Volume 32, Issue 1, January 1960
Index of content:
32(1960); http://dx.doi.org/10.1121/1.1907872View Description Hide Description
Part A gives the results of remanent ,magnetization tests made under ordinary and anhysteretic conditions, and shows that the major anhysteretic properties of a recording tape can be expressed in terms of three easily measured constants. The design of the test equipment is discussed and test results are listed for thirteen representative types of tape. Part B reviews some of the theories of fine particle magnets that can be applied to recording tape, and gives an extensive treatment of remanent magnetization based upon the Preisach diagram. Some aspects of the Preisach diagram treatment may be of interest to workers outside the magnetic recording field. The anhysteretic properties are important in hf biased recording and a second paper describes how these properties can be used to predict certain recording performance characteristics.
32(1960); http://dx.doi.org/10.1121/1.1907870View Description Hide Description
This paper discusses and enlarges on a recent theoretical paper by W.R. MacLean on the acoustics of cocktail parties. The discussion is supported by experimental evidence accumulated during the past two years. MacLean's analysis suggests that there is a critical density of participants above which a “quiet” cocktail party becomes abruptly “noisy.” It would appear that one might actually plan a quiet or noisy party as required (assuming control over the number of participants). Unfortunately the cases studied experimentally do not show this quiet‐noisy transition, and it is believed that factors not considered in the theory result in a blurring of the distinction. Indications are that there is a gradual increase in sound level to a saturation value that is independent of the properties of the room, the beverages served, and the number of participants. There is, however, dependence on the sex of the participants.
32(1960); http://dx.doi.org/10.1121/1.1907871View Description Hide Description
In a turbulent boundary layer, the effective velocity fluctuations amount to about 4% of the free‐stream velocity, independent of the curvature of the body and within wide limits independent of the velocity of the flow. The fluctuations in pressure can then be computed from those of the velocity by an equation similar to the standard Bernoulliequation, except that the numerical constant is different. Vortices pass along the receiving hydrophone; they represent pressure pulses that generate a constant power spectrum at low frequencies that decreases approximately inversely proportional to the cube of the frequency at higher frequencies. This information, in conjunction with the pressureequation, makes it possible to compute the power spectrum of the flownoise as a function of the frequency, the boundary layer thickness and the speed of the flow. The flownoise at greater speeds and at higher frequencies turns out to be predominantly generated by the surface roughnesses.
A small hydrophone records the local fluctuations at the pressure in the boundary layer; on the other hand, a large hydrophone is very insensitive to the small‐scale turbulence. It indicates the true sound pressure that is produced by the generation and the decay of the turbulence and by the vibrations of the walls of the vessel. A large hydrophone therefore reads the same pressure, whether it is placed inside the boundary layer or outside the near field region, not too far away from the turbulent layer. The theoretical conclusions are borne out, qualitatively and quantitatively, by the experimental results obtained with the aid of a rotating cylinder and through measurements in the test section of the Garfield Thomas Water Tunnel.
32(1960); http://dx.doi.org/10.1121/1.1907873View Description Hide Description
The studies reported in this paper have dealt with the responses of human observers to speech stimuli transmitted in a background of white Gaussian noise. In all cases the listeners attempted to identify the transmitted items and then made a second response in an attempt to convey additional information. It was found that when the listeners were allowed a second‐choice identification response, very little information was contained in these responses which was not already contained in the listeners' first identification response. When the second response was a confidence rating, significant amount of information was added to that which was carried by the identification response.
The rating which followed each identification response was assigned by the observers in an attempt to estimate the probability that their identification response was, in fact, correct. For message sets of four items and for sets of sixteen items, it was found that the observers were quite capable of making such estimates over a wide range of speech‐to‐noise ratios. Their estimates did appear to be affected to some extent by the size of the message set and by the speech‐to noise ratio, but this interaction was slight.
The observers' rating responses were used to generate ROC curves. These curves were adequately fit by straight lines when the data were plotted on normal‐normal probability paper. Regardless of the size of the message set, all curves, for all speech‐to‐noise ratios, were fit by a single slope. However, the point at which these curves intersected the abscissa was a function of both variables.
