摘要:

The minimal interaural noise correlation discriminable by trained listeners was examined as a function temporal sampling parameters of the noise. The prime determinant of the listeners' performance is the total integrated duration of the noise samples, irrespective of the other temporal parameters. Within the restriction of a constant integrated duration, superior performance is attained with an interval of about 2 msec between successive brief noise samples.

ISSN:0001-4966    

DOI:10.1121/1.1908221

出版商:Acoustical Society of America    

年代:1960

数据来源: AIP

摘要:

A group of 111 naval recruits was examined in order to establish reference hearing level against which to assess the effects of noise exposure in serving men. The pure‐tone thresholds were taken in a quiet ordinary room with a commercial audiometer. After exclusion of some subjects for reasons given, the thresholds of the main group of 74 men were analyzed. Precautions taken to insure constancy of performance of the audiometer are described and its calibration was obtained relative to both the appropriate British and American standards. The results of the survey showed good agreement with the British standard as regards the main group and additionally for a group of 15 men selected for minimal exposure to gunfire (less than seven rounds of small‐arms fire). A comparison was made with the hearing level of a small group of more experienced listeners when an improvement of about 2.5 db was found. The relation of the work to other hearing surveys and laboratory measurements is discussed.

ISSN:0001-4966    

DOI:10.1121/1.1908223

出版商:Acoustical Society of America    

年代:1960

数据来源: AIP

摘要:

Theoretical models for frequency sensitivity in human observers are discussed. One decision procedure for a multiple‐filter model is considered in some detail as a general model for decision situations in which each available response is tied to more than one of the possible signal alternatives. Two experiments were conducted in an attempt to choose between a sweeping‐filter model and a multiple‐filter model. Detection in a two‐alternative forced‐choice experiment in which the signal could be one of two possible signals was measured over a range of frequency separations. The data yield further support for a multiple‐filter model.

ISSN:0001-4966    

DOI:10.1121/1.1908225

出版商:Acoustical Society of America    

年代:1960

数据来源: AIP

摘要:

When the axes of two directional pressure gradient receivers providing inputs to a steerable correlator receiving system deviate from the desired broadside direction, the system output is altered. An analysis of the improvement in output signal‐noise ratio shows that, in general, the maximum improvement decreases, the direction of maximum improvement shifts from broadside, and the range on source direction over which there is improvement decreases. Simplified expressions are given for these changes in the important case of small deviation angles. The system output is then studied under the condition that the deviation angles are random variables and probability expressions are derived for the maximum improvement, the direction of maximum improvement, and the improvement at a given source direction. An example is included for the case of independent deviation angles having identical, even, uniform distributions.

ISSN:0001-4966    

DOI:10.1121/1.1908227

出版商:Acoustical Society of America    

年代:1960

数据来源: AIP

摘要:

The problem of operating condenser microphones in a vibration environment is considered, and it is pointed out that there is a precise relation between the vibration and pressure response for any given transducer. This relation is shown to be dependent on the mass per unit area of the microphone diaphragm, and graphs are given from which the suitability of a given microphone to perform its function in a vibration environment can be assessed. The results of measurements on a well‐known type of condenser microphone are presented to support the theory, and the usefulness and disadvantages of vibration‐isolating mounts and of systems using two transducers are discussed. A proposed design of a condenser microphone to be insensitive to vibration is described.

ISSN:0001-4966    

DOI:10.1121/1.1908229

出版商:Acoustical Society of America    

年代:1960

数据来源: AIP

摘要:

