摘要:

This is a new field concerning the aerodynamicist and acoustician, the vibration specialist, the metallurgist, and the structural designer: each aspect has to be investigated and welded together to form a general picture. Apart from purely scientific interest, a principal result of the work in this field should be guidance to the aircraft designer. The paper briefly considers these various aspects in the light of theoretical and experimental work that has been done or is in progress, and shows how a careful coordination between theoretical and experimental work is necessary, since several aspects appear to be beyond present theoretical methods. Essentially the problem as visualized at present necessitates (1) an experimental study to obtain certain pertinent quantities in the near pressure field of a jet (where “hydrodynamic” turbulent effects are important): the quantities are spectra of space‐time correlations which at present are only calculable far from the jet; (2) a theoretical step to estimate the effectiveness of the pressure field in exciting vibration, whose modes and damping are calculable in simple structures (but an experimental approach is necessary for most practical aircraft structures); and (3) finally use has to be made of cumulative damage concepts to estimate the fatigue life under these random vibrations, this step again involving much experimental work.

ISSN:0001-4966    

DOI:10.1121/1.1905133

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

An empirical correlation of the noise field of aircraft propellers with operating conditions and spatial position is described. The results are useful for estimating the propeller‐noise levels over the surface of an aircraft fuselage or other surface in the near‐field of a rotating propeller at points separated from the propeller axis by a distance greater than the propeller radius. Data for this study came principally from three sources: (a) NACA Reports, (b) the work of Beraneket al.published circa 1947, and (c) recent measurements by Bolt Beranek and Newman Inc. The parameters considered include the power delivered to the propeller, the propeller diameter and nmnber of blades, the propeller tip Mach numbers (both rotational and helical), and the distance from the propeller tips to the measurement point. The results of the study are presented in a number of charts giving the over‐all level, spatial distribution, and frequency spectrum of the propeller noise. The use of the charts is described, and a comparisou of predicted and measured levels is made.

ISSN:0001-4966    

DOI:10.1121/1.1905137

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

The velocity of ultrasonics in compressed gases was measured by the optical method using the diffraction of monochromatic light. Our investigations may be summarized as follows: (1) Measurements on various compressed gases (A,N2, CH4) at pressures up to 1200 atmos, temperatures from 20°C to 200°C, and at frequencies ranging from 0.5 to 3.5 Mc. No definite dispersion was found in that range. The velocity increases as a function of pressure; at high densities its temperature coefficient is negative. The velocities calculated from the experimental PVT data of other authors are in good agreement with our measured velocities. The ratio of specific heats and the adiabatic compressibilities have been calculated. Calculations have been made for A,N2, andCH4using a series expansion of the velocity of sound with respect to densities; the coefficients of this expansion are related to the first four virial coefficients and their first two derivatives. The usual (6–12) spherically symmetric intermolecular potential has been assumed. It is found that a satisfactory agreement between calculated and observed velocities can be achieved at densities up to 300 amagat units; this agreement is obtained for both PVT and sound velocity data, by taking into account only a constant value of the fourth virial coefficient in our temperature range. It is concluded that a purely thermodynamical interpretation is sufficient to explain the observed velocities in our range and that sound velocity data seem a convenient means to test, jointly with the equation of state, the accuracy of a given intermolecular potential law. (2) Measurements of ultrasonic velocity near the critical point: The decrease of the velocity, previously observed forCO2andN2O, has been studied inC2H6andC3H8. Systematic measurements onC3H8show that no dispersion is present in our frequency range (0.585 to 3.2 Mc). Values of the velocity have been calculated on a purely thermodynamical basis from some semiempirical equations of state. The measured values are a little lower than the calculated values in the immediate vicinity of the critical point. Such a possible disagreement has been tentatively ascribed to the effect on the bulk viscosity of a structural relaxation process. (3) Measurements by a pulse method of the ultrasonic attenuation in nitrogen at pressures from 80 to 1400 atmos are presently in progress. Preliminary results indicate that the attenuation is strongly decreased under pressure.

ISSN:0001-4966    

DOI:10.1121/1.1905142

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

Nondestructive testing in the broad sense refers to methods whereby internal characteristics of solid structures can be examined without permanently affecting the structure. Thus, parts that prove to be satisfactory under specified test conditions are not degraded by the test procedures. We are concerned primarily with the question “Will the structure satisfactorily perform its intended function throughout its service life?” Implications of this question as related to the interpretation of the results of nondestructive test methods will be discussed. Elastic waves are presently employed to detect gross internal mechanical defects such as voids, fractures, and impurities. Many internal characteristics that bear on the question of structural performance are not tested by ultrasonic techniques. These include such properties as grain structure and size, variation in elastic moduli, variation in chemical composition, presence of permanent strains, structure of domains, depth of surface treatment produced by hardening procedures, effects of radiation, etc. A discussion of the importance of these additional internal characteristics will be given, and the applicability of ultrasonic procedures to the measurement of these properties will be treated.

