Index of content:
Volume 58, Issue 4, October 1975
58(1975); http://dx.doi.org/10.1121/1.380748View Description Hide Description
The document is a review of information which deals directly and indirectly with the effects of noise on children. Potential problem areas are identified and recommendations for research are stated.
Subject Classification: 10.60; 50.70; 65.10.
58(1975); http://dx.doi.org/10.1121/1.380749View Description Hide Description
The analysis of sound radiation from a time‐varying simple source in uniform circular and helical motion is made. The expressions for the mean‐square farfield sound pressure and the overall sound‐power output are derived. The retarded time approach is used and illustrates the relative significance of the time rate of pulsations and the acceleration due to rotation of the source.
Subject Classification; 20.15, 20.55; 30.50.
58(1975); http://dx.doi.org/10.1121/1.380750View Description Hide Description
It is shown that the efficiency of the parametric transmitting array may be improved over that obtained where two equal amplitude‐frequency components are employed in the primary wave. Through suitable periodic shaping of the envelope of the primary wave, available improvements in the output at the ’’difference’’ frequency of between 2.1 and 6.0 dB are possible in principle. The degree of available improvement depends upon whether the transmitter is peak power or average power limited and whether finite‐amplitude attenuation of the primary wave is significant. It is proposed that the on–off modulation of a single‐frequency carrier provides a simple, practical method through which near optimum efficiencies may be obtained under most circumstances. A bandwidth greater than the ’’difference’’ frequency is required at the transducer in order to achieve this improvement.
Subject Classification: 25.35; 30.75.
58(1975); http://dx.doi.org/10.1121/1.380751View Description Hide Description
The effect of flow on the acoustic resonances of an open‐ended, hard‐walled duct is analyzed. The flow produces acoustic losses both in the interior of the duct and at the ends. Unless the duct is very long, typically 100 times the diameter, the losses at the ends dominate. At flowMach numbers in excess of 0.4 the losses are so large that axial duct resonances are almost completely suppressed. The plane‐wave Green’s function for the duct with flow is expressed in terms of the (experimentally determined) pressure reflection coefficients at the ends of the duct, and the flow dependence of the complex eigenfrequencies of the duct is obtained. Some observations concerning the noise produced by the flow in the duct are also reported.
Subject Classification: 28.60.
Perturbation solution of the Navier–Stokes equations and its relation to the Lighthill–Curle solution of aerodynamic sound58(1975); http://dx.doi.org/10.1121/1.380752View Description Hide Description
The Lighthill–Curle solution of sound generated aerodynamically is examined by a perturbation approach. Based on the method of matched asymptotic expansions, the full Navier–Stokes equations are expanded for a small Mach number. To the first order, the nearfield is generally nonisentropic. The pressure field (pseudosound) is generated by the incompressible Reynolds stresses in the turbulent flow and the velocity, pressure perturbation, and their derivatives on the boundaries. In the farfield, the acoustic field is obtained by matching the nearfield and determined by the first‐order sound sources. A uniformly valid first‐order solution for the pressure field is obtained. The solution shows that this kind of aerodynamic soundgeneration does not depend on the viscous, thermal, and entropyeffects in the adiabatic flow nor on the shear stress on a rigid smooth boundary. Comparisons with the Lighthill–Curle solution confirm some approximations used in current farfield aerodynamic noise studies.
Subject Classification: 28.65; 30.65; 50.55.
58(1975); http://dx.doi.org/10.1121/1.380726View Description Hide Description
A turbulent boundary layer on the fuselage of a glider was used to excite a Helmholtz resonator. The resonator orifice was flush with the surface and smaller than the boundary‐layer thickness. Resonator frequencies were chosen so they would tune with different portions of the boundary‐layer wall pressure spectrum. The resonators were excited at both the Helmholtz frequency and a standing wave frequency. The results show a shift in the Helmholtz frequency when the boundary layer is present. This shift indicates the degree to which the turbulence interacts with the acoustic motion in the orifice and modifies the end correction. Four of the nine resonators tested were ’’strongly’’ excited and radiated considerable acoustic sound. This phenomenon occurs when turbulenteddies about the size of the orifice are convected past the orifice with the proper velocity to impose a frequency that matches a resonant frequency of the resonator.Turbulenteddies which are either smaller or larger than the orifice will not produce strong excitation.
