摘要:

An improved instrumentation system has been developed to analyze the dynamic pressure field produced by jet aircraft. The system is automatic and utilizes analog computer techniques to obtain an accuracy of ±0.2 db in its readout. The method used is to secure a true rms value of octave band segments of the jet noise spectrum and to convert this to decibels by the use of a dc logarithmic amplifier. The transition to acoustical decibels is made by obtaining the difference between this voltage and another voltage obtained from a reference oscillator. The value of this reference oscillator in acoustical decibels is obtained by comparing its output voltage to that of a calibrated microphone. The dynamic range of the system has proved more than adequate for greater than 98% of the data processed by this unit during the time this system has been in operation. The output is in the form of punched cards for utilization with a digital computer.

ISSN:0001-4966    

DOI:10.1121/1.1908390

出版商:Acoustical Society of America    

年代:1961

数据来源: AIP

摘要:

In a classical study of the vowel sounds of English, G. E. Peterson and H. L. Barney collected a large body of experimental data which related perceived vowel quality to measurements of the first three formant frequencies and the voice pitch. It is difficult for a computer to use this raw data to interpret vowel quality because the vowel types have complicated boundaries in the coordinate system of the physical measurements. This paper describes a coordinate transformation which simplifies these boundaries.

ISSN:0001-4966    

DOI:10.1121/1.1908410

出版商:Acoustical Society of America    

年代:1961

数据来源: AIP

摘要:

The frequently used pulse method of measuring ultrasonic wave velocities in solids involves a high‐frequency quartz crystal transducer cemented to one end of a specimen having parallel end faces. The phase shift for waves reflected from the transducer must be considered for highest accuracy. It is shown that combining several measurements of phase delay (at two frequencies differing by approximately 10%) with a theoretical analysis of the reflection phase angle makes possible a determination of velocity to within one part in 5000 for round trip delays greater than 5 μsec. Indirectly, the approximate thickness of the cement bond between transducer and specimen can be determined. The advantages of the method for making measurements as a function of temperature or pressure are discussed.

ISSN:0001-4966    

DOI:10.1121/1.1908386

出版商:Acoustical Society of America    

年代:1961

数据来源: AIP

摘要:

The problem of the diffraction of a plane compression pulse by a rigid, incompressible semi‐infinite body in lubricated contact with a homogeneous, isotropic, perfectly elastic half‐space is formulated by a potential function and separation of variables approach. The boundary conditions are expressed by dual integral equations that are solved by a function‐theoretic technique. The exact solution is obtained, as a function of space and time variables, in a closed form.

ISSN:0001-4966    

DOI:10.1121/1.1908388

出版商:Acoustical Society of America    

年代:1961

数据来源: AIP

摘要:

In conducting experiments on waves in an elastic rod, J. Oliver discovered a new mode of symmetric vibration in which the motion was predominantly at the end of the rod. It is shown that this “end mode” arises when pure extensional waves are coupled with waves with complex wave numbers. Approximate equations are used to study the reflection of pure extensional waves at the end of a semi‐infinite rod. The properties of the resulting end mode are studied in detail and are compared with the phenomena reported by Oliver.

ISSN:0001-4966    

DOI:10.1121/1.1908392

出版商:Acoustical Society of America    

年代:1961

数据来源: AIP

摘要:

The propagation of acoustic waves in a layered medium is considered for the case of an elastic plate submerged at a certain depth below the surface of a liquid‐filled half‐space. It is shown that there exist unattenuated modes for the plate‐liquid layer system which have horizontal phase velocities greater than the sound velocity in the liquid. In spite of such greater phase velocities, radiation into the liquid below does not take place because the lower surface of the plate exhibits no vertical motion. Thus, energy can be trapped in and above the plate by vibrations which leave the underlying liquid undisturbed. It is pointed out that radiation in the horizontal direction may also be very small when the above conditions of total reflection are approximately satisfied, as is indicated by the existence of low group velocities for the modes of a free plate. Attention is called to the three‐dimensional nature of such trapping, vertically by an approximate condition of no transmission across the plate, and horizontally by the simultaneous vanishing of the group velocity at nonvanishing angles of incidence. The present modes are in contrast with the usual case of total reflection and “wave guide propagation” in layered elastic media, where the horizontal phase velocity is less than the sound velocity in the liquid (lower half‐space), and the signal below the plate decays exponentially with distance from the interface. Conditions under which the present modes can arise have been found and evaluated numerically. Relations between phase velocity, wave number, and ratio of fluid layer to plate thickness are presented for a lucite plate is water. These have been compared with the results of much more elaborate calculations for the point source “singing” problem.

ISSN:0001-4966    

DOI:10.1121/1.1908394

出版商:Acoustical Society of America    

年代:1961

数据来源: AIP

摘要:

A series of capacitive accelerometers with frequency‐response characteristics governed by the resistive and elastic properties of a thin air film are described. The accelerometers have been designed, in accordance with theory developed previously to have response constant within ±5% at frequencies up to 1.48 times the undamped natural frequency. Experimental data are given to show that the accelerometer response agrees closely with that predicted theoretically.

ISSN:0001-4966    

DOI:10.1121/1.1908396

出版商:Acoustical Society of America    

年代:1961

数据来源: AIP

摘要:

The theory of electrical lines and filters is based on the concept of a characteristic impedance. Similar concepts are permissible for mechanical systems. It thus becomes possible to derive a generalized theory of vibrating systems that is free from insignificant characteristics of the vibrator. The conclusions are illustrated by computations and by measurements for flat plates. The results are used to develop a theory of the propagation of bending waves in plates and shells on lines similar to the theory of reverberation in room acoustics.

ISSN:0001-4966    

DOI:10.1121/1.1908398

出版商:Acoustical Society of America    

年代:1961

数据来源: AIP

摘要:

The root mean square response of a simple mechanical resonator mounted on another, which is excited by vibration of a flat motional power spectrum, is investigated theoretically for the case of no loading effect between resonators. While the solutions are not general for any pair of coupled resonators, the condition of zero loading is of particular interest, as it leads to extreme vulnerability of electronic and electromechanical equipment to shock and vibration when there is a coincidence of resonance frequencies of coupled resonators. Approximate formulas are obtained and computed by hand. The exact equation is solved on a digital computer to yield a series of curves for comparison. The approximate formulas hold surprisingly well, except when the resonators are close in frequency and highly damped. Curves of the derivative of the rms response, as one resonance frequency or the other is varied, are obtained by use of the digital computer. They illustrate the importance of mechanical tolerances in relation to detuning as a factor in design changes for improved reliability.

ISSN:0001-4966    

DOI:10.1121/1.1908400

出版商:Acoustical Society of America    

年代:1961

数据来源: AIP

摘要:

The axisymmetric extensional modes of free vibrations of a thin prolate spheroidal shell separate into two classes, one class with displacements perpendicular to the meridional planes and one class with displacements in the meridional planes. This paper concerns itself with the former, the torsional modes. The differential equation for the mode shapes, obtained by application of Hamilton's principle, is found to be satisfied by single prolate spheroidal angle functions of the first kind, and the transcendental frequency equation is readily solved with the aid of tabulated eigenvalues. Numerical and graphical nondimensional results are presented for the first eight modes.

ISSN:0001-4966    

DOI:10.1121/1.1908402

出版商:Acoustical Society of America    

年代:1961

数据来源: AIP