The ultrasonic pulse‐echo technique has been employed to study scattering and attenuation in polycrystalline copper and α‐brass. Specimens in the form of cylindrical rods were annealed to various grain sizes and tested at frequencies between 100 kc and 18 Mc, corresponding to scatterer circumference to wavelength ratios (πD/Λ) from 10−3to 1.6. The primary objectives of this investigation were to study the scattering process by direct observation of the scattered radiation, and to study the associated attenuation under conditions not previously amenable to measurement; these included cases where pulse patterns were complicated by excessive scattering, and by mode dispersion. A preliminary investigation of the complications attending the propagation of acoustic pulses was performed for steel rods over a range of specimen radius to wavelength ratios (a/Λ) similar to that to be covered in the later work. Pulse patterns were found to be relatively free from complication fara/Λ10, approximately. In the intermediate range, the distribution of the input energy among the various vibration types was found to vary witha/Λ, and with the details of the excitation. At lowa/Λ, (<0.2), where solutions of the Pochhammer frequency equation predict only one mode of vibration for the rod, an additional mode was observed. This made corresponded in timing to a shear mode generated upon reflection of the predicted made, and can be accounted for by considering those roots of the frequency equation corresponding to Bessel functions of imaginary argument. To avoid the complications due to mode dispersion, the scattering experiments were designed to measure critically backscattered radiation, i.e., the scattered sound that arrived at the detector before any of the modes could effect a round trip. The observed scattering levels were in reasonable agreement with theoretical expectations (based on Bhatia's theory for scattering in the long wavelength region), except at low πD/Λ where the backscattering was seen to decrease with increasing frequency. The reason for this discrepancy is not yet definitely established, but it is believed to be a manifestation of propagation phenomena associated with the presence of attenuated vibrations in lossless media. Attenuation measurements were performed with pulses of duration comparable to the length of the specimen. This, combined with the high scattering ability of the medium, results in a pulse pattern whose envelope masks the details of the mode pattern. The rate of decay of this envelope corresponds to the attenuation of scattered sound and hence characterizes the intrinsic absorption properties of the metal. At πD/Λ≈10−1the energy attenuations were approximately 3×10−3db/grain for copper and approximately 3×10−4db/grain for α‐brass. Comparison of the experimental data with the principal attenuation mechanisms operative at these frequencies showed the observed attenuation to vary over most of the range in the manner expected for elastic hysteresis damping.

ISSN:0001-4966    

DOI:10.1121/1.1908231

出版商:Acoustical Society of America    

年代:1960

数据来源: AIP

摘要:

Ultrasonic velocities in aqueous solutions ofNaNO3, KNO3, Ca(NO3)2⋅4H3O, Sr(NO3)2, Pb(NO3)2, andBa(NO3)2are determined making use of a variable path interferometer. These solutions have a common nitrate ion. The effects of different cations on the compressibilities are studied and discussed on the basis of coulomb forces.

ISSN:0001-4966    

DOI:10.1121/1.1908233

出版商:Acoustical Society of America    

年代:1960

数据来源: AIP

摘要:

The displacement distribution of the vibrator of the Langevin type was observed by means of multibeam interferometry. The displacement at various portions of the surface was measured as a shift of the contour fringes. Besides the fundamental mode at 84 kc, several modes of vibration were found between 50 and 400 kc.

ISSN:0001-4966    

DOI:10.1121/1.1908236

出版商:Acoustical Society of America    

年代:1960

数据来源: AIP

摘要:

The canonical problem of scattering by a circular cylinder is expressed in integral form. The integrals are evaluated asymptotically to yield the first motion approximations. The geometrical theory of diffraction, developed by Keller, is combined with the first motion approximations to solve the problem of scattering by a smooth convex cylinder. The results are valid for pulses whose durations are short compared to the travel time across the cylinder.

ISSN:0001-4966    

DOI:10.1121/1.1908238

出版商:Acoustical Society of America    

年代:1960

数据来源: AIP

摘要:

Improvement in detected signal‐to‐noise ratio is obtained for a periodic signal masked by additive noise and turbulent noise backgrounds. Comparisons are made between autocorrelation, crosscorrelation, and a combination of frequency filtering and crosscorrelation. Although the latter method provided the greatest improvement, the crosscorrelation technique was the most successful single method. It turned out that the maximum improvement obtainable was limited by the dynamic range of the correlator computer and not by errors due to finite averaging time and scanning the delay. The improvement for signals masked by turbulent noise was found to be about 5 db less than that obtained for additive noise.

ISSN:0001-4966    

DOI:10.1121/1.1908240

出版商:Acoustical Society of America    

年代:1960

数据来源: AIP