesized for several English vowels. Breathy versions of each series were created by increasing spectral tilt. Results from identification tasks indicated that, in general, breathy phonation led to more high vowels being identified as tense. In addition, the effect of breathiness was greater for vowels modeled after female productions. These results suggest that the covariation in African languages may be consistent with the general principle of auditory enhancement and adaptive dispersion. The findings may also be relevant to gender differences in voice quality. [Work supported by NIDCD Grant No. DC-00719 and NSF Grant No. DBS-9258482.]
Effects of lexical status on native and non-native English-speaking adults’ vowel perception
Authors: Michela Victoria L. , Randazza Lauren A. , Walley Amanda C. , Flege James E.
Published: October 2005

Monolingual, English-speaking adults and native Chinese (Mandarin) speakers who learned English as a second language heard stimuli from two ‘‘native,’’ synthetic continua, in which the vowels ranged from English /i/ to /i/ in the context /b–b/ or /b–p/. Thus the end points of the first continuum constituted an English word and a nonword (‘‘bib’’ vs *‘‘beeb’’); the reverse held for the second continuum (*‘‘bip’’ vs ‘‘beep’’). These same subjects also heard stimuli from two ‘‘foreign’’ continua, where the vowels ranged from English /i/ to a foreign (non-English) vowel /Y/ in the contexts described above. Thus the end points of the first continuum corresponded to a word and a nonword (‘‘bib’’ vs *‘‘bYb’’); both end points of the second continuum corresponded to nonwords (*‘‘bip’’ vs *‘‘bYp’’). After training on end points, subjects’ identifications of the nine stimuli of a given continuum were examined to assess whether: the Chinese speakers, like native English speakers, exhibit a ‘‘lexical bias’’ effect for English vowels (from the native continua); vowel categories not bounded by another native/English vowel (as in the foreign continua) expand outward or become better defined with increased exposure to English and/or lexical status. [Work supported by NIH.]
Helicopter field data compared with fast field program
Authors: Barnes Michael T.
Published: October 2005

A study was conducted to compare the fast field program (FFP) to data obtained from helicopters. The purpose of the study was to see how well FFP predictions would compare to helicopter data over ranges out to 20 km and determine if the FFP could be used to reliably predict the propagation conditions for acoustic arrays listening for helicopters. Over a period of 2 weeks acoustic data from several helicopters with various flight paths out to a range of 20 km was recorded. At the same time meteorological conditions were also noted. These included temperatures up to 400 m, relative humidity, barometric pressure, wind speed, and wind direction up to 2000 m. The results were very promising and in most cases provided a good comparison until the helicopter signal was buried in background noise.
When is downwind propagation really upwind propagation?
Authors: Noble John M.
Published: October 2005

For a long time, atmospheric acoustic propagation has used phases like ‘‘upwind propagation,’’ ‘‘downwind propagation,’’ and ‘‘crosswind propagation’’ to describe the type of propagation conditions present in the atmosphere. For short-range propagation (<3 km), the portion of the atmosphere between 50 and 100 m will primarily influence the propagation of sound. This region of the atmosphere is called the surface layer. Typically, through this region there is not much change in the mean wind direction. Therefore the terms mentioned earlier have some meaning for short-range propagation. This is due to the effective speed of sound for a given direction not varying much due to the small changes in the mean wind direction with height. For longer range propagation, this terminology could have little to do with the actual propagation conditions due to the effect of wind speed and wind direction variations with height above the surface layer. The purpose of this presentation is to discuss why this terminology is not appropriate and for what conditions it is appropriate for use in atmospheric acoustics.
Auditory-evoked potentials for assessment of hearing in marine animals
Authors: Ridgway S. H. , Carder D. A.
Published: October 2005

Brain-wave activity (EEG) even at minute levels recorded from the dolphin head surface may be processed in synchrony with sound to reveal an auditory-evoked potential (AEP). AEPs can provide objective information about the auditory system and many features are consistent across species so that experience with common laboratory animals and humans may be of help in evaluating responses. Although AEPs do not require a behavioral response, they may be compared with behavioral responses as sound is attenuated toward threshold. Some components of the AEP are unaffected by level of consciousness, allowing their use to evaluate hearing in sleeping infants and to determine brain damage or brain death. Auditory thresholds, and related information such as temporary threshold shifts, are critical for evaluating the potential impacts of ocean noise pollution on marine animals. Most species that are of concern, such as the great whales will not likely be brought into the laboratory so that their auditory system can be studied; however, many opportunities exist for brief studies when such animals become stranded or entrapped. Physiological studies, including AEPs, could go a long way toward providing critical information needed to define some limits for safe noise exposure for marine animals.
Progress in acoustic transducer design and calibration
Authors: Rasmussen Gunnar
Published: October 2005

The accuracy of acoustic measurements will play an increasing role, if legislation, prediction, and noise control are to be taken seriously. If the traffic intensity is doubled by year 2000, one should expect a 3 dB higher noise level, unless the emitted sound power from each vehicle is reduced by 3 dB. It is unlikely that emitted power will be cut in half, unless a number of sources the sum of which is half are attacked. This calls for better and more accurate measurements. Better monitoring equipment and better source location and source ranking equipment is needed. Outdoor monitoring has undergone a development to more rugged, stable, and accurately calibrated systems. Intensity measurements are getting out of the crib. Standards have evolved and calibration procedures are starting to be evolved. Instrumentation has been improved. It is important that intensity probes can now be produced to narrow specifications to fulfill the demand for precise and reproducible measurements; they are no longer selected from large populations, but manufactured to exact specifications. It is important to make proper calibrations, not just assume that certain parts of a probe are working properly. If intensity measurements shall gain in popularity it is important to develop the instrumentation from magic machines into practical tools for the practitioner. Practical systems, calibration, and applications will be discussed.
Free-field calibration and characterization of microphone systems
Authors: Nedzelnitsky Victor
Published: October 2005

Significant discrepancies occurred in results from the recent European intercomparison of free-field calibration of IEC type LS2aP (13.2 mm nominal diameter) laboratory standard microphones [K. Rasmussen and E. Sandermann Olsen, The Acoustics Laboratory, Technical University of Denmark (DTH) Report PL-07 (1993)]. Consequently, Dr. Richard Barham of the National Physical Laboratory, U.K. (NPL) visited NIST to resolve one such discrepancy between NPL and DTH by obtaining a NIST calibration of a microphone that also had been calibrated by NPL and DTH in the European comparison. All laboratories used the reciprocity method, with independently implemented apparatus at each laboratory. The NIST calibration agreed relatively well with that of DTH, but not NPL, if all laboratories used the same values for the frequency-dependent acoustic center positions of the microphones. The problematical DTH and NPL determinations of acoustic center positions for the type LS2aP microphone, which disagree with the values obtained by scaling the positions standardized for another microphone type, are considered. Various implications for the current draft IEC standard on primary free-field calibration are discussed. Selected NIST free-field comparison calibration methods and transducer characterization procedures, recently applied to relatively novel devices such as micro-machined silicon microphones, are described.
Noise exposures in U.S. coal mines
Authors: Seiler John P. , Valoski M. P. , Crivaro M. A.
Published: October 2005

Mine Safety and Health Administration (MSHA) inspectors conduct full-shift environmental noise surveys to determine the occupational noise levels to which coal miners are exposed. These noise surveys are performed to determine compliance with the noise standard promulgated under the Federal Mine Safety and Health Act of 1977. Data from over 60 000 full-shift noise surveys conducted from Fiscal Year 1986 through 1992 were entered into a computer database to facilitate analysis. This paper presents the mean and standard deviation of over 60 000 full-shift noise dose measurements for various underground and surface coal mining occupations. Additionally, it compares and contrasts the levels with historical noise exposure measurements for selected coal mining occupations that were published in the 1970s. The findings were that the percentage of miners surveyed that were subjected to noise exposures above 100%, neglecting personal hearing protectors, were 26.5% and 21.6% for surface and underground mining, respectively. Generally, the trend is that the noise exposures for selected occupations have decreased since the 1970s.
The impact of using a 3-dB vs 5-dB exchange rate on the predicted employee’s 8-h equivalent A-weighted sound-pressure level in three different types of industrial work environments
Authors: Royster Larry H. , Royster Julia Doswell
Published: October 2005

