Volume 31, Issue 12, December 1959
Index of content:
31(1959); http://dx.doi.org/10.1121/1.1907664View Description Hide Description
Measurement on experienced listeners of interaural time difference(ITD) thresholds for wide‐band random noise indicates that the threshold varies systematically with duration of stimulation. In order to determine the point at which increase duration no longer decreases ITD threshold, stimulus (noise burst) duration was varied between 0.01 and 1.94 sec. A given ITD was maintained throughout any particular burst, starting time included. All stimuli were presented at a level of 65 db SPL to each phone. The “duration” versus “ITD threshold” function reaches asymptote at approximately 0.7 sec, indicating that the binaural system which effects the comparison necessary for a lateralization judgment may integrate information over that period for the kind of stimulus used.
31(1959); http://dx.doi.org/10.1121/1.1907665View Description Hide Description
In analyzing interaural temporal relations, the binaural system may receive information from one or more of three separate stimulus aspects: (1) difference in time of the start of stimulation, (2) difference in time between similar portions of the continuing wave form at the two ears, and (3) difference in time of the end of stimulation. In this study, the first and third kinds of difference were combined for convenience as “transient disparity”; the second was called “ongoing disparity.” The relative effectiveness of these two temporal relations in producing changes in auditory localization was investigated by finding, for various values of transient disparity, a value of ongoing disparity that brought the sound back to center. For a given value of transient disparity, the necessary ongoing disparity value varies as a function of stimulus duration. Transient disparity loses its effectiveness for stimulus durations greater than about 150 msec. For a duration of 100 msec, it takes roughly 35 times as much transient disparity as ongoing disparity to bring the sound to center; for a duration of 30 msec, it takes about 7 times as much; and for a duration of 10 msec, 4 to 5 times as much. From the working hypothesis that the relative values of transient and ongoing disparities are directly proportional to the durations over which each cue is operative, an “effective onset duration” appears to lie between 2 and 4 msec.
31(1959); http://dx.doi.org/10.1121/1.1907666View Description Hide Description
Experiments are reported on the effect of a tone in one ear on the localization of a tone in the other ear. Localization effects were systematically explored with low frequencies at 30‐db sensation level, the frequency in one ear being a simple n/1 multiple of that in the other. With this procedure, localization effects can be obtained between low‐frequency tones (below 1000 cps) of widely differing frequencies. If multiples are not used, localization effects occur only within narrower frequency limits for low frequencies. Thresholds for difference between the frequencies in the two ears within which the localization effect occurs increase systematically as one goes from low‐ to high‐frequency regions. The localization effects on a given tone cause this tone to be perceived as “pulled in” from the side location that it had when sounded alone.
On the Influence of the Diffraction of Sound Waves around the Human Head on the Characteristics of Hearing Aids31(1959); http://dx.doi.org/10.1121/1.1907667View Description Hide Description
A small hearing aid, hanging in an anechoic room is made to drive an AVC circuit, the output signal of which is conducted to a power amplifier and loudspeaker and can be recorded on a tape. During playback of this tape, with the output of the recorder connected to the power amplifier, the same sound field as existed around the hearing aid is reproduced. If the apparatus is placed on a person in the position where it is to be worn and that person is situated so that the hearing aid is at the same point as during the recording, the output of the hearing aid during playback of the tape will indicate the influence of the diffraction around the human head.
Three specimens of hearing aid were measured on different people. The results showed that there exists a large difference between the hearing aids but no fundamental differences between the persons. The curves plotted for males and females showed the same trend, and no correlation was found with the hairdress. We did not succeed in an attempt to replace the human head by a simple model, such as a wooden sphere or a wooden box, the agreement of the diffraction phenomena between model and head being too poor.