Data from one set of observers in the rating experiments were used in an attempt to predict the performance of a different group of observers whose task was to monitor subsets of messages. While predictions were fairly good, discrepancies were noted. An internal check in the monitoring experiment strongly suggests that these discrepancies arose because of differences between the two groups of observers.
32(1960); http://dx.doi.org/10.1121/1.1907874View Description Hide Description
An approach to the design of a machine for the recognition and synthesis of speech is proposed, with particular emphasis on problems of acoustical analysis. As a recognizer, the proposed machine accepts a speechwave at its input and generates a sequence of phonetic symbols at its output; as a synthesizer it accepts a sequence of symbols at its input and generates speechwave. Coupling between the acoustical speech signal and the machine is achieved through two peripheral units: one an analog filter set or equivalent, and the other a model of the vocal tract. Between the analog filters and the phonetic output the signal undergoes an intermediate form of representation that is related to vocal‐tract configurations and excitations but is not necessarily described specifically in these terms. Each stage of analysis is performed by synthesis of a number of alternative signals or patterns according to rules stored within the machine and by comparison of the synthesized patterns with the input signals that are under analysis. Possible advantages of the proposed method of analysis are discussed. An experimental study based on the general analysis approach is described in an Appendix. In this study a method for the determination of the frequencies of vocal‐tract resonances from the speechwave is simulated on a digital computer.
32(1960); http://dx.doi.org/10.1121/1.1907875View Description Hide Description
Piezoelectric data on a mixed titanate system (82.0% , 9.1% , 3.6% , and 4.4% were obtained with an apparatus intended to measure the dynamic compressibility of materials. The calibration constant, determined from measurements on specimens of known compressibility, involves the piezoelectric and dielectric constants of the ceramics used as transducers. The method is discussed and an operating equation for the apparatus derived. Piezoelectric and dielectric constants have been measured as a function of temperature and pressure over the ranges: −25 to 37.5°C and 0 to 1000 kg/cm2. An apparant phase transition was observed near 5°C. A qualitative discussion of the results is given.
32(1960); http://dx.doi.org/10.1121/1.1907876View Description Hide Description
Diffraction from a fixed sphere is treated as an initial value problem rather than as a boundary value problem. By requiring only boundedness of the diffracted wave potential rather than the stronger Sommerfeld radiation condition, it is shown via use of the Laplace transform and complete inversion integral that the diffracted wave potential consists of the usual steady‐state term plus transient terms. The magnitude of the transient terms are governed by β = ka where a is the radius of the sphere and k is the wave number; however, the rate of decay is governed by the dimensionless decay parameter ωt. Both Dirichlet and Neumann boundary conditions are discussed in detail. Finally, the transient force on the sphere is computed in the Neumann case and the behavior examined for the long‐wave (β≪1) and short‐wave (β≫1) approximations.
32(1960); http://dx.doi.org/10.1121/1.1907877View Description Hide Description
The relation between spatial and temporal absorption for acoustic waves in a viscous medium is discussed by considering the problem of a viscous‐liquid filled tube with a piston at one end and a reflecting termination. The solution of the problem is found in terms of traveling waves from image sources, and standing waves in a tube. The phase velocity and attenuation of the steady‐state terms for both solutions are the same. The transient part of the standing wavesolution gives rise to the usually plotted velocity and temporal attenuation; it is shown that these do not have the same meaning as in the spatial absorption case, since they depend upon the allowed wave numbers of the system, that is, upon the geometry and boundary condition.
32(1960); http://dx.doi.org/10.1121/1.1907878View Description Hide Description
A method and apparatus are described for calibrating bonded wire resistance strain gauges in the frequency range from 2 to 20 kc. The gauges are mounted on an electromagnetically excited longitudinally resonant bar whose amplitude of vibration is measured by an interferometer. Results are given for one type of bonded wire strain gauges. Factors affecting the frequency sensitivity relationship of bonded wire strain gauges are discussed. Application for calibration of accelerometers is discussed.
32(1960); http://dx.doi.org/10.1121/1.1907879View Description Hide Description
The problem of collision between a simple mechanical oscillator (with damping) and an adjacent rigid member, or between two adjacent simple oscillators, when both are subjected to random vibration is solved in terms of probability of occurrence and the mean time between occurrences. A complete solution is obtained for white Gaussian excitation. This solution can be applied to such problems as the mechanical design of electronic components, bottoming of resilient mounts, and collision between an equipment item and adjacent structure, when the representations of a single degree of freedom system can be used.
Guided Waves in a Fluid with Continuously Variable Velocity Overlying an Elastic Solid: Theory and Experiment32(1960); http://dx.doi.org/10.1121/1.1907880View Description Hide Description
Satisfactory agreement between theory and experiment has been secured for the propagation of very low frequency sound (10 and 20 cps) emitted by a point source in a thick sedimentary layer (607 m) overlying a solid elastic basement. In the theory the sediment is treated as a fluid of sound velocity varying with the depth z like , and the basement is represented by a solid elastic half space of Poisson's ratio. The wave equation has been integrated numerically on an electronic computer. The physical theory, and, in particular, the interesting coupling effects between sound waves and Stoneley or interface waves are discussed. The experimental procedure was the same as the one used in a previously published account of experiments in a different frequency range. Theoretical predictions of the behavior of the sound field in the range of 1 to 5 km are in substantial agreement with the measurements.
32(1960); http://dx.doi.org/10.1121/1.1907881View Description Hide Description
To study acoustic propagation in shallow water without the idealization of conventional theory or the many unknowns in field trials, a model with known realistic parameters is used. A water layer 0.5 to 2 cm deep overlies a 2‐in. slab of Hycar rubber with greater density and sound speed. With frequencies from 55 to 600 kc, a small probe in the water allows measurements of phase and group velocities, vertical pressure distributions, mode interferences, and attenuation coefficients. Theoretical predictions about the effects of compressional absorption and shear waves in the bottom, and of sloping bottoms, are supported. An analysis of the additional attenuation caused by rough surfaces is made by spreading small metal spheres on the bottom. This attenuation is proportional to area density of the spheres, the free‐field scattering cross section of a sphere, and the acoustic intensity at the depth of the spheres. An additional factor is the fraction g of scattered power actually lost; g correlates empirically with the ratios water depth to sphere radius and sphere radius to acoustic wavelength. A phenomenological theory ties these results together.
32(1960); http://dx.doi.org/10.1121/1.1907859View Description Hide Description
Equations are presented for the computation of sound speed that are designed to agree with Kuwahara's tables. These are in general use for the mass reduction of oceanographic data. The equations give the temperature, salinity, depth, and latitude dependence and the interaction effects due to the simultaneous variation of temperature, salinity, and depth. Some in situ measurements utilizing the bathyscaph are presented.
32(1960); http://dx.doi.org/10.1121/1.1907860View Description Hide Description
Measurements of the backscattering of 60‐kc sound from the surface of the sea have been made in Puget Sound in an effort to relate the strength of the scattered sound to the character of the surface. In addition to providing a means of predicting reverberation levels, an attempt has been made to obtain a better understanding of the fundamental mechanism of scattering at the air‐water boundary.
The reverberation measurements are presented, along with observations of several oceanographic and meteorological parameters. An examination of the data allows the following conclusions: (1) reverberation pulses from an area of several square yards follow a Rayleigh distribution; (2) reverberation cannot be directly related to the wave height, but is closely correlated with wind speed; (3) reverberation increases with wind speed to a speed of 14 knots, and remains constant for higher wind speeds; and (4) reverberation is independent of the angle with the surface for angles from 20 to 60 degrees, but drops off rapidly as the angle is decreased below 20 degrees.
