Studies of the threshold of the acoustic stapedius reflex to determine normative values and measures of stability revealed a small (approximately 2 dB), but statistically significant threshold improvement with repeated stimulation for a population of 30 normal ears. To investigate further and define the finding that acoustic stimulation may result in an increase in sensitivity of the stapedius reflex, two studies were designed. In the first, 500-msec bursts of white noise were presented in an ascending series of 2-dB increments. Contractions of the stapedius muscle were monitored by the impedance method, using a Zwislocki acoustic bridge and a specially designed electroacoustic readout system. Each of the nine normal-hearing subjects was exposed to three consecutive runs of the ascending white-noise stimuli from about 50-dB sensation level to about 10 dB above the reflex threshold; the mean threshold for the first run was 60.44 dB, while the mean for the third run was 56.67 dB. This difference was found to be significant at the 0.001 level. Phase 2 involved 10 normal ears which were exposed to 120 dB SPL of white noise for a period of 1 min. Measurements of the reflex threshold were obtained before and after the noise exposure, again by means of an impedance-monitoring technique. Threshold decreases averaging approximately 5 dB were recorded for this group.
are derived in normalized form in order to effect optimum simplification and generalization of equations. Thus, the four parameters, mass, elastance, damping and frequency are replaced by two parameters, normalized damping and normalized frequency. The generalization effected by normalization makes equations (1) and (2) more readily applicable to all subjects.The amplitude response curves shown in figure 1, plotted from equations (1) and (2), graphically demonstrate the variation of body motion with frequency. This variation of body response with frequency is the basic cause for distortion in the recorded ballistocardiogram.One method of correcting this distortion consists of analysing the recorded ballistocardiogram and resynthesizing a new tracing by the Fourrier method.2 The corrected tracing is free of body resonance distortion. However, this method does not yield a continuous corrected record and is, therefore, unsuitable for clinical ballistocardiography.A second method of correcting the distortion introduced by the body's resonant response consists of recording the ballistocardiogram by means of specially designed beds. Talbot and his coworkers8 approximate a tracing free of distortion due to the body, by recording the ballistocardiogram from a board floating in a mercury filled tank. vron Wittern3 records the ballistocardiogram from a pendular bed adjusted so as to mechanically filter the distortions introduced by the body and the bed. Both technics can approximate undistorted ballistocardiograms. However their application
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