摘要:

In a previous paper [J. Acoust. Soc. Am.28, 1158 (1956)], it was shown that the gravest mode of contour vibration of anisotropicthin circular plate has the circumferential order of two and is mostly equivoluminal in nature. Although the exact expression of the displacements includes Bessel functions, it can be reasonably approximated by a simple combination of trigonometric functions, which makes it possible to handle theanisotropiccase. Since the mode is the gravest, Rayleigh's principle is applied in a procedure of approximation, taking an angle of rotation and a wave number as adjusting parameters.The results are compared with experiments of quartz vibrators. Calculated frequencies show agreement better than 1% with an exact value for isotropic plates and an observed value for a rotated Y cut of quartz, which has a limited anisotropy. The accuracy decreases as the degree of anisotropy increases. For the X cut of quartz, the calculated value is 4.7% higher than an observed value.One of the significant characteristics of the mode of vibration of an anisotropic plate is the fact that the direction of maximum displacement makes a certain angle with a crystallographic axis. The calculated value of this angle agrees exactly with the observed angle for the rotated Y cut of quartz and within a few degrees for the case of the X cut of quartz.Equivalent electric circuit constants and surface patterns of piezoelectrically induced charges are also calculated and compared with experiments.

ISSN:0001-4966    

DOI:10.1121/1.1909720

出版商:Acoustical Society of America    

年代:1958

数据来源: AIP

摘要:

This paper discusses the advantages of studying fluctuations of underwater sound in vertical transmission and points out the drawbacks of the horizontal transmission studies. Certain experiments eliminating avoidable known causes of fluctuations have been carried out at the Indian Naval Physical Laboratory and are described. The results show that the variation coefficient of fluctuation has a higher value than those reported by other workers in other areas. The suggestion put forward by Nanda that scattering by zooplankton may be responsible for this large fluctuation is supported.

ISSN:0001-4966    

DOI:10.1121/1.1909722

出版商:Acoustical Society of America    

年代:1958

数据来源: AIP

摘要:

This note describes a short series of experiments investigating the nature and magnitude of the differences in the noise field from subsonic cold air jets having circular and elliptic orifices. An anticipated increase in the noise level occurs in directions normal to the major axis, but other changes having more obscure origins are also revealed.

ISSN:0001-4966    

DOI:10.1121/1.1909724

出版商:Acoustical Society of America    

年代:1958

数据来源: AIP

摘要:

Four message‐sets were constructed on the basis of phonemic confusion patterns with the aim of modifying the role of frequency‐of‐occurrence in the determination of speech intelligibility in noise. Deliberate selection of words may reinforce, nullify, or negate word‐frequency effects for known message‐sets.

ISSN:0001-4966    

DOI:10.1121/1.1909726

出版商:Acoustical Society of America    

年代:1958

数据来源: AIP

ISSN:0001-4966    

DOI:10.1121/1.1909728

出版商:Acoustical Society of America    

年代:1958

数据来源: AIP

摘要:

The noise reduction afforded by ducts or parallel baffles depends not only on the physical properties of the duct and its lining, but also upon the angle of incidence of the sound waves impinging on the ducts. The noise reduction per duct width or per unit length provided by a duct for sound waves impinging at several angles of incidence is found to vary with the length of the duct. The variation of noise reduction per duct width decreases with increasing duct length. For sufficiently long ducts, however, the noise reduction per duct width approaches a constant value For a fairly wide range of duct lengths, the noise reduction can be expressed asa+bl, wherebis the noise reduction per duct width,lis the length of the duct measured in duct widths, andais a source distribution correction factor. Several sets of experimental data are presented to show thatamay be comparable in size toblfor typical duct lengths. Several implications of a noise reduction of the forma+blare discussed. Procedures are given for estimating the noise reduction for ducts of finite length by use of Morse's theory and the data presented here. Some unsolved problems are discussed. [Supported in part by the U. S. Air Force under Contract Nos. AF 33(616)‐2151, AF 33(616)‐3335, and AF 33(616)‐3938.]

