Modeling of Ordnance-Induced Pyrotechnic Shock Testing
Design of an ordnance-induced pyrotechnic shock test to meet a specific acceleration based Shock Response Spectrum (SRS) test requirement for a given test article has traditionally been an empirical process. Based on experience, the test engineer will determine a potential test configuration and then, through a trial-and-error process, modify the test parameters and configuration until acceptable SRS levels have been achieved. As a complement to this approach, National Technical Systems (NTS) has developed an explicit finite element based modeling approach to simulate an ordnance-induced pyrotechnic shock test. This tool may be used to assist with test configuration design for particularly challenging test requirements or to streamline the process of arriving at acceptable test levels during the calibration phase of a test program.
Improved recursive formula for calculating shock response spectra
NOMENCLATURE x(t) = base input displacement of a single-degree-of-freedom system y(t) = response displacement of a single-degree-of-freedom system) t (x & & = base input acceleration) t (y & & = response acceleration z(t) = relative displacement y(t) – x(t) ζ = fraction of critical damping n ω = natural frequency of a single-degree-of-freedom system, rad/sec s = complex variable H = transfer function L[ ] = Laplace transform L-1 [ ] = Inverse Laplace transform Z = z transform Z-1 = Inverse z transform d ω = damped natural frequency, 2 1 n ζ − ω T = sample interval) t (δ = delta function;) t (δ = 1 for t = 0 ,) t (δ = 0 elsewhere d m = digital delta function; d m = 1 for m = 0 , d m = 0 all other m SDOF = single degree of freedom u(t) = units step function; u(t) = 1 for t > 0 , u(t) = 0 for t < 0 t = time
High-frequency hearing thresholds: effects of age, occupational ultrasound and noise exposure
AbstractPurpose
It has been suggested that high-frequency audiometry (HFA) could represent a useful preventive measure in exposed workers. The aim was to investigate the effects of age, ultrasound and noise on high-frequency hearing thresholds.MethodsWe tested 24 industrial ultrasound-exposed subjects, 113 industrial noise-exposed subjects and 148 non-exposed subjects. Each subject was tested with both conventional-frequency (0.125–8 kHz) and high-frequency (9–18 kHz) audiometry.ResultsThe hearing threshold at high frequency deteriorated as a function of age, especially in subjects more than 30 years old. The ultrasound-exposed subjects had significantly higher hearing thresholds than the non-exposed ones at the high frequencies, being greatest from 10 to 14 kHz. This hearing loss was already significantly evident in subjects with exposure <5 years and increased with years of exposure and advancing age. The noise exposure group had significantly higher hearing thresholds than the non-exposed group at the conventional frequencies 4 and 6 kHz and at the high frequency of 14 kHz. After stratification for age, there was a significant difference between the two groups at 9–10 and 14–15 kHz only for those under 30 years of age.ConclusionMultivariate analysis indicated that age was the primary predictor, and noise and ultrasound exposure the secondary predictors of hearing thresholds in the high-frequency range. The results suggest that HFA could be useful in the early diagnosis of noise-induced hearing loss in younger groups of workers (under 30 years of age).
