Volume 29, Issue 1, January 1957
Index of content:
29(1957); http://dx.doi.org/10.1121/1.1908671View Description Hide Description
Ultrasonic propagation is affected by elastic properties of material distribution of material and stress continuity. Since these are properties that determine structural strength of manufactured parts, ultra sound is useful for nondestructive testing. Variations in internal properties of structure can be detected, but it is often not possible to evaluate the effect that detected structural variations have on the ability of a part to function properly. This paper takes a critical view of the nondestructive testing field in the light of this objective.
29(1957); http://dx.doi.org/10.1121/1.1908680View Description Hide Description
This paper reports experimental investigations of the behavior of single cavitation bubbles produced in water. In the first set high intensity sound of frequency 14.6 kilocycles is focused on a hydrophone. Oscillograms were made of the natural oscillations of the cavitation bubbles. In the second series of observations motion pictures were made of cavitation bubbles at the rate of 105 000 frames a second. This enables the actual oscillations of the bubbles to be observed visually.
29(1957); http://dx.doi.org/10.1121/1.1908696View Description Hide Description
A cylindrical bariumtitanate element has been placed at the distal end of a specially designed catheter and used in studies of the heartsounds of dogs and humans. Heartsounds have been recorded with the element located in the four chambers of the heart as well as in the great vessels leading from the heart. X‐ray photographs monitored the location of the catheter tip in each case. The sounds have been recorded on tape and simultaneously on a photographic galvanometer recorder, all studies were continuously monitored by electrocardiogram. Sound spectrographic analysis of the several sounds was made and analyzed.
29(1957); http://dx.doi.org/10.1121/1.1908662View Description Hide Description
Torsional waves are propagated in a uniform diameter rod with the shear velocity. On the other hand, the effective velocity is slower in a disk‐loaded rod with alternate large‐diameter and small‐diameter sections. Because of the slower velocity, a disk‐loaded rod will delay a signal longer than conventional ultrasonic delay lines of the same path length. This paper describes the construction of these disk‐loaded torsional wave delay lines, and of electromechanical transducers for generating and detecting the torsional waves. The method of testing and the results of tests on the lines are presented. For example, a brass line having the disk diameter five times the diameter of the sections between the disks provided a delay of 114 μsec/cm at 32 kc. For contrast, the delay of a uniform diameter brass rod is only 4.5 μsec/cm.
Disk‐Loaded Torsional Wave Delay Line. II. Theoretical Interpretation of Results and Design Information29(1957); http://dx.doi.org/10.1121/1.1908666View Description Hide Description
In order to account for the magnitude of the delays reported in Part I, the effects of distributed mass and elasticity in the disk must be included in the analysis. Formulas for the torsional impedance at the inner circle of a thin annular elastic disk with a free outer boundary were first derived by H. T. O'Neil. One finds from these formulas that the structure can properly be represented by lumped electrical elements over a relatively wide frequency range. This electrical analog is a tandem arrangement of identical low‐pass filter sections for which formulas are conveniently available.
Complicated elastic effects near the sudden change of diameter are not included in the theory, and these effects may account for the discrepancy between theory, and experiment. Relatively simple empirical corrections should be possible after a sufficient number of tests.
The design of disk‐loaded delay lines is discussed in the light of the aforementioned analysis.
29(1957); http://dx.doi.org/10.1121/1.1908669View Description Hide Description
An expression obtained (1951) by the author for the radiation force on a scattering obstacle with arbitrary normal impedance is now shown to be valid for any scattering obstacle. The derivation of the final expression for the force in terms of the asymptotic scattering function for the obstacle in the field of an incident plane wave is accomplished by taking into account the interaction of the incident wave with the scatteredwave. Thus the former assumption of a perfectly collimated beam (1951) is avoided by considering the incident plane wave to be of infinite extent. The result for the force in the direction of the incident wave is where c 0 is the velocity of sound, Γ s is the magnitude of the mean scattered intensity, θ is the angle formed between the incident and scatteredwaves. This is the same result as that obtained in 1951.
The expression given in 1951 for the force perpendicular to the incident wave is correct only for an object which scatters in a restricted way. The correct general expression for the perpendicular component of the force is in which β is the angle formed between F ⊥ and Γ s .
