1
TECHNICAL NOTES AND RESEARCH BRIEFS iterative technique is good for long wave- lengths (kR•<<l) and any ratio of wave- numbersin the scatteringand surrounding media. For shorter wavelengths and small ratio of wavenumbers (e.g., k•/k=l.1, where k• is the wavenumber in the scatter- ing medium), it givesa good approximation to a0 for the entire rangeof kR• considered (0_<kR•_<2•r). For shorter wavelengths and larger ratioofwavenumbers (e.g., k•/k - 1.5, 2.0), it gives a good approximationto a0 out to approximately kR•=3,r/4. More general problems using this method are also discussed. Available from Clearing- house FSTI, Springfield, Virginia 22151, as No. AD 696528. Acoustical Characteristics of Fibrous Absorbent Material. M. E. Delany and E. N. Bazley, National Physical Labora- tory, Teddington, England. An extensive investigation into the acoustical properties of fibrous absorbent materials has been carried out. Measured values of character- istic impedance and propagation coefficient are each normalized in terms of power-law functions of frequency divided by flow resistance.The resulting empirical rela- tionsare interpretedin termsof parameters characterizing the material and are each comparedwith theoretical predictions for an idealized material. For most practical applications, a knowledge of the flow resistance allows use of the normalized data and eliminates the need for routine impedance-tube measurements. Available from Clearinghouse FSTI, Springfield, Virginia 22151, as No. N 69-40148. Wave Propagation in an Inhomogeneous Medium. H. Watson, Jr., Southern Metho- dist University, Dallas, Texas. The effects of a high concentration of particulate matter held in suspension in water on the progrationof an acoustic wave in water is discussed. Expressions for the average wave speed, average pressure,and incoherence of the pressurefield are obtained for an incidentharmonic pressure wave on a large number of discrete, suspended scatterers in water. The wavelength of the wave is assumed to be largecompared to a scatterer diameter. Results are compared with experiments of acousticwave propagation in sand-water mixtures. Available from Clearinghouse FSTI, Springfield, Virginia 22151,as No. AD 698296. The Dynamic Response of Various Media. M. Anliker and Chi-Chang Chao, StanfordUniversity, Palo Alto, California. This report contains a summary of the research performed under the following topics: (1) a theoretical analysis of waves propagatingin a bounded,heterogeneous elastic medium, (2) the transient response of an elastic half-space to a spreading surface load, (3) propagation of stress waves in a homogeneous isotropic wedge, (4) seismic wave radiation from sources moving over a finite distance, (5) the dynamic response of a fluid-filledcircularcylindrical shellto pressure waves while subjected to a uniform external pressure, and (6) trans- mission of sounds and pulse wavesin large arteries and veins. Seven publications relevant to the research effort are also listed.Available from Clearinghouse FSTI, Springfield, Virginia 22151 as No. AD 696459. Acoustic Characteristics of a Glass- Filament-Wound Pressure Vessel. A. M. Young and J. F. Prandoni, Naval Research Laboratory, Underwater Sound Reference Division, Orlando, Florida. Acoustic insertion loss of a glass-filament- wound pressure vessel intended for trans- ducer calibration varies significantly as a function of frequency, position, and hydro- static pressure. The variationsare believed to be due to voids in the glassresin and in the glass-resin/rubber-liner interface, which give rise to large changes in the character- istic impedance of the composite walls as a function of the same variables. Mechanical behavior of the vessel under hydrostatic pressure required a reduction of the specified maximum operating pressurefrom 2000 to 1000 psig. Available from Clearinghouse FSTI, Springfield, Virginia 22151, as No. AD 698282. Hearing Levels of Children by Age and Sex, United States. National Center for Health Statistics, U.S. Department of Health, Education, and Welfare, Wash- ington, D.C. This new report from the National Center for Health Statistics presents national estimates of the hearing levels of noninstitutionalized children in the United States. These estimates are based on data collected aspart of the second cycle of the Health Examination Survey, conducted in 1963-1965. A probabilitysam- ple of 7417 children were selected to repre- sentthe 24 million children 6-11 years of age in the noninstitutionalized population of the United States. Hearing threshold levelsfor the right ear and left ear of each child examined were determinedindividually by air conduction with standard pure-tone audiometers at eight frequencies-250,500, 1000, 2000, 3000, 4000, 6000, and 8000 cps. Testing was done under carefully controlled con- ditions, with respect to equipment and acoustical environment, by trained tech- nicians in an acoustically treated room in the specially constructed trailer in the mobile examiningcenter. More than 70% of the children have, at least for their better ear, hearingthreshold lower (better) than the 1951 American Standard audiometric zeroat all frequencies tested. In general,hearing thresholds were similar for the right and left ears. It was found that hearing sensitivity generally tended to increase with an increase in tonal frequency.Also, hearing sensitivity increased with increasing age, particularly at the lower tonesof 2000 cps or less. Hearing thresholds of boysand girls were similar. Comparison of the findings from the presentstudy with thosefrom someof the previous large-scale hearing surveys, such as the 1935-1936 National Health Survey and the Pittsburgh Surveys,are included. Tablesgiving the medians andpercentage distributionsof children by hearing thresh- old levelsat each test frequency, age,and sex are shownfor the right, left, and the "better" ear. Also shown are estimates of hearing levels for speech. Appendixes are also provided which contain a description of the survey designand reliability of the estimates as well as the recording formsand standards for reference (audiometric) zero usedin the testing. Copies of this report, PHS Publication No. 1000, Series 11, No. 102, may be purchased for $0.55 from the Superinten- dent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.• 434 Volume 48 Number2 (Part 1) 1970 Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 155.33.16.124 On: Mon, 24 Nov 2014 10:01:38