31(1959); http://dx.doi.org/10.1121/1.1907668View Description Hide Description
An experiment was carried out to explore the auditory masking effect of a noise burst on a preceding weak stimulus. A short 1000‐cycle tone preceded a burst of white noise by a variable silent interval. The threshold intensity level of the tone was taken as a measure of the masking provided by the noise burst. The effects were examined of combinations of the following conditions: tone durations of 5, 10, 15, 20, 25, and 50 msec; silent intervals of 0, 2, 5, 10, 25, and 100 msec; and noise burst levels ranging from 50 to 130 db SPL. Appreciable elevations of tone threshold were observed for silent intervals less than 25 msec. Threshold elevations increased progressively as noise burst level was increased. The latter effect increased with shorter silent intervals. The tone‐noise interval was a more critical factor than tone duration.
31(1959); http://dx.doi.org/10.1121/1.1907669View Description Hide Description
The identification of interaural noise correlations was examined as a function of the: duration, sound level, frequency range, and interaural balance of the noise. Progressive changes in identification performance were observed with changes in the individual variables.
31(1959); http://dx.doi.org/10.1121/1.1907670View Description Hide Description
The influence of one pure tone on the threshold of another was investigated. In contrast to previous experiments, masking in the present experiment was studied by determining the level of the masker necessary to mask a signal as a function of the frequency of the masker. The level and frequency of the signal served as parameters. The general forms of the masking functions are similar to those reported previously, but vary in several details. The maximum masking effect occurred when the frequency of the masker approximated that of the signal. When the masker frequency was greater than that of the signal the slope of the masking function was very steep, 150–280 db/octave depending upon signal level and frequency. This slope tended to be steeper at higher signal levels and, although less marked, also at higher signal frequencies. When the masker frequency was about 0.85 that of the signal an irregularity appeared in the masking curves. The size of this hump increased as signal frequency and level increased. Its frequency location seemed to rule out aural harmonics as a cause and its presence was tentatively related to perception of envelope modulation.
The data are discussed from the points of view of band‐pass filter analog and compared to studies of previous investigators. In general the present findings represent an extension of pure tone masking data with wider ranges of stimuli and a different method than those previously used.
31(1959); http://dx.doi.org/10.1121/1.1907671View Description Hide Description
A theoretical and experimental investigation into the causes and mechanism of playback surface and groove noise of recorded media has been made. It is shown that such noise can be considered as arising from the random superposition of voltage pulses produced by local changes in the number and size distribution of welded friction junctions or asperities supporting the playback stylus load. A generalized noise equation, involving the physical parameters of the recorded medium and the playback stylus, has been derived and shows reasonable agreement with the measured playback noise spectra for several cold‐flowing and rigid plastic surfaces. From the noise equation emerges a concept of modulation noise and prediction of its dependence on modulation velocity. Examination of the playback noise level as a function of recording and playback conditions has also been included. It is also shown that surface noise measurements can be used to give information concerning the n/a ratio associated with the theory of friction.
The measured lower limit of wide band playback noise for most plastic surfaces corresponds to an equivalent rms lateral noise velocity of 10−2 cm/sec or equivalent lateral noise amplitude of 1 microinch peak at a linear velocity of 15 cm/sec. The equivalent roughness is thus half of that associated with an optical flat. Using this information, it is shown that at 1000 lines/in. information density a signal/noise ratio of 50 db at a linear velocity of 15 cm/sec should be realized.
31(1959); http://dx.doi.org/10.1121/1.1907672View Description Hide Description
From the application of information theory, it is found that for low signal‐to‐noise ratios, there is no better method of increasing the information content of a signal than to add the outputs of the elements of an array to improve the signal‐to‐noise ratio. When the signal‐to‐noise ratio is already high, however, the array should be designed so that independent information is supplied by each element when associated with a reference element.
The possibility of increasing directionality by nonlinear operations is then discussed. In particular, it is shown that, for the noiseless case, a two element array can be made to yield patterns equivalent to those produced by an n‐element linear array. Linear maximum directivity arrays (in the sense of Pritchard) may also be synthesized with three omnidirectional elements and a number of nonlinear operations which remains invariant as the order of the equivalent linear army is increased.
Finally linear methods designed to minimize the mean squared error are considered and it is found that array rotation is capable of giving optimum results under certain circumstances.
