From Current Literature

Acoustics

MEASUREMENT OF REVERBERATION. The work of V. Kreisler and W. Snyder describes a reverberometer. The sound source is a dynamic loudspeaker fed by a heterodyne generator; the receiver is a condenser microphone. The time-counting apparatus is switched on automatically after a steady state has been reached and is switched off after the density of the sound energy has fallen to a certain preset value. (V. L. Chrisler and W. F. Snyder, Journ. Soc. Mot. Pict. Eng., 18, 479—487, 1932.)

ACOUSTICS OF LARGE ROOMS. In rooms of very large size, according to the data of S. Wolf’s article, large discrepancies are observed between theoretically predicted data and experimentally determined values. For example, when the reverberation time in Madison Square Garden (New York) was measured, the calculation gave a figure of 35.5 sec., whereas from measurements for a frequency of 500 hertz a value of 7.6 sec. was determined. The paper describes a method for correcting the acoustics of the Madison Square Garden room, as a result of which the reverberation was reduced to 3.5 sec. (S. K. Wolf, Journ. Soc. Mot. Pict. Eng. 18, 517—525, 1932.)

CALIBRATION OF A CONDENSER MICROPHONE BY THE “SUBSTITUTING” CAPACITANCE CIRCUIT. As is known, at frequencies lying below resonance, a condenser microphone is characterized by proportionality between the amplitude of the pressure in the sound field and the amplitude of displacement of the membrane. Further, there is a simple relation between the displacement of the membrane and the change in the capacitance of the microphone caused by this displacement. The method proposed in W. Lange’s paper for calibrating a condenser microphone is based on experimental determination of the amplitude of capacitance oscillation and recalculation of this amplitude into the value of the effective pressure in the sound field. In Lange’s circuit the condenser microphone is replaced by a variable capacitor made of two glass disks, rotating relative to one another, with radially applied 100 sectors of metal foil on each disk. When the disk rotates, the capacitance between the disks changes approximately sinusoidally, and the amplitude of the change

of capacitance is easily determined by calculation. For each amplitude given by the substituting capacitor, a deflection of the oscillograph is noted in the final stage of the amplifier; after this it is not difficult to determine the amplitude of the microphone capacitance at a given frequency by connecting it in place of the substituting capacitor. Comparison of the results of this method with data obtained by Rayleigh’s disk showed that the discrepancy does not exceed 13%; the experiments were carried out with a high-frequency Riegger microphone. (W. Lange, H.-F. Techn. u. Eb. Ak. 39, 133—136, 1932).

TRANSVERSE VIBRATIONS OF TUBES were studied by H. Kröncke using glass tubes excited by organ pipes, the frequency range extending from 200 to 60,000 hertz. To observe the vibrations, a little dry sand was poured into the tube; this made it possible to observe overtones up to the 20th and higher. The investigations showed that, so long as the wavelength is comparable with the diameter of the tube, the observed vibration frequencies are lower than the theoretically calculated ones. In thin-walled tubes, still other transverse vibrations, with nodal lines parallel to the axis of the tube, are superposed on the transverse vibrations of the ordinary type. (H. Kröncke, Z. f. techn. Phys., 13, 196—198, 1932).

FORCED VIBRATIONS OF A CIRCULAR PLATE, experimentally studied by Elsaß (1883) and Schulze (1907), are theoretically interpreted in the work of W. Flügge. It is shown that a plate, like any other oscillatory system, can execute forced vibrations at any frequency. The repeatedly observed asymmetric arrangement of the nodal lines can be accounted for analytically by introducing special conditions for the application of the forcing forces. (W. Flügge—Z. f. techn. Phys., 13, 199—204, 1932).

MANUFACTURE OF MICROPHONE MEMBRANES BY THE METHOD OF CATHODIC SPUTTERING. The method of cathodic sputtering of metal in vacuum has until recently been used only in laboratory practice (manufacture of thin metallic films, conducting quartz filaments, etc.). In G. Frese’s article the application of the cathodic sputtering method for industrial purposes in the manufacture of membranes for Western Electric carbon microphones is described: cathodic sputtering is used to deposit a thin layer of gold on those parts of the duralumin membrane which come into contact with the carbon grains. This eliminates difficulties associated with the previously used electrolytic deposition of gold—unevenness, peeling of the layer, etc. The apparatus described by Frese consists of a bell-shaped vessel (7.5 l), serving as the working chamber, 6 aluminum cathodes coated with gold, a spiral anode, a vacuum regulator (bleeder valve), and a pumping unit. The discharge current is taken from a 1:90 transformer with a power of 0.5 kVA. The vacuum in the chamber is 0.1 mm Hg. The deposited layer of gold, not more than 1 μ thick, is quite uniform and does not peel away from the duralumin. The produc-

conductivity of operation of three apparatuses, serviced by one worker,—about 90 membranes per day. A detailed bibliography on the subject is appended to the article. (H. F. Fruth, Bell. Syst. Techn. Journ., 11, 283—292, 1932).

ARTIFICIAL VOICE AND ARTIFICIAL EAR. In studying telephone apparatus from the standpoint of the quality of speech transmission, it is not always convenient to use the natural voice and ear, owing to the impossibility of taking into account the individual characteristics of the speaker and the listener. In addition, a whole series of measurements (for example, taking characteristics) is impossible under conditions of normal functioning of the apparatus. In the work of A. Inglis, C. Gray and R. Jenkins, artificial voice and ear devices developed for telephone measurements are described. In the first apparatus the sound radiator is a loudspeaker radiating through a system that includes an active acoustic resistance of about 41 mechanical ohms per 1 cm². The loudspeaker is connected through an electrical system either to a sound generator, or to an adapter, or, finally, to a microphone. The article gives exhaustive characteristics of the radiator and its individual elements. The artificial ear is made in the form of a cavity reproducing the shape of the outer ear; an opening is made in the cavity, replacing the acoustic load caused by the tight fitting of the telephone to the auricle. The cavity passes into a channel replacing the auditory canal. The eardrum is replaced by the membrane of a condenser microphone; near it is located an acoustic system including mass, compliance and resistance, the magnitudes of which are chosen in accordance with the data of an experimental investigation of the natural ear. In the appendix to the work a description is given of microphone calibration and of the method of measuring pressure in the ear with a telephone fitted to it. (A. H. Inglis, C. H. Gray a. R. T. Jenkins, Bell. Syst. Techn. Journ., 11, 293—317, 1932).

DETERMINATION OF THE ABSORPTION COEFFICIENT. The usually employed methods for determining the coefficient of sound absorption by various materials require samples of sufficiently large size, which is often inconvenient and associated with excessive expense. In the work of R. Norris, the application of the Norris–Andree reverberometric method to the determination of absorption coefficients is described; moreover, for measurement a sample of the material under test of small size (1.5 m²) is required. The paper describes in detail the measurement procedure and gives the theory of the method. (R. F. Norris, Journ. Acoust. Soc. Amer. 3, 361—370, 1932).

STUDY OF NOISES OF RAPID URBAN RAILWAY TRANSPORT. The development of rapid railway transport in large cities brings to the fore the struggle against the noise caused by it, which primarily applies to elevated lines on viaducts. In exactly the same way, in the design and construction of new lines, there must be taken into account

conditions for the possibility of reducing the noise level. It is clear that all these measures can be implemented only after a careful study of the mechanism of origin and the physical nature of noises. This topic is addressed by the work of G. Stanton and J. Tweddall, which presents the results of a study of the noises of New York elevated and underground railway lines. The aim of the work was to take into account, from the standpoint of the “noise problem,” the design features of cars, trestles, the permanent way, as well as the effects connected with the character of the surrounding locality, with buildings on it, etc. Noise was measured by an objective method (measurement of the strength of rectified noise pulses in the final stage of a microphone amplifier); automatic recording of the noise level over a certain interval of time during the passage of a train was also used. The measurements showed that the average noise level of an elevated railway varies, depending on the type of cars and trestle, from 83 to 91 db; in streets adjacent to the line, the level is 60–70 db. It is characteristic from the standpoint of occupational hygiene that in institutions and enterprises located near the line the noise level with closed windows reaches 75 db, and with open windows rises to 85 db. Noises in tunnels and subway cars also reach a high level—about 90–95 db. In the concluding part of the work, possibilities for reducing the noise level in adjacent streets and buildings are briefly considered (G. T. Stanton and J. E. Tweeddall, Journ. Acoust. Soc. Amer., 3, 371–387, 1932).

PASSAGE OF SOUND THROUGH OPENINGS. In E. Ritchie’s work a question of great technical interest is considered: the acoustic conductance of openings. The author carried out a large number of experimental determinations, the results of which are summarized as follows: 1) The value of the conductance of an opening in an infinitely thin wall, theoretically derived by Lamb, is in sufficient agreement with experiment. 2) With increasing wall thickness the conductance decreases according to an exponential law; for the region of middle frequencies (450–1800 hertz) the author proposes the empirical formula: \(x = 73.308 - 7.4084 \lg y\), where \(y\) is the thickness of the wall in inches, and \(x\) is the conductance in percent. 3) With an increase in the number of openings the total conductance increases; the dependence of conductance \((y)\) on the number of openings \((x)\) is expressed, for openings of diameter \(1/4''\), by the empirical formula: \(y = 0.06439 x^3 - 1.74 x^2 + 17.09 x - 4.93\); for openings of diameter \(1/2''\), by the formula: \(y = 66.94 + 39.89x - 4.1702xx^2 + 0.15447x^3\). 4) The conductances of openings of different depth and size are always proportional to the area of the openings. (E. Ritchie, Journ. Acoust. Soc. Amer. 3, 402–414, 1932).

MEASUREMENTS OF SOUND-ABSORPTION COEFFICIENTS IN THE REGION OF HIGH FREQUENCIES. F. Gosher’s work is concerned with establishing the frequency characteristics of various sound-absorbing materials in

over a wide range of frequencies, extending up to 8000 hertz. The measurements were carried out by the reverberation method with warbling tones, in a room measuring \(1.67 \times 1.67 \times 1.53\) m, with walls about 30 cm thick. In the measurements, the humidity of the air, the hygroscopicity of the materials studied, and other similar factors were taken into account. From the curves presented, characterizing various sound-absorbing materials, it is evident that there are no typical forms of characteristics for different materials: some materials have a fairly even characteristic (tile facing containing mineral wool), while others increase absorption with frequency (hair felt). Therefore the choice of material must be made in accordance with the intended purpose, on the basis of knowledge of the frequency characteristics, precisely in the region of high frequencies (F. L. Hopper, Journ. Acoust. Soc. Amer., 3, 415—427, 1932).

PROPAGATION OF SOUND IN SEA WATER. Knowledge of the peculiarities of the propagation of sound waves in sea water is of great importance in a whole series of problems of applied acoustics (hydroacoustic sounding, direction-finding of ships and submarines, etc.). The work of H. Dorsey is devoted to the physical questions connected with these peculiarities. The principal conclusions reached by the author after 15 years of experimental work are as follows: 1) The hypothesis of multiple reflections of sound propagating in sea water must be abandoned and replaced by the hypothesis that sound propagates in water in the same way as in air. 2) The speed of sound and the acoustic resistance of sea water increase with temperature. 3) The speed of sound does not depend on its intensity. (H. G. Dorsey, Journ. Acoust. Soc. Amer., 3, 428—442, 1932)