Wave Propagation in an Inhomogeneous Medium

  • Upload
    h

  • View
    216

  • Download
    2

Embed Size (px)

Citation preview

Page 1: Wave Propagation in an Inhomogeneous Medium

TECHNICAL NOTES AND RESEARCH BRIEFS

iterative technique is good for long wave- lengths (kR•<<l) and any ratio of wave- numbers in the scattering and surrounding media. For shorter wavelengths and small ratio of wavenumbers (e.g., k•/k=l.1, where k• is the wavenumber in the scatter- ing medium), it gives a good approximation to a0 for the entire range of kR• considered (0_<kR•_<2•r). For shorter wavelengths and larger ratio of wavenumbers (e.g., k•/k - 1.5, 2.0), it gives a good approximation to a0 out to approximately kR•=3,r/4. More general problems using this method are also discussed. Available from Clearing- house FSTI, Springfield, Virginia 22151, as No. AD 696528.

Acoustical Characteristics of Fibrous Absorbent Material. M. E. Delany and E. N. Bazley, National Physical Labora- tory, Teddington, England. An extensive investigation into the acoustical properties of fibrous absorbent materials has been carried out. Measured values of character-

istic impedance and propagation coefficient are each normalized in terms of power-law functions of frequency divided by flow resistance. The resulting empirical rela- tions are interpreted in terms of parameters characterizing the material and are each compared with theoretical predictions for an idealized material. For most practical applications, a knowledge of the flow resistance allows use of the normalized data and eliminates the need for routine

impedance-tube measurements. Available from Clearinghouse FSTI, Springfield, Virginia 22151, as No. N 69-40148.

Wave Propagation in an Inhomogeneous Medium. H. Watson, Jr., Southern Metho- dist University, Dallas, Texas. The effects of a high concentration of particulate matter held in suspension in water on the progration of an acoustic wave in water is discussed. Expressions for the average wave speed, average pressure, and incoherence of the pressure field are obtained for an incident harmonic pressure wave on a large number of discrete, suspended scatterers in water. The wavelength of the wave is assumed to be large compared to a scatterer diameter. Results are compared with experiments of acoustic wave propagation in sand-water mixtures. Available from

Clearinghouse FSTI, Springfield, Virginia 22151, as No. AD 698296.