31(1959); http://dx.doi.org/10.1121/1.1907673View Description Hide Description
Unison strings of a concert grand piano were tuned to five “unison” conditions. The conditions were “zero‐beat” tuning and the upper string of three string unison groups tuned sharp and the lower string tuned flat by , 1, 2, and 3 cents relative to the center string. Magnetic tape recordings were made of the piano tuned under these conditions. These recordings in the form of a paired comparison preference test were presented to musically trained and untrained subjects. The most preferred tuning conditions for three string unison groups as recorded and reproduced from magnetic tape, are 1 and 2 cents maximum deviation among strings. Musically trained subjects prefer less deviation in tuning among unison strings than do untrained subjects. Close agreement was found between the subject's tuning preferences and the way artist tuners actually tune piano unison strings.
31(1959); http://dx.doi.org/10.1121/1.1907674View Description Hide Description
The aerodynamic noise resulting from the subsonic flow over a flat rigid plate at zero incidence has three origins. “Surface” noise due to fluctuating surface pressure is postulated to vanish by the author's image argument, except near the edges of the plate, where it is more appropriately called edge noise. Of dipole nature, its acoustic power depends on the velocity raised to between the fourth and fifth power, and consequently is to be expected to be of prime importance at low enough speeds. The contribution from fluctuating shear stresses is likely to be much smaller and so has been neglected. Quadrupole radiation takes place from from the turbulence of the boundary layer, producing layer noise and also from the turbulent wake, producing wake noise. Together, the latter two are suggested to have a spectrum with a single peak, bounded by slopes like f 2 and f −7/4. Their noise power depends on nearly the eighth power of velocity, so is of increasing importance with speed. Analytical details rest on similarity concepts; the spectra in particular are subject to certain conditions. Also, the convection effects on the acoustical power and spectra are excluded on empirical grounds stemming from considerations of jet noise.
31(1959); http://dx.doi.org/10.1121/1.1907675View Description Hide Description
A theoretical discussion is presented on the fundamental processes by which pulsating gas bubbles in liquids dissipate their energy. The survey is limited to the case where the amplitude of the volume pulsations are assumed to be sufficiently small that the pulsations may be described by linear equations. A portion of the energy of the bubble system is lost by the radiation of spherical sound waves, a part is lost by heat conduction due to the polytropic compressions and expansions of the enclosed gas, and a portion is lost by viscous dissipation attributed to viscous forces acting at the gas‐liquid interface. A survey is made of the procedures for measuring the resonant damping constant as described in the methods of successive oscillations, width of the resonance response, standing‐wave ratios, and resonance absorption. Experimental results verify that the damping at resonance is due to thermal and radiation, and possibly viscous damping.
31(1959); http://dx.doi.org/10.1121/1.1907676View Description Hide Description
The ability of a dynamic absorber of optimum design to reduce the transient motion of resiliently mounted equipment is discussed theoretically. The motion of the mounted item is assumed to result from step‐like foundation displacements possessing a wide range of rise times. The absorber mass is attached to the mounted item by a viscously damped spring. It is shown that the transient motion of the mounted item is characterized by the rapid manner in which it decays with time.
In general, if the contents of a resiliently mounted item are to receive the greatest possible protection from damage due to shock, the maximum acceleration and the maximum displacement and relative displacement of the mounted item should be simultaneously small. These are conflicting requirements, but they are fulfilled more closely than is possible with a simple mounting when a dynamic absorber of mass comparable to that of the mounted item is utilized.
31(1959); http://dx.doi.org/10.1121/1.1907677View Description Hide Description
A new calibration standard hydrophone for use in the frequency range 5 cps to 150 kc is described. The active element is a lithium sulfate crystal on a tungsten backing plate. The free‐field voltage sensitivity (end‐of‐cable) is −90±3 db re 1 v per μbar throughout most of the frequency range. The sensitivity is little affected by temperature changes in the range 1° to 25°C, and is not affected by hydrostatic pressure changes in the range 0 to 1000 psig. Directivity patterns are very nearly those to be expected from a 1‐in. piston.
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