Volume 20, Issue 1, January 1948
Index of content:
20(1948); http://dx.doi.org/10.1121/1.1906343View Description Hide Description
The development of an ultrasonic delay line for the storage of pulses 1 microsecond long for periods up to 2000 microseconds is discussed. Theoretical considerations of the piezoelectric transducer involving the quartz crystals and acoustical constants of the medium developed by Emslie, Huntington, and Benfield, are applied to determine the band width and power loss of the delay line. Problems, peculiar to the use of solids because of the presence of more than one mode of propagation and the distribution of energy between the different modes on reflection from an air surface, are evaluated for a wide range of Poisson ratios.Measurements of velocity and attenuation in some polycrystalline metals, single crystals, and glasses are given. Fused quartz was found to be the most promising material.
Delay lines constructed using a light source and photo‐elastic pick‐up were not completely developed, but several types using crystal pick‐up as well as transmitter were satisfactory. For delay times longer than 300 microseconds, multiple reflection paths in two and three dimensions were used and their preparation from fused quartz discussed.
20(1948); http://dx.doi.org/10.1121/1.1906344View Description Hide Description
The available methods for electrical phase measurement are reviewed, and the usefulness of one using a standard lag line as a reference is pointed out. Ways of using such a line for various types of measurements are discussed. The construction of two particular lines for measurements in the 10‐kc to 80‐kc range is described, and a method of calibration is outlined. An investigation of errors indicates that accuracies to within can be attained.
20(1948); http://dx.doi.org/10.1121/1.1906345View Description Hide Description
With the increasing importance of acoustical impedance and its relation to flow resistance and porosity in homogeneous porous materials, a simplification in the technique of porosity measurement seems in order. It is the purpose of this paper to describe a dynamic method of measuring porosity. The essential feature of the dynamic method is the use of the compliance of the air in the pores of the material to control the period of the free damped oscillation of a mechanical system. The dynamic method has the advantage of small temperature dependence and minimal time for a single measurement.
Effects of Differentiation, Integration, and Infinite Peak Clipping upon the Intelligibility of Speech20(1948); http://dx.doi.org/10.1121/1.1906346View Description Hide Description
Previous experiments on the effects of distortion in voice communication circuits have shown that intelligibility is impaired surprisingly little by the type of amplitude distortion known as peak clipping. It has been found, in fact, that conversation is possible even over a system that introduces “infinite” peak clipping, i.e., that reduces speech to a succession of rectangular waves in which the discontinuities correspond to the crossings of the time axis in the original speech signal.
The intelligibility of the rectangular speechwaves depends critically upon the frequency‐response characteristics of the speech transmission circuits used in conjunction with the “infinite clipper.” In the present experiments, resistance‐capacitance circuits with sloping frequency‐response characteristics (tilting circuits) were introduced into the system at points preceding and/or following the clipping circuit. The interactions of the nonuniform frequency characteristics of the resistance‐capacitance circuits with the nonlinear amplitude characteristic of the clipping circuit were studied by means of articulation tests. That there was strong interaction is evidenced by the fact that word articulation scores of 97 and 15 percent were obtained with two systems consisting of the same components in different orders. The components were (1) a tilter with a frequency‐response characteristic rising 6 db per octave (a “differentiating” circuit), (2) an infinite peak clipper, and (3) a tilter with a frequency‐response characteristic falling 6 db per octave (an “integrating” circuit). When these distorters were cascaded in the sequence 1‐2‐3 the speech output consisted of triangularly shaped waves which sounded very much like normal speech and which were highly intelligible (97 percent). When the reverse sequence (3‐2‐1) was used, the speech output consisted of sharp pulses giving rise to extremely poor quality and very low intelligibility (15 percent).
Tests with single distorters and with pairs of distorters indicated that: (1) In the absence of frequency distortion, infinitely clipped speech is of poor quality but moderate intelligibility (50 to 90 percent depending on the listeners skill and familiarity with the test words). (2) A differentiator or an integrator preceding the clipper determines the degree to which intelligibility is impaired by infinite clipping. (3) A differentiator or an integrator following the clipper (or used alone in a linear system) affects the quality but not the intelligibility of the speech transmitted by the system.
20(1948); http://dx.doi.org/10.1121/1.1906347View Description Hide Description
Monaural and binaural thresholds of ten listeners were determined for a 1000‐cycle tone and for a white noise. For each of the observers, the binaural threshold was found to be significantly lower than the monaural threshold when the two ears were “equated” in sensitivity, i.e. when the difference in sensitivity between the ears of a given observer was effectively canceled by experimental procedures. Furthermore, the difference between the monaural and binaural thresholds was found to be significantly greater for the pure tone than the corresponding difference for the noise.
Monaural and binaural thresholds were also determined with the two ears “equated” in sensitivity and with the two ears “mismatched” in sensitivity by fixed ratios. It was found that the difference between the binaural threshold and the threshold of the better ear decreased as the difference in the effective stimulation at the two ears was increased. The difference between the binaural threshold and the threshold of the better ear was found to be not statistically significant when the two ears were stimulated at sensation levels more than 6 db apart.
The results of the present study fail to confirm the hypothesis that the auditory threshold is constant and equal to the sum of the effective acoustic powers at the two ears. For most of the observers, the difference between the monaural and binaural thresholds was found to be significantly less than would be predicted by this hypothesis.
20(1948); http://dx.doi.org/10.1121/1.1906348View Description Hide Description
Although a sufficiently intense noise in one ear will mask speech heard in the contralateral ear a weaker noise has the opposite effect: it enhances the loudness of speech heard in the other ear. Procedure: one earphone is used to introduce speech at a constant intensity into one ear. Another earphone introduces a white noise into the other ear. The listener first listens to the speech heard monaurally (ear 1) with no noise in the contralateral ear (ear 2). If noise is now introduced into ear 2, the loudness of speech heard in ear 1 increases. Most listeners report a change in the localization of the speech: it is localized nearer to the center of the head when noise is introduced into the opposite ear. This paper concerns these phenomena.
20(1948); http://dx.doi.org/10.1121/1.1906349View Description Hide Description
Some experiments have been conducted to determine the effect of intense high frequency airborne sound on mice and a variety of insects. The sound source was a high frequency siren. The frequency used was about 20 kc and its acoustic level, in the region where the subjects were placed, was between 160 and 165 db (relative to 10−16 watts/cm2). With sufficient exposure—from 10 seconds for flies and mosquitoes to 3 or 4 minutes for roaches and caterpillars—the sound proved lethal in all cases. More detailed work was performed on mice and the roach, Periplaneta americana. In both cases it was definitely established that the heating produced by sound absorption was sufficient to cause death. In addition to the heating there are other effects, notably tissue rupture, as evidenced by the almost complete destruction of the wings on flies and mosquitoes and the rapid deterioration and final disappearance of the external pinna of a mouse which had received a sub‐lethal dose. During observations it has been impossible to completely avoid personal exposure to the sound field and some of the effects observed under these conditions will be described. These include momentary dizziness, and heating of exposed parts of the hand.
- LETTERS TO THE EDITOR
20(1948); http://dx.doi.org/10.1121/1.1906351View Description Hide Description