A noise exposure database was previously collected using a 5-dB exchange rate for three different types of industrial work environments: paper manufacturing, aluminum casting and rolling, and meat packaging. This database will be reanalyzed to estimate the impact on the percentage of additional employees that would have to be included in each company’s hearing conservation program if a 3-dB exchange rate replaces the present OSHA 5-dB exchange rate. In addition, the effect of using different combinations of criterion and action levels in conjunction with the use of a 3-dB exchange rate will be discussed.
Contribution of off-the-job noise to hearing thresholds of employees with occupational noise exposure
Authors: Royster Julia D. , Royster Larry H. , Calliari Pamela S. , Tennyson Kelly L.
Published: October 2005

Employers often assert that it is fruitless to protect employees from noise at work because workers’ off-the-job noise exposures are sufficient to cause significant hearing loss. Certainly, the lower the action level for including employees in a hearing conservation program (HCP) is set, the greater the potential contribution of nonoccupational exposures becomes. This paper will describe the types of nonoccupational noise exposures reported by employees in HCPs at several industries on annual auditory history questionnaires and in interviews. In order to evaluate the contribution of nonoccupational noise exposure to workers’ hearing thresholds, employees were grouped according to their self-reported amount of off-the-job noise. Results of analyses on age-corrected hearing levels will be presented.
Nonoccupational noise exposures and estimated daily Leq values for attendance at college basketball games and shopping centers
Authors: Royster Larry H. , Royster Julia Doswell
Published: October 2005

Noise exposure data were collected during 1988–1989 home basketball games at North Carolina State University. Thirteen exposure samples averaging 2 h in duration were collected utilizing Larson–Davis 700 noise dosimeters. Measured noise level results were as follows: average unweighted peak pressure was 133.1 dB, average maximum sound level (LA) was 115.2 dB, and average LAeq2 h was 97.1 dBA. The equivalent 40-h exposure level LAeq40 h was 84 dBA. As another example of nonoccupational noise exposure, between 1982 and 1994 seven shopping malls in four different cities were visited to establish the sound level distributions. Over 800 noise level samples were obtained. When the data were restricted to malls with at least 500 000 sq ft, it was determined that measured noise levels LA could be approximated utilizing the following equations (where N is the number of shoppers within a radius of 7.6 m of the measurement location): LA=61+10 log(N0.55), for N≤10, and LA=54+10 log(N1.20), for N≳10.
Investigation of lawn-maintenance equipment noise
Authors: Lepley Gordon T. , Bruce L. Marcelyn
Published: October 2005

As the lawn-maintenance industry grows in suburban residential areas, it has become a new and significant source of environmental noise and occupational noise exposure. Due to the short duration at any one location and public acceptance of this activity, there are few community complaints about the noise (except at unreasonable hours). This paper will concentrate on recreational and occupational exposure, where individuals are exposed to high noise levels for long periods of time. Most lawn-maintenance workers spend from 8–10 h per day exposed to A-weighted sound levels greater than 85 dB, and it appears that few employees wear hearing protection. Sound levels were measured and monitored at the operator’s ear and measured at a distance of 10 ft for the following equipment: lawn mowers, gas and electric edgers, gas and electric trimmers, electric blowers, and an electric hedge trimmer. A-weighted sound levels at the operator’s ear ranged from 82 to 102 dB. A comparison of equipment sound levels will be presented as well as a comparison of noise levels for gas versus electric lawn equipment. Finally, daily noise exposure for lawn-maintenance employees will be developed using the data.
Active noise reduction for circumaural hearing protectors by digital feedforward control
Authors: Pan G. J. , Brammer A. J. , Ryan J. , Zera J. , Goubran R.
Published: October 2005

A commercial analog active noise reduction (ANR) communication headset, constructed using a circumaural hearing protector, has been modified to explore the performance of a digital ANR system for this application. The apparatus retains one ear cup and cushion, together with the earphone and attached microphone of the commercial device. The sound field outside the ear cup is sensed by a miniature microphone attached to the exterior of the ear cup, and forms the ‘‘reference’’ signal. This signal is fed forward to an adaptive controller, which drives the earphone within the ear cup. Control of the sound pressure within the volume enclosed by the hearing protector is maintained by an ‘‘error’’ microphone attached to the earphone, using the filtered-X LMS algorithm. The error-path impulse response is identified off-line. A Texas Instruments TMS320c31 floating point digital signal processor has been used to implement the control system in real time. The noise reduction of the adaptive digital system will be discussed when the hearing protector is subjected to bandlimited noise in the frequency range from 50 to 1000 Hz. [Work done in collaboration with the Defence and Civil Institute of Environmental Medicine, Toronto, Ont.]
Weakly nonlinear propagation of N waves through turbulence
Authors: Lipkens Bart , Blanc-Benon Philippe
Published: October 2005

