摘要:

If a system comprising a water column bounded by a steel cylindrical shell is driven at a fixed frequency, the pressure amplitude has been found to vary along the axis in a complex manner. On the hypothesis that this phenomenon results from the excitation of two or more modes of vibration having different speeds of phase propagation, an investigation has been made of waves in liquid‐filled cylindrical shells. It is shown that, in addition to the modes associated with boundaries of high impedance, there is also a possible mode associated with each type of free wave in the shell. The phase velocities of several symmetrical free waves are calculated by an extension of the method employed by Love for solid cylinders. A comparison with measurements appears to confirm the hypothesis.

ISSN:0001-4966    

DOI:10.1121/1.1906918

出版商:Acoustical Society of America    

年代:1952

数据来源: AIP

摘要:

Some frequencies for the symmetrical normal nodes of a finite, isotropic disk are calculated by satisfying the boundary conditions exactly at the flat surfaces and approximately at the curved surface. It is found that two sets of frequencies occur for each vibration pattern. One set approaches the odd order thickness compressional frequencies, whereas the other approaches the even order thickness shear frequencies, as the radius of the disk increases indefinitely. As a test of the validity of the approximations, the maximum residual stresses at the curved surface are compared with the compressional component of the stress at the center of the disk.

ISSN:0001-4966    

DOI:10.1121/1.1906919

出版商:Acoustical Society of America    

年代:1952

数据来源: AIP

摘要:

The forces due to acoustic radiation in a beam of finite cross section in a nonviscous medium striking a plane reflector at oblique incidence are derived from simple mechanical considerations. The formulas are applied to a wedge‐shaped vane. For a vane, the wings of which include an angle of 90°, the force turns out to be quite independent of the coefficient of reflection at the boundary between vane and medium.

ISSN:0001-4966    

DOI:10.1121/1.1906920

出版商:Acoustical Society of America    

年代:1952

数据来源: AIP

摘要:

A new method for the observation of ultrasonic field distributions described, utilizing a starch plate in a dilute solution of iodine in a manner analogous to the use of photographic emulsions. Near‐field diffraction patterns are shown as illustrative of results, which appear to be superior to those of other methods, and the advantages of the new method over previous techniques are described.

ISSN:0001-4966    

DOI:10.1121/1.1906921

出版商:Acoustical Society of America    

年代:1952

数据来源: AIP

摘要:

The velocity, dispersion, and absorption of ultrasonic waves in dry,CO2‐free air have been measured at 32°C, and at two and three Mc/sec, and at pressures ranging from 0.020 to one atmos. Dispersion of the velocity has been found beginning at 30 Mc/atmos, increasing by five percent at 100 Mc/atmos, accompanied by a large increase in absorption such that at the higher limits off/p, reached measurements became nearly impossible with the equipment used. The ratio αexp/αclassdecreased from about 2.4 to 1.3 andCv/Rfrom 2.5 to nearly 1.5 asf/pincreased. The changes in velocity, absorption, and internal specific heat are interpreted as the result of the slowing of energy exchange between translational and rotational states. Assuming that relations for relaxation of translational‐vibrationai exchange also hold for this case, the relaxation time for translational‐rotational exchange as derived from the dispersion measurements has been found to be 2.29×10−9sec. This corresponds to a frequency of the midpoint of the dispersion curve of 116 Mc/atmos, and to 16 as the number per molecule of collisions required for an energy exchange between translational and rotational states. Absorption results were more difficult to secure; using low frequency values a relaxation time of about 3×10−9sec is indicated, giving 87 Mc/atmos as thef/pvalue of the midpoint of the dispersion curve, and 21 as the number of collisions required for the energy exchange.

ISSN:0001-4966    

DOI:10.1121/1.1906922

出版商:Acoustical Society of America    

年代:1952

数据来源: AIP

摘要:

The criteria for an “optimum” hydrophone receiving system for underwater acoustic signals are considered, and the equilibrium thermal‐noise pressure spectrum expression is shown.Experimental ambient sea‐noise spectrum levels are in excess of the thermal noise at all frequencies below 25 kc. Extrapolation of experimental curves (−5 db/octave) shows intersection with thermal‐noise spectrum (+6 db/octave) between 50 and 200 kc, depending on sea state. (For a definition of “sea state,” seeInstructions to Marine Meteorological Observers(U. S. Department of Commerce Weather Bureau, U. S. Government Printing Office, Washington, D. C., June, 1941), seventh edition, circular M.) The ambient‐noise level is expressed as a function of the equivalent temperature.Formulas are then derived for the required plane‐wave signal level for which electrical signal‐to‐noise ratio is unity. The operating noise factor is defined in terms of the electroacoustical efficiency of the linear reversible hydrophone, the amplifier noise figure, and the equivalent ambient‐noise temperature for the sea condition and frequency under consideration.

