Volume 25, Issue 2, March 1953
Index of content:
25(1953); http://dx.doi.org/10.1121/1.1907018View Description Hide Description
It is shown that the mathematical solution of problems involving the propagation of noise is materially aided by the introduction of the space correlation function ψ(x 1, x 2, τ), defined as the average, over t, of p(x 1 t)p(x 2 t−τ), p being the acoustic pressure in the noise field. The differential equations satisfied by ψ are derived. Its relation to Ψ(α, β, γ) is discussed, α, β, γ being the propagation vector of a sinusoidal wave, and Ψ/2ρc 2 being the density of potential energy in the α, β, γ space.
The theory of uniform noise fields, both isotropic and anisotropic, is developed in detail. The anisotropy caused by a reflecting plane is discussed. The radiation of noise by a vibrating plane is discussed, neglecting the reaction of the radiation on the motion of the surface. In particular, it is shown that to this approximation the sea surface cannot radiate subsonic energy into the high atmosphere because the velocity of the surface gravity waves is less than the velocity of sound in air.
The theory of noise, as here developed, is analogous to the theory of turbulence, and the relation between noise and turbulence is briefly discussed.
25(1953); http://dx.doi.org/10.1121/1.1907019View Description Hide Description
The force‐time history required to produce a step in the velocity of a circular piston mounted in an infinite baffle is determined with the aid of the Laplace transform. Working from this result, the inverse problem of the velocity response to a step force is calculated approximately. These two results suffice to treat arbitrary distributions of applied force or velocity by Duhamel superposition. The application of the results to a loudspeaker model shows that a system designed for critical damping on the “steady‐state” approximation for the air loading will actually be slightly overdamped in its initial motion, but that the characteristic time during which the design assumption is inadequate is of the order of 10−3 second.
25(1953); http://dx.doi.org/10.1121/1.1907021View Description Hide Description
Diffraction of an acoustic wave by an aperture in a plane reflecting screen has been investigated experimentally at a wavelength of about 3 cm. The pressure amplitude distribution within a circular aperture illuminated by a plane, progressive sound wave exhibits oscillations which do not conform to theoretical requirements. A possible explanation for the disagreement offered. Diffraction measurements made in the near‐field of the circular hole were found to be in fair agreement with the Fresnel‐Kirchhoff theory. A modified theoretical expression, valid for apertures large as compared to the wavelength, is also presented. It is concluded that this theory is capable of predicting the diffraction effects to a greater degree of accuracy and over a larger region of space than Kirchhoff's classical formula.
25(1953); http://dx.doi.org/10.1121/1.1907022View Description Hide Description
Exact solutions of the one‐dimensional gas dynamic equations, representing periodic sound waves of finite amplitude, are obtained for a particular medium. The progressive wave from a vibrating piston and the standing wave in a closed tube are examined in detail. Limits on the amplitude of the sound wave are found in order to avoid shocks or cavitation.
25(1953); http://dx.doi.org/10.1121/1.1907023View Description Hide Description
Well‐known laws which govern the reflection of elastic waves that strike free surfaces obliquely are used to deduce particle motion at the free surface of a body. The surface particle motion is presented in the form of diagrams and graphs for the case of an incident longitudinal wave. Considerations indicate that, for oblique incidence, the particle motion at the surface will not, in general, be perpendicular to the surface but will depend on the angle of incidence and the Poisson's ratio of the material. The data are expected to be of value in the solution of problems connected with impulsively loaded bodies such as metal‐explosive systems.
25(1953); http://dx.doi.org/10.1121/1.1907024View Description Hide Description
A relatively tractable expression for the transmittivity of steel plates immersed in a fluid medium is formulated. Criteria for total transmission and total reflection are inherent in the expression.
An investigation of the discrepancies between calculated and observed transmittivity indicates a probability that the assumption of negligible losses associated with shear waves in steel is not tenable.
The Acoustic Wave Guide. I. An Apparatus for the Measurement of Acoustic Impedance Using Plane Waves and Higher Order Mode Waves in Tubes25(1953); http://dx.doi.org/10.1121/1.1907025View Description Hide Description
The (1 0) and (2 0) modes of acoustic waves in rectangular wave guides have been excited to the virtual exclusion of plane waves. The experimental techniques depend on the use of acoustic sources equivalent to two or more pistons with appropriate relative phases and amplitudes, precise adjustment of which is accomplished with the aid of an accurately located probe microphone. The standing wavepattern which arises when the wave guide is terminated by a partially absorbing surface may be used to determine the specific normal impedance of the surface, the value of which is characteristic of waves having a particular oblique angle of incidence and may be compared with normal incidence values obtained from plane wavemeasurements in the same wave guide and at the same frequency; the angle of oblique incidence may be varied by changing the frequency. The apparatus described operates in the 1–3 kc/sec frequency region and the accuracy of impedance measurement with (1 0) and (2 0) waves is comparable with that attainable with the usual plane wave tube techniques. Equivalent angles of incidence at the absorbing surface up to 84° have been used. The principles underlying the design of the apparatus and some of the distinctive problems which arise with higher order mode waves are discussed.
The Acoustic Wave Guide. II. Some Specific Normal Acoustic Impedance Measurements of Typical Porous Surfaces with Respect to Normally and Obliquely Incident Waves25(1953); http://dx.doi.org/10.1121/1.1907026View Description Hide Description
The experimental techniques described in Part I have been applied to the measurement of some acoustic properties of specimens of rockwool, hairfelt and acoustic tile, backed by a rigid wall. The more porous surfaces show substantial increases resistance at oblique angles of incidence, an increase of nearly 100 percent at 80° a incidence being observed in one case; smaller changes in reactance also occur. The absorption coefficients corresponding to impedance measurements at oblique incidence are compared with those “predicted” from normal incidence impedance measurements and significant differences are found. The measurement of surfaces having structure of finite dimensions is discussed with particular reference to an acoustic tile specimen, the impedance of which varies only slightly with angle of incidence. In some cases slight scattering of acoustic energy from one mode to another provides evidence of lack of uniformity of impedance over the surface of the specimen.
25(1953); http://dx.doi.org/10.1121/1.1907027View Description Hide Description
Depending upon the kind of electrode system employed, a sinusoidal electric current gives rise to at least five phenomena. (1) With a moving electrode on the skin or on the roof of the mouth (fricative effect), the subject receives the strongest sensation of hearing. He may hear either the first harmonic or the first and second harmonics, or the second harmonic alone, depending upon the applied voltage and upon the placement and properties of the electrode. (2) With a large‐area electrode any place on the skin of the head, if the skin is dry the subject hears the second harmonic; if it is wet, he hears nothing. (3) When the electrode is immersed in salt solution in the ear (the most common method), the subject hears a complex tone containing mostly second harmonic, suggesting the action of a square‐law transducer. (4) When the electrode is on the mucous tissue inside the middle ear (with eardrum removed), the subject hears the first harmonic and/or a noise. (5) When the electrode is in contact with the epidermis of the meatus, the subject hears the first harmonic, and at low frequencies he may also hear a noise. The experimental results seem to indicate that the hearing of a tone under any of these five conditions is probably due to vibrations set up outside the cochlea, although there appear to be at least four different transducing mechanisms. Contrary to earlier hypotheses, the tympanic membrane is apparently not involved in the conversion of the electrical energy into mechanical vibration.
25(1953); http://dx.doi.org/10.1121/1.1907028View Description Hide Description
The Royal Festival Hall was opened in London in May, 1951. This paper describes in some detail its acoustical design, the test concerts held in it before the opening, the objective measurements made in it, and the comments about its acoustics that have been made during the first eighteen months since its opening. These comments show that the “definition” is excellent, but that for some types of music more “fullness of tone” would be desirable. It is concluded that the reverberation time is the only objective measurement which, at the present stage of development, is of practical use. Its value in the Royal Festival Hall when full is 1.5 seconds (at 500 cps), which is 0.2 second shorter than the optimum value given by Knudsen and Harris for a hall of this size. It seems probable that the “fullness” would be adequate if the reverberation time could be lengthened to 1.7 seconds or somewhat longer.
25(1953); http://dx.doi.org/10.1121/1.1907029View Description Hide Description
An apparatus is described which may be used to measure the ultrasonicabsorption coefficient in liquids. The transducer is one‐hundred percent modulated at an audiofrequency. This produces the same frequency variation in the radiation pressure. A condensermicrophone is used to detect these low frequency pressure changes. The apparatus has been tested in water over the frequency range 9–27 mc and possesses an accuracy which is comparable to that of pulse and optical methods.
25(1953); http://dx.doi.org/10.1121/1.1907030View Description Hide Description
The amplitude absorption coefficient α for pulsed sound waves in argon, nitrogen, and oxygen has been measured over a range of pressure p from 1 to 10 mm Hg for frequencies f between 60 and 70 kc, the temperature in all cases being held nearly constant at 20°C. For the three gases used, the following experimental values of (pα/f 2)107 (cgs units) were obtained: (1) for argon 1.86 with an rms deviation of 0.03, (2) for nitrogen 1.64 with an rms deviation of 0.04, and (3) for oxygen, 1.92 with an rms deviation of 0.03. The corresponding values computed from the classical absorption equation are (1) for argon, 1.87, (2) for nitrogen, 1.31, and (3) for oxygen, 1.61. Thus the absorption in argon is classical while for nitrogen and oxygen the absorption exceeds the classical value. These excesses are attributed to rotational relaxation and the associated relaxation times are calculated in accordance with the thermal relaxation theory of Herzfeld and Rice, and Kneser. For nitrogen, the relaxation time (reduced to STP conditions) is 4.85×10−10 second and for oxygen is 4.95×10−10 second. Both these values are significantly smaller than the values obtained by other workers at higher frequencies.
25(1953); http://dx.doi.org/10.1121/1.1907031View Description Hide Description
The subject of this paper, which is mainly descriptive, grew out of the desire of a biologist to measure the thickness of the human bowel wall in the living intact subject. [J. J. Wild, Surgery 27, 183 (1950)]. The object was to justify certain methods of treatment for the suspected underlying mechanisms at work, the results of which are lumped together by surgeons under the title of bowel distension.