Reported here is a numerical investigation to explain experimental observations of the effect of turbulence on N wave propagation [B. Lipkens, Ph.D. thesis, Mech. Eng. Dept., Univ. of Texas at Austin (1993)]. An adaptation of Von Kármán’s spectral model for incompressible, isotropic turbulence is used to generate a statistical realization of a turbulent field. The 2-D, random, isotropic velocity or temperature fields consist of a collection of discrete Fourier velocity modes [Ph. Blanc-Benon etal., Theoret. Comput. Fluid Dynamics 2, 271–278 (1991)]. The nonlinear propagation model consists of two parts: (1) Linear geometric acoustics is used to trace rays through each realization of the turbulent field, and (2) a nonlinear transport equation is derived for the propagation along the eigenrays and solved by a Pestorius-type algorithm. The input waveform to the algorithm is a plane N wave similar to that used in the model experiment to simulate sonic boom propagation through a turbulent atmosphere. Statistics of peak pressure and rise time, parameters that determine the loudness of sonic booms, are calculated over 100 realizations. The effect of turbulence is to reduce the nonlinear distortion of the N wave. On average, turbulence reduces peak pressure and increases rise time. Qualitatively, the same conclusions are observed as in the model experiment.
A numerical study of random focusing for plane waves propagating in scalar or vectorial turbulent media
Authors: Blanc-Benon Philippe , Juvé Daniel
Published: October 2005

A numerical simulation has been developed to investigate the random focusing for an acoustic plane wave propagating through scalar or vectorial turbulent fields. The turbulence is represented as a set of realizations of random fields (temperature or incompressible velocity) generated by a limited number of random Fourier modes [M. J. Karweit etal., J. Acoust. Soc. Am. 89, 52–62 (1991)]. Through each realization the propagation of a plane wave is considered in either the geometric or the parabolic approximation. Using the ray equations the ray trajectories and the location of the instantaneous caustics are computed. The acoustic pressure field and the intensity fluctuations are obtained by solving a wide-angle parabolic equation. Ensemble averaging is then performed to evaluate the probability distribution of the occurrence of caustics and the variance of acoustic intensity fluctuations. For 2-D and 3-D scalar simulations the results demonstrate that the position of caustics as well as the maximum peak in the scintillation index are governed by universal parameters related to the transverse spatial derivatives of the correlation function of the fluctuating components of the turbulent medium. Results will be given to generalize the definition of these universal parameters for sound propagation in random velocity fields.
Modulation detection interference on presentation of two amplitude-modulated tones
Authors: Dolphin William F.
Published: October 2005

Psychophysical studies have shown that the processing of modulation in one frequency channel is interfered with when a similar modulation pattern is present in another frequency channel. To test for physiological correlates to this phenomena a sinusoidally amplitude-modulated (SAM) probe tone was presented in the presence of a simultaneously gated, modulated, or unmodulated interfering tone while recording from the scalp of anesthetized gerbils. Responses were recorded from the scalp, amplified, digitized, and averaged. The amplitude of the response was determined as the magnitude of the Fourier transform measured at the frequency corresponding to the probe fmod and fc. Responses in the presence of the interfering tone were compared to responses obtained to the SAM probe tone alone. The shape of the interference pattern was highly dependent on the modulation frequency of the interfering tone. With some modulation frequencies an increase in interference relative to control responses was observed at the probe fmod, while at other modulation frequencies either no interference, or even enhancement of the response was apparent. In contrast, at the probe fc interference patterns were very similar for all interference tone modulation frequencies.
Atmospheric sound propagation in a turbulent atmosphere over irregular terrain
Authors: Di Xiao , Gilbert Kenneth E.
Published: October 2005

