摘要:

An experimental investigation of three couplers, suitable for testing circumaural earphones, was made to determine the limitations on their use. Two of these couplers were modified forms of standard U.S. couplers, the ASA‐type 1 and the NBS‐9A, while the third was a simple flat plate with a hole in which the test microphone is flush mounted. Results presented are based on tests made using the Telephonics TDH‐39 earphone with NAF‐48490‐1 cushion and the Philips model HA‐10 circumaural earphones. Below 2000 Hz, all three couplers are reliable and useful for measuring the responses of circumaural earphones. Above 2000 Hz, small variations in the electroacoustic system, defined by the earphone, cushion, coupler, and measurement microphone, cause large variations in the measured response. Variations in earphone responses are shown to be caused by variations in (1) earphones of a particular type, (2) cushions of a particular type, (3) applied force on the earphone, and (4) placement of the earphone on the coupler. These variations are in addition to the expected variations in the measured responses of different types of earphones when used with different types of cushions and when measured on different types of couplers.

ISSN:0001-4966    

DOI:10.1121/1.1909850

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

Research and development leading to an electropneumatic acoustic generator capable of producing in excess of 6 KW of acoustic power is reviewed. The modulation of pneumatic power through an electrodynamically controlled valve provides a transducer capable of producing either random or complex periodic signals. The development of this valve required considerable investigation of materials to establish the necessary compliance and optimized moving valve structure to obtain a reliable sound generator. The efficiency of this generator with respect to the pneumatic power supply is as high as 17%. The evaluation of the power spectrum, amplitude distribution, and nonlinear characteristics of the transducer is performed in a progressive‐wave tube. Current developments include units that produce considerably more acoustic power or extend the useful frequency range of the sound generator.

ISSN:0001-4966    

DOI:10.1121/1.1909851

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

Transducers used for the excitation and/or detection of acoustic waves in laboratory plasmas must be compact and dimensionally compatible with the structure that contains the plasma; and, they must be made of materials that do not outgas in high‐vacuum environments, and are capable of withstanding temperatures of 500°C. This paper considers such requirements and establishes the important acoustic and constructional specifications for transducers that are housed in long, cylindrical glass discharge tubes with diameters of approximately212in. After pointing out the limitations of conventional designs, details of satisfactory moving‐armature and solid dielectric‐type transducers are presented, along with brief descriptions of their measured performance. Although the moving‐armature refit has had the greatest use to date, the solid dielectric version promises to be quite useful as a microphone in detecting plasma sound waves. Therefore, an explanation of techniques for building such a device is included, with particular emphasis on methods for coating the stationary electrode with a dielectric material.

ISSN:0001-4966    

DOI:10.1121/1.1909852

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

Acoustic‐wave propagation in gases is studied for the case where the ratio of the transmitter‐receiver separationsto the mean free path Λ of the gas molecules is sufficiently small so that the propagation constant is independent of Λ (free‐molecule propagation). Experimental results, obtained in air and in argon, are found to be in agreement with the existing theories. Free‐molecule propagation is distinguished from ordinary sound propagation that takes place ifs/Λ is sufficiently large in order that the propagation constant is independent ofs. The ranges of free‐molecule propagation and sound propagation are found to be approximatelys/Λ5, respectively.

ISSN:0001-4966    

DOI:10.1121/1.1909853

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

When a submerged solid body is subjected to a translational velocity step, it radiates oscillatory, transient acoustic pressures, whose natural period is proportional to the travel time of a sound pulse around the accelerated solid. Transient velocities displaying more‐complicated distributions over the radiating surface give rise to acoustic‐wave harmonics that, with the exception of the breathing mode, are also oscillatory, and characterized by one or more “natural” frequencies proportional to the ratio of the sound velocity to the characteristic dimension of the radiator. These pressures are analogous to room reverberation attenuated by radiation damping but their phase velocities and attenuation are reminiscent of creeping waves. If the initial velocity impulse is modified by subsequent gradual accelerations (or decelerations) as embodied, for example, in an exponentially decaying or “pulsed CW” velocity, the “reverberant” oscillatory‐sound field is superimposed on an acoustic field whose time history is identical with that of the motion of the radiating surface. The phenomenon of oscillatory transients radiated by nonoscillatory motions of submerged bodies appears to be characteristic of any finite solid geometry. The effect of these reverberant transients on the radiation and radiation loading of spherical sources generating CW pulses is analyzed. Finally, the effect of these transients on the performance of large arrays is explored. It is shown that, at the beginning of a CW pulse, radiation impedance is uniform over the array, the difference with the CW impedance that fluctuates strongly over the array surface being accounted for by the transient. Consequently, arrays in which circuits of individual elements have been adjusted for fluctuations in CW radiation impedance will display a non‐uniform response at the beginning of the pulse, which may result in fatigue failures.