ISSN:0001-4966    

DOI:10.1121/1.1906924

出版商:Acoustical Society of America    

年代:1952

数据来源: AIP

摘要:

The high noise level associated with aircraft gas turbine operation creates two noise problems: (1) The reduction of the noise in the neighborhood of the installation to an acceptable level, and (2) the protection of operating personnel from excessive noise. This paper reports work done by the Aircraft Gas Turbine Division of the General Electric Company at their plants in Lockland, Ohio, and in Lynn, Massachusetts. Permissible sound levels were established for surrounding residential areas. The attenuation due to spherical divergence, and also the probable deviation due to varying atmospheric conditions, was determined. Thus, the sound level permissible at the exhaust stacks of the test cells was obtained. Knowing the noise level inside the test cell, the required sound attenuation of the stacks was calculated. Various acoustical treatments were investigated. An acoustical design is proposed which makes use of a combination on various duct treatments, and which meets the established design objectives. The permissible noise level inside the control room is discussed, and the required attenuation through the barrier between test cell and control room is calculated. A design for a control room meeting the established noise‐level criterion is proposed.

ISSN:0001-4966    

DOI:10.1121/1.1906925

出版商:Acoustical Society of America    

年代:1952

数据来源: AIP

摘要:

In a previous paper by Fletcher and Galt [J. Acoust. Soc. Am.22,89 (1950)] a method was described for measuring experimentally and also for calculating the interpretation aspect of the perception of speech. The listeners were considered to have normal hearing. In the present paper the same principles are applied to persons having abnormal hearing.

ISSN:0001-4966    

DOI:10.1121/1.1906926

出版商:Acoustical Society of America    

年代:1952

数据来源: AIP

摘要:

The unexplained difference in sound pressure in the ear canal which appears to exist when equally loud low frequency tones are presented alternately from an earphone and from a loudspeaker has bedeviled acousticians for many years and, unfortunately, still continues to do so. There are presented here the results of some of the measurements carried out at the Bell Telephone Laboratories which show the magnitude of the effect and various attempts at explaining it. While no satisfactory explanation has been found, it is hoped that publication of these results will stimulate interest in the problem.

ISSN:0001-4966    

DOI:10.1121/1.1906927

出版商:Acoustical Society of America    

年代:1952

数据来源: AIP

摘要:

Pairs of very small electrodes were placed in two or more turns of the cochlea of the guinea pig. The cochlear microphonic from a short segment (about 1 mm) of the cochlear partition can thus be recorded, and without contamination by action potentials. The outputs of the second, third, and fourth turns were compared with that of the first turn with respect to both amplitude and phase as a function of frequency. The space‐time pattern thus revealed is a traveling wave which passes up the cochlea to a distance that depends on the frequency. The pattern agrees well with that of mechanical movement (Békésy) except that the cochlear microphonic shows relatively greater amplitude (voltage) in the basal turn. Low frequencies travel farther up the cochlea than do high. The velocity (for a 750 cps tone) is about 100 m/sec in the basal turn and about 2 m/sec in the fourth turn. Phase differences of as much as 5π were observed, by means of Lissajous patterns, between the responses of the first and the third turn. Certain discontinuities in the input‐output relationships and in phase relationships were found to be a function of frequency and of position along the cochlear partition. These discontinuities occurred at or near the frequencies that showed a phase difference of 2π from the basal end of the cochlea. The relation of these critical frequencies to position along the cochlea constitutes a new “frequency‐map” of the cochlea.The space‐time pattern of the cochlear microphonic proved to be very little affected by removal of portions of the bony wall of the cochlea or delivering acoustic energy through a hole near the apex. It is concluded that, in addition to “direct driving” of the cochlear partition by pressure differences between the two scalae and to “surface waves” arising from such drying, the transmission of transverse waves along the solid structures of the cochlear partition must also be included for a satisfactory interpretation of all of the available data.

ISSN:0001-4966    

DOI:10.1121/1.1906928

出版商:Acoustical Society of America    

年代:1952

数据来源: AIP