Volume 28, Issue 1, January 1956
Index of content:
28(1956); http://dx.doi.org/10.1121/1.1908210View Description Hide Description
This paper reports on a continuing study of the mass physical properties of surficial, shallow‐water marine sediments off San Diego, California. Sound velocity measurements in situ at 100 kc were made by pulsing between small transducers inserted into the bottom by diver. Density, porosity, and size analyses were determined on relatively undisturbed samples taken by diver. The following averaged representative values have been obtained:
sediment at Impedance
Density Porosity Med. diam 60°F (105 g/
Sediment type (g/cc) (%) (mm) (ft/sec) cm2 sec)
Fine sand 1.93 46.2 0.19 5520 3.25
Very fine sand 1.92 47.7 0.12 5435 3.18
Silty very fine sand 1.68 61.3 0.05 5075 2.60
Medium silt 1.69 60.9 0.03 4825 2.49
Clayey fine silt 1.60 65.6 0.02 4800 2.34
Laboratory measurements of velocity and attenuation 25 to 35 kc were made by a resonant‐chamber method using diver‐taken samples; average attenuation values of about 0.5 db/ft (silt) to about 5 db/ft (fine sand) were obtained. At three stations the sediment sound velocity was less than the velocity in the bottom water.
28(1956); http://dx.doi.org/10.1121/1.1908208View Description Hide Description
Recent in situ and laboratory determinations of density and velocity of sound in fine‐grained, high‐porosity sediments of the sea floor off San Diego, California, reveal several stations at which the velocity of sound in the sediment was 2% to 3% less than the velocity of sound in the water just above the bottom. Comparison of the compressibility of the sediment computed in two ways and of the ratio: velocity in sediment/velocity in water computed and actually measured indicates that these high porosity sediments are approximately described acoustically by the velocity formula which applies to a suspension. The theoretical explanation for this phenomena was apparently made by R. J. Urick, [J. Appl. Phys. 18, 983 (1947); J. Acoust. Soc. Am. 20, 283 (1948)] and R. J. Urick and W. S. Ament [J. Acoust. Soc. Am. 21, 115 (1949)].
28(1956); http://dx.doi.org/10.1121/1.1908212View Description Hide Description
Gas bubbles, when entrained in water or other liquid, can generate high sound pressures in the liquid. Significant sound pressures are associated only with volume pulsations of the bubble, whereas oscillations in the shape of the bubble do not result in appreciable sound. Calculations have been made of the sound pressures resulting from excitation of volume pulsations by the following mechanisms: by bubble formation,coalescence, or division; by the motion of a free stream of liquid containing entrained bubbles past an obstacle; and by the flow of liquid containing entrained bubbles through a pipe past a constriction. The calculation of the sound pressuregenerated by bubble formation has been verified by measurements with bubbles formed at a nozzle.
28(1956); http://dx.doi.org/10.1121/1.1908214View Description Hide Description
An optical method for the quantitative study of ultrasonicwave fields at frequencies below 1 Mc/sec has been developed and applied to stationary waves. The method is based on the effect of mirage. A detailed theoretical analysis shows the possibility of determining wave forms and sound pressure amplitudes. Some experimental results of sound velocity and pressure amplitude determinations are given. The records of wave forms in several liquids indicate the distortion of waves during propagation.
28(1956); http://dx.doi.org/10.1121/1.1908216View Description Hide Description
28(1956); http://dx.doi.org/10.1121/1.1908218View Description Hide Description
An optical interference technique employing illuminated multiple beam Fizeau fringes has been used to study the surface motion of bariumtitanate disks. In a properly polarized, accurately shaped cylindrical disk only normal modes having symmetry with respect to the axis and to the central plane are observed. A particular disk having radius/semithickness ratio (a/l) of 3.86 has been studied in some detail, and the displacement patterns for three important modes in the thickness resonance region have been accurately measured. Stress distributions for these three modes have been calculated from theory and these show that the wave motion is of a complex character in all three cases; this question is discussed in the light of elementary conceptions of thickness resonance. A more comprehensive experimental survey covering 25 different values of a/l within the range 1.14 to 6.63 has now been completed, and vibration patterns of 12 modes are given together with graphs of resonance frequency and values of electromechanical coupling coefficient. At relatively low frequencies the observed modes are obviously of the radial type but as the frequency approaches a value such that where Λ s is the wavelength of Rayleigh waves, take the form of a surface waveresonance with maximum motion occurring at the edge of the disk. There is no single mode which can be uniquely identified as the fundamental dilatational thickness resonance Rather, as a/l changes, each mode in turn passes through a maximum or minimum of electromechanical coupling the thickness resonance region. In general, for any given value of a/l two or three modes have high coupling this region, and the vibration patterns have certain well‐defined forms but none approaches uniform piston‐like motion. Nevertheless, there is some evidence suggesting an optimum value of a/l in transducer design.
28(1956); http://dx.doi.org/10.1121/1.1908220View Description Hide Description
The theory of radial vibrations in short, hollow cylinders of bariumtitanate has been developed. This development results in an expression which relates the radial coupling coefficient to the measurement of the resonant and antiresonant frequencies of the first harmonic of the radial mode of vibration. The important parameter in this development was found to be the ratio of outside diameter to inside diameter, and once this ratio was given, the expression relating coupling coefficient to resonance measurement took a particularly simple form. An interesting result of theory was that for any value of the ratio of diameters, many of the higher harmonics are forbidden in this mode of vibration.
Experiments were devised to test some of the theoretical conclusions and were found to be in excellent agreement with the theory.
Propagation of Elastic Waves in Cylindrical Shells, Including the Effects of Transverse Shear and Rotatory Inertia28(1956); http://dx.doi.org/10.1121/1.1908222View Description Hide Description
Two systems of equations of motion, designated as (I) and (II), for thin elastic cylindrical shells are derived which include the effects of both transverse shear deformation and rotatory inertia. The character of the two systems of equations is such that, upon the neglect of transverse shear deformation and rotatory inertia, Eqs. (I) will reduce to those known as Love's first approximation, while (II), which enjoys a considerable mathematical simplicity as compared to (I), will correspond to those given by Donnell. Both systems of Eqs. (I) and (II) are employed in a study for propagation of axisymmetric waves in an infinite cylindrical shell. The agreement between the predictions of the two systems of equations, in all modes of motion, for phase velocities of propagated waves in the complete range of wavelengths is found to be excellent. The results, with reference to the nature of the modes of motion according to both (I) and (II), are further examined and the relative merit of the present paper to the work of other authors is discussed.
28(1956); http://dx.doi.org/10.1121/1.1908224View Description Hide Description
Integral expressions are derived for the mean‐square fluctuationpressure,pressure correlation, and certain moments of the pressure correlation within general anisotropic,homogeneous turbulence. Application is made to idealized flowmodels exhibiting certain anisotropy characteristics similar to those observed in boundary layer turbulence.Pressurefluctuations arising from the turbulence itself and from interaction of the turbulence with a mean flow exhibiting constant shear are compared with the isotropic case. It is found that, in contrast to the completely isotropic case, the anisotropicpressure correlation is negative for some difference coordinate values and exhibits a “tail” which falls off with distance x as x −3 at separations for which the velocity correlations have fallen essentially to zero. It is concluded that in general the principal normalized moments of the pressure correlation will tend to be less than the corresponding velocity correlation moments. For isotropic turbulence this implies that the pressure correlation must fall off with distance faster than the velocity correlation. For the models of anisotropicturbulence treated it is found that departure from isotropy results in lower mean‐square pressurefluctuations for a given mean kinetic energy of turbulence.
28(1956); http://dx.doi.org/10.1121/1.1908226View Description Hide Description
A method of measuring the mechanical impedance of the earth is described, and some experimentally measured impedances as functions of water content, soil character, and source diameter are given. A comparison of the measured impedance with some previous theoretical work is also included.
28(1956); http://dx.doi.org/10.1121/1.1908228View Description Hide Description
The correlation properties of noise fields when considered as a random superposition of elementary sources are derived as an extension of Rice's work on the shot effect [S. O. Rice, Bell System Tech. J. 34, 282 (1944); 24, 46 (1945)]. Next a formalism is derived which calculates the correlation properties of the response of a continuous, linear system when subject to an applied noise field. The requirements for solution are a knowledge of the impulse response of the system and the correlation function of the source. The latter may be obtained experimentally or from calculation.
28(1956); http://dx.doi.org/10.1121/1.1908230View Description Hide Description
The dispersion of a rectangular pulse with many cycles of carrier frequency propagated along an acoustic wave guide of rectangular cross section has been observed experimentally. The main signal is propagated with the group velocity and is observed to separate from the initial and final transient portions which travel with the greater, free medium velocity. In the dispersion process, the beating of the initial transient with the main signal causes fluctuations in the envelope of the carrier which compare favorably with the mathematical formulation given. This experiment serves, essentially, as a calibration of the wave guide so that it can be used qualitatively as a type of analog computer to predict the effects of similar dispersion on the transmission of pulses of other shapes that are not amenable to actual computation.
28(1956); http://dx.doi.org/10.1121/1.1908232View Description Hide Description
A method is developed for easily determining the directivity pattern in an arbitrary distance from the emitter from measured instantaneous values of the field in other distance. Numerical computation is facilitated by the tables put together in this paper.
28(1956); http://dx.doi.org/10.1121/1.1908234View Description Hide Description
28(1956); http://dx.doi.org/10.1121/1.1908235View Description Hide Description
The shock spectrum for an acceleration impulse applied to an electronic equipment or other component is a measure of severity of mechanical shock in terms of possible peak response at each frequency within the component structure. It is commonly measured by a reed gauge, which is intended to simulate the possible resonant characteristics of the internal structure. The reed gauge is bulky, however, and provides data at a limited number of frequencies.
The shock spectrum computer operates on an accelerometer signal, either direct or recorded, and permits measurement of as many points in the spectrum as desired. It is a device which solves the differential equation of a simple undamped resonant mechanical appendage for any applied impulse of acceleration to yield the response. In the present version, the response is made available as an output signal which is applied to a commercial cathode‐ray oscilloscope. Peak accelerations and their time of occurance may be obtained readily from the oscilloscope display. The computer can be tuned to any frequency up to 2000 cps. Control circuitry is used to damp the computer shortly after each impulse and to undamp it immediately before the next so that the impulse under study can be applied repeatedly from a recirculating loop of magnetic tape. Shock spectra obtained from the computer and oscilloscope display are used as a measure of the severity of shocks experienced by electronic apparatus and as a basis for deriving shock specifications for use in design approval tests.
28(1956); http://dx.doi.org/10.1121/1.1908184View Description Hide Description
Data on the absorption of upholstered theater chairs as measured during the construction of a 496‐seat auditorium and a 738‐seat auditorium are presented. These data obtained on the seats after placement in each auditorium indicate a much lower absorption per seat over the entire audio‐frequency band than has previously been reported. Before the seats were installed in the 496‐seat auditorium, 1080 ft2 of Axminster carpet were laid on the floor of this room, and the absorption characteristics of this full scale sample were determined. These data are also reported in this paper.
28(1956); http://dx.doi.org/10.1121/1.1908186View Description Hide Description
Difference‐limens for the discrimination of intermittence were obtained at ten interruption rates of random noise between 1 and 320 cps. The duty cycle of the intermittent noise was 0.5 and the sound pressure level was 75 db. Data from four listeners give average deviations from the standard frequency (ΔF) of 0.02 to 9.74 cps. The graph relating ΔF and F from 1 to 320 cps shows ΔF to be a monotonically increasing function of F. When the data are plotted with logarithmic coordinates, two functions are revealed, suggesting a change in the method of frequency discrimination at about 5 cps. This change appears due to the listener's ability to count the noise bursts from 1 to 5 cps, but not above about 5 cps. The relationship between 5 and 320 cps may be described by a parabolic function of the form . Below 5 cps, the form of the relationship cannot be determined from the data available. The relative difference‐limen, ΔF/F, lies between 0.008 and 0.03, and graphic integration of 1/ΔF yields 560 just noticeable differences in the range studied. These data show difference‐limens for flutter that are smaller than those previously reported.
28(1956); http://dx.doi.org/10.1121/1.1908188View Description Hide Description
Two electronic devices for automatically extracting the first three formant frequencies from continuous speech are described. Both devices are designed to yield three continuous dc output voltages whose magnitudes, as functions of time, represent the formant frequencies of the input speech. The principles of operation and the design features of both devices are discussed, and typical examples of the operation of each are presented.
28(1956); http://dx.doi.org/10.1121/1.1908190View Description Hide Description
A procedure for evaluating the operation of two electronic formant extractors is described and results of the evaluation are given. The formant extractors are designed to yield electrical outputs representing the formant frequencies of continuous input speech. These outputs are compared in a prescribed manner to formant data derived from sound spectrograms. The results show that the more reliable extractor of the two follows the first formant of vowel sounds in speech within ±150 cps greater than 93% of the time; it follows the second formant within ±200 cps greater than 91% of the time. The first and second formant outputs of the same extractor are simultaneously within the above tolerances greater than 85% of the time.
- LETTERS TO THE EDITOR
28(1956); http://dx.doi.org/10.1121/1.1908192View Description Hide Description
If one in the study of transmission of sound into a moving medium allows flow speeds larger than twice that of sound, the analysis indicates at least formally the existence of two critical angles between which total reflection occurs. A discussion of the corresponding behavior of the angle of refraction and the reflection coefficient is presented.