摘要:

The theory of the scattering of plane waves of sound by isotropic circular cylinders and spheres is extended to take into account the shear waves which can exist (in addition to compressional waves) in scatterers of solid material. The results can be expressed in terms of scattering functions already tabulated. Scattering patterns computed on the basis of the theory are shown to be in good agreement with experimental measurements of the distribution‐in‐angle of sound scattered in water by metal cylinders. Rapid changes with frequency in the distribution‐in‐angle of the scattered sound and in the total scattered energy are found to occur near frequencies of normal modes of free vibration of the scattering body.

ISSN:0001-4966    

DOI:10.1121/1.1906780

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

Subharmonic oscillations at one‐half the frequency of excitation may appear in certain types of oscillating systems, among which is the direct‐radiator loudspeaker. These oscillations occur at very nearly the resonant frequency of the system when the parameters of the system are made to vary at twice this frequency. The rate of growth of the subharmonic depends upon the amount of variation of the parameters relative to the dissipation in the system. If the dissipation is small, the rate of growth may be large. In the loudspeaker, conditions are such that the rate of growth is usually small for typical conditions of operation.

ISSN:0001-4966    

DOI:10.1121/1.1906781

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

The expression for the normal velocity of sound in a fluid becomes indeterminate at the critical point since the coefficient of elasticity tends to zero and the specific heat at constant pressure approaches infinity. An expression has been obtained for the isentropic slope which does not become indeterminate at the critical point. Observations were made with a Pierce type variable path acoustic interferometer placed completely inside a pressure tank with provisions for external control. The isothermal variation of velocity and absorption was measured as a function of pressure at 572 kc in carbon dioxide. Above the critical temperature it was found that, as the pressure increases, the velocity decreases steadily, goes through a sharp minimum, and rises abruptly. Below the critical temperature a discontinuity in velocity was observed between liquid and vapor. The velocity in the liquid at 29°C was 208 meters/second, and in the vapor 179 meters/second. The velocity variations away from the critical point agree with previously published results. The attenuation decreases steadily with increasing pressure and shows an abrupt rise as the critical point is reached, which would be expected from critical fluctuation scattering. Using isotherm data two values of the velocity at the critical point were calculated, 154 and 146 meters/second. The measured value was found to be 141.6 meters/second.

ISSN:0001-4966    

DOI:10.1121/1.1906782

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

A pulse technique particularly suited to dissipative materials is described for measuring attenuation and phase shift constants of plastics, using either transverse or longitudinal waves in the frequency range of 5–50 Mc.A thin wafer of the material under test is placed between two identical fused silica buffers; and waves generated by quartz crystals at the ends of the assembly are transmitted simultaneously through the specimen in both directions. Comparison of transmitted and reflected components by means of a special balancing circuit provides information from which the complex propagation constant can be calculated, and hence dynamic rigidities and viscosities.Illustrative data for polyethylene and Nylon are given.

ISSN:0001-4966    

DOI:10.1121/1.1906783

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

A survey of city noise in the Chicago area was initiated in 1947. This report describes traffic, industrial, and residential area noise which was investigated as a part of the work. Levels of traffic noise were 35 to 45 db, 45 to 65 db, and 65 to 75 db in the 400–800‐cps band for light, average, and heavy traffic conditions, respectively. Industrial noise ranged from 50 to 60 db in the same band for fifty percent of the cases measured. Ninety percent were below 65 db. A limiting spectrum for noise which is considered not objectionable is presented. Residential area noise ranged from 38 to 47 db in the mentioned band for fifty percent of the cases measured. At night and for winter conditions, traffic noise was 10 db lower in the mentioned band than in the daytime. At night, industrial area noise dropped to levels of existing traffic conditions, and in winter it was lower by about 5 db in octave bands mainly because factory windows were closed. At night, residential area noise was 5 to 10 db less in octave bands than during the day, and for winter conditions there was a drop of 6 to 8 db in octave bands due to the modified character of distant traffic.

ISSN:0001-4966    

DOI:10.1121/1.1906784

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

The formant regions of vowels sustained by well‐known male operatic singers were compared with each other and with those of untrained voices. Since vowel samples from commercial phonograph records were too short for electrical harmonic analysis, these samples were re‐recorded on circular grooves. Analysis consisted of measuring the energy content of the individual harmonic partial tones. The result of this method seemed to be more informative in many ways than those obtained by using the sound spectrograph which shows only the general formant regions. Three important formants were found in the male voice, although the third of these was more prominent in trained voices. A fourth of minor importance occurred in trained voices only. The frequency positions of the three important formants are found to be in dose agreement for the various voices analyzed.

