Index of content:
Volume 33, Issue 1, January 1961
33(1961); http://dx.doi.org/10.1121/1.1908390View Description Hide Description
An improved instrumentation system has been developed to analyze the dynamic pressure field produced by jet aircraft. The system is automatic and utilizes analog computer techniques to obtain an accuracy of ±0.2 db in its readout. The method used is to secure a true rms value of octave band segments of the jet noisespectrum and to convert this to decibels by the use of a dc logarithmic amplifier. The transition to acoustical decibels is made by obtaining the difference between this voltage and another voltage obtained from a reference oscillator. The value of this reference oscillator in acoustical decibels is obtained by comparing its output voltage to that of a calibrated microphone. The dynamic range of the system has proved more than adequate for greater than 98% of the data processed by this unit during the time this system has been in operation. The output is in the form of punched cards for utilization with a digital computer.
33(1961); http://dx.doi.org/10.1121/1.1908410View Description Hide Description
In a classical study of the vowel sounds of English, G. E. Peterson and H. L. Barney collected a large body of experimental data which related perceived vowel quality to measurements of the first three formant frequencies and the voice pitch. It is difficult for a computer to use this raw data to interpret vowel quality because the vowel types have complicated boundaries in the coordinate system of the physical measurements. This paper describes a coordinate transformation which simplifies these boundaries.
33(1961); http://dx.doi.org/10.1121/1.1908386View Description Hide Description
The frequently used pulse method of measuringultrasonic wave velocities in solids involves a high‐frequency quartz crystal transducer cemented to one end of a specimen having parallel end faces. The phase shift for waves reflected from the transducer must be considered for highest accuracy. It is shown that combining several measurements of phase delay (at two frequencies differing by approximately 10%) with a theoretical analysis of the reflection phase angle makes possible a determination of velocity to within one part in 5000 for round trip delays greater than 5 μsec. Indirectly, the approximate thickness of the cement bond between transducer and specimen can be determined. The advantages of the method for making measurements as a function of temperature or pressure are discussed.
33(1961); http://dx.doi.org/10.1121/1.1908388View Description Hide Description
The problem of the diffraction of a plane compression pulse by a rigid, incompressible semi‐infinite body in lubricated contact with a homogeneous, isotropic, perfectly elastic half‐space is formulated by a potential function and separation of variables approach. The boundary conditions are expressed by dual integral equations that are solved by a function‐theoretic technique. The exact solution is obtained, as a function of space and time variables, in a closed form.
33(1961); http://dx.doi.org/10.1121/1.1908392View Description Hide Description
In conducting experiments on waves in an elastic rod, J. Oliver discovered a new mode of symmetric vibration in which the motion was predominantly at the end of the rod. It is shown that this “end mode” arises when pure extensional waves are coupled with waves with complex wave numbers. Approximate equations are used to study the reflection of pure extensional waves at the end of a semi‐infinite rod. The properties of the resulting end mode are studied in detail and are compared with the phenomena reported by Oliver.
33(1961); http://dx.doi.org/10.1121/1.1908394View Description Hide Description
The propagation of acoustic waves in a layered medium is considered for the case of an elastic plate submerged at a certain depth below the surface of a liquid‐filled half‐space. It is shown that there exist unattenuated modes for the plate‐liquid layer system which have horizontal phase velocities greater than the soundvelocity in the liquid. In spite of such greater phase velocities, radiation into the liquid below does not take place because the lower surface of the plate exhibits no vertical motion. Thus, energy can be trapped in and above the plate by vibrations which leave the underlying liquid undisturbed. It is pointed out that radiation in the horizontal direction may also be very small when the above conditions of total reflection are approximately satisfied, as is indicated by the existence of low group velocities for the modes of a free plate. Attention is called to the three‐dimensional nature of such trapping, vertically by an approximate condition of no transmission across the plate, and horizontally by the simultaneous vanishing of the group velocity at nonvanishing angles of incidence. The present modes are in contrast with the usual case of total reflection and “wave guide propagation” in layered elastic media, where the horizontal phase velocity is less than the soundvelocity in the liquid (lower half‐space), and the signal below the plate decays exponentially with distance from the interface. Conditions under which the present modes can arise have been found and evaluated numerically. Relations between phase velocity,wave number, and ratio of fluid layer to plate thickness are presented for a lucite plate is water. These have been compared with the results of much more elaborate calculations for the point source “singing” problem.
33(1961); http://dx.doi.org/10.1121/1.1908396View Description Hide Description
A series of capacitive accelerometers with frequency‐response characteristics governed by the resistive and elastic properties of a thin air film are described. The accelerometers have been designed, in accordance with theory developed previously to have response constant within ±5% at frequencies up to 1.48 times the undamped natural frequency. Experimental data are given to show that the accelerometer response agrees closely with that predicted theoretically.
33(1961); http://dx.doi.org/10.1121/1.1908398View Description Hide Description
The theory of electrical lines and filters is based on the concept of a characteristic impedance. Similar concepts are permissible for mechanical systems. It thus becomes possible to derive a generalized theory of vibrating systems that is free from insignificant characteristics of the vibrator. The conclusions are illustrated by computations and by measurements for flat plates. The results are used to develop a theory of the propagation of bending waves in plates and shells on lines similar to the theory of reverberation in room acoustics.