The Dynamic Response of Various Media. M. Anliker and Chi-Chang Chao, Stanford University, Palo Alto, California. This report contains a summary of the research performed under the following topics: (1) a theoretical analysis of waves propagating in a bounded, heterogeneous elastic medium, (2) the transient response of an elastic half-space to a spreading surface load, (3) propagation of stress waves in a homogeneous isotropic wedge, (4) seismic wave radiation from sources moving over a finite distance, (5) the dynamic response of a fluid-filled circular cylindrical shell to pressure waves while subjected to a uniform external pressure, and (6) trans- mission of sounds and pulse waves in large arteries and veins. Seven publications relevant to the research effort are also

listed. Available from Clearinghouse FSTI, Springfield, Virginia 22151 as No. AD 696459.

Acoustic Characteristics of a Glass- Filament-Wound Pressure Vessel. A.

M. Young and J. F. Prandoni, Naval Research Laboratory, Underwater Sound Reference Division, Orlando, Florida. Acoustic insertion loss of a glass-filament- wound pressure vessel intended for trans- ducer calibration varies significantly as a function of frequency, position, and hydro- static pressure. The variations are believed to be due to voids in the glass resin and in the glass-resin/rubber-liner interface, which give rise to large changes in the character- istic impedance of the composite walls as a function of the same variables. Mechanical

behavior of the vessel under hydrostatic pressure required a reduction of the specified maximum operating pressure from 2000 to 1000 psig. Available from Clearinghouse FSTI, Springfield, Virginia 22151, as No. AD 698282.

Hearing Levels of Children by Age and Sex, United States. National Center for Health Statistics, U.S. Department of Health, Education, and Welfare, Wash- ington, D.C. This new report from the National Center for Health Statistics

presents national estimates of the hearing

levels of noninstitutionalized children in the United States. These estimates are

based on data collected as part of the second cycle of the Health Examination Survey, conducted in 1963-1965. A probability sam- ple of 7417 children were selected to repre- sent the 24 million children 6-11 years of age in the noninstitutionalized population of the United States.

Hearing threshold levels for the right ear and left ear of each child examined were

determined individually by air conduction with standard pure-tone audiometers at eight frequencies-250, 500, 1000, 2000, 3000, 4000, 6000, and 8000 cps. Testing was done under carefully controlled con- ditions, with respect to equipment and acoustical environment, by trained tech- nicians in an acoustically treated room in the specially constructed trailer in the mobile examining center.

More than 70% of the children have, at least for their better ear, hearing threshold lower (better) than the 1951 American Standard audiometric zero at all frequencies tested. In general, hearing thresholds were similar for the right and left ears.

It was found that hearing sensitivity generally tended to increase with an increase in tonal frequency. Also, hearing sensitivity increased with increasing age, particularly at the lower tones of 2000 cps or less. Hearing thresholds of boys and girls were similar.

Comparison of the findings from the present study with those from some of the previous large-scale hearing surveys, such as the 1935-1936 National Health Survey and the Pittsburgh Surveys, are included.

Tables giving the medians and percentage distributions of children by hearing thresh- old levels at each test frequency, age, and sex are shown for the right, left, and the "better" ear. Also shown are estimates of

hearing levels for speech. Appendixes are also provided which contain a description of the survey design and reliability of the estimates as well as the recording forms and standards for reference (audiometric) zero used in the testing.

Copies of this report, PHS Publication No. 1000, Series 11, No. 102, may be purchased for $0.55 from the Superinten- dent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.•

434 Volume 48 Number 2 (Part 1) 1970

Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 155.33.16.124 On: Mon, 24 Nov 2014

10:01:38