ISSN:0001-4966    

DOI:10.1121/1.1906785

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

A speech analyzer is described which measures the degree of fit between the energy distribution of any speech signal and a set of “standard” energy distribution patterns stored in the machine. This technique is based on the assumption that a phoneme can be characterized, at least for a single speaker, by specifying the points of energy concentration in the frequency spectrum and by specifying the energy envelope, i.e., energyvstime characteristic, of selected portions of the spectrum. A set of contiguous band filters covering the frequency range 100–7000 cycles/sec are used in combination with rectifiers and integrators to produce polarized signals which are functions of the energy distribution of the speech input signal. Improved selectivity and discrimination against noise and noise‐like signals are obtained by combining the outputs from adjacent filters to measure thedifferencebetween signal levels, and further combining signals to measure thesecond differencebetween signals in adjacent filter bands. “Optimum filters” for phonemes are constructed by adding output signals corresponding to frequencies of energy concentration and subtracting signals corresponding to energy minima; this summation generates a signal proportional to the degree of “fit” between the speech signal and the specified pattern. Separate summations of the low and high frequency filter outputs provide measures of intensity, voiced‐unvoiced detection, and pitch extraction signals. Some results are presented.

ISSN:0001-4966    

DOI:10.1121/1.1906786

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

When a narrow band of noise, of constant level (thesignal) is masked by a wider band of noise (themasker), the amount of noise required to drown out the signal often depends significantly upon the interaural phase relations of signal and masker. Thus, more masking noise is required if the signal is 180° out of phase at the two ears than if it is in phase: provided that the masker remains in phase. The out‐of‐phase signal, in other words, shows an advantage in audibility. The advantage is very striking for a signal near 250 cps; and falls off progressively toward 1400 cps, above which it is small. At all frequencies between 250 and 6000 cps, the level of masking, for a signal in phase at the ears, remains relatively constant.An important property of physical stimuli that produce an audibility‐advantage is an interaural phase‐deviation caused by the signal. When the signal is added to a corresponding masking component, the combined pressure‐wave, on the average, leads in one ear and lags in the other. The resulting phase‐deviation depends upon the relative levels and phase‐positions of the masking and masked components. It is maximum when these components are 90° apart. The interauraltime‐deviation thus produced depends also upon frequency. If the frequency is increased, the level of the masking component must be decreased in order to hold the time‐deviation, associated with the different frequencies, constant.The present experiments showed that, with the out‐of‐phase signal, the decline in level of masking, as a function of frequency, is just such as to hold the interaural time‐deviation (for the 90° position) constant at about 125 microseconds. Tonal signals tend to show a decreasing critical time‐deviation with increasing frequency.

ISSN:0001-4966    

DOI:10.1121/1.1906787

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

Calculations of the informational capacity of the human ear are made by computing the number of discriminable sound patterns per second, and applying the Shannon information theory. A maximum of 104bits/sec transmission found. This is compared with the capacity of existing auditory channels and recording media, and with the rate of actual information perception from speech and music. It is shown that a capacity of upwards from 5 × 104bits/sec, depending on the informationalmatchto the ear, is necessary for high fidelity transmission or recording. It is also shown that the brain can utilize less than 1 percent of the information transmitted by the ear. Finally, an average capacity of about 0.3 bit/sec, or of 40 tones/sec, is calculated for an individual cochlear fiber.

ISSN:0001-4966    

DOI:10.1121/1.1906788

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

By means of the interferometer, the velocity and absorption of ultrasonic waves in nitrogen were measured at the frequency of 2.992 mc/sec, in the pressure range of 2.09 to 76 cm of Hg at a temperature of 29°C. A dispersion of velocity was found ranging from 354.3 meters/sec to 364.4 meters/sec. The ratio αexp/αclassdropped from 1.40 to 1.32, and the corresponding value ofCv/Rdropped from 2.50 to 2.08.Theoretical values for the change in velocity and in the absorption ratio, calculated by applying the equations for the exchange of energy between translational and vibrational degrees of freedom, show good agreement with the observed values. The increase in velocity and the decrease in αexp/αclassis interpreted as due to the slow exchange of energy between the translational and rotational degrees of freedom. The relaxation time for rotation was found to be 1.2×10−9second.

ISSN:0001-4966    

DOI:10.1121/1.1906789

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP