摘要:

Underwater ambient noise has been measured in the central Arctic Ocean on three separate occasions with the use of a horizontal and a vertical array. These data help to identify various noise mechanisms. Of principal interest is noise caused by various ice cracking events which depend upon stress levels in the ice and which seem to have varying temporal and spectral characteristics. I review these mechanisms and show how the data relate to them. While spatial analyses have been carried out only at very low frequencies, spectral analyses cover the range from 1 to 10 000 Hz. In this frequency range noise mechanisms other than pack ice are sometimes important, such as wind‐generated noise and earthquake noise. [Work supported by ONR.]

ISSN:0001-4966    

DOI:10.1121/1.2020845

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

A generic class of sonar receivers are modeled in an attempt to describe average receiver performance in the Arctic channel. A ray‐theoretic model, discussed in a companion paper, describes the average reverberation intensities within an Arctic channel in the presence of pressure ridge scattering. Receiver performance is evaluated in terms of signal detectability as a function of range, ridge location, and source/receiver depth. The models are extremely general in that arbitrary pulse shapes and sensor locations are allowed. As an illustration, two pulse shapes are considered—a transient and a CTFM pulse. Their effect on receiver performance is dependent on the source/receiver geometry as well as ridge location. In general, however, the longer CTFM pulse often leads to a reverberation‐limited condition as a result of surface as well as pressure ridge scattering. [Work supported by ONR.]

ISSN:0001-4966    

DOI:10.1121/1.2020899

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

The literature on digital signal processing is based on four kinds of transform: Fourier transform (FT),z‐transform, Fourier series, and discrete Fourier transform (DFT). Digital implementation uses only the DFT, usually in the form of an FFT algorithm. Some popular texts attempt to describe digital results by using analog (usually FT) analysis. Some have presented analytical results for infinite length random signals and then used illustrations based on finite length periodic signals. What “windows” do and their different effects on random and periodic signals seldom are clearly identified. There are two implementations of the DFT that differ by a constant factor. Fast convolution and correlation analyses require special handling to avoid “circular” effects. The DFT treats each finite length sample from a random signal as if it were a sample from a deterministic signal. The purpose of this tutorial is to illuminate these and other potentially troublesome aspects of digital signal processing pertinent to applications in sound and vibration analysis.

ISSN:0001-4966    

DOI:10.1121/1.2020949

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

When used for frequency analysis, an FFT analyzer produces a line spectrum on the amplitude‐frequency plane. Each spectrum line represents the output of a filter centered at the frequency associated with the line, the shape of the filter being determined by the effective time window applied to the data being analyzed. While it is generally recognized that the use of an explicit window function is often desirable—e.g., to control spectral “leakage”—many FFT users seem unaware thatuse of a window implies a compensatory calibration adjustment that is dependent upon the character of the data and the objective of the analysis. Effective filter shapes for rectangular, Hanning, flat top, and Kaiser‐Bessel windows are discussed along with the concept of filter amplitude response and effective noise bandwidth. Appropriate calibration for rms, power, power spectral density, and energy spectral density spectra is explained.

ISSN:0001-4966    

DOI:10.1121/1.2021003

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Arrays of microphones have long been used in underwater acoustics, but only recently has it been appreciated that the directivity of microphone arrays makes them superior to conventional microphones in many audio applications as well. In this paper we describe a particular microphone array, the Quorum TM teleconferencing microphone, whose directivity is achieved by the nonuniform spacing of the transducers. We present equations for the array sensitivity assuming plane and spherical wave radiation, and compare these results with measurements.

ISSN:0001-4966    

DOI:10.1121/1.2021052

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

A simple procedure is described for finding the direction to the source of an acoustic signal—primarily the human voice. The method, in effect, constructs the cross correlation of severely distorted signals from two microphones. The distortion, a combination of time‐domain processing and digital logic, is such to give strong preference to direct‐path early arrivals over reflections that follow, and to produce streams of pulses whose cross correlations can be computed very simply. The method has been implemented as a combination of custom hardware, and a simple microprocessor. The implementation operates in real time (with ∼0.2‐s delay) and produces outputs to indicate either the azimuth angle, or inter‐microphone delay. Possible uses include self‐aiming directional microphones, automatic aiming or switching of video cameras, and remote identification of talkers in nonvideo teleconferencing.

ISSN:0001-4966    

DOI:10.1121/1.2021106

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

In nearly quiescent seismic environments, many bullfrog (Rana catesbeiana) saccular afferent axons exhibit ongoing activity, typically ranging from 10 to 80 spikes/s. Low‐level vibration of the whole animal [see H. Koyamaet al., Brain Res.250, 168–172 (1982) for methods] produces linear modulation of this activity, allowing one to define small‐signal linear transfer ratios and to observe tuning curves for such axons. Each tuning curve to date reflects multiple resonances, sometimes coalescing with no intervening antiresonances, sometimes separated by simple antiresonances. The latter typically are much sharper than the resonances on either side, indicating that they are not simply concomitants of additive parallel resonances. Phase lag typically increases by two cycles or more with increasing frequency through the tuning range (roughly 10 to 200 Hz), with abrupt half‐cycle declines through the antiresonances. Resonant and antiresonant frequencies vary from axon to axon, usually lying between 20 and 120 Hz. The amplitudes of peak transfer ratios vary approximately from 50 to 200 spikes/s per cm/s2. At 100 Hz, where it occasionally is found, the latter corresponds to approximately one spike/s response for whole‐animal displacement of 10 pm. [Supported by NSF Grant BNS‐8005834.]

ISSN:0001-4966    

DOI:10.1121/1.2021154

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Electrophysiological and psychophysical measures of threshold as a function of sinusoidal stimulus frequency deviate from those predicted by Hill's excitation model. Several membrane models (e.g., the Frankenhauser‐Huxley model and the Hodgkin‐Huxley model) suggest possible mechanisms responsible for the deviations. For example, by using a modified Hodgkin‐Huxley model more accurate threshold “predictions” were obtained over the 50–500‐Hz frequency range. The Hodgkin‐Huxley model was modified by increasing the sodium inactivation rate constant, beta‐h, by a factor of 4. This modification caused the hyperpolarizing phase of the stimulus to become more effective at increasing the simulated membrane's excitability to the subsequent depolarizing phase, particularly when the hyperpolarizing phase was on the order of 10 ms in duration. Consistent with this model, recordings in the AVCN of eat indicate that a hyperpolarizing electrical stimulus applied to the auditory nerve significantly increases the excitability of the nerve to a depolarizing stimulus that follows. [Work supported by NIH.]

ISSN:0001-4966    

DOI:10.1121/1.2021210

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

We studied the perceptual integration of two complex tones, each sent to both ears and formed by amplitude modulation (AM) of a carrier frequency CF by a sinusoid with modulation frequency MF. One tone always had CF = 1500 Hz and MF = 100 Hz. The other had CFs around 500 Hz and MFs around 100 Hz. Both harmonic and inharmonic partials, produced by AM, were employed. The method involved studying the competition between two organizations: (a) the fusion of the two tones, and (b) the tendency of the higher to be stripped out of the mixture by a competing sequential organization. Fusion was best when the both tones had the same MF, even when the resulting partials did not form part of the same harmonic series. Fusion was enhanced when AM applied to the two tones was in phase. Results relate to the perceptual separation of simultaneous voices and favor a theory in which basilar membrane outputs that are amplitude modulated by the same glottal pulse will be allocated to the same voice.

ISSN:0001-4966    

DOI:10.1121/1.2021268

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP

摘要:

Twenty‐four speakers prolonged the vowels /a e i o u/ and read the passage “My Grandfather.” The speakers were divided into four groups of six (three males, three females) based on chronological age, 41–50 years, 51–60 years, 61–70 years, and 71 years and over. Naive listeners provided perceptual judgements of each speaker's age based on the first two sentences from “My Grandfather.” The speakers' vowel productions were measured to determine time required to initiate and terminate each vowel. These ten measures plus time to articulate the four syllables /ma ɪ grændfaðɚ/ and speaker sex were regressed against speaker chronological and perceived ages. The best significant model for chronological age yielded anR‐Square of only 0.428; while for the best significant model for perceived ageR‐Square was 0.647. Results indicate that speaker temporal control of speech exerts a strong influence on listener perception of speaker age, but has a relatively weaker relationship with speaker's chronological age. These findings suggest that temporal aspects of speech are critical in estimation of speaker age.

ISSN:0001-4966    

DOI:10.1121/1.2020731

出版商:Acoustical Society of America    

年代:1983

数据来源: AIP