ISSN:0001-4966    

DOI:10.1121/1.1909854

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

Three experiments were performed to demonstrate the presence of the first‐order diffracted ray from a periodic surface. In these experiments, the incident wave was at grazing incidence and the direction of surface periodicity was in the plane of incidence. The direction of propagation of the first‐order diffracted ray agreed with the prediction of Y. P. Lysanov [Soviet Phys.—Acoust.1, 60–72 (1955)]. In particular, when λ (wave‐length) =L(surface period), the diffracted ray was normal to the periodic surface. One experiment used CW spherical waves in an anechoic chamber with a periodic floor. As the microphone traversed the floor, the pressure dipped once per period. The dips were symmetric and smooth only when λ=L. The other two experiments used pulsed plane waves of ultrasound. In one, the longitudinal ultrasonic waves traveled in a rectangular aluminum block that had grooves milled into one face parallel to the beam axis. The opposite, parallel face supported a longitudinal receiving transducer. When λ=L, the plane first‐order diffracted wave produced multiple echoes between the grooved surface and the opposite face. In the other ultrasonic experiment, the waves traveled in liquids parallel to the grooved bottom of a tank. An immersion transducer with its axis normal to the grooved surface detected a maximum signal when the ultrasonic frequencyfwas adjusted to make λ=L. The velocities found by using the relationv=λfagreed with pulse‐echo measurements to 2%–3%.

ISSN:0001-4966    

DOI:10.1121/1.1909855

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

A complete solution for the response of an arbitrary shell subjected to time‐dependent surface loads is derived by means of the classical method of spectral representation. The solution is expanded in terms of the modes of free vibration, and their orthogonality is proved for an arbitrary shell. As an example, the response of a spherical shell to a suddenly applied uniform normal load is calculated in detail.

ISSN:0001-4966    

DOI:10.1121/1.1909856

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

This paper concerns the use of a canonical transformation in finding a solution to the problem of the relativistic perturbed oscillator. The transformation is performed so that the final solution is obtained in a certain desired form. The ultimate nature of the relativistic perturbed oscillation is revealed.

ISSN:0001-4966    

DOI:10.1121/1.1909829

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

Flexural, axisymmetric, and torsional vibrations of a transversely isotropic, simply supported, cylindrical shell of finite length are considered. Comparatively simple forms of the equations of motion have been obtained by introducing three auxiliary variables. The solution of these equations is given in terms of Bessel functions. Applying suitable boundary conditions, the frequency equations have been derived. Three‐dimensional theory of elasticity is used in the analysis.

ISSN:0001-4966    

DOI:10.1121/1.1909830

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

Normal‐mode theory is used to calculate the propagation of sound at moderate‐to‐long ranges in a realistic, constant‐depth, shallow‐water channel. The channel is made up of two isovelocity water layers lying over a layered, viscoelastic, solid bottom. The equations for the sound intensity are written in rather simple form by using Brekhovskikh's formulation in which the effect of the channel bottom enters the equations as a single reflection coefficient. Because the bottom sediments absorb some sound energy at each bottom reflection, each of the modes has art exponential attenuation as well as the usualr−12spreading loss. In this paper, these mode‐attenuation constants are calculated exactly—i.e., to any desired accuracy—by an iterative process. Reasonable agreement is shown between calculated values of propagation loss and some experimental values from a recent sea test by the U.S. Navy Electronics Laboratory. The introduction of a directional source into the calculations is briefly discussed.

ISSN:0001-4966    

DOI:10.1121/1.1909831

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP