Volume 28, Issue 3, May 1956
Index of content:
28(1956); http://dx.doi.org/10.1121/1.1908320View Description Hide Description
A very small condensermicrophone has been developed which exploits materials and methods of construction only recently made available by the post‐war developments in plastics. These techniques have made possible a microphone with extremely regular voltage amplitude and phase responses as functions of frequency and with a reasonably high sensitivity. An analysis is given for the resonance frequencies of a circular membrane stretched over a closed cavity with its center fixed over a small area.
28(1956); http://dx.doi.org/10.1121/1.1908322View Description Hide Description
A systematic determination of the electromechanical coupling coefficient k has been made for Fe‐Al alloys containing aluminum percentages between 12 and 14 by weight and annealed at temperatures between 600°C and 1100°C. For annealing temperatures in the neighborhood of 1000°C, k 2 is about equal to 0.05 and is nearly independent of composition, a result in agreement with earlier measurements of Masumoto. For lower annealing temperatures, k 2 becomes sensitive to composition, the highest value measured being equal to 0.12 for an alloy containing 12.3% Al and annealed at 650°C. In all cases eddy current losses are lower for Fe‐Al than for soft annealed “A” nickel.
Elastic, Piezoelectric, and Dielectric Constants of Polarized Barium Titanate Ceramics and Some Applications of the Piezoelectric Equations28(1956); http://dx.doi.org/10.1121/1.1908324View Description Hide Description
A complete set of the elastic,piezoelectric, and dielectric constants of polarized bariumtitanateceramics is presented. The various electromechanical coupling factors, as derived from the piezoelectric equations, and their numerical values are also given. The determination of the elastic and piezoelectric constants using the resonant method applied to small bars, square plates, and disks is discussed.
28(1956); http://dx.doi.org/10.1121/1.1908326View Description Hide Description
The theory for a plane wave method of measurement of acoustic impedance in a tube is worked out. The effect on the measurement of irregularities in the surface of the sample has also been investigated. It is shown that the measured impedance differs from that of the average impedance evaluated at the surface of the specimen. A number of recommendations for the method of preparation of a sample have also been given.
28(1956); http://dx.doi.org/10.1121/1.1908328View Description Hide Description
The use of a high impedance acoustical load for the type B diaphone is capable of raising the efficiency of the fog horn system to values of 10 to 16% for normal operating pressures. The high impedance is provided by a resonant exponential or catenoidal horn. To provide the high degree of frequency stability required by high Qresonant loads the mechanical impedance of the diaphone is reduced to permit it to lock in with the horn frequency. The reduction of the mechanical impedance is achieved by reducing the mass of the piston through the use of aluminum and reducing and stabilizing the frictional forces by using a central shaft for supporting the piston instead of allowing it to float in the liner. The resulting frequency stability is such that the efficiency under field conditions equals that obtained in the laboratory. An incidental benefit arises from the fact that the horns are smaller and made of aluminum reducing the weight by a factor of six.
28(1956); http://dx.doi.org/10.1121/1.1908330View Description Hide Description
The Fourier series solution for the radiation field of a circular pressure ring is applied to the synthesis of a sound field expressed in the form of a Tchebyscheff polynomial. This leads to an exact solution in closed form for the amplitude and phase of excitation of point source elements on a circle when the main lobe width of the radiation pattern, the side lobe suppression, and the array circle diameter are specified. Calculations are carried out for two sample beam patterns to show the effects of different choices of array circle diameter and number of point source elements.
Fourier Transform Method for the Treatment of the Problem of the Reflection of Radiation from Irregular Surfaces28(1956); http://dx.doi.org/10.1121/1.1908334View Description Hide Description
A method is presented which can be used for the calculation of the distribution of energy reflected from irregular surfaces. The formulation is useful for the first boundary value problem and can be used in either two‐ or three‐dimensional problems with any given incident field. The solution is reduced to quadrature with negligible error when the average square of the slope of the reflecting surface is small and when the wavelength of the incident radiation is not small compared with the displacement of the surface from its average value. A numerical example is worked, the sinusoidal surface, and is compared with experiment and with a method due to Rayleigh. It is found that the Fourier transform method is preferable to previous methods, notably those which can be classified physical optics (such as Rayleigh's), since the error in the transform method is of second order in the surface slope whereas the error in previous methods is of first order in the same quantity.
