摘要:

A theory is presented that permits the study of the effects of horizontal winds on the dispersion and amplitudes of acoustic‐gravity waves in the atmosphere. It is shown that the effective horizontal group velocity for a given frequency in a given normal mode depends on direction of propagation as well as on frequency and that it is not necessarily in the same direction as the horizontal‐wavenumber vector. A number of useful integral theorems are derived from a variational principle and one is subsequently applied to the development of a perturbation method for the computation of wind effects on dispersion. Application of the method to a realistic example indicates that winds can appreciably alter the dispersion of the normal modes and that they should be considered in any quantitative interpretation of experimental microbarograms.

ISSN:0001-4966    

DOI:10.1121/1.1909317

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

This paper is concerned with the long‐wavelength expansion of the scattered field in the ratio body dimension to the wavelength. For scalar scattering by spheroids, prolate and oblate, hard as well as soft, the radius of convergence of the series is determined. A method is given whereby the coefficients in the expansion for a general body can be obtained from the solutions of a set of specific and classical potential problems. It is shown how the required solutions can be found for a body that is the intersection of two or more bodies, for each of which the exterior electrostatic Green's functions are known, and as an illustration the procedure is applied to a hard finite cone with a spherical cap.

ISSN:0001-4966    

DOI:10.1121/1.1909318

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

The behavior of linear systems with parameters that vary as “physical” white noise (very‐wide‐band Gaussian) is developed. The Fokker‐Planck equation is used to develop moment equations, and “equivalent” systems are derived.

ISSN:0001-4966    

DOI:10.1121/1.1909319

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

Discussed theoretically is the manner in which the driving‐point and transfer impedances of a free‐free beam depend upon the position of the driving force and upon the position along the beam at which the velocity is monitored. Also considered is the effect of one or more arbitrarily situated forces upon the driving‐point impedance that is presented to a primary force at a separate location. The influence of internal damping upon the response of the beam is always considered. The influence of rotary inertia and shear displacement is usually disregarded, however, except in one example that also describes the manner in which the observed frequency dependence of the mechanical properties of the beam material—especially rubberlike materials and plastics—may be inserted into the beam equations.

ISSN:0001-4966    

DOI:10.1121/1.1909320

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

The acoustic stiffness of a liquid within a small enclosure may be reduced greatly by the introduction of compressible matter, which may be in the form of compliant tubes. This effect is due in part to the added compressibility of the compliant tubes and in part to volume resonance. The mixture can be tuned or detuned by changing either the number of compliant tubes or the type of liquid in the fill. Such flexibility permits the resonant frequency of an acoustic transducer to be altered. Additional investigations demonstrate that a simple liquid fill can also be employed successfully if the diaphragm compliance of a transducer is designed to be comparable to that presented by the fluid. The resonance width of a transducer may be broadened by reducing the thickness of a metallic diaphragm and using the liquid fill for the essential stiffness with a lesser amount of mass. However, the effective electromechanical coupling factor may increase or decrease by an amount that depends on both the relative stiffness and the specific transducer design.

ISSN:0001-4966    

DOI:10.1121/1.1909321

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

Measurements have been made of the acoustic losses of a resonator with a small vent and of their dependence both on the speed of steady flow through the system and on the position of the vent. The resonator is physically modeled on the “Tburner” used to test solid propellants for combustion instability, but measurements are made at room temperature and pressure. It is a closed circular cylinder, resonant in the fundamental plane‐wave mode, with a vent through the wall near the center. Steady gas flow is introduced at one end through a slightly porous piston whose vibration also generates sound. Losses are determined from the rate of decay of pressure. Significant flow‐dependent loss attributable to the vent is found for flow speeds in the vent exceeding about 0.02 times the sound speed. The loss depends on vent position. Good quantitative prediction of this loss at flow velocities above a critical value is achieved by a theory based on the characteristics of quasisteady fluid efflux from an orifice (total pressure quadratic in total flow).

ISSN:0001-4966    

DOI:10.1121/1.1909322

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

A noncoherent optical‐correlation detector of the stored reference type is described. The correlator performs real‐time correlation of multiple delayed signals with the stored reference withouta prioriknowledge of the signal delays. The correlator makes use of real‐time integration, thereby making it unnecessary to record received signals prior to correlation. An experimental version of the correlator is described, its performance is analyzed, and the predicted performance is compared with the observed performance and found to agree to within 3 dB. A signal‐processing gain of 20 dB is obtained with the experimental version of the correlator.

ISSN:0001-4966    

DOI:10.1121/1.1909323

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

Air spaces between a rigid termination and a sample of foam material give absorption effects similar in some respects to the results obtained from surface coatings. Air spaces require larger thicknesses to produce equal improvement at lower frequencies. Surface coatings of increasing thickness decrease the absorption at higher frequencies, but the combination of an air space and surface coating may improve the absorption at these frequencies. The effects of air space were attributed to a considerable extent to their similarity to an equivalent thickness of foam.

ISSN:0001-4966    

DOI:10.1121/1.1909324

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

An approximate solution for the diffraction of a plane sound wave indicent normally on a circular aperture in a plane rigid wall of finite thickness is obtained by postulating rigid, massless, infinitely thin plane pistons in each end of the aperture, whose motions simulate the movement of the air particles at these positions under acoustic excitation. Plane longitudinal waves are assumed inside the aperture. Numerical solutions obtained on an IBM‐7090 computer very closely coincide over a wide range of frequencies with those of much more complicated existing exact solutions. Furthermore, extensive experiments were conducted on the transmission of reverberant sound through steel pipes of various lengths and diameters inserted individually into a heavy wall between two concrete chambers of 1750 ft3each. Measured values of reverberant sound‐transmission loss through these apertures were found to agree within experimental error with those calculated from the approximate theory. Experimental results indicated that the assumption of plane‐wave propagation in the aperture was valid up to values ofd/λ exceeding unity, wheredis the pipe diameter and λ is the wavelength of incident sound.

ISSN:0001-4966    

DOI:10.1121/1.1909325

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

To provide experimental verification of an approximate solution for the diffraction of sound waves by a circular aperture in a plane rigid wall of finite thickness, a series of experiments was conducted on the transmission of reverberant sound through steel pipes inserted individually into a thick wall between two reverberation chambers. The usually recommended methods of evaluating the transmission loss proved inadequate because long, narrow apertures have sharp resonant peaks. By electronically averaging and equalizing the outputs of four microphones in each chamber and using a slowly sweeping sine wave with a small super‐imposed warble, it was possible to record continuous plots of the aperture transmission loss with high resolution of frequency and amplitude. Experimental accuracy of 1 dB was possible over most of the frequency range of 300–12 000 cps, diverging to 2 dB at the frequency extremes, and for apertures with sharp resonances, diverging to about 4 dB at the peaks. The accuracy was limited by reading the “average” of the recorded fluctuating sound level, which was most difficult at resonant peaks. For larger‐diameter apertures, which gave data relatively uninfluenced by flanking sound, the agreement with theoretical results was within 1 dB over most of the frequency range.

ISSN:0001-4966    

DOI:10.1121/1.1909326

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP