Volume 20, Issue 5, September 1948
Index of content:
20(1948); http://dx.doi.org/10.1121/1.1906413View Description Hide Description
Ultrasonics was conceived and came into being in France in a wartime academic atmosphere. Its physical principles were explored in England and in this country during World War I. Piezoelectricquartz was the primary, and for many years the only, active transducer element which was utilized in Europe. However, as soon as the work was started in this country, vigorous innovations were introduced. Synthetic Rochelle salt crystals were investigated and applied. Some theoretical work and several experimental techniques were developed in the United States concerning transducers. Of the number of outstanding physicists identified with this effort in America, most returned to their universities as soon as the emergency was over, and the subject of ultrasonics become dormant during the twenties. The Navy maintained an interest in ultrasonics and sponsored classified projects in the field. The subject of piezoelectricity was advanced, new synthetic crystals were brought forth, and large‐scale processing, cutting, grinding, and lapping of crystals were started and pursued by the Navy. An American version of the steel sandwich was produced for operational tests. The Pierce magnetostrictive devices were incorporated in sonar systems. Similarly, the electrodynamic principle was implemented into a useful transducer. A variety of absolute and secondary standards were developed.
20(1948); http://dx.doi.org/10.1121/1.1906414View Description Hide Description
Ultrasonic and also sonic waves of high intensity may produce discomfort and even injury. The dominant biological effects of various parts of the frequency spectrum are summarized. “Discomfort” implies that some sense organ is affected; but some combinations of intensity, frequency, and duration of exposure may possibly injure without the usual warning of discomfort. For the benefit of personnel working in high intensity sonic or ultrasonic fields we should establish both discomfort and danger contours, and there are wide gaps in our present knowledge. The Ultrasonics Panel of the Aeronautical Board desires particularly to receive any relevant well‐authenticated observations, either positive or negative, particularly concerning the effects of high‐intensity sound or ultrasonics on the sense organs and the nervous system. Casual publication of isolated, uncontrolled but spectacular “observations” may cause widespread popular apprehension and hinder the adoption of methods and instruments that involve ultrasonics of any sort.
20(1948); http://dx.doi.org/10.1121/1.1906415View Description Hide Description
Absorption of ultrasonic waves in water has been measured at temperatures from 0°C to 80°C at six frequencies between 12.25 megacycles and 40.50 megacycles. The experimental values are shown to agree within experimental error with the theoretical values calculated by L. H. Hall on the basis of a structural absorption.
20(1948); http://dx.doi.org/10.1121/1.1906417View Description Hide Description
20(1948); http://dx.doi.org/10.1121/1.1906418View Description Hide Description
The paper deals with phenomena associated with the interactions between flexural vibrations in a plate and the sound field in an ambient fluid medium. An equation is developed in tractable form which determines the propagation function of the free and forced flexural waves which can exist under stated assumptions. It is found that the reaction of the fluid on the plate makes possible a wave which cannot exist in the free plate in steady state. For a steel plate in water, the potential effect of the additional wave is relatively great when the product of frequency and plate thickness lies between 6 and 20 kilocycle‐inches.
20(1948); http://dx.doi.org/10.1121/1.1906419View Description Hide Description
It has been found that an appreciable reflection occurs in the direction of the incident wave when high frequency underwater sound strikes a submerged steel plate at an oblique angle of incidence. The magnitude of this “non‐specular” reflection, together with the associated transverse vibrations in the plate, has been studied experimentally as a function of frequency, plate dimensions and angle of incidence. Significant non‐specular reflection occurs from bounded plates between about one‐quarter inch and two inches thick, when the product of thickness in inches and frequency in kilocycles per second lies between about 20 and 60 in.‐kc. Within this range, the angle of incidence for maximum non‐specular reflection, η is described by the empirical relation: . This relation has been found to apply also to 0.002 inch plates at 15 megacycles.
The results of the present study are in agreement with a theoretical analysis by Fay, and with the experimental results obtained by Sanders at frequencies of several megacycles.
20(1948); http://dx.doi.org/10.1121/1.1906420View Description Hide Description
The reciprocity method for the absolute calibration of vibration transducers is reviewed, and the design and calibration of a set of primary standard transducers is described. Resulting calibrations are shown to be both reliable and accurate up to at least 700 cycles per second.
20(1948); http://dx.doi.org/10.1121/1.1906421View Description Hide Description
Obtaining a suitable mechanical resistance usually presents considerable difficulties when constructing electromechanical apparatus, such as microphones and loudspeakers. This article describes how a mechanical resistance—or, in general, a mechanical impedance—can be achieved in an electrical way. Special artifices such as compensation coils or bridge connection, which are required to overcome secondary difficulties, are illustrated by some practical applications.
20(1948); http://dx.doi.org/10.1121/1.1906422View Description Hide Description
The natural frequencies of longitudinal vibration of a rod fixed at one end and carrying a concentrated mass at the other end can be found by solving the transcendental frequency equation. Rayleigh showed that a good approximation to the fundamental frequency is got by replacing the distributed system by a simple harmonic oscillator having one degree of freedom, whose stiffness equals the original stiffness of the rod and whose mass equals the end‐mass increased by one‐third the mass of the rod alone. The factor one‐third may be replaced by some other “lumping coefficient” λ. It is shown that Rayleigh's lumping coefficient is the best possible one when the end mass is large compared with the mass of the rod alone, and leads to a maximum error in the frequency of 10.3 percent. This maximum error takes a minimum value of 1.68 percent when a lumping coefficient is used; this “best” coefficient yields a better approximation to the fundamental frequency than does the Rayleigh coefficient when the end mass is less than 0.511 times the mass of the rod alone. The variable lumping coefficient, which is a function of the masses involved, is discussed.
20(1948); http://dx.doi.org/10.1121/1.1906423View Description Hide Description
The radiation admittance of a semi‐infinite space, as seen by a cylindrical tube through an aperture in an infinite plane, is computed. The pressure fields in the tube and half space are developed in terms of the axial velocity in the aperture, and an integral equation for this velocity is determined. It is shown that the velocity function which satisfies this integral equation makes the real and imaginary parts of the radiation admittance absolute minima. An asymptotic expression for the field at a distance from the aperture is developed, and reflection and transmission coefficients are determined in terms of the radiation admittance.
The general theory is applied to a circular tube and an infinite slot. Numerical results are given in the form of curves, and the results for the circular tube are compared with those obtained by Rayleigh.
20(1948); http://dx.doi.org/10.1121/1.1906424View Description Hide Description
The radiation from a plane piston into a rigid circular tube is treated. Consideration is given to the propagation of higher order sound waves. Curves of radiation impedance and pressure distribution are given together with an analysis of the elements of the equivalent impedance circuits. In addition to the closer study for special values of the tube's end impedance, some outline of the general case is given. Simplified measurements verify the calculations in the case of a tube closed by a rigid wall. Finally, the resonance frequency of cylindrical resonators is discussed.
20(1948); http://dx.doi.org/10.1121/1.1906425View Description Hide Description
Expressions are obtained for the energy radiated by a clamped circular diaphragm excited by periodic impacts. The radiation consists of a line spectrum with intensity maxima in the neighborhood of the resonant frequencies of the various vibrational modes of the diaphragm. The energy in the spectrum lies almost entirely below a frequency equal to twice the reciprocal of the time of contact of the hammer with the diaphragm at each impact.
Expressions for the efficiency of the system as a converter of mechanicai energy into acoustical energy are derived, and the conditions resulting in maximum efficiency are determined. Curves are presented to assist the calculation for the case of a steel diaphragm in water.
20(1948); http://dx.doi.org/10.1121/1.1906426View Description Hide Description
The masking of tones by continuous and interrupted noise was determined for three noise intensities, three noise‐time fractions, and rates of interruption varying from 0.5 to 512 interruptions per second. The decrease in masking effectiveness as a result of the discontinuity of the masking noise depends principally upon the duration of the silent interruption, and secondarily upon the frequency of the masked tone and the intensity of the noise. A computational device is presented which will permit a first approximation of the masking produced by interrupted noises.
20(1948); http://dx.doi.org/10.1121/1.1906427View Description Hide Description
A combination speech‐pure tone group test technique is described in which standard phonograph speechaudiometer equipment is readily converted for dual test use through the addition of a commercial discrete frequency audiometer and an inexpensive coupling unit. The group pure tone section of the test is rapidly scored. The group pure tone section of the test is shown to possess a marked superiority over the phonograph record test in terms of correlation with individual pure tone test criteria.
20(1948); http://dx.doi.org/10.1121/1.1906428View Description Hide Description