ISSN:0001-4966    

DOI:10.1121/1.1905148

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

As shown by C. L. Schaefer and L. Bergmann in 1934, ultrasonic elastograms may be obtained by diffraction of light transmitted through crystal samples that are excited to high‐frequency vibrations. The elastic constants of a number of cubic crystals (KCl, KBr, Kl, NaCl, NaBr, NaF, RbCl, RbBr, LiF, andCaF2) are determined by this method. An attempt is made to interpret the specific form of the resulting elastograms from the lattice structure of the crystals.

ISSN:0001-4966    

DOI:10.1121/1.1905152

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

Vibratory drilling refers to the process of drilling into rock by vibrating the drilling tool at audio‐frequencies. The basic mechanism of vibratory drilling was ascertained by preliminary laboratory experimentation to consist of a series of impacts on the rock at the frequency of vibration. A fundamental study of this basic mechanism, made by dropping weighted chisels on rock, showed that the primary parameter which determined the rate of penetration was the mechanical power input to the rock per unit cross section of hole; the values of the vibration frequency and of other variables were of minor consequence over wide ranges. A theoretical analysis was made of the vibration of an elongated magnetostriction transducer capable of generating the required power level, taking into account the distributed nature of the generation of vibrations. Intermediate power transducers have been built and tested and a high‐power transducer for down‐hole operation is under construction. [The material for this presentation is based on work carried out at the Battelle Memorial Institute under the sponsorship of Drilling Research, Inc., an organization formed by a group of major companies engaged in various phases of oil production for the purpose of investigating novel methods of rock drilling.]

ISSN:0001-4966    

DOI:10.1121/1.1905156

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

A method is given for calculating the scattered sound field from plane inhomogeneous surfaces having normal impedance or admittance independent of the angle of incidence. The expression for the scattered field is represented in the form of power series of a parameter determining the relative inhomogeneity of the surface. A recurrent expression is given for thenth term of the series. Several particular cases are considered in more detail. The proposed method is valid for any ratio λ/Lwhere λ is the wavelength of the incident radiation andLthe spatial period of inhomogeneity. A comparison is made with the approximate solution obtained on the assumption that the change of admittance is small at the distance of the order of one wavelength; it means that it is possible to neglect diffraction effects in the immediate vicinity of the surface. A criterion is given for the validity of this approximate method.

ISSN:0001-4966    

DOI:10.1121/1.1905161

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

If the signal from an acoustic source consists of arbitrary pulses which are separated in time by an interval large compared to the length of the individual pulses, a series of simple mathematical operations performed on the pressure and particle velocity as measured at a point will yield an expression from which one can determine the distance between the source and the field point. The term containing the distance is measurable only in the near field and becomes less than 10% of other terms at distances greater than the wavelength corresponding to the dominant frequency of the pulse. The theory is presented for spherical and dipole sources. Experimental measurements made in air are presented for various pulse shapes from a dipole source and are compared with the theory. The effect of a noise background on the method is also considered.

ISSN:0001-4966    

DOI:10.1121/1.1918269

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

Bach's familiar Toccata in D minor has been performed and recorded by E. Power Biggs in many different churches and concert halls, on many different organs. Performance times and the duration of pauses in the performance are studied in relation to reverberation times measured from the recordings.

ISSN:0001-4966    

DOI:10.1121/1.1918280

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

Response curves of loudspeakers have irregularities, mostly not visible in the case of a loudspeaker of good quality and measured with normal equipment. By enlarging the ordinate scale these small peaks and dips of ±2 db become visible, but with the design of a cone it is considered that these irregularities are more or less inevitable and not so serious. This opinion is strengthened because by listening tests with a variable audio‐frequency nothing abnormal can be detected. It is also impossible to measure any distortion in the region of these peaks and dips. By introducing short and sharp bursts of sine waves, however, all sorts of spurious oscillations between the bursts are visible on a cathode‐ray tube. The amplitude of these phenomena depends on the internal damping of the diaphragm, the choice of the fiber, and the degree of beating of the pulp. The purpose of this paper is to raise the following question: Is this phenomenon the cause of the apparent harshness, etc., when listening to high‐level high‐note reproduction?

ISSN:0001-4966    

DOI:10.1121/1.1918292

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP