Volume 60, Issue 6, December 1976
Index of content:
60(1976); http://dx.doi.org/10.1121/1.381237View Description Hide Description
The physical optics farfield inverse scattering (POFFIS) identity relates the phase‐ and range‐normalized farfield backscattering amplitude to the spatial Fourier transform of the characteristic function of the scattering obstacle. The characteristic function is equal to unity in the region occupied by the obstacle and zero elsewhere. The original identity was derived by Bojarski for impulsive point sources. Here we extend the result to sources of arbitrary time dependence. We obtain an alternative form of Bojarski’s POFFIS identity. We also derive a POFFIS identity in the time domain. Numerically synthesized checks on the method are provided.
Subject Classification: 20.30.
60(1976); http://dx.doi.org/10.1121/1.381238View Description Hide Description
The fluid radiation loading exhibited by a finite cylindrical shell with rigid immovable end plates is investigated by implementing the methods of Fourier integral transforms. The solution is obtained for a generalized velocity distribution on the cylindrical surface. The numerical solution for a radially pulsating surface is utilized as the basis for comparison to previous investigations. In addition, the radiation loading displayed by the low‐order modes of a simply supported shell section is obtained, and the relative effect of the imposed condition of an infinite cylindrical baffle embodying the shell is ascertained. It is shown that the classical solution for a vibrating band on an otherwise rigid surface offers a reasonable approximation in the determination of the fluid radiation loading imparted to a finite cylindrical shell with rigid end closures.
Subject Classification: 20.60.
60(1976); http://dx.doi.org/10.1121/1.381239View Description Hide Description
Experimental measurements of the backscattering of a plane acoustic wave by a rigid cylinder parallel to an elastic plate are compared with theoretical calculations. Two sets of calculations, one which includes the effects of multiple scattering between the cylinder and the plate abd one which ignores multiple scattering between the cylinder and the plate and one which ignores these effects, were performed. The measurements were done in fresh water these effects, were performed. The measurements were done in fresh water using a 5.08‐cm‐diam solid steel cylinder at various distances from a 0.635 cm‐thick steel plate. The frequency of the incident wave was varied from 50 to 100 kHz. Comparisons between the theories and experiment were made both for the case of the cylinder in front of the plate and for the case of the cylinder behind the plate. The results confirm that multiple scatteringeffects should be included in these scattering calculations.
Subject Classification: 30.40; 20.30.
60(1976); http://dx.doi.org/10.1121/1.381240View Description Hide Description
The pure‐water high‐pressure sound‐speed data of Wilson have been analyzed, and it is concluded that all Wilson’s values should be shifted by the apparent error of his 1‐atm values. A new equation has been determined to represent the shifted pressure data of Wilson, as well as the best 1‐atm data of Del Grosso and Mader.
Subject Classification: 30.25.
60(1976); http://dx.doi.org/10.1121/1.381241View Description Hide Description
An analysis is presented for coherent multipath propagation between a moving source and a fixed receiver. The major mulitpath effects of source motion are frequency spreading range or emission‐coordinate spreading, and travel‐time spreading of the multipath arrival set. These are analytically related to the signal propgation as functions of time and velocity. Computer simulations with a shallow moving source and a fixed receiver below the SOFAR channel are presented which clearly illustrate the contributions of source motion developed in the analysis.
Subject Classification: 30.20 20.20..
60(1976); http://dx.doi.org/10.1121/1.381242View Description Hide Description
Equations are derived for the power transferred between normal modes due to scattering from weak, random variations in the horizontal strati‐ fication of the ocean.Perturbation theory is used to obtain results for a horizontally isotropic medium. The validity of these results is discussed for the case of dorward scattering. The modal power transfer given by two‐ dimensional model will, in many cases, suffice to estimate modal power transfer.
Subject Classification: 30.20; 20.15.
60(1976); http://dx.doi.org/10.1121/1.381243View Description Hide Description
Infrasonic and low‐frequency sonic sea noise was recorded for seven days in the 5‐ to 32‐Hz band in the seismically active rift valley of the Mid‐Atlantic Ridge near the Azores. Two ocean bottom seismographs (OBS) in water depths of about 2400 m made continuous, simultaneous recordings of acoustic and seismic signals generated by seaquakes. The dominant source of deep sea noise averaged over the recording interval was distant shipping traffic, deduced by comparing mean OBS levels to Wenz’s shipping noise estimates. Mean spectrum levels between 5 and 10 Hz did not exceed 80 dB//μPa, while mean levels at frequencies above 10 Hz were higher. The mean spectrum level at 5 Hz was about 9 dB lower than at 30 Hz, indicating that distant shipping traffic noise does not contribute to the noise field below 5 Hz. Radiated noise due to nearby ship passages comprised about 13% of the recording time and produced spectral lines at frequencies as low as 6.5 Hz. Over 515 seismic events were detected by one OBS unit; epicenters were estimated to be from 3 to 10 NM from the instrument. Seaquakes took place at a mean rate of 2.9 events/h, persisted from 12 sec to about 2 min, varied in Richter magnitude from 0 to 2.5, occurred as a swarn on at least one occasion and often raised broadband acoustic levels in excess of 20 dB.
Subject Classification: 30.70; 28.30.
60(1976); http://dx.doi.org/10.1121/1.381244View Description Hide Description
During a 1957 field study at the Chesapeake Bay, abnormal attenuation was detected in 18% of the 1.0‐MHz data, 22% of the 2.0‐MHz data, and 45% of the 3.0‐ and 5.0‐MHz data. This ranged from 0.4 to 7.4 dB/m above the attenuation of fresh water. The interpretation of the data was facilitated by stable water conditions and the strong location orientation of results which permitted a separation of variables. The added attenuation appears to arise wholly from the presence of microbubbles which enter principally from atmospheric aerosols. The absence of abnormal attenuation in regions that receive significant drainage from residential and industrial areas is attributed to trace chemicals that modify of manner the microbubble stabilization. In regions of heavy ship traffic, on the other hand, wake bubbles may transport these trace chemicals to the surface, leaving a residue of smaller microbubbles that produce a high level of excess attenuation.
Subject Classification: 30.20.
60(1976); http://dx.doi.org/10.1121/1.381222View Description Hide Description
An interferometer method for the measurement of the ultrasonic propagation velocity and absorption coefficient in solids is described. The method provides a simple procedure to permit accurate evaluation using standard laboratory test instruments. Impedance and frequency measurements at successive maxima and minima are made using a half‐wave transducer coupled to a short sample. Experimental results of velocity versus temperature and absorption coefficient for polystyrene at 2 MHz are given and the results compared with other published values.
Subject Classification: 35.80, 35.26.
60(1976); http://dx.doi.org/10.1121/1.381223View Description Hide Description
The shock spectrum of the foundation or isolator mass of a two‐degree‐of‐freedom vibratory system was studied. The system had a cubic hardening elastic nonlinearity in the foundation or isolator restoring force. The system was impulsively shocked, and analytical, experimental, and numerical methods used to determine the resulting shock spectrum. The system was studied theoretically in two ways. An analytic solution was developed using a perturbation expansion of the nonlinear equations of motion, combined with an analytic solution for the shock spectrum due to the motion. A numerical solution to the nonlinear equations of motion was developed and used to verify the range of validity of the solution developed from the perturbation expansion. The nondimensional parameter of the perturbation expansion was found to be directly dependent on the coefficient of the cubic hardening spring and the square of the initial velocity of the foundation or isolator mass, and inversely dependent on the generalized mass and the fourth power of the natural frequency of each linear mode. The numerical solutions of the equations of motion verified that variations in this parameter defined the degree to which the behavior of the nonlinear system departed from the behavior of the linear system. The theoretical studies predicted that the cubic hardening spring would cause the peaks in the shock spectrum corresponding to the linear natural frequencies to be shifted to higher frequencies, and that significant additional peaks would be introduced at combinations of the shifted frequencies. An experimental study of a two‐degree‐of‐freedom system with a cubic hardening spring was undertaken. The experimental results verified shifting of the frequencies of the peaks and introduction of additional peaks in the shock spectrum.
Subject Classification: 40.40, 40.30, 40.60.