28(1956); http://dx.doi.org/10.1121/1.1908336View Description Hide Description
The pressurefluctuations exerted on a flat plate by a turbulent boundary layerflow are investigated. The approximate dependence of the mean‐square intensity, spatial scale, and frequency scale on Mach number and distance from transition point are estimated through the use of similarity arguments. The wave number spectrum of the pressurefluctuation distribution over the surface of the plate is expressed in terms of transforms of two‐point velocity correlations and expressions are derived for the driving force exerted on a rectangular piston set in the surface of the plate. It is found that the integral over the boundary surface of the two‐point quadratic correlation function of the pressurefluctuation should vanish, with the result that the mean‐square force per unit area exerted on a large area of the surface should tend to zero as the area increases indefinitely. An idealized model of turbulent boundary layerflow is constructed and used to relate the spectrum and correlation function of the surfacepressure distribution to the corresponding functions for a homogeneous turbulent flow. Application of the theory to Laufer's data on turbulent channel flow indicates that the dominant contribution to the pressurefluctuations should depend only on the mean velocity profile and the two‐point quadratic correlation of the fluctuating velocity component perpendicular to the boundary surface. On the basis of the seriously incomplete data available the rms pressurefluctuation from this source is tentatively estimated. The treatment of the present paper is based on the assumptions of (1) adiabatic conditions, (2) small compressibility, and (3) slow rate of growth of the boundary layer.
28(1956); http://dx.doi.org/10.1121/1.1908338View Description Hide Description
A formulism for predicting the motion of continuous systems excited by random noise fields is applied to the special case of a one‐dimensional string. Several examples are attacked: (1) Brownian motion of finite strings, (2) response to a spatially stepped‐temporally delayed noise field, (3) response of finite ribbon to moving, random pressure fluctuations, (4) response of an infinite ribbon to moving, random pressure fluctuations. Cases (2) and (3) are investigated experimentally, the latter by using flowing turbulence.
In case (2) appears the interesting phenomenon of the capability of the string “remembering” or “forgetting” a signal which it received in the past. The larger the damping of the string, the sooner it will forget a previous signal. In cases (3) and (4) a coincidence effect appears between the flow velocity of the forcing field and the velocity of waves on the ribbon. Here again, the phenomenon of forgetting appears and sharply determines the magnitude of any coincidence effect which may appear.
28(1956); http://dx.doi.org/10.1121/1.1908340View Description Hide Description
The Vobanc VOiceBANd Compressor) is a speech band‐width reduction system which provides a reduction of two in transmission channel band width, without a comparable loss in articulation. A description is given of an experimental system and results obtained with it are described.
28(1956); http://dx.doi.org/10.1121/1.1908342View Description Hide Description
This study concerns the intelligibility of speech under various periodic switching operations. In diphasic speech the signal is separated into successive segments which are reversed in phase. The intelligibility of this speech was found to be high for switching frequencies below about 100 cps; in the intermediate audio‐frequency region the intelligibility fails to very low values. Throughout this intermediate frequency region, the intelligibility of diphasic speech is markedly less than that of interrupted speech. Some explanation for this difference is found in the Fourier expansion for the two cases. In interrupted speech the original spectrum is retained, but in diphasic speech terms for the spectrum in its normal form do not appear. It was experimentally observed that throughout the intermediate frequency region binaural diphasic speech, in which the signal is switched alternately from one ear to the other, is more intelligible than monaural diphasic speech or interrupted speech.
28(1956); http://dx.doi.org/10.1121/1.1908344View Description Hide Description
The “identification” and “discrimination” of components of elementary auditory displays are here differentiated in terms of the relative contiguity between the unknown signal and the set of possible signals that may be chosen. Listeners attempted to match arbitrary designations to sound levels of a tone under three conditions of relative contiguity between the unknown sound level and the set of possible sound levels. Significant improvements in matching are associated with increasing contiguity, especially under finely subdivided conditions encompassing a wide range of sound levels.
28(1956); http://dx.doi.org/10.1121/1.1908346View Description Hide Description
Many of the phenomena of masking can be explained on the basis of two models, one for monaural listening, and the other for binaural. The monaural model is the familiar narrow band‐pass filter followed by a detector responsive to changes in output level. The binauralmodel is a series of coincidence detectors associated with a delay‐network capable of matching delay in the stimulus with a delay in the neural path.
The two models have proved helpful in understanding the phenomena of tonal masking, and have led to a number of predictions which have been subsequently verified by experiments reported here. Some of the new findings are related to monaural masking and some to binaural. Among the latter are the fact that masking‐level differences can be observed in the masking of one pure tone by another when a short signal is employed, and that a binaural signal can be heard in the presence of uncorrelated noise at the two ears better than a monaural signal can be heard against noise presented monaurally, again provided that a short signal is employed.
Many studies of the masking of tones have appeared during the past few years. They have been characterized by close agreement in results wherever they have had common conditions, and they provide a large body of dependable quantitative information. With this body of fact there also grew, at least in the minds of present writers, a considerable state of bewilderment. Many of the facts appeared to contradict inferences drawn from others. The present paper discusses a number of these facts and attempts to encompass them under a few explanatory principles.
28(1956); http://dx.doi.org/10.1121/1.1908348View Description Hide Description
Noise during jet aircraft launching operations seriously interferes with communications in important island spaces aboard aircraft carriers. Measurements of sound pressure levels made in these spaces during normal jet operations at sea are reported. It was found that these levels could be estimated from a knowledge of jet engine operating conditions [A. C. Pietrasanta, J. Acoust. Soc. Am. 28, 434 (1956)] and the physical properties of the structures involved. The problem of the selection of criteria for speech communication is discussed. Noise reduction requirements for island spaces, based on estimated noise levels of present and future jet aircraft, are presented.
28(1956); http://dx.doi.org/10.1121/1.1908350View Description Hide Description
The noise fields around several jet aircraft have been measured for various engine operating conditions. Directivity patterns as a function of octave bands of frequency are presented. Acoustic power levels have been computed and found to agree with a previously published correlation of power level with engine operating conditions [O. K. Mawardi and I. Dyer, J. Acoust. Soc. Am. 25, 389 (1953)]. Analysis of these data has led to the development of a procedure for estimating the characteristics of the noise fields around non‐afterburner jet aircraft operating at military power.
28(1956); http://dx.doi.org/10.1121/1.1908352View Description Hide Description
A hydrophone monitoring a constant level sound source in the open ocean records a sound whose intensity fluctuates over short periods of time. Previously it has been demonstrated that much of this fluctuation can be accounted for by the temperature microstructure. It has been suggested that another cause of the observed fluctuation could be inhomogeneities the velocity field (turbulence). This paper is an attempt to estimate the magnitude of turbulence in the ocean. The conclusion is reached that turbulence contributes very little to the fluctuation observed in the intensity of underwater sound signals.
28(1956); http://dx.doi.org/10.1121/1.1908354View Description Hide Description
The pressurewave produced by the collapse of an electrically generated spherical cavity (∼1 cm in radius) in water was measured by means of a small electroacoustic hydrophone ( in diameter) at a distance of 50 cm. The pressure was found to increase as the bubble collapsed according to the t −4/5 (time) law in the interval corresponding to subsonic flow. The pressure then suddenly jumped to a higher value and rapidly decayed to zero. This rapid increase in pressure is assumed to be a shock wave. Using Gilmore's theory for the collapse of the cavity and finite amplitude‐wave theory for the pressurewave, we find that the value of the pressure‐amplitude characteristic at the shock front corresponds to a cavity‐wall velocity approximately equal to the velocity of sound. Since the shock wave has been attenuated, the peak wall velocity must be greater than this value.
28(1956); http://dx.doi.org/10.1121/1.1908356View Description Hide Description
The effect on the vibrational and rotational relaxation time of de‐excitation by photon emission in four gases possessing a permanent electric dipole moment is investigated. Such radiative energy losses are independent of pressure, and may be expected to control the relaxation process at low pressure when collisions become relatively infrequent. Calculations are shown for , OCS, and . Estimates of the efficiency of collisions to deactivate a vibrational level in HCl, indicate that the radiation effect on the vibrational relaxation time of this gas should be the easiest to measure; at a pressure of 10−2–10−3 atmos, de‐excitation by radiation should be as important as collisional de‐excitation. Radiative de‐excitation will become increasingly important at low temperatures because of the consequent increase of the collisional relaxation time.
28(1956); http://dx.doi.org/10.1121/1.1908358View Description Hide Description
Ultrasonicabsorptionmeasurements were made to compare the relative effectiveness of and molecules in influencing thermal vibrational relaxation in molecules. The displacement rates of the absorption per wavelength maxima induced by and were compared in and mixtures. Experimental apparatus was constructed for these measurements which utilized the “direct” absorption technique in conjunction with pulsed sound waves, having a frequency of approximately 113 kc. The parameter of frequency/pressure was varied by changing the gas pressure.
The data were taken at a temperature of 21°C in the frequency/pressure range of 100–700 kc/atmos. The maximum amplitude of the molecular absorption coefficient per wavelength in dry was 0.153 and this occurred at 210 kc/atmos. Measurements were taken in and mixtures at ‐vapor and ‐vapor concentrations of 0.39%, 0.77%, and 1.14%.
The usual single relaxation time theoretical absorption curve could be fitted to the data for dried . However, in the mixtures there was some evidence of two separate relaxation times, one possibly associated with collisions and the other with or collisions. The maximum absorption coefficient in the mixtures was somewhat less than that observed in the dried . The molecules were 1.72 times more effective than the molecules in shifting the frequency associated with the absorption maximum.