Subject Classification; 28.65, 28.60; 85.32; 20.40.
58(1975); http://dx.doi.org/10.1121/1.380753View Description Hide Description
An experimental study was conducted to investigate directivity measurements made in the farfield of a conventional sonartransducer with a planar array of probe transducers. The principal questions investigated were (1) how closely must measurement probes be spaced for directivity measurements to be of useful accuracy and (2) how does one process and display the data in a useful format. Full‐scale experiments were conducted with a probe transducer positionable over a grid of measurement points to simulate a multiprobe array. Measurements were made at probe spacings varying from 0.1 to 0.4 times ψ H P , the half‐power beamwidth of the sonartransducer. The data were processed by computer and displayed with contour plots, which proved quite easy to use and easy to interpret. The data obtained from the experimental measurements were compared with conventional beam patterns of the transducer; results were encouraging and, in fact, somewhat better than expected, generally. Discrete data taken at 0.1 and 0.2 times ψ H P defined all features of the directivity function to within the measurement accuracy of conventional directivity measurements. Data obtained at the spacing equal to 0.4 times ψ H P resolved major characteristics of the pattern to within ±2 dB and ±2°. It is believed that the use of more sophisticated interpolation techniques in processing the data prior to contour plotting would improve the accuracy.
Subject Classification: 30.85, 30.82; 85.40.
58(1975); http://dx.doi.org/10.1121/1.380727View Description Hide Description
Transmitted energy levels for wide‐band SOFAR transmission are calculated for arbitrary sound‐speed profiles using ray‐mode analysis [R. P. Porter, J. Acoust. Soc. Am. 54, 1081–1091 (1973)]. The analysis of the received field has been generalized to include arbitrary normal modes and range‐varying, sound‐speed profiles. It is shown that computations of wide‐band propagation based on ray‐mode analysis require an order of magnitude fewer calculations than do calculations by harmonic analysis. A computer code has been developed and used to estimate the wide‐band loss for profiles from the Mediterranean and the North Atlantic. Comparisons with continuous tone, normal‐mode calculations yield agreement to within 3 dB.
Subject Classification: 30.20, 30.25; 20.20, 20.40.
58(1975); http://dx.doi.org/10.1121/1.380728View Description Hide Description
A procedure has been proposed by Cook [J. Acoust. Soc. Am. 53, 330(A) (1972)] through which a cross‐sectional mapping of a progressive sound beam could be obtained by conventional light diffraction. This technique requires that a series of optical diffractionmeasurements be taken laterally across the field, for a large number of rotations about the sound field’s major axis. Since the number of measurements could become excessive, and investigation of the data requirements of this technique has been performed. Computer‐generated data were calculated of the integrated optical effect (both magnitude and phase) produced by the pressure field of a circular plane piston in an infinite rigid baffle. From these data a cross‐sectional map was obtained using an effective two‐dimensional Fourier transform, as well as the two‐dimensional fast Fourier transform algorithm. The cross‐sectional mapping was evaluated at different axial positions, using one set of data obtained by varying the data set and comparing the results to those obtained from the diffraction integral. The data requirements were found not to be excessive.
Subject Classification: 35.65.
58(1975); http://dx.doi.org/10.1121/1.380729View Description Hide Description
The problem of free extensional (in plane) vibration of thin plates of Hookean material of various shapes was studied. Solutions for dilatational and rotational vibrations were obtained separately. The boundary conditions were satisfied in a least‐square sense. Numerical computations were performed for circular, elliptical, triangular, square, and hexagonal plates. The nodal pattern corresponding to each natural frequency for different modes was obtained. The frequency parameters computed for the circular plate are found to be within 0.05% of published results. For plates where known results were not available, the accuracy was checked by taking additional terms in the series solution and by dividing the boundary perimeter into finer intervals. It was found that the circular plate had the lowest fundamental frequency (k 1) at 2.049. The value increases as the shape is changed from a circle, with the triangular plate having the highest value at 6.733 in dilatational vibration while the frequencies in rotational vibration (h 1) were found to be 3.737 and 16.364, respectively.
