摘要:

Causes and effects of nonlinearity in structures are surveyed and illustrated by means of examples. Geometrical and material nonlinearities, and nonlinearities due to bearings, friction, clearance, stops, etc., are described. Nonlinearity in stiffness is contrasted with nonlinearity in damping. Dynamic effects of nonlinearity, including jump phenomena, subharmonic resonance, and nonlinear coupling between modes, are illustrated. A discussion of criteria for smallness of motion to justify linearized treatment of nonlinear systems is given. Illustrative examples including vibration of flat plates, vibration of antenna mounts, and earthquake response of multi‐story buildings are described. A structural earthquake isolation system utilizing friction‐controlled foundation pads is presented. Analytical and numerical techniques for predicting the statistical response of the system are demonstrated. [Supported by NSF Grant GI‐34945. ]

ISSN:0001-4966    

DOI:10.1121/1.1919508

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

The interaction of flutter and random vibrations of clamped‐clamped beam is studied. The external loads are composed of pressure fluctuations in the field of random forces, which are assumed to be random space‐time functions, and the pressure perturbation which depend on the beam deformations according to the “piston theory.” Corcos's concept of the cross‐spectral density of turbulent fluctuations is used with Bull's correction for low‐frequency range. The Galerkin's procedure with two‐ and three‐mode approximations is used for solving the stochastic boundary‐value problem.

ISSN:0001-4966    

DOI:10.1121/1.1919514

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

The paper presents the results of a study of turbulent flow typically found in subsonic inlets. Space‐time correlations measured at the inlet‐engine interface plane were used to represent the structure of the turbulent flow. The results have application in the analysis of rotor noise and performance. It was observed that turbulence which originated outside and was convected through the inlet had elongated shapes with stream correlation lengths much longer than those in transverse directions. However, when inlet originated turbulence was measured, the lengths were about the same in each direction. Thus, the contraction effect on the flow through the inlet strongly affects the shape of the turbulent eddies. The basis for the observations were high‐frequency response total pressure transducers mounted on rakes in the inlet near the fan plane. The inlets used were representative of current technology in subsonic aircraft. 15‐in.‐diameter models were used in a 9 × 9‐ft wind tunnel. Flow through the inlets was established using a downstream engine. Mach numbers studied were representative of the full‐scale inlet. Conditions represented in the results are static, crosswind, and angle of attack.

ISSN:0001-4966    

DOI:10.1121/1.1919517

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

Farfield noise radiated from low‐tip‐speed propellers on the YO‐3A quiet observation aircraft and measured in flyover tests exhibited unexpected levels and trends that have been called the “quiet airplane paradox.” Levels of rotational propeller noise were found to be higher than predicted by conventional axisymmetric theory, and as propeller tip speed was decreased below a given value, at constant thrust, both harmonic and broad‐band noise levels actually increased. Thus a “bucket” was formed in the SPL vs tip speed curve. In addition, the harmonic character of the flyover propeller noise was markedly different than that observed in static propeller tests. These data were declassified in 1973, and a program was conducted to investigate the paradoxical experimeutal results and to develop a theoretical explanation. The theoretical study considered propeller blade aerodynamics, nonuniform inflow through the propeller, chordwise blade loading, and, finally, propeller blade wake/wing interaction. It was concluded that the principal explanation of the paradox lies with the non‐uniformity of inflow through the propeller disk causing circumferential variations of blade loads. The blade wake/wing interaction rated second in important. [Original experiments with the YO‐3A aircraft were supported by the Army and this research program was supported by the Air Force.]

ISSN:0001-4966    

DOI:10.1121/1.1919521

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

With the extensive and ever‐increasing use of ultrasound in clinical medicine and the development of progressively more sophisticated systems of tissue visualization, the importance and urgency of specifying the safety standards and the possible hazards of over‐exposure to adult and embryonic tissues cannot be overemphasized. The implications of the results of several pertinent interaction studies and epidemiological surveys to the problem of realistic risk assessment are discussed.

ISSN:0001-4966    

DOI:10.1121/1.1919526

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

Knowledge of the mechanism of tissue damage by ultrasound of low‐megahertz frequency is essential for the prediction of safe dosage levels for diagnostic and theraputic use. Both ultrasonically induced heating and cavitation can cause or contribute to damage in organized tissues. Separating the effects of the two mechanisms is difficult, since temperature rise and cavitation are concurrent; also, synergism between thermal and mechanical effects has been proposed. A series of experiments was conducted under sufficiently high ambient pressures to suppress cavitation. Since all absorbed energy is dissipated as heat, elevated ambient pressure would not affect a thermal mechanism. Cat brain was irradiated with focused ultrasound at ambient pressures up to 40 atm and the acoustic emission was measured to assess the presence and magnitude of cavitation. The relation between ultrasonic dosage and the extent of damage in the cat brain under elevated ambient pressures was observed. The results are discussed in terms of the proposed mechanisms. [This work was supported in part by USPHS Grant No. NIH‐5‐P01‐HL14322.]