Array processors for simultaneous noise and interference suppression or simultaneous signal‐to‐noise and resolution enhancement60(1976); http://dx.doi.org/10.1121/1.381224View Description Hide Description
A symmetrical treatment is described of signal and interference arrivals, both spatially coherent across the aperture of a receiving array and appearing in a generalized ambient‐noise background. By separating the coherent interference from the rest of the noise background, it is possible to implement a simple form of adaptive suppression of the interference, particularly when the latter is time varying and the remainder of the noise is stationary. Optimal processors are derived for simultaneous suppression of noise and interference, and for enhancement of the signal‐to‐noise ratio after elimination of interference. The processor for the latter case is shown to be a suboptimum version of the former. Both deterministic and stochastic interference are considered, as well as interference whose arrival time is synchronized with that of the desired signal (e.g., unwanted signal multipath arrivals, reverberation, etc.). Processors for two or more simultaneous and independent interference sources are described. Finally, the interference descriptor can be reinterpreted as a generalized resolution measure, so as to describe processors that simultaneously maximize the signal‐to‐noise ratio and the signal time resolution, or the signal‐to‐noise ratio and a combination of arrival angle and time resolution.
Subject Classification: 60.30.
60(1976); http://dx.doi.org/10.1121/1.381225View Description Hide Description
A design procedure is developed for digital bandpass filters having quadrature sampling. The design process is developed from Nth‐order bandpass sampling reconstruction theory. These filters possess properties that satisfy the requirements of a class of digital filter problems.
Subject Classification: 60.20; 85.32.
60(1976); http://dx.doi.org/10.1121/1.381226View Description Hide Description
Two sets of studies were conducted to determine the perceived auditory roughness of pure tones and narrow bands of noise modulated in amplitude. The signals used as carrier frequencies were 500‐ and 2000‐Hz tones and two narrow bands of noise centered at the same two frequencies. Auditory roughness was measured by the psychophysical methods of magnitude estimation and a combination of magnitude estimation and magnitude production procedures. The average of the numerical estimates given by 10 Ss and the adjustments made by each one of the 40 Ss were plotted versus the degree of modulation m=Δ p/p of the SP. Results indicate that the perceived auditory roughness functions can be described by a power function with an average exponent of about 1.4. The exponent was about the same for the amplitude fluctuations of tones and noises.
Subject Classification: 65.75.
60(1976); http://dx.doi.org/10.1121/1.381227View Description Hide Description
In recent years there has been an increased interest in the use of interaural time as a cue for lateralization at high frequencies. Our study examines this question using AM complexes with carriers of 1800, 2400, 3000, 3600, 3900, 4200, and 4500 Hz, modulated at 300 Hz and interaurally delayed. The findings indicate that (a) the ability to lateralize these stimuli does not depend on low‐frequency distortion, since this ability remains despite the presence of intense low‐frequency masking; (b) the task is easier when the entire waveform, rather than just the modulation envelope, is delayed, though this difference seems to decline with practice; (c) the effects of stimulus amplitude and duration on the detection of interaural time differences are reminiscent of those observed with pure tones, although ongoing time differences may be more fully utilized with AM signals; and (d) lateralization declines with interaural carrier frequency discrepancy. We were unable to obtain lateralization of AM complexes at one ear against pure tones (at the modulation frequency) at the other. This strengthens the argument that all stimulus information used is carried in high‐frequency channels.
Subject Classification: 65.62.
60(1976); http://dx.doi.org/10.1121/1.381228View Description Hide Description
Several experiments were performed in which brief tonal target bursts centered around 820 Hz were preceded and followed by interference signals. These interference signals included tones and noise bursts having a variety of durations, intensities, and center frequencies. Interference signals were ipsilateral or contralateral to the ear receiving the tonal targets. Backward recognition interference was observed with tonal maskers when target‐masker frequency separations were within 10 Hz. No interference was observed with noise maskers. Aggregate results from these experiments suggest that disruptions in tonal recognition must occur after a preliminary perceptual analysis.
Subject Classification: 65.54, 65.75.
60(1976); http://dx.doi.org/10.1121/1.381229View Description Hide Description
The perceptual role of the temporal fine‐structure of vowel waveforms was investigated in five experiments. The interaction of the fundamental frequency and the first formant (F1) was shown to result in temporal patterns consisting of a number of cycles of F1 per fundamental frequency period. Changes in these patterns were shown to correlate with shifts in the perceptual boundary between /i/ and/I/. The results indicate that the perceptual system was responding to either a change in number of cycles of F1 per fundamental period or a change in the harmonic structure of the sounds. The hypothesized temporal cue was then used in synthesizing vowel‐ like sounds which, while not differing in formant center‐frequency or harmonic structure, did differ in temporal structure. Subjects were able to match different sequences if the vowel‐like sounds with sequences of natural vowels as predicted by their temporal properties. In a subsequent experi‐ ment, two pure tones were used as building blocks for synthesizing the vowels a, e, i, o, and u. With careful temporal modeling, the two tones proved sufficient fpr synthesizing intelligible tokens of the five vowels. Although the possibility exists that all the results may be explained in terms of spectrum, the indications are that temporal properties play a considerable role in vowel perception.
Subject Classification: 70.30; 65.75.
60(1976); http://dx.doi.org/10.1121/1.381230View Description Hide Description
The human vocal fold is treated as a continuous, transversally isotropic, bounded medium. The theory of viscoelasticity is applied to formulate the stresses, strains, glottaltissue impedance, and propagation characteristics for sinusoidal oscillation within the vocal‐fold tissues. Incompressibility of the tissues leads to a biharmonic differential equation, the solution of which represents commonly observed modes of vibration. The energy dissipated in the tissues and the associated damping factors are calculated. Mode impedances of the tissues are calculated and lumped‐model approximations of the system evaluated. The usual mass‐spring models are shown to be inadequate in certain important cases, especially during vocal‐fold closure and for modes in which vertical phase differences occur.
Subject Classification: 70.20.
60(1976); http://dx.doi.org/10.1121/1.381231View Description Hide Description
The ’’phoneme‐boundary effect’’ refers to the observation that discrimination of stop consonants across a boundary between phoneme categories is superior to discrimination of comparable stimulus differences within a phoneme category. The present experiment employed signal detection methodology (a) to distinguish between changes in discriminability and response bias; and (b) to reevaluate the role of phonetic categorization in the phoneme‐boundary effect. A S A M E–D I F F E R E N T discrimination task compared discrimination of 10− and 20−msec differences in voice onset time (VOT) in a synthetic stimulus continuum ranging from [ba] to [pa] (VOT’s from −50 to +70 msec). Both a clear increase in discriminability and a marked shift in response bias from S A M E to D I F F E R E N T occurred near the voiced–voiceless boundary. When variations in VOT were isolated from syllable context so that they were not categorized as phonemes, discriminability increased near the voiced–voiceless boundary in a manner comparable to the full‐syllable stimuli. These data suggest that the phoneme‐boundary effect in VOT discrimination reflects a genuine increase in discriminability near the phoneme boundary. However, in agreement with the recent data of J. D. Miller and his colleagues, these results suggest further that the phoneme‐boundary effect for VOT is not due exclusively to phonetic categorization but may instead reflect acoustic and auditory properties which are distinct from phonetic processing.
Subject Classification: 70.30; 65.75; 70.35..
60(1976); http://dx.doi.org/10.1121/1.381232View Description Hide Description
Dimensional analysis of the orifice impedance problem produces several sets of nondimensional variables which may be used to correlate nonlinear orifice behavior. Published experimental data on thin orifices were examined and it was found that tests with oil, water, and air can be correlated. Graphs of orifice resistance are given in nondimensional forms that are useful over a wide range of conditions. The form for resistance is R/ρ (νω)1/2 as a function of an aspect ratio parameter l/d, an orifice velocity parameter V/(νω)1/2, and a diameter‐viscous parameter d/(ν/ω)1/2. For thin orifices the resistance is independent of d/(ν/ω)1/2 when this parameter is greater than 10. Nonlinear effects begin when V/(ν/ω)1/2=3. This parameter compares the displacement amplitude to the viscousdiffusion length. The reactance end correction δ was normalized by the Rayliegh value of 8d/3π and correlated using the variables l/d, V/ωd, and d/(ν/ω)1/2. Again thin orifices show no dependence on d/(ν/ω)1/2 for values greater than 20 and the nonlinear reactance region begins at V/ωd=0.1. Thus, different parameters govern the onset of nonlinear resistance and reactance.
Subject Classification: 85.20, 85.50; 28.60.