Subject Classification: 40.24.
Transmissibility across simply supported thin plates. I. Rectangular and square plates with and without damping layers58(1975); http://dx.doi.org/10.1121/1.380730View Description Hide Description
The transmissibility across thin, simply supported, rectangular, and square aluminum plates has been determined experimentally and found to agree closely with theoretical prediction through the frequency range of measurement, which extended from 25 to 3025 Hz. Both the rectangular and the square plates considered were uniquely supported by spring‐steel flanges, which were designed to provide, for the first time, true simple supports that simulated almost identically the idealized supports assumed as boundary conditions in theoretical analyses for more than a century. For example, the first 20 resonant frequencies of the rectangular plate with these novel supports differed, with only one exception, by less than 0.55% from their predicted values. One important result obtained for the damped plates considered—which comprised aluminum plates plus applied tiles of high‐damping compound to yield composite plates having damping factors of essentially 0.3—was that the measured transmissibility across the plates could be duplicated closely by an expression for transmissibility developed for an internally damped homogeneous plate having damping factors equal to those of the composite plates.
Subject Classification: 40.24, 40.60, 40.20.
58(1975); http://dx.doi.org/10.1121/1.380731View Description Hide Description
A method is shown which will provide natural frequencies and mode shapes for the free vibrations of a simply supported polar orthotropic sectorial plate. The special case of a wedge‐shaped plate is also treated. The solution applies to any sectorial plate with radial edges simply supported and arbitrary boundary conditions along the circular edges. A series solution which converges rapidly for many natural frequencies is obtained for radial, circular, and mixed modes. An evaluation of the affect of plate orthotropy on the modes is presented. The solution is easily reduced to the isotropic case.
Subject Classification: 40.24.
58(1975); http://dx.doi.org/10.1121/1.380732View Description Hide Description
The distribution functions are considered for the random sampling in space of a three‐dimensional acoustic mode. Expressions are obtained for the probability density and cumulative functions for a single mode excited in an ideal elastic fluid contained in a right‐circular cylindrical enclosure. In earlier work, axisymmetric modes of lower order were treated; here the work is extended to some modes of higher order, and in addition the rms values of the modal function are sampled, as well as the mean‐square vales. The rms modal function ‖J 0(⋅) ‖ contains cusps on the abscissa, and these cause the distribution functions to differ from those pertaining to the square of the modal function. In both cases, however, the density functions have a number of poles which increases with the order of the mode. Computed values of the functions are presented, together with values of the variances of the distributions.
Subject Classifications: 45.30; 55.20.
58(1975); http://dx.doi.org/10.1121/1.380733View Description Hide Description
In this paper, subjective studies made on nonexponentially decaying sounds are described. The various decay conditions are realized by changing the position of the pickup microphone in a reverberation room with a highly absorbing sample on the floor. It is shown, by means of articulation tests, that intelligibility of speech is more close to a highly absorbing sample than away from it. It is also shown that the perception of decay is mainly due to the initial portion of a nonexponential decay. The significance of these studies is determining the acoustics of halls is explained.
Subject Classification: 55.20; 70.35; 55.30.
58(1975); http://dx.doi.org/10.1121/1.380734View Description Hide Description
The response of single cells in the cochlear nucleus of the cat to noise with a cosinusoidal power spectrum (cosine noise) was studied. Cosine noise is obtained by adding white noise to the same noise delayed over a time τ; maxima in the spectrum occur at n/τ (n=0,1,2,⋅⋅⋅). The spike rate of a unit was continuously registered as a function of τ (τ diagram). This registration shows a periodic fluctuation having maxima at τ=n/CF and troughs at τ= (n+1/2)/CF, with a decreasing peak–trough ratio for increasing n (CF is the characteristic frequency of the unit). For the majority of the units investigated, a maximal peak–trough ratio was observed not for n=0 but for about n=3. This is ascribed to lateral inhibition. By introducing the mathematical concept of a weighting function in the frequency domain, a quantitative relation with the response of the unit to a pure tone in white noise (iso‐intensity curve) could be established. The results are compared with psychophysical results on the internal representation of cosine noise. Possible implications for the existence region of (repetition) pitch and the phenomenon of spectral dominance are discussed.