ISSN:0001-4966    

DOI:10.1121/1.1919528

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

It has previously been reported [Smoorenburg, J. Acoust. Soc. Am.52, 615 (1972)] that for some stimulus conditions the psychophysically observed distortion product2f1 − f2behaves nonmonotonically. As either the frequency or the intensity of the primary tones is changed, the amplitude of the distortion product goes through an abrupt null and the phase goes through a sudden shift. We have observed a similar effect in a non‐linear model of the basilar membrane. The basilar membrane is modeled as a 175‐section transmission line, and nonlinearities are introduced into the viscous damping term of each section [Hall, J. Acoust. Soc. Am.53, 324 (1973); Kim, J. Acoust. Soc. Am.53, 324 (1973)]. In the model, the nulls are caused by standing waves at frequency2f1 − f2produced by reflections from the basal end of the membrane. Primary‐tone frequencies for which the nulls occur can be determined analytically [Schroeder, J. Acoust. Soc. Am.53, 429 (1973)]. If the same mechanism is operative in humans, then psychophysical investigation of these nulls should provide information about mechanical properties of the basilar membrane.

ISSN:0001-4966    

DOI:10.1121/1.1919534

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

The pattern‐transformation model of pitch perception [Wightman, J. Acoust. Soc. Am.54, 407–416 (1973)] can successfully predict the pitch of many types of complex‐tone stimuli. In addition, the model provides numerical estimates of pitch “strength.” In order to test these latter predictions we assumed that performance in a musical interval‐recognition paradigm [Houtsma and Goldstein, J. Acoust. Soc. Am.51, 520–529 (1972)] is a direct indicator of pitch strength; the better the performance, the stronger the pitch. Thus, Houtsma and Goldstein's data can be used to evaluate the pitch‐strength predictions of the model To do this, a set of stimuli was defined such that all stimuli in the set led to the same level of performance in the interval‐recognition paradigm; the stimuli lay on one of Houtsma and Goldstein's “constant‐performance contours.” Then pitch‐strength predictions were generated for these stimuli. If our assumption is valid, all the stimuli should have the same predicted pitch strength. Except for stimuli of low fundamental frequencies, the predictions follow this pattern. The failure with low‐fundamental stimuli seems to be a result of our initial assumption that spectral resolution (critical bandwidth) is a constant percentage of frequency (i. e., that the peripheral analyzer has constantQ). If the constantQassumption is relaxed, so that our assumed critical bandwidths agree more closely with available data, the performance of the model is much improved.

ISSN:0001-4966    

DOI:10.1121/1.1919538

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

The performance of six absolute‐pitch and six control subjects was examined in three studies involving recognition memory for tone frequency. When the standard and comparison were 1/10 semitone apart, absolute‐pitch subjects reported using a sensory coding strategy and did not differ from the controls in overall performance or rate of forgetting. When the stimulus difference was 3/4 semitone, absolute‐pitch subjects shifted to a verbal coding strategy, and their memory for pitch was significantly better than the controls. With a one‐semitone difference, the absolute‐pitch group showed no significant forgetting over a 15‐sec retention interval filled with interference tones. These findings support the hypothesis that possessors of absolute pitch show superior pitch memory only when they can differentially label the stimuli with musical‐note names.

ISSN:0001-4966    

DOI:10.1121/1.1919544

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

An attempt will be made to clarify what is meant by “degree of lateralization” and to discover the conditions under which one can conclude that “Xis more lateralized thanY.” These notions are made explicit by reference to idealized “criterion experiments” for each of two different concepts of lateralization: hemispheric ability lateralization and hemispheric use lateralization. A person's “true” degree of lateralization on a task is then defined in terms of the scores he would get on a criterion experiment with that task. Indirect procedures such as dichotic listening or dichoptic viewing are normally necessary to provide a practical means of testing degree of laterization in the laboratory. The problem of finding the “best” scoring procedure for such tasks can be redefined in terms of finding functions which will transform the obtained scores on these tasks into good estimates of what the degree of lateralization scores on the appropriate criterion experiment would have been, had it been performed instead. Usually, the “best” lateralization estimating function is not known. It is therefore suggested that any conclusions of relative degrees of lateralization based on indirect (e.g., dichotic) experimental data be tested for “robustness,” i.e., tested to see that the conclusions remain the same regardless of which of the various plausible scoring procedures is used. A good rule is: if the conclusion remains valid for both POE and POC measures of laterality, it will remain valid across a broad family of plausible measures and is “robust.”

ISSN:0001-4966    

DOI:10.1121/1.1919545

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP