Volume 8, Issue 4, April 1937
Index of content:
8(1937); http://dx.doi.org/10.1121/1.1915897View Description Hide Description
Most of the previous theoretical work on acoustic filtration has dealt with idealized infinite structures. The present paper is a theoretical study of finite filters such as are used in practice. Considering an acoustic structure with air as the medium with main line impedance Z and with n branch impedances Zb (pure reactances) equally spaced at intervals 2l, it develops that one can express the power transmission ratio for the structure in the simple form , (1) where , (2) the familiar parameter of the transmission theory of filtration (reference 2). The plot of Pr in (1) as a function of frequency shows the presence of transmission and attenuation bands which approach those of the corresponding ideal infinite case as n becomes large compared with unity. Comparison of (1) with the actual transmission measurements of Stewart (reference 2, pp. 170, 175) shows satisfactory agreement.
The phase change which occurs between successive sections in a filter is also computed and it is shown that unlike the situation in the infinite case the phase change is a function of the section. However, at relatively high frequencies and for large n this approximates to the total phase change along the structure divided by n. Comparison with such experimental results as are at present available yields reasonably satisfactory agreement.
8(1937); http://dx.doi.org/10.1121/1.1915899View Description Hide Description
Laminations in the plating and other members of fabricated steel structures cause weaknesses and failures in present welded junctions that did not occur in the case of riveted joints. Thus the need has arisen for some effective method and means for detecting such flaws in structural steel material. Simple theoretical considerations predict that the presence and location of such laminations can be determined by the distortion which they produce in sand patterns formed on the plates when they are thrown into various types of mechanical resonance. Practical tests have substantiated these predictions in the case of plates of different form and thickness.
8(1937); http://dx.doi.org/10.1121/1.1915900View Description Hide Description
An analogy is drawn between the piano string and an electrical transmission line, utilizing a different correlation than is customary in usual electroacoustical analogs. The current in the line is compared with the displacement and the voltage is compared with the momentum. With this correspondence the highly developed treatment of transmission lines becomes applicable to the piano string. The various cases discussed include: Open circuited line with and without attenuation (i.e., ideal and actual string with ends rigidly fixed), terminated line (string affixed to bridge of soundboard), electrical impulse introduced (impact of piano hammer). A relation is derived giving the optimum position of striking point for a given ratio of hammer mass to string mass, and this is shown to agree with the results of George and Beckett. The appearance of the nth harmonic when the string is struck at 1/nth of its length is explained, likewise the advantages of the modern practice of high string tensions. Conclusions are drawn concerning impedance matching (string to soundboard) and the velocity of propagation for different frequencies.
8(1937); http://dx.doi.org/10.1121/1.1915901View Description Hide Description
Recently materials for direct recording and reproducing work have been improved so that they are now suitable for many uses. These materials, as they are available on the market, are classified chemically into five groups and measurements are given of frequency characteristic, surface noise, life, distortion, etc. These data have been taken with both lateral and vertical recording. A short aural demonstration was given of typical recording on materials of each group.
8(1937); http://dx.doi.org/10.1121/1.1915902View Description Hide Description
8(1937); http://dx.doi.org/10.1121/1.1915903View Description Hide Description
Instruments for sound analysis may be grouped into five classes, graphic, resonance, heterodyne, stroboscopic, and diffraction analyzers. The operation of each type of instrument is briefly described. From the point of view of analysis sounds may be classified roughly into four groups: (1) Steady state sounds or sounds which may be maintained at constant fundamental frequency, constant intensity and unvarying quality for long enough to carry out the analysis; (2) Sounds which are essentially transient in nature; (3) Sounds which may be maintained constant on the average but whose intensity, frequency, and wave form are modulated at a constant frequency; (4) Noise, or sounds which are entirely random in form but which are continuously maintained. The sound spectra corresponding to each group are described and the application of the various types of instruments to the analysis of these spectra is discussed. It is pointed out that a graphic analyzer may not be used to analyze single waves taken from a short transient sound.