A phase screen method for atmospheric turbulence and a cascaded conformal mapping technique for irregular terrain are used together to investigate scattering of sound by turbulence into a terrain-generated shadow zone. Comparisons with and without atmospheric turbulence are made for frequencies from 50 to 500 Hz. It is shown that with the near-ground turbulence model of Daigle and a hill similar to that in the Joint Acoustic Propagation Experiment (JAPE), turbulence effects are negligible below approximately 125 Hz. Above 125 Hz, there is considerable scattering into the shadow zone behind the hill. For the data currently available from the JAPE measurements, the theoretical predictions are in good agreement with experiment. [Work supported by the Army Research Laboratory.]
Developing standards for protecting marine mammals from noise: Lessons from the development of standards for humans
Authors: Bowles Ann E.
Published: October 2005

Models for protecting marine mammals from noise have been suggested that are analogous to human noise criteria, specifically (1) weighting functions that model species-specific auditory threshold functions (analogous to A-weighting); (2) threshold models for predicting the proportion of individuals that avoid a noise (analogous to the Schulz model of annoyance); and (3) the equal-energy hypothesis for predicting hearing loss. Models for reducing sleep interference, speech interference, and attention deficits might also be applicable. All these models will be reviewed. Unfortunately, human noise criteria do not apply to a number of effects that could occur in free-ranging marine mammals. Noise could affect nonauditory physiology. Noise could also mimic natural sounds (e.g., seismic impulses that are similar to tail beats), or attract marine mammals into dangerous areas (e.g., attracting killer whales to fishing gear). Given the paucity of research available on noise effects in marine mammals, standards may be difficult to establish, although they are badly needed. At present, management agencies have adopted extremely conservative noise criteria. The experience of regulating noise in human communities suggests that such stringent criteria cannot be enforced consistently. Solutions that have proved practical for human communities will be reviewed.
Marine mammals and ocean-acoustic experiments: A personal view from Monterey Bay
Authors: Flatté Stanley M.
Published: October 2005

Experience in presenting technical descriptions of the relationship between marine mammals and the Acoustic Thermometry of Ocean Climate project to environmental groups, business groups, students, and researchers is described. It is pointed out that the greatest gap in the knowledge base of the general public is the lack of realization that low-frequency acoustic noise in the ocean is at present dominated by anthropogenic sources. [Work supported by ONR Ocean Acoustics.]
Estimation of time-varying phase for multiple tones in atmospheric sound propagation
Authors: Burgess John C. , Havelock David I.
Published: October 2005

Atmospheric turbulence affects both the amplitude and phase of transmitted sound. Tones at different frequencies will be affected differently. In an earlier presentation [J. C. Burgess and D. I. Havelock, ‘‘Estimation of time-varying phase (instantaneous frequency) in atmospheric sound propagation,’’ J. Acoust. Soc. Am. 95, 2838(A) (1994)], the time-varying phase of a single tone was estimated by two methods, one based on an FFT with optimum data windows, the other based on the discrete Hilbert transform. In this presentation, the optimum data window method is extended to multiple tones. Accuracy of the method is examined using a synthesized signal with and without additive noise.
Adaptive enhancement of microphone array signals
Authors: Sydow Carsten
Published: October 2005

The signal-to-noise ratio of a speech signal picked up by a microphone array can be improved by adaptive post processing. Enhancement techniques known from single microphone or dual microphone signal processing, like noise canceling and spectral subtraction can be extended to a multimicrophone array system. The noise canceling technique and derived structures try to model the room impulse response by an adaptive transversal filter. Thus the performance of these algorithms is limited by the ratio of filter length to reverberation time and by the capability to track the nonstationary impulse response. Reduction of the noise of approximately 8 dB can be achieved with acceptable filter length in a stationary environment, but precautions must be taken to avoid canceling of the desired speech signal. The spectral subtraction method yields higher improvements in signal-to-noise ratio up to 12 dB, but due to false estimation of magnitude short time spectra the processed speech signal contains distortion known as ‘‘musical tones.’’ Different structures for adaptive post processing of microphone array signals based on both enhancement techniques are presented and discussed. Their performance in stationary and nonstationary environments is characterized by means of improvement of the signal-to-noise ratio and subjective speech quality.
Crosscoupling effects on large wideband arrays
Authors: Johnson Richard K.
Published: October 2005

