摘要:

To show the effect of the temporal pattern of acoustic stimulation on TTS15 min, guinea pigs were subjected to isoenergetic noises with the same spectrum. The exposures in a first experimental series were continuous noises and noise bursts. The continuous noise was presented with different durations and levels but always with the same energy. The noise burst stimulation consisted of a constant number of bursts with different interstimulus intervals. Both duration and repetition rate were shown to affect the TTS15 minmeasured for these isoenergetic stimuli. A duration of 225 to 1800 s and a repetition rate of one per second produced the greatest TTS15 min. In a second experimental series continuous noise and acoustic impulses with the same spectrum and 100‐Hz repetition rate were presented at different levels. In this case the waveform of the stimulus (phase spectrum) was shown to have an effect on TTS15 min.

ISSN:0001-4966    

DOI:10.1121/1.391401

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

This paper is concerned with the effects of time‐variant (including time‐invariant) linear systems (filters) upon the nonstationarity of an acquired random process. The basis approach is the frequency domain description and, therefore, the relationship between the modulation function (MF) and the power spectral density function is emphasized though the both are time‐dependent for nonstationary processes. In particular, this paper proposes a general series form of the input–output relationship expressed in terms of the relevant modulation functions of the processes. Based upon this form, the problems of filtering a nonstationary input can be conveniently handled. The system performance is also described by a time‐variant differential equation and the technique proposed in this paper is the so‐called Modulation Function Equation (MFE) method which is an alternative time‐variant differential equation, derived from an ordinary one, yet with the randomness excluded. The principle of deriving a class of MFE is presented and the technique is applied to some practical problems, e.g., trolley wires, in order to describe their responses to a random nonstationary excitation (noise and/or vibration). The solutions of the formed MFE are, however, essentially those of the equivalence of the frequency implementation which is now defined as being the frequency/time two‐dimensional function for the time‐variant systems (filters). By invoking the MFE technique the implementation can be conducted numerically or analytically. This paper discusses such possibilities and, in particular, develops two approximations: namely the System Iterative Method (SIM) and the Power Series Method (PSM) for a class of time‐variant MFEs; they both may be usefully applied to certain nonstationary problems.

ISSN:0001-4966    

DOI:10.1121/1.391402

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

The flight parameter of an aircraft moving without acceleration can be estimated by exploiting the target‐generated noise received by a directional sensor consisting of three or four microphones. The Cramér–Rao lower bound is calculated to investigate the theoretical accuracy of estimates. Moreover, an algorithm is proposed which computes the maximum‐likelihood estimate. Extended simulations prove the algorithm to work reliably and fast, and the estimate to be nearly unbiased and efficient when averaged over a large region of flight parameters. Results obtained with real data are included.

ISSN:0001-4966    

DOI:10.1121/1.391403

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

A class of consistent frequency‐wavenumber spectral estimators of the ambient noise field from the output of a totally randomn‐dimensional array is introduced. The performance of these spectral estimators is established. In particular, precise asymptotic expressions for the bias, variance, and covariance of the estimators are obtained together with exact rates of quadratic‐mean convergence. It is shown that the results hold for any positive value of the density of the number of sensors per unit volume and are thus valid even for sparse arrays. An application to a representative planar array is given.

ISSN:0001-4966    

DOI:10.1121/1.391404

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Coherent detection processors for sonar (e.g., matched filters for active systems) require continuous time series as input and have conventionally been coupled to delay‐and‐sum time‐domain beamformers. As an alternative, we have extended the FFT beamforming technique to be functionally equivalent to a time‐domain beamformer. The procedure utilizes the ‘‘analytic’’ representations of the hydrophone signals as input and produces continuous beam time series as output, thereby allowing for complete flexibility in post‐beamformer processing. The method will be applicable when the array elements are uniformly spaced, and when a large fraction of the possible beams are to be computed. In such cases, the algorithm offers the following advantages relative to a conventional time‐domain beamformer: (1) elimination of the need for a high input sampling rate to achieve acceptable beam patterns—the frequency‐domain approach is insensitive to the sampling rate provided this exceeds the (bandpass) Nyquist rate; (2) reduction of high beam sidelobe levels due to dead or malfunctioning array elements—the response of missing array elements is readily estimated in the frequency domain by interpolating from the spectra of neighboring sensors; and (3) potential reductions in the computational load when the array is large or the center‐frequency/bandwidth ratio of the signal is high. Using a general purpose array processor we have demonstrated near real‐time evaluation of a complete beam set for a 128‐channel linear array. Computer‐generated ‘‘acoustic’’ data were used to confirm that the software preserves the envelope, spectrum, and temporal correlation properties of signals while yielding near‐theoretical performance in reducing beam sidelobe levels.

ISSN:0001-4966    

DOI:10.1121/1.391405

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

The sonar equation is the basis for the prediction of sonar performance. Under certain assumptions concerning the signal‐to‐noise probability density function, one may calculate the probability of detection for a system with known processor gain. Herein, the detection process is assumed to be applied to a series of nonoverlapping time intervals. The signal‐to‐noise ratio in dB is assumed to be the sum of two independent Gaussian random variables, one of which is perfectly correlated from interval to interval, the other of which is uncorrelated. Analytic expressions for probability of detection are derived and evaluated numerically for various cases. In addition, the probability of signal excess being greater than zero is calculated and compared to the probability of detection.

ISSN:0001-4966    

DOI:10.1121/1.391406

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

If new vehicle noise levels cannot be predicted accurately, manufacturers must overdesign for quietness in order to be sure of meeting noise standards. To determine factors affecting sound levels, nine truck–tractor configurations were tested according to SAE Standard J366b, Exterior Sound Level for Heavy Trucks and Buses during a variety of weather conditions at three, 2‐week intervals over a 6‐month period. Tests made according to the standard produce reasonably repeatable results when a vehicle is tested repeatedly at the same test site (σ≤0.83 dB). There was a statistically significant but weak effect of temperature on the A‐weighted sound level located 15 m from the center line of vehicle travel for three of nine truck test configurations, showing decreasing sound level with increasing temperature. The coefficient of this temperature effect averaged about −0.020 dB per degree Fahrenheit. Sound emission from truck–tractors was affected by temperature for eight of nine truck configurations. Humidity, barometric pressure, wind, and wind and temperature gradients have no consistent pattern of effect on J366b noise levels. The SAE J366b passby test sound levels cannot be practically corrected for environmental effects, and probably need not be if the requirements of the standard are carefully adhered to. Coastby and stationary tests were not notably more repeatable than the J366b passby test and one type of test cannot be used to predict the result of another.

ISSN:0001-4966    

DOI:10.1121/1.391407

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

There have been nearly a dozen papers published in the last 5 years which compare community response to noise from several different sources such as road and rail traffic. This paper draws on those and other studies to address three questions. First, is the variation in results simply due to random sources or measurement error, such that use of an ‘‘average’’ dose‐response curve is appropriate, or is there some identifiable systematic variation? Second, what characteristics of the noise or community might lead to systematic variation, if there is any? Third, under what conditions is it reasonable to use an average function, even if there are systematic variations? Both of the first two questions receive only tentative answers on the basis of available studies. Although the evidence shows different functions for different sources (e.g., rail and road noise), for different types of one source (e.g., air carrier airports and general aviation airports), and even for different studies at the same location (e.g., Heathrow), the evidence is not sufficiently strong to totally reject the idea that all of this is just random variation about an ‘‘average’’ response. As yet, there is no clear identification of what acoustical or community factors explain the differences in response, so it is sensible simply to use the type of source (i.e., road, rail, or air traffic noise) to categorize the differences. The answer to the third question, then, is that an ‘‘average’’ dose‐response curve is useful in the face of limited information—that is, when we cannot specify precisely the conditions calling for different functions.

ISSN:0001-4966    

DOI:10.1121/1.391408

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

The role of group invariance in nonlinear motion of strings and rods is discussed. Problems relating to propagation of waves in rods with nonlinear elastic, elastoplastic, and viscoplastic materials are considered. A classification of linear and nonlinear equations of motion of rods and strings is given by the study of their invariance under inspectional groups, namely, dimensional, translational, and spiral groups of transformations.

ISSN:0001-4966    

DOI:10.1121/1.391409

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

The ECOLOG dual‐beam echo‐counting and sizing system is used in both a laboratory and a field situation with moderate densities of demersal fish. Free swimming fish in a large tank give estimates of backscattering cross section (σbs) with low variance. Application in field surveys confirms that dual‐beam acoustic data provide information on both size composition and density distribution, which is different than single‐beam estimates but similar to and at least as precise as that provided by trawl nets.

ISSN:0001-4966    

DOI:10.1121/1.391410

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP