摘要:

Natural resonant frequencies of oblate acoustical resonators with either perfectly soft or perfectly rigid boundaries are calculated by locating the zeros ofRmn(1)(−ih, iξ)andRmn(1)′(−ih, iξ). These frequencies are normalized to those of the corresponding modes of a spherical cavity with the same volume and presented for various ratios of major to minor axes.

ISSN:0001-4966    

DOI:10.1121/1.1912817

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

The sensitivity and frequency response characteristics of type‐L standard laboratory microphones were measured with an electrostatic actuator at surface and at 12 different levels to 31 atm absolute (ATA) in a hyperbaric chamber. The microphones were measured in compressed air and a helium‐air environment, with and without the capillary pin. In addition, one microphone remained at a depth of 31 ATA for 24 h to measure its stability under pressure. Results indicated a loss in sensitivity in compressed air and helium‐air as a function of pressure. In a compressed‐air environment a resonant peak occurred at 4000 Hz, which shifted to 8000 Hz in the helium‐air environment. The microphones showed extremely close agreement between compression and decompression, between different pressurizations, and between different microphones. Also, there was no change in sensitivity over a 24‐h period of pressurization at 31 ATA. Calibration data at several pressures are given.

ISSN:0001-4966    

DOI:10.1121/1.1912823

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

Experiments were conducted on propeller fan blades at free‐flow conditions to determine the noise reduction associated with the use of rigid porous materials as the blade material in lieu of solid material. The results suggest that significant sound reduction can be achieved with no loss of pumping efficiency when the pitch angle is increased from normal.

ISSN:0001-4966    

DOI:10.1121/1.1912815

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

There is a significant excess attenuation of sound traveling downwind within a typical turbulent shear profile due to the downwind refraction and spreading of the sound rays. This attenuation is in excess of that due to absorption by turbulent eddies, spherical spreading, and ground reflection. When the source and receiver are both located on the ground, the excess attenuation of the directly transmitted sound due to refraction is predicted to be uniform at −11 dB beyond a short range downwind of the source until a shadow zone is reached at a much longer range. This predicted excess attenuation decreases markedly with the elevation of the source and receiver, particularly at short range. There can be an indefinitely large number of reflected rays transmitted to a receiver downwind of a ground‐level source, and the shallow reflected rays which skip off the ground a large number of times may have little excess attenuation, since they travel through the laminar sublayer where the velocity gradient is constant. There are several caustics formed downwind of a ground‐level source. The results show that wind can easily cause anomalous recording of the sound‐pressure level from freeways, airports, and other urban noise sources.

ISSN:0001-4966    

DOI:10.1121/1.1912816

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

Passive processing is interpretation of the noise and noise like signals received by an array of hydrophones. Our model assumes that there is an important noise component about which very little is known, that the data are stationary, that a long data sample is available, that signal wavefronts are planar, and that the response of the array to plane waves is known. We propose a processor and compare it with high‐resolution frequency‐wavenumber spectrum analysis. LetF̂denote an estimate of the spectral density matrix, and letHdenote any spectral density matrix that corresponds to a noise field composed of uncorrelated plane waves. A reasonable estimate of the spectral matrix of the signals and ambient noise is a value ofHfor whichF̂‐His positive semidefinite and traceHis maximized. The rationale for this estimate is closely related to a rationale for the estimate obtained by high‐resolution frequency‐wavenumber spectrum analysis (which is also called adaptive beamforming). The difference is that the former extracts the contribution from all directions at once, whereas the latter extracts the contribution from each direction separately. The former estimate rejects more unwanted noise than the latter.

ISSN:0001-4966    

DOI:10.1121/1.1912818

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

This paper considers the bearing estimation performance of a monopulse processor for passive linear arrays. The variance of the bearing estimator for this process is derived and compared with computer simulation results for different integration times and input signal‐to‐noise (S/N) ratios. The simulation results illustrate how and why the bearing estimate degrades for very poor S/N ratios or very short integration times.

ISSN:0001-4966    

DOI:10.1121/1.1912819

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

Some of the results obtained by Yu are compared with values obtained by other investigators. In the case of a square plate, Yu's results are approximately 8% higher than the exact values.

ISSN:0001-4966    

DOI:10.1121/1.1912821

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

The distinction between categorical and continuous modes of perception has played a central role in discussions of speech perception. Recent findings of Fujisaki and Kawashima [J. Acoust. Soc. Amer.47, 57 (1970)] suggest that these two modes are not necessarily dichotomous but may be related to the degree to which some brief auditory short‐term memory store is employed during discrimination. The present experiment examined discrimination performance within and between phoneme boundaries using a delayed comparison AX same‐different task. The delay interval between presentations of the standard and comparison sounds varied over a range from 0–2.0 sec for each of four types of synthetic speech stimuli. For steady‐state vowels accuracy of discrimination both within and between boundaries was related to changes in the delay interval. Discrimination of stop consonants remained relatively stable both within and between boundaries. The results suggest that vowels (even as short as 50 msec in duration) are stored differently than stop consonants in auditory short‐term memory. [Research supported in part by grant to Haskins Laboratories from NICHD.]

ISSN:0001-4966    

DOI:10.1121/1.1981614

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

Are meaningless sounds learned and identified in a speech mode? Sixteen sounds were generated differing on four dimension, each having two values. These were: the source waveform contained either all harmonics or odd harmonics; the fundamental frequency was either 90 or 142 Hz; there were either one or two formant frequency regions; and the formant(s) was (were) either low, 600 (and 1550) Hz or high, 940 (and 2440) Hz. Twenty‐four listeners identified these sounds. Fewer confusions were made between sounds (1) as the number of dimensions on which they differed increased, and (2) as the dimension(s) changed, from formant number to ferment frequency region, to fundamental frequency, to source waveform. Similarly, fewer confusions were made between sounds whose formant frequencies and fundamental frequency were in a fixed ratio than between those with the same formant frequency. In essence fixed ferment sounds were more often called the same sound than were fixed ratio sounds. The data fit a hypothesis that classification was made on a generalized “pitch” dimension.

ISSN:0001-4966    

DOI:10.1121/1.1981619

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP

摘要:

Acoustic Radiation from a spherical source eccentrically positioned within a fluid sphere embedded in another fluid is evaluated. This configuration is an idealization of a spherical acoustic lens used as a sound projector. To effect a solution, the translational addition theorem for spherical wavefunctions is used to express series of wavemodes centered at one sphere in terms of modes centered at the other, thereby facilitating the task of satisfying the boundary conditions. Because spherical wavefunctions centered at one point are not orthogonal over the surface of a sphere centered elsewhere, each mode associated with the first sphere couples into every one of the modes associated with the other. Hence, the evaluation of the modal coefficients in the series for the radiated pressure necessitates solving an infinite set of equations. Approximate solutions are obtained by truncating the set to finite size and solving numerically. The source radiation impedance and farfield directivity have been calculated for representative values of the many parameters that characterize the problem, such as wavelength size of fluid sphere and source, relative characteristic impedances and sound velocities of the two fluids, and position and velocity distribution of the source. [Research was supported by the Ordnance Research Laboratory under contract with the U. S. Naval Ordance Systems Command.]

ISSN:0001-4966    

DOI:10.1121/1.1981624

出版商:Acoustical Society of America    

年代:1972

数据来源: AIP