摘要:

From the point of view of their use in supersonic wind tunnels in which it is desired to obtain a high Mach number without great expenditure of power, the substitution of the air by vapors having a low velocity of sound is of interest. The freons (halogen‐substituted methanes) suggest themselves for this. It is shown that these vapors have ultrasonic relaxation times in the neighborhood of 10−7sec coupled with high absorption coefficients in the ultrasonic range, a fact which may limit their usefulness in wind tunnels. The relation between relaxation times measured by other methods and by the present one are discussed.

ISSN:0001-4966    

DOI:10.1121/1.1908541

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

In recent experiments the compositions and methods of preparation of various ferrites were varied with the aim of achieving optimum mechanical and piezomagnetic performance. Essential improvements were obtained by small cobalt substitutions in the chemical composition and by suitable modifications of the mechanical and thermal treatments.In CW operation, appropriate ferrites can excite radiation intensities up to at least 10 w/cm2, with an electroacoustic efficiency of more than 75% up to 50 kc/sec. So they are highly promising for underwater transmission and processing, in spite of their brittleness (ultimate tensile strength up to about 6 kg/mm2). Wide‐band hydrophones with linear frequency response and high pressure‐sensitivity can also be built from such ferrites having magnetomechanical coupling coefficients at remanence up to almost 0.25, like Permendur.The total variation of the mechanical resonant and antiresonant frequencies of ferrite filter elements in the temperature range from +20 to +50°C, being 0.1–0.25% for existing commercial ferrites, has been lowered to 0.03% and less. Since the mechanical quality factors are usually much better than 2000, such ferrites can be profitably applied to the construction of electrical and electromechanical band filters.

ISSN:0001-4966    

DOI:10.1121/1.1908543

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

Results of experiments conducted to determine the dependence of the ultrasonic machining rateVin Ketos tool steel on the peak‐to‐peak amplitude 2ξ on the frequency of vibrationfon the abrasive particle diameterdand on the ratio of the mass of abrasive to the mass of water used in compounding the slurry are given. It is shown that within the range under investigation the quotientV/2ξfdis a constant for a critical ratio of the mass of abrasive to that of water. It is also shown that events occurring within the volume of a cube having sides equal todare of fundamental importance in the ultrasonic machining process. A phenomenological equation is derived from which the most probable machining rate can be calculated for the range under investigation.

ISSN:0001-4966    

DOI:10.1121/1.1908545

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

In many applications of underwater sound, the “reverberation,” or backscattered return, from the sea surface is the limiting background in which the desired echo appears. Little has been known about sea‐surface backscattering in spite of its importance to the prediction of reverberation levels in practical applications.Using two transducers—one for transmitting, one for receiving—mounted so as to be rotatable in a vertical plane and suspended from a buoy, measurements have been made of the backscattering of sound pulses from the surface of the sea as a function of grazing angle between 5 and 90 degrees, wind speed between 3 and 18 knots, and pulse length. When converted to a scattering coefficient per unit area, the results provide some hints as to the processes by which sound is scattered by the ocean surface, and give the basic data for computing the reverberation to be expected with transducers of arbitrary beam width and orientation.

ISSN:0001-4966    

DOI:10.1121/1.1908547

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

The natural oscillations of membranes can be studied by reflection of light from an acoustically excited soap film. In order to obtain a uniform soap film, it proves advantageous to rotate the film so that its thickness is uniform. The oscillations may be obtained even when the films have become extremely thin after long rotation. In addition, one finds strange streaming phenomena with certain natural oscillations.

ISSN:0001-4966    

DOI:10.1121/1.1908549

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

When a turbulent boundary layer is produced by air flow past a solid surface, the turbulence in the boundary layer can generate a sound field in the free stream and will also induce fluctuating loads on the solid surface. If the surface is flexible, this motion will generate an additional sound field on both sides of the surface.In an initial investigation of the latter form of sound generation, suitable equipment has been developed to measure the fluctuating wall pressure in the turbulent boundary layer. The equipment includes a specially designed low noise and turbulence level wind tunnel and a small barium titanate transducer and preamplifier combination for frequencies up to 50 kc. The transducer and preamplifier may be useful for other applications.Using this equipment, some of the properties of the wall pressure fluctuations in a turbulent boundary layer have been measured. It was found that the spectrum of the wall pressure fluctuations extends to 50 kc and that the root‐mean‐square wall pressure was a constant portion (0.0035) of the free stream dynamic pressure for0.2 

ISSN:0001-4966    

DOI:10.1121/1.1908551

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

Formulations of one‐dimensional sound propagation in a homogeneous, progressively moving medium are applied to calculations of recurrent filter characteristics of side branch tube attached to the side wall of an air duct. The complex propagation constants and characteristic impedance are functions of the flow Mach number. Therefore, the insertion loss is dependent upon flow but relative changes with flow depend upon the terminating impedance levels as well as geometry. Neglecting dissipation the linear system is found nonreciprocal with respect to phase shift.

ISSN:0001-4966    

DOI:10.1121/1.1908553

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

Studies of the insertion loss of a side branch resonant filter in moving air reveal that nonlinear excitation of the resonator and turbulence may influence the measured characteristic. These effects occur at relatively low Mach numbers for particular flow geometry patterns. The insertion loss is found to be destroyed for sufficiently high flow velocities. An interpretation of insertion loss values as averaged over a finite band of frequencies is also given some attention.

ISSN:0001-4966    

DOI:10.1121/1.1908555

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

The origin of the magnetic driving forces which are responsible for magnetic noise and a method of calculating the mechanical response of the motor structure are reviewed in this paper. A general method is developed to calculate the sound pressure levels of magnetic noise at a specified point near the motor. The paper gives numerical results of calculations, and the corresponding measured sound pressure levels of the magnetic noise for a particular experimental 4‐pole, 30‐hp, polyphase induction motor under five different operating conditions; calculations agree with measurements within 3 db.

ISSN:0001-4966    

DOI:10.1121/1.1908557

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

Basic criteria are given for the incorporation of many reactive components in sound absorbers. Forms of absorbers, determined by these criteria, are derived. Design parameters are treated in relation to techniques for determining their optimum values. Many reactive components are shown to have the effect of enhancing uniformly high absorption over extended frequency ranges.

ISSN:0001-4966    

DOI:10.1121/1.1908559

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP