摘要:

The acoustic response of a semireverberant enclosure with two interacting, velocity prescribed source distributions was analyzed using standard modal analysis techniques with a view towards a better understanding of active noise control. Different source and enclosure dimensions, source separations, and single wall admittances were studied over representative bandwidths of 10 Hz, and between 40 and 300 Hz, with source relative phase as a parameter. Results indicate that power radiated into the enclosure correlates with the spatial average of the mean‐square pressure even though the reverberant field is nondiffuse. Power reductions can therefore be used to estimate global noise reduction in a nondiffuse semireverberant environment. Parametric studies indicate that maximum power reductions of up to 25 dB can be achieved when secondary and primary sources are compact and closely spaced, as might be expected. Although there is less reduction with increasing frequency and source separation or size, significant reductions of up to 8 dB still occur over 1‐ or 2‐Hz bandwidths.

ISSN:0001-4966    

DOI:10.1121/1.2022283

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

A brief tutorial presentation of modal analysis methods and experimental techniques with examples and comparisons to finite element results. Samples of animated mode shapes will be shown.

ISSN:0001-4966    

DOI:10.1121/1.2022284

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

The acoustic behavior of the three‐dimensional cavities of musical instruments can be studied using the acoustical finite element method. For some cases, one‐ and two‐dimensional finite element models may be used, resulting in more efficient computations and a simplification of the input. The 1‐D model is used for long, cone shaped cavities found in flutes, woodwinds, and brass instruments. Guitars, violins, and instruments with a small, but variable, change in height can be studied by the 2‐D model. Analytical models incorporating elemental geometries with lossy behavior have been used in the past for the quasi 1‐D case of woodwind instruments. A 1‐D finite element program was used for similar problems where the axisymmetric cavity was long with respect to its cross section. Tone holes are included in the model by using empirically determined impedance functions applied as boundary conditions. A 2‐D finite element program was written which incorporates the height of the cavity as a parameter at the nodes. The procedure correctly predicts frequencies and mode shapes of cavities with gradual changes in height. Limitations for the permissible rate of change in height are demonstrated.

ISSN:0001-4966    

DOI:10.1121/1.2022287

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

We will demonstrate the peculiar behavior of nonlinear oscillations with a collection of experiments: (1) A stretched rubber band, driven by a loudspeaker, exhibits hysteresis due to its bent resonance curve. (2) A doubly bent resonance curve and hysteresis are exhibited by a parametrically driven pendulum bouncing against stops. (3) Subharmonics can be heard when a loudspeaker, laid on its back, causes a pencil to bounce. (4) A hanging chain undergoes quasiperiodically modulated vibrations, where the modulations have a frequency independent of that of the drive. (5) A pendulum, free to rotate 360°, crosses the transition to deterministic chaos when driven with the proper oscillating torque. (6) Two metronomes on the same platform, initially ticking independently, pull each other into synchronism and phase lock, as seen by Christian Huygens in 1665. (7) A parametrically driven rigid pendulum will defy gravity by standing on end and oscillating upside down. (8) We will show the nonpropagating soliton, seen by the authors in a trough of water [Phys. Rev. Lett.52, 1421 (1984)], and two of these solitons oscillating about each other. [Work supported by ONR.]

ISSN:0001-4966    

DOI:10.1121/1.2022292

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

Young chickens suppress their regular peeping when they hear a change in the frequency of a tone. Gray and Rubel [J. Acoust. Soc. Am. Suppl. 176, S13 (1984)] estimated frequency difference limens in newborn chickens using peep suppressions and a nonstandard adaptive procedure. The definition of a correct response depended on an initial series of control trials used to estimate the distribution of suppressions expected when there was no change in frequency. Consistent improvement in frequency difference limens, approximately 1% in 4 days, was observed. A more controlled procedure involves the presentation of a stimulus and a control trial in pairs. When the duration of suppression is greater on the stimulus than control trial then the response is defined as correct, and transformed up‐down rules can be applied as usual. Unfortunately, the data from this procedure are inconsistent. Any increase in frequency selectivity over age is probably greater at high than low frequencies. Difference limens appear to be affected by intensity, but not by the direction of deltaFnor temporal characteristics of the stimulus. In conclusion, high variability makes frequency difference limens difficult to estimate in neonates. [Work supported by NIH.]

ISSN:0001-4966    

DOI:10.1121/1.2022301

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

Some researchers have indicated that substantial improvements in the sonar audio detection threshold may be achieved by a technique referred to as phase‐altered audio. The technique described in the literature, sometimes referred to as the masking level difference phenomenon, involves phase shifting the signal 180° and presenting this to one ear while the unphase shifted signal is presented to the other ear. The noise background is presented in phase at each ear. Alternatively, the noise to one ear may be phase shifted (180°) and the signal presented in phase at each ear. The degree of improvement is a function of the frequency of the signal and the degree of noise correlation. While it is clear from the literature that large improvements in audio detection threshold may be achieved under laboratory conditions where the signal and noise can be independently controlled, it is not clear how this phenomenon can be utilized to improve the audio presentation to sonar operators. A series of tests with a cylindrical array sonar was conducted at a US Navy Ashore Training Facility and a contractor test facility to determine if a phase‐altered approach offers any substantial improvements to sonar operators on a passive sonar. The tests showed no significant benefit from the masking level difference phenomena for any phase‐altered audio mode in the normal operating range of passive sonar half‐beam noise correlation. But there was a significant improvement insum(right half‐beam plus left half‐beam) beam audio performance for high negative half‐beam noise correlation and withdifference(right half‐beam minus left half‐beam) beam audio performance for high positive half‐beam noise correlation. Any improvement in performance at high half‐beam noise correlations was due primarily to thesumordifferencebeam gain and not the masking level difference phenomena. [Work supported by NAVSEA.]

ISSN:0001-4966    

DOI:10.1121/1.2022305

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

What is the relationship between syntactic constituent structure and the intonation of a sentence? Psycholinguistic answers have varied, some viewing intonation as a close derivative of syntactic composition. The present study of this issue comparedF0 and perceptual attributes of a number of syntactically and lexically identical sentence pairs. In each pair produced by a single talker, each sentence had the same pragmatic, thematic, and semantic function within a larger monologue of at least 40‐s duration, but one member had been produced spontaneously, and the other was read from a text prepared from the spontaneous corpus. Our linguistic results of the comparison of two kinds of fluent speech are discussed in terms of intonation in reading and spontaneous speech, as well as in terms of the phonological control of intonation. Our perceptual results are discussed with respect to the listener's sensitivity to the spontaneous/read distinction, and the implications for the potential contribution of intonation to sentence perception.

ISSN:0001-4966    

DOI:10.1121/1.2022306

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

Knowing the position of lexical stress in a polysyllabic word may considerably limit the number of lexically possible responses. Thus perceptual cues may contribute to a higher probability of correct recognition and faster recognition of words in the perception of speech of less than optimal quality. Probability correct was assessed for 30 three‐syllable Dutch words, synthesized from diphones. All words were of CVCVCVC phonemic structure, containing no consonant clusters. Ten words each had initial, medial, or final lexical stress. Stress position was either not cued at all, cued by durational cues only, cued by a conspicious pitch movement, or cued by both durational and pitch cues. Each word in each condition was responded to by five listeners. Scores correct, averaged over stress position, were 66% with no stress cues, 74% with durational cues, 81% with pitch cues, and 81% with durational and pitch cues combined. The contribution of stress cues was strongest for words with initial stress (from 54%–76%), and minimal for words with final stress (from 80%–88%). To investigate the effect of stress cues on the rapidity with which listeners tune in on the correct response, a gating‐paradigm‐type of experiment was performed on the same material. However, in this task errors against stress position were much more frequent, and positive effects of stress position were small.

ISSN:0001-4966    

DOI:10.1121/1.2022315

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

Comparison of the Biot theory with experimental results over a broad range of frequencies has proven to be difficult because the coefficients in Biot's equations are functions of frequency whose forms depend upon the microstructure of the medium. In this work, Biot's equations are used to analyze the propagation of plane waves parallel to the layers of a laminated material consisting of alternating layers of an elastic material and a compressible viscous fluid. For this case, all of the coefficients in the theory can be evaluated. In particular, the drag and virtual mass coefficients can be evaluated as functions of frequency using the method of Bedford, Stern, and Costley [J. Acoust. Soc. Am. Suppl. 174, S57 (1983)]. Furthermore, the phase velocity and attenuation of plane waves in the layered medium can be determined exactly. Although it is for a special geometry, this comparison provides a definitive test of the ability of the Biot theory to model a two‐phase material. It is shown that the Biot theory matches the first two modes of the exact results with great accuracy over a large range of frequency. However, this result is shown to depend upon the use of an appropriate frequency‐dependent expression for the virtual mass coefficient.

ISSN:0001-4966    

DOI:10.1121/1.2022327

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP

摘要:

In this paper, a procedure is described for finding the backscattered response of an elastic, solid particle in the nearfield of a high‐frequency ultrasonic transducer using aTmatrix to characterize the response of the scatterer itself. Numerical results are presented for spherical particles for both shaded and unshaded transducers. The particular application of interest is the monitoring of the vertical profile of sedimentary suspensions up to a meter above the abyssal ocean floor. For a 5‐MHz transducer, particles at distances of less than 50 cm from the transducer are in the complicated nearfield. The backscattered acoustic response of the sediment particles in a given volume as measured by an ultrasonic transducer is directly related to the concentration and size distribution of the particles.

ISSN:0001-4966    

DOI:10.1121/1.2022330

出版商:Acoustical Society of America    

年代:1985

数据来源: AIP