The effects of crosscoupling on array performance can be well described by spatiotemporal or spatiofrequency correlation matrices. The data acquisition and analysis required for this approach, however, are very demanding tasks for large wideband arrays. The most important effects of crosscoupling within an array are latent in single element beam patterns which are relatively simple to measure. This paper describes the use of these single element beam patterns to predict array performance and to establish crosscoupling requirements.
A model for the holographic reconstruction of sound fields disturbed with temperature gradients
Authors: Gutiérrez V. Martín , Arenas B. Jorge P.
Published: October 2005

Holographic techniques are used to detect noise sources in a wide range of devices. Some of these techniques are commonly employed in the reconstruction of sound fields produced by automobile or motorcycle engines. These sound fields are characterized by having several, rather than single, temperatures; in fact, temperature gradients are frequently found in regions near the engine. These gradients produce changes in every point of the sound wave velocity. The mathematical model presented in this paper was designed to minimize the effect of assuming a constant sound velocity in the entire space in reconstruction of the sound field. It is based on the space transformation of sound field (STSF) technique [Hald, Jo/rgen, B&K Tech. Rev. 1, 1–50 (1989)], which uses a microphone array to measure pressure or determine the principal component of cross spectra over a hologram plane. The model scans the sound field in two planes. The first plane is used as an STSF initial condition (hologram plane), and the second plane is considered the boundary condition to Rayleigh integral’s propagation.
High-frequency acoustic bottom reverberation
Authors: Stanic S. , Kennedy E. , Goodman R.
Published: October 2005

A series of high-frequency bottom backscattering experiments were conducted off the coast of Panama City, Florida. Reverberation results will be presented as a function of frequency (20 to 180 kHz) and grazing angle (5°–30°). Geoacoustic parameters taken during the experiments will be used as inputs to an existing seafloor backscattering model initially developed by D. R. Jackson etal. [J. Acoust. Soc. Am. 79, 1410–1422 (1986)], and modified by A. Lyons etal. [J. Acoust. Soc. Am. 95, 2441–2451 (1994)]. The experimental data will then be compared to these model simulations.
Modeling the sonar backscatter by objects buried in very shallow water
Authors: Lim Raymond
Published: October 2005

Conventional sonar systems for bottom searching underwater coastal environments have limited long-range classification capabilities. This is especially true when objects searched for are significantly buried so that imaging is difficult. To investigate the feasibility of nonimaging methods of classification (e.g., based on isolating resonances in the echo from the desired object), models that faithfully predict the acoustic response of known objects in various states of burial are required. Because the response of the object can be strongly modified by the structure of its local environment, an appropriate model must account for effects due to propagating the acoustic field to and from the object, the scattering itself, and the interaction of the object with its local environment. The present talk will discuss how these issues can be handled in an exact fashion via a transition-matrix formulation both for fully buried and partially buried objects. In either case, the solution involves identifying a suitable set of global basis functions. [Work supported by ONR and the CSS IR program.]
Moderate frequency scattering from objects in an elastic bottom with rough interfaces
Authors: Lee Jaiyong , Schmidt Henrik
Published: October 2005

The diffuse reverberation from small scale bottom roughness severely affects the spatial coherence of the scattered field from near-bottom buried objects. To investigate this phenomenon theoretically, a numerical model is being developed for moderate frequency scattering from an elastic inclusion in a stratified, rough bottom. The model combines theories for object scattering and small-scale rough interface scattering. For the numerical implementation, a global approach is introduced, which combines three different methods into a consistent hybrid scheme. Boundary elements are used to compute the effects due to an elastic inclusion [P. Gerstoft and H. Schmidt, J. Acoust. Soc. Am. 89, 1629–1642 (1991)], wave-number integration provides numerical Green’s function for the horizontally stratified medium, and finally the rough interface scattering is handled using a self-consistent perturbational method [W. A. Kuperman and H. Schmidt, J. Acoust. Soc. Am. 86, 1511–1522 (1989)]. Compared to earlier implementations, the efficiency of the boundary element component of the model has been significantly improved by introducing a new analytical integration approach to the computation of the influence matrices. The modeling approach will be described and its application to realistic sonar scenarios will be demonstrated. [Work supported by ONR.]