33(1961); http://dx.doi.org/10.1121/1.1908400View Description Hide Description
The root mean square response of a simple mechanical resonator mounted on another, which is excited by vibration of a flat motional power spectrum, is investigated theoretically for the case of no loading effect between resonators. While the solutions are not general for any pair of coupled resonators, the condition of zero loading is of particular interest, as it leads to extreme vulnerability of electronic and electromechanical equipment to shock and vibration when there is a coincidence of resonance frequencies of coupled resonators. Approximate formulas are obtained and computed by hand. The exact equation is solved on a digital computer to yield a series of curves for comparison. The approximate formulas hold surprisingly well, except when the resonators are close in frequency and highly damped. Curves of the derivative of the rms response, as one resonance frequency or the other is varied, are obtained by use of the digital computer. They illustrate the importance of mechanical tolerances in relation to detuning as a factor in design changes for improved reliability.
33(1961); http://dx.doi.org/10.1121/1.1908402View Description Hide Description
The axisymmetric extensional modes of free vibrations of a thin prolate spheroidal shell separate into two classes, one class with displacements perpendicular to the meridional planes and one class with displacements in the meridional planes. This paper concerns itself with the former, the torsional modes. The differential equation for the mode shapes, obtained by application of Hamilton's principle, is found to be satisfied by single prolate spheroidal angle functions of the first kind, and the transcendental frequency equation is readily solved with the aid of tabulated eigenvalues. Numerical and graphical nondimensional results are presented for the first eight modes.
33(1961); http://dx.doi.org/10.1121/1.1908404View Description Hide Description
The intense vibration accompanying the extension of dive brakes and other high‐drag devices on aircraft and missiles has been observed for many years. Recently, considerable vibration has been experienced at the fundamental longitudinal mode and at other internal resonance frequencies of a vehicle which had a blunt base during the maximum dynamic pressure phase of flight. In both cases the vibration is believed to result from the turbulent fluctuations in the wake of the base or drag device.
The base pressure fluctuations have been measured at two base positions on a small body of revolution at eleven points in the velocity range of 68 to 352 ft/sec. The results show that: (a) The ratio of varies between 0.007 at the center of the base to 0.015 at 65% radius; (b) The spectrum of the pressure fluctuations is a function of the body Strouhal number; (c) The variation of observed vibration response with flight parameters can be predicted from the base pressure fluctuation data.
33(1961); http://dx.doi.org/10.1121/1.1908406View Description Hide Description
Variations in underwater acoustic ambient noise spectra, from 20 to 1250 cps, were analyzed using a statistical filter technique to examine the spectral density functions in the range from 1 to 80 cycles per week. At frequencies below 100‐cps narrow band components at seven cycles per week were observed in the spectral density functions. In several cases harmonics of the seven‐cycle‐per‐week component were also evident. The period of the cyclic variation agreed more closely with the solar day than with tidal period. The maxima occurred at approximately midnight local zone standard time, and at one location additional maxima were observed at approximately noon. The magnitudes of the periodic variation were small, 1.5 to 5.0 db, showing little evidence of seasonal dependence, except that at one location daily changes of 10 to 20 db were observed during the period of the summer solstice while hardly any periodic change was evident during the winter solstitial season. These periodic changes in the ambient noise levels were not correlated with changes in local wind speed.
33(1961); http://dx.doi.org/10.1121/1.1908408View Description Hide Description
The development of equipments and techniques for making measurements on underwater transducers is described. The Naval Air Development Center used a circular tank with no special linings, and directed its effort to develop a suitable electronic pulse test equipment. It is concluded that the accuracy and reliability of the calibration facilities, measurement techniques, and the use of the round wooden tank are satisfactory for most underwater transducer evaluations.
33(1961); http://dx.doi.org/10.1121/1.1908413View Description Hide Description
Hydrographic data (temperature, salinity, depth) and directly measured sound velocities were obtained from a section made at 6°E across the Mediterranean Sea in June and July, 1959. Sound velocities were calculated from the hydrographic data and compared with the measured values. Two methods were used to calculate the sound velocity from the hydrographic data. One method used Del Grosso's zero depth values with the depth correction from Matthew's tables. (This has been the usual method of calculation at the Woods Hole Oceanographic Institution until now.) The other method used the empirical equation of Wilson derived from laboratory measurements made at the Naval Ordnance Laboratory. The significant difference observed in the comparisons appears below 200 m. Here the corrections using Matthew's tables result in too large values for the velocities at depth, the magnitude of the difference being about 2.5 m/sec at 2500 m depth. The calculated values from Wilson's equation agree with the experimental values more closely (about 0.5 m/sec difference at 2200 m depth).
33(1961); http://dx.doi.org/10.1121/1.1908415View Description Hide Description
A study is reported which examined the efficiency of human observers in the detection of a stimulus waveform which is a train of damped sinusoids. The signal duration and degree of damping (or spectral bandwidth) were varied, with the energy of the signal held constant. Increased bandwidth is shown to decrease detection of relatively long duration signals. A negligible effect of bandwidth was found for short durations. Detection of an amplitude difference, in contrast to detection of the presence of weak signal in noise, is shown to be relatively unaffected either by signal duration or bandwidth over the range of values studied.