Subject Classification: 65.44, 65.40.
58(1975); http://dx.doi.org/10.1121/1.380735View Description Hide Description
Fourier analysis of discharge patterns in response to sinusoidal acoustic stimulation provides a consistent and repeatable measure of response phase and amplitude. The distribution of the fundamental components of response for large populations of fibers as a function of their characteristic frequency provides a link between the spatiotemporal characteristics of basilar membrane vibration and single fiber response.
Subject Classification: 65.42, 65.40.
58(1975); http://dx.doi.org/10.1121/1.380736View Description Hide Description
Intense cues detract from performance in the monaural detection with contralateral cue (MDCC) situation. The addition of noise to the cue, so as to partially mask the cue, restores this performance and at the same time makes the intense cue sound softer. The performance level with the masked cue is roughly the same as that of the equally loud unmasked cue, but the phase effects are the same as those of the equally intense unmasked cue. The phase relations of binaural signals appear to be related to intensity, whereas loudness is probably a more central effect.
Subject Classification: 65.50, 65.58, 65.62, 65.75.
58(1975); http://dx.doi.org/10.1121/1.380737View Description Hide Description
Effort‐dependent speechloudness (of sustained vowels) was investigated with the general intention of making a comparison between experimental loudness data and calculated loudness data (based on the Zwicker model). The adjustment method was used in the listening experiment as an alternative to the magnitude‐estimation technique used in earlier experiments. The stimuli varied with respect to (1) vocal effort, (2) vowel quality, and (3) initial structure. Intensity, fundamental frequency, and duration were held constant. The speech stimuli, produced by five male speakers, were presented to 25 listeners. The equal loudness data of the listeners indicate that the speechloudness varies significantly with effort and vowel quality, while the initial structure does not have any significant influence. In the second part of the investigation, loudness was calculated on the basis of spectral information. The calculated loudness data were compared with the listener loudness data (phon level). With respect to the relative level, listener loudness and calculated loudness agree to a very large extent in effort‐dependent loudness differences and loudness differences dependent on vowel quality. With respect to absolute loudness levels, the listener loudness and the calculated loudness differ almost constantly (by 9 phons). The comparative loudness data suggest that effort‐dependent speechloudness changes are due to pure spectral changes, and that a special perceptual mechanism in the loudness perception of speech is questionable.
Subject Classification; 70.30, 70.20.
58(1975); http://dx.doi.org/10.1121/1.380738View Description Hide Description
As a first step toward automatic phoneticanalysis of speech, one desires to segment the signal into syllable‐sized units. Experiments were conducted in automatic segmentation techniques for continuous, reading‐rate speech to derive such units. A new segmentation algorithm is described that allows assessment of the significance of a loudness minimum to be a potential syllabic boundary from the difference between the convex hull of the loudness function and the loudness function itself. Tested on roughly 400 syllables of continuous text, the algorithm results in 6.9% syllables missed and 2.6% extra syllables relative to a nominal, slow‐speech syllable count. It is suggested that inclusion of alternative fluent‐form syllabifications for multisyllabic words and the use of phonological rules for predicting syllabic contractions can further improve agreement between predicted and experimental syllable counts.
Subject Classification: 70.40, 70.60.
58(1975); http://dx.doi.org/10.1121/1.380739View Description Hide Description
Subjects were asked to identify dichotically competing pairs of CV syllables and two types of acoustically comparable nonspeech sounds. Dichotic signals were presented with onset asynchronies ranging from 0 to 150 msec. Subjects identified lagging signals more accurately and reported them as clearer than the leading signals at asynchronies between 30 and 75 msec. This advantage for the lagging signal was larger, more reliable, and had a different form for the speechsounds than for the nonspeech sounds. Similarities in the pattern of results for speech and nonspeech were interpreted in terms of the forward‐, backward‐, and simultaneous‐masking effects occurring early in the central processing of auditory events. The differences in speech and nonspeech results, on the other hand, were seen as reflecting the particular sensitivity of later stages of speech processing to the masking effects. This sensitivity may be related to the way in which the phonetic message is encoded in the sound stream and the special speech processing occasioned by this encoding.
Subject Classification: 70.30; 65.58; 65.75.