ISSN:0001-4966    

DOI:10.1121/1.1929930

出版商:Acoustical Society of America    

年代:1958

数据来源: AIP

摘要:

In order to take best advantage of the speed and flexibility of the digital computer as a research tool in speech analysis and recognition studies a real‐time input system is needed. Such a system, which makes power‐spectrum data directly available to the computer, has been constructed at the Lincoln Laboratory for use with the TX‐0 and TX‐2 computers. A speech signal is fed to 35 band‐pass filters covering the range 100 to 10000 cps. The output of each filter is envelope‐detected, and the envelopes are sampled periodically by a 128‐position mercury commutator switch. The output of the commutator is converted to binary digital form and fed directly to the computer, At present the system is capable of making 7620 samples per second with a signal‐to‐noise ratio of over 40 db (7 bits). The filters, made up of high‐pass and low‐pass sections in cascade, can be connected to give a variety of band widths and center frequencies Patch‐cord connections between envelope‐detector outputs and commutator inputs permit some frequency channels to be sampled at higher rates than others when desired. Utilizing the maximum sampling rate of the input system and the full storage capacity of the computer, up to 20 sec of continuous speech may be read into the computer and held for analysis. [The research reported in this talk was supported jointly by the U. S. Army, Navy, and Air Force under contract with the Massachusetts Institute of Technology.]

ISSN:0001-4966    

DOI:10.1121/1.1929934

出版商:Acoustical Society of America    

年代:1958

数据来源: AIP

摘要:

The present American standards for normal hearing for audiometric purposes imply that the difference between audiometric reference zero for a 1000‐cps pure tone and audiometric reference zero for spondee words is 6 db (i.e., approximately 16 db SPL for a 1000‐cps tone and 22 db SPL for spondee words). An accumulation of evidence from several sources suggests, however, that the difference between the two thresholds is considerably larger than 6 db. The present study was designed to assess the effects of sex, ear, order of test presentation, method of threshold measurement, and degree of prior familiarity with spondee vocabulary on the difference between the threshold for a 1000‐cps tone and the threshold for spondee words, as measured with W‐1 recordings. Subjects were 126 young adult college students. Results indicate that the difference varies from 12 to 14 db, depending slightly on degree of prior familiarity with the spondee words, and on the method used to measure the pure tone threshold. The average speech threshold SPL in the present group agreed closely with the current standard reference level of 22 db, while the average 1000‐cps pure tone threshold SPL was 6 to 8 db lower than the current standard reference level of 16 db. [This research was supported by the U. S. Air Force under Contract No. AF 41 (657)‐185, monitored by the School of Aviation Medicine, USAF, Randolph Air Base, Texas.]

ISSN:0001-4966    

DOI:10.1121/1.1929936

出版商:Acoustical Society of America    

年代:1958

数据来源: AIP

摘要:

Studies have indicated that dc airflow through an orifice creates acoustic resistance. The creation of this resistance has been investigated by the measurement of the impedance of a perforated plate in an impedance tube. A method of calculating the acoustic resistance due to dc airflow has been prepared. The close agreement of theory and measurements points the way toward the use of this phenomenon for the improvement of all types of silencers where airflow is present.

ISSN:0001-4966    

DOI:10.1121/1.1929944

出版商:Acoustical Society of America    

年代:1958

数据来源: AIP

摘要:

The exact linear equations for propagation of a disturbance in a viscoelastic material are reviewed, together with the usual small‐loss approximation. For tan δ⩽0.165 the approximate relations are within about 1%. Many plastic materials, however, have loss tangents considerably greater than 0.165 and appreciable errors result if the exact expressions are not used. A table of corrections to the small‐loss approximate propagation constants is presented. The case of large‐loss materials (tan δ>>1) is also considered and approximate equation derived. For tan δ ⩾ 10 the large‐loss approximate relations are within 5%. An interesting result from the large‐loss approximation is that the product of the propagation velocity and attenuation is equal to 2 times the frequency (cα = ω). Some experimental data, both shear and longitudinal, for typical plastic material are given.

ISSN:0001-4966    

DOI:10.1121/1.1929946

出版商:Acoustical Society of America    

年代:1958

数据来源: AIP