The approach to the subject has been almost entirely biological, not ignoring, however, the theory of sound where applicable. It will be seen that biological methods of control are used as a first approximation in order to justify more detailed examination of the system. Some of the difficulties of the exact approach from theory will be discussed after presentation of the experimental facts. It may be of interest to the more exact‐thinking members of the audience to note that a system having a nightmare of complexity to the physicist may be considered simple by the biologist in his blissful ignorance. This situation is perhaps fortunate because otherwise the biologist might be tempted to give up too easily. It is the duty of the biologist to try to simplify complex phenomena so that his more able colleagues in more exact branches of science will be stimulated to “pitch in” and help. We hope that today we shall be able to bring about this desirable end and succeed in stimulating more workers to investigate this field. It should, however, be realized at the outset that the increasing exploitation of the observed phenomena does not necessarily need to wait upon whatever fundamental explanations may be forthcoming. The experimental results will not necessarily be presented in chronological sequence but an attempt will be made to present some of our collected experimental data in as logical a manner as possible.
25(1953); http://dx.doi.org/10.1121/1.1907032View Description Hide Description
The results of a histological study of the changes produced by high intensity ultrasound on tissue of the central nervous system are presented. The study was restricted to the nontemperature effects of the sound since these are of particular significance for neurology. The results obtained show that nerve cell bodies are particularly sensitive to the action of the ultrasound, while blood vessels and nerve fibers are much more resistant.
The use of a focused beam of ultrasound to produce discrete lesions deep in the brain without disturbance of the vascular system or through‐going nerve tracts in the region of the lesion is demonstrated by a study on the bulboreticular inhibitor formation of the cat medulla.
Physical factors which require consideration are discussed in terms of the histological work.
25(1953); http://dx.doi.org/10.1121/1.1907033View Description Hide Description
Measurements of absorption and velocity of sound in blood, plasma, and solutions of albumin and hemoglobin have been carried out in the frequency range 800–3000 kc and temperature range 5–45°C. The absorption departs only slightly from a linear dependence upon frequency. Absorption for the various solutions is in direct proportion to protein content. It is concluded that the acoustic properties of blood are largely determined by the proteins which it contains.
25(1953); http://dx.doi.org/10.1121/1.1907034View Description Hide Description
Pepsin preparations were sonically treated for various periods of time in a 9‐kc magneto‐striction oscillator with the temperature maintained at 13°C–16°C during treatment. After sonic treatment the residual proteolytic activity of the enzyme was determined by measuring the turbidity decrease during the digestion of an albumin substrate. It was found that dilute solutions of Merck U.S.P. pepsin were rapidly inactivated by sonic treatment whereas highly concentrated solutions were refractory to ultrasonic destruction. Sonic irradiation of a pure Armour crystalline pepsin solution yielded slight enzyme inactivation only. In no instance was enzyme activation observed in this study.
25(1953); http://dx.doi.org/10.1121/1.1907035View Description Hide Description
After a short description of mechanical transducers such as the siren in air and the Pohlman Whistle in liquids, some fundamentals of electromechanical transducers using electric and magnetic fields are given, and magnetostrictive and piezoelectric transducers for the irradiation of liquids which are available on the market are mentioned.
In the second part transducers made of barium titanate (“ceramic transducers”) are described. Polycrystalline ceramic material can be shaped and thereafter polarized in such a way as to produce convergent waves, with the possibility of focusing ultrasonic energy and thus creating zones of extremely high ultrasonic intensities. Electrical and acoustic data on spherical ceramictransducers are given. Fair agreement is found between O'Neil's theory of spherical transducers and measurements of the pressure distribution in a sound field along the central axis in front of the transducer. The diagram of motional impedance for a 400 kc spherical bowl and power limitations of ceramictransducers are discussed. Finally, a tubular ceramictransducer is described which seems to offer good possibilities for efficient treatment of large liquid volumes in a continuous flow process.
25(1953); http://dx.doi.org/10.1121/1.1907036View Description Hide Description
Transducers were constructed using thin‐walled bariumtitanate tubes cut in short lengths. The outside walls of the tubes were covered with pressure relief material. Radial motion of the tubes developed motion along their axes in an enclosed fluid medium. If one end of the tube was closed by a heavy metal block, the tube became the equivalent of a quarter wave piston, the open end being a vibrating surface moving parallel to the axis of the tube. Various mosaic patterns were formed of these tubes to give directional transducers. Their operating characteristics were quite dependent on the kind of fluid enclosed by the tube.
25(1953); http://dx.doi.org/10.1121/1.1907037View Description Hide Description
A device is described for the measurement of the lateral complex mechanical impedance of phonograph pick‐ups in the frequency range between 30 and 10 000 cps. The device employs four electromagnetically driven, calibrated vibrators resonating different frequencies. The theory of operation is outlined, and results obtained for a typical phonograph pick‐up are presented.