29(1957); http://dx.doi.org/10.1121/1.1908673View Description Hide Description
The equation for the radiated sound associated with body forces in a fluid is applied to the flow past a circular cylinder. The sound field is found to be related to the oscillating lift and drag forces which act on the cylinder. Quantitative predictions are made of the directionality and intensity of the field. Some experiments were conducted in a subsonic wind tunnel, in order to test the theory. The microphone was located both inside and outside the tunnel. Over‐all sound pressure levels and sound spectra were measured with various cylinders mounted in the test section. There is qualitative agreement between the theory and the experimental results.
29(1957); http://dx.doi.org/10.1121/1.1908675View Description Hide Description
This is an analysis of dispersive properties of elastic plates in vacuo. For low modes there exists conclusive experimental verification of these properties. Model studies show prominent arrivals having the proper spectra and velocities for group velocity maxima and minima corresponding to several symmetric and anti‐symmetric modes. In addition, detailed calculations based upon exact formulas predict some new and as yet unconfirmed properties of plates, e.g., negative phase velocities. New results concerning high modes of propagation are also displayed. These modes are of considerable theoretical interest since they belong to the transition region between the domains of validity of the wave and ray theories. The structure of the symmetric and antisymmetric fiftieth mode dispersion curves illustrates that for a given angle of incidence the propagating energy will be either chiefly shear or chiefly compressional, passage from one to the other corresponding to energy transfer between symmetrical and antisymmetrical modes or propagation. Comparison with multilayered liquid wave guides are mentioned.
29(1957); http://dx.doi.org/10.1121/1.1908676View Description Hide Description
The exact equations of elasticity are employed in an investigation of the flexural vibrations of a solid circular cylinder. Contrary to previous work, it is shown that the phase‐velocity‐wavelength relation has an infinity of branches, thus overcoming objections, on physical grounds, which have been made to the earlier work. The three lowest branches of this dispersion relation are calculated, and these are used to study the rate of energy transmission in terms of group velocity.
29(1957); http://dx.doi.org/10.1121/1.1908678View Description Hide Description
Aerodynamic mixing is of fundamental importance in many fluid flow processes, some of which have intense superimposed sound fields. The effect of a transverse sound field on the temperature field downstream from a line source of heat was investigated both analytically and experimentally. In moderately intense sound fields, the temperature field is affected only in the time‐mean sense. In intense sound fields, the heat wake is substantially deformed so that both the time‐mean and instantaneous temperatures are considerably reduced.
29(1957); http://dx.doi.org/10.1121/1.1908682View Description Hide Description
The standard mathematical procedure formally describes scattering by the superposition of a scattered pressure on the unscattered sound field. At low frequencies, because of the irregular distribution of the inhomogeneities, the phases of the scatteredwaves are at random and scattering is an interference phenomenon. As the frequency increases, scattering becomes highly collimated in the forward direction and the phase differences decrease to zero. At this point ray theory starts to apply. The scattered pressure, then, essentially describes only a phase change caused by the different sound velocities and the focusing and defocusing by the lens action of the patches.
The medium in the neighborhood of the receiver can be shown to contribute only by focusing, the medium farther away only by interference fluctuations. Focusing leads to normally distributed amplitude fluctuations. The distribution of the interference fluctuations, however, passes from normal to Rayleigh with increasing values of range.
29(1957); http://dx.doi.org/10.1121/1.1908684View Description Hide Description
In this preliminary investigation the electromagnetic transducer method of Fitzgerald and Ferry is used to obtain the complex shear compliance and shear modulus of muscletissue taken from a freshly killed cow at room temperatures and in a frequency range from 25 to 2500 cps. Both the level and the shape of the compliance‐frequency curves are found to depend on the elapsed time after slaughtering. While there are some difficulties due to the rapid changes in viscoelastic properties with time, the results indicate that the method can be successfully applied to determine the viscoelastic properties of animal tissues.
