摘要:

A method of measuring the phase of vibrational displacement, utilizing a Michelson interferometer with a laser source, is described. Results of several applications are presented and discussed.

ISSN:0001-4966    

DOI:10.1121/1.1911658

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP

摘要:

Experimental methods are needed for determining the characteristics of small acoustic filters used in systems with pulsating gas flows. These characteristics could then be used to predict the performance of proposed filter designs in a particular system, according to plane‐wave acoustic theory. Dependence on trial‐and‐error experimentation in solving noise‐control problems would thus be minimized. A literature survey revealed only a few earlier attempts to evaluate the acoustic performance of small filters and filter elements. Three methods for determining reflection and transmission factors are described, evaluated, and compared. A method employing a standing‐wave tube of unique design is recommended for determination of reflection factors. Transmission factors are obtained from reflection factors and pressure measurements at the filter inlet and outlet. Use of an anechoic termination simplifies the calculations and increases accuracy; design and evaluation of such a termination is described.

ISSN:0001-4966    

DOI:10.1121/1.1911661

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP

摘要:

The amplitude of the surface elastic wave produced by the application of an alternating voltage to an interdigital arrangement of electrodes on the surface of a piezoelectric medium is determined. The electric field produced by the surface electrodes is calculated subject to the assumption that the piezoelectric coupling of the material can be neglected. This electric field acts as the forcing term for the inhomogeneous elastic equation, which is then solved to obtain the amplitude of the surface wave generated by the transducer. A reciprocal relationship between the excitation and detection problems is used to obtain the power extracted from the surface wave by an interdigital arrangement of surface electrodes. Measurements made on single crystals of quartz and cadmium sulfide are found to be in good agreement with theoretical predictions. The maximum value of the product (efficiency×fractional bandwidth) for a surface‐wave transducer is calculated. For an interdigital surface‐wave transducer on the basal plane of CdS, the maximum value of this product is found to be 7.8×10−2. It is shown that one can connect linear pairs of surface electrodes in an appropriate binary code so as to obtain high efficiency and large bandwidth transducers.

ISSN:0001-4966    

DOI:10.1121/1.1911656

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP

摘要:

“Toroidal” microphones are defined as microphones having toroid‐shaped directional characteristics, i.e., they have minimum sensitivity along the rotational axis of the toroid and maximum sensitivity in the plane perpendicular to that axis. Several practical designs of toroidal microphones based on second‐order gradient receivers are proposed and analyzed as to directional characteristics and frequency response. Experimental microphones are presented, some of which use a Helmholtz resonator tuned to the lower end of the frequency range to compensate the ω2dependence of the second‐order gradient receivers and to improve the over‐all signal‐to‐noise ratio of the microphones. Measurements show that the microphone sensitivity in a plane through the rotational axis obeys approximately the expected cosine‐squared law. In the direction of the rotational axis, the sensitivity at midfrequencies was found to be typically 20 dB lower than the sensitivity in the plane normal to that axis. The equalized frequency response in this plane is within ±3 dB from 0.3 to 2.5 kHz. Sensitivities are about −80 dBV/μbar. Using existing low‐noise pre‐amplifiers, this corresponds to a signal‐to‐noise ratio of about 35 dB for speech at conversational levels.

ISSN:0001-4966    

DOI:10.1121/1.1911657

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP

摘要:

This paper studies the minimum bearing error attainable with a linear passive array. Signal and noise are stationary Gaussian processes with arbitrary power spectra, and the noise is assumed to be statistically independent from hydrophone to hydrophone. The Cramér‐Rao technique is used to set a lower bound on the rms bearing error and the results are compared with the bearing error of a slightly modified split‐beam tracker. The latter reaches the lower bound for a two‐element array and comes very close to reaching it for a linear array with an arbitrary number of equally spaced hydrophones. Thus, the modified split‐beam tracker is very nearly optimal for the uniformly spaced array. Comparisons of split‐beam tracker error with the Cramér‐Rao lower bound are also obtained for nonuniform hydrophone spacings.

ISSN:0001-4966    

DOI:10.1121/1.1911659

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP

摘要:

It is experimentally demonstrated that scanned‐receiver acoustical holography is useful for mapping or “imaging” those parts of a complex vibrator responsible for the energy radiated to the far field.

ISSN:0001-4966    

DOI:10.1121/1.1911660

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP

摘要:

Viscoelastic materials are extensively used to damp flexural vibrations of metallic structures; it has been known for some time that the energy dissipation due to shear strain in the viscoelastic layer can be increased by constraining it with a stiffer covering layer. In this report, we discuss a method for increasing this damping by cutting the constraining layer into appropriate lengths. The analysis for a single layer of this treatment is relatively straightforward. The damping can be increased still further by using several layers; in this case, the analysis is based upon effective complex elastic moduli of an equivalent homogeneous medium. One result found from this analysis is that, for optimum spacing of cuts, the damping depends primarily upon the stiffness of the constraining layer and only slightly on the shear modulus of the viscoelastic layer. Experimental data are presented for comparison with the theoretical predictions.

ISSN:0001-4966    

DOI:10.1121/1.1973457

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP

摘要:

Acoustic techniques have a long history of use in the sea. As man becomes more involved in this major portion of his environment, his use of acoustics increases both in quantity and variety. A number of acoustic devices played key rôles in the search for the lost submarines,THRESHER(SSN 593) andSCORPION(SSN 589), as well as in the location and recovery of the nuclear device lost off the coast of Spain. These mishaps have spurred research and development of acoustic and other types of equipment for use on the deep ocean floor and, while considerable progress has been made, there remains much room for improvement. As a result of its successful participation in these search efforts, the Naval Research Laboratory has become deeply involved in man's quest for mastery over this hostile portion of his habitation.

ISSN:0001-4966    

DOI:10.1121/1.1973461

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP

摘要:

The development of an aircraft noise‐suppression system is essential to the relief of communities adjacent to commercial airports that are adversely affected by jet‐aircraft noise. This paper is a brief review of the structural aspects of current turbofan noise‐suppression designs presented in conjunction with a summary of the environmental conditions that determine the design criteria of duct‐lining materials. Also included are: (1) prospective drawings of the various acoustical sandwich configurations; (2) descriptions of the porous sheet, honeycomb core, and adhesive materials that compose the composite configurations; and (3) a discussion of the special structural and evironmental tests required to develop the mechanical properties of acoustical materials.

ISSN:0001-4966    

DOI:10.1121/1.1973467

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP

摘要:

The ultrasonic absorption in a number of liquids whose molecular structure is a six‐membered ring has been measured over the frequency range 0.6–45 MHz at temperatures in the range −100 to +150°C. In piperidine and piperazine derivatives, a large relaxation with a low relaxation frequency is found at the higher temperatures of measurement. The activation energy of the relaxation indicates a barrier to inversion of the chair form of the ring of ∼10 kcal/mole. In cyclohexene and cyclohexanone derivatives, a different relaxation is found at the lower temperatures with a high relaxation frequency. Its molecular mechanism is believed to be the interconversion of chair and boat configurations of the ring, and these measurements represent, apparently, the only direct physical measurements of the elusive and important boat form in relatively simple molecules.

ISSN:0001-4966    

DOI:10.1121/1.1973481

出版商:Acoustical Society of America    

年代:1969

数据来源: AIP