32(1960); http://dx.doi.org/10.1121/1.1907861View Description Hide Description
The range dependence of variation coefficients has been investigated for each of eight selected experiments performed in New Zealand waters during midsummer for a variety of meteorological conditions. These experiments have employed 1.3‐msec pulses of 14.5 kc sound from a projector of variable depth (40 to 100 ft). The pulses have been simultaneously received by a vertical string of six equidistant hydrophones (extreme depths 40 and 165 ft) and the square of the average variation coefficient V for the six hydrophones has been plotted against the range R to test for the eight individual experiments the applicability of the Mintzer fluctuation formula. This predicts for a medium exhibiting temperature microstructure that , where f is the frequency and α and a are, respectively, the rms values of the refractive index and the correlation length of fluctuation in the refractive index. An area was selected where the sea is fed by no rivers of any size, the depth varying with experiment from 40 to 200 fathoms. Drought conditions prevailed over the whole period of the work, and, as far as is known, no confluence of local ocean currents here produces microstructure. The latter if present should presumably be confined to surface layers and be due to diurnal insolation. One has hence selected for analysis those experiments performed during small swell and sea state in. (a) the early morning, (b) hot sunshine during the mid and late afternoons, (c) the mid afternoon during cool days with a completely covered sky, and (d) early evening after hot cloudless days. Close correlation exists between insolation and intensity fluctuations which follow the Mintzer dependence on range and the values of α2 a obtained [(5.0–11.0)×10−7] are near the mean value reported by Sheehy (5.0×10−7.
The fact that the Mintzer formula is obeyed by case (b) only and the small values of V and independence of V upon R for the aforementioned conditions (a), (c) and (d) are interpreted to mean a diurnal cycle of growth and decay of insolation‐produced microstructure in the surface layers of the sea in summer.
32(1960); http://dx.doi.org/10.1121/1.1907862View Description Hide Description
Measurements of the detectability of a noise signal in noise are reported in this paper. Parameters of the noise signal such as the band width, duration, and center frequency are investigated. The results are compared with an optimum‐detection model. For some constant detectability the equation generated by the model and one constant, an attenuation factor, closely fit the experimental data over the major range of the experimental parameters. The major area of discrepancy between model and data is the shape of the psycho‐physical function. Implications of the data for the critical‐band mechanism are also discussed.
32(1960); http://dx.doi.org/10.1121/1.1907864View Description Hide Description
Thresholds for interaural difference between the intensities of dichotic tone pulses were measured on five subjects by the method of constant stimuli, at frequencies between 250 and 10 000 cps and at a sensation level of 50 db. The just noticeable dichotic difference in intensity is about 1 db at 1000 cps, a little smaller at lower frequencies, and still smaller (0.5 db) at higher frequencies. This function is compared with the interaural difference in intensity produced by the just noticeable deviation from the median plane of an actual source of tone pulses. At low frequencies, where phase or time differences are generally considered more important than intensity differences for auditory localization, these two functions differ greatly. At frequencies between 1500 and 6000 cps, the threshold for a dichotic difference in intensity matches the interaural difference in intensity that is produced by the just noticeable deviation from the median plane of an actual source. The relation between the discrimination of dichotic differences in phase and intensity and the discrimination of actual direction is shown in a graphical summary.
32(1960); http://dx.doi.org/10.1121/1.1907865View Description Hide Description
Three experiments designed to examine latent effects of low noise‐level stimulation gave completely negative results. It is concluded that noises that do not themselves produce TTS (temporary threshold shift) have no effect on the growth or decay of TTS produced by higher level exposures.
- LETTERS TO THE EDITOR
32(1960); http://dx.doi.org/10.1121/1.1907867View Description Hide Description
The fading of the color of the congo red dyestuff at concentrations 0.016. 0.01, and 0.005 g per liter under the influence of ultrasonic radiation from a Mullard generator with bariumtitanate crystal having a frequency of 1 Mc/sec and power output 225 w has been studied and the reaction is found to be unimolecular, the average velocity constants being respectively, 0.129, 0.209, and 0.193. The fading of congo red is not due to the production of hydrogen peroxide, and it is neither an oxidation nor a reduction process in the ordinary sense. The high cavitation energy appears to be responsible for the breaking down of the molecule of congo red resulting in the color fading.