29(1957); http://dx.doi.org/10.1121/1.1908686View Description Hide Description
A theoretical study is made of noise spectra radiated by the vibration of thin, stiff, flat plates under the action of turbulent boundary layerpressurefluctuations. The transmitted radiation investigated arises from the streamwise convection of quasi‐static pressurefluctuations by the steady flow and is critically influenced by the dispersive character of transverse wave propagation in the plates. At moderate subsonic Mach numbers and typical parameter values the total transmitted power varies approximately as the fifth power of Mach number, for sufficiently thin boundary layers. At lower Mach numbers the dependence may be flatter and near certain transonic velocities should be steeper. The dominant transmitted frequencies increase with Mach number, varying, under certain conditions, as Mach number squared. Plate parameters which importantly influence sound transmission included mass/unit area, size of independent plate sections, stiffness, and damping factors for the various vibrational modes. For sufficiently thin boundary layers, the transmitted spectrum shape is essentially independent of boundary layer thickness, but the total transmitted power varies approximately as the fourth power of the thickness. For thicker boundary layers, the dependence is flatter and the Much number dependence is also flattened. Several possible procedures for reducing the noise transmission are mentioned briefly.
29(1957); http://dx.doi.org/10.1121/1.1908688View Description Hide Description
Methods used in the measurement and evaluation of traffic noise in cities in West Germany are summarized. Particularly useful are the values of (1) the background level, (2) the mean peak levels, and (3) the average level (as obtained by a planimeter), evaluated for one‐minute periods. For spectral considerations, the “summation spectrum” is used; this is the sum of the spectra of a variety of vehicles which therefore averages out the variations between individual sources. Typical measurements are presented. Steps taken to reduce the disturbance from traffic noise by redesign of public vehicles and by town planning are described.
29(1957); http://dx.doi.org/10.1121/1.1908690View Description Hide Description
The steady‐state sound transmissioncharacteristics in model rooms of various shapes were measured. The shape of the model was varied from rectangle to trapezoid and to quadrangle with nonparallel walls, while the interior volume of the room was kept constant. In the results, it was observed that the trapezoidal rooms in which the ratio of l to a is about 10%, where a is the mean length of the parallel walls, and l is the difference between them, are effective in decreasing frequency irregularity in the transmission characteristics.
On the other hand, the sound characteristics of a fan‐shaped room, adopted as an example of splayed rooms, were analyzed mathematically. The fan‐shaped room with has good characteristics in low‐frequency region, where ρ0 is the ratio of the outer and inner radius of the room. Such a fan‐shaped room corresponds to a splayed room with . The results are in good agreement with the experimental results and our experience.
29(1957); http://dx.doi.org/10.1121/1.1908692View Description Hide Description
This arena‐type auditorium with a stage at one end has a volume of 3.24 million cubic feet, seats 5774 persons in stationary chairs and a total of nearly 10 000 with additional, portable chairs. Both the stationary and the portable chairs have upholstered seats. The acoustical hazard of concave rear wall and opposite wall containing the proscenium opening was ameliorated by special treatment. A combination structural‐acoustical ceiling was used. The design objective of a reverberation time of about 2.5 seconds in the unoccupied room at 500 cps was attained without adjustment. The results of reverberation‐time measurements are shown and discussed. The public‐address system is described and the subjective assessment of the auditorium is mentioned.
29(1957); http://dx.doi.org/10.1121/1.1908694View Description Hide Description
Most speech sounds may be said to convey three kinds of information: linguistic information which enables the listener to identify the words that are being used; socio‐linguistic information, which enables him to appreciate something about the background of the speaker; and personal information which helps to identify the speaker. An experiment has been carried out which shows that the linguistic information conveyed by a vowel sound does not depend on the absolute values of its formant frequencies, but on the relationship between the formant frequencies for that vowel and the formant frequencies of other vowels pronounced by that speaker. Six versions of the sentence Please say what this word is were synthesized on a Parametric Artificial Talking device. Four test words of the form b‐(vowel)‐t were also synthesized. It is shown that the identification of the test word depends on the formant structure of the introductory sentence. Some psychological implications of this experiment are discussed, and hypotheses are put forward concerning the ways in which all three kinds of information are conveyed by vowels.
29(1957); http://dx.doi.org/10.1121/1.1908631View Description Hide Description
29(1957); http://dx.doi.org/10.1121/1.1908634View Description Hide Description
The two major cues for stop consonants, the burst of the stop release and the formant transitions in the adjacent vowel, were investigated. Detailed energy density spectra of the bursts were prepared. The transitions were studied by means of sonagrams. Possible criteria for identification were developed and tested. In order to assess the efficacy of the two types of cue, perceptual tests were conducted with isolated segments that contained either stop bursts or vowel transitions alone. Common acoustical properties of bursts and formant transitions are noted; differences as well as similarities are discussed in the light of different varieties of pitch judgments.