Showing papers on "Impulse noise published in 1976"

In-Flight Far-Field Measurement of Helicopter Impulsive Noise

Abstract: : An in-flight technique for measuring UH-1H helicopter impulsive noise (sometimes called 'blade slap') by stationkeeping with a quiet instrumented lead aircraft was found to be highly successful. Far-field quantitative acoustic waveforms and radiation patterns were easily obtained over a wide, continuous range of UH-1H flight conditions, including several areas known to produce annoying acoustic radiation. The data collected using this technique were not (to any significant degree) contaminated by transmission path distortions that have hindered measurement efforts in the past. The two major finding of this initial measurement program were: (1) Judging the occurrence and severity of a helicopter's radiated impulsive noise signature from cabin-based noise measurements can be misleading. For the UH-1H helicopter, reduction in cabin audible impulsive noise levels may constitute a necessary but certainly not sufficient to indicate that far-field impulsive noise radiation has been reduced. The following three distinct types of impulsive noise are radiated by the Un-1H helicopter while flying between 80 and 115 knots at descent rates from zero to 1000 ft/min: (a) A series of positive pressure pulses believed to be related to blade-tip vortex interaction. These pulses are responsible for the crisp popping sound of the radiated noise; (b) A negative pressure disturbance that rapidly increases in amplitude with forward velocity, becoming quite intense and sawtoothed in shape at 115 knots IAS; and (c) A narrow positive pressure spike that closely follows that sawtooth-shaped negative pressure pulse at high airspeeds (115 knots).

On noise in digital recording

Abstract: A set of rules, equations and relations are given for the effects of coherent and statistical noise on digital recorders. Simple formulas, intended for laboratory use, are given for the change in signal to noise ratio (SNR) caused by the channel. Bit error probability is related to SNR and channel parameters for the zero-crossing detector channels typical of digital computer disk and tape recorders, and many digital instrumentation recorders. Valid and invalid ways to calculate error rate statistics are discussed for situations where bit errors are correlated, i.e. not statistically independent.

Impulse Noise Performance Bounds for Binary Correlation Receivers

Abstract: A general performance equation is derived for digital receivers in the presence of impulse noise. Two bounding techniques are employed to simplify the results. First, the Schwarz inequality removes the dependence on both the impulse noise waveforms and the signal-set basis functions. Second, the Chebyshev inequality is applied to the Schwarz results to additionally eliminate the statistical dependence on the probability density function of the noise waveform weighting factor. The bound curves are compared to specific theoretical results where the noise waveforms and probability density functions have been assumed. The tightness of the bound curves is a function of the signal-energy-to-noise-weighting-factor variance ratio and converges for the Schwarz bound as the ratio increases. Considerably looser bounds are obtained by the Chebyshev technique, but rough estimates of system performance can be obtained when the signalenergy-to-signal-energy-to-noise-variance ratio is the only known parameter.

A Review of the Nature and Problem of Impulse Noise Damage to the Soldiers Hearing and its Prevention

Abstract: This paper reviews the historical background to and nature of gunfire induced impulsive noise damage to the hearing. The implications for the soldier and his employment and for the Army Medical Services are discussed. It is concluded that, despite much progress in recent years to safeguard the soldier's hearing, a more intense effort based upon valid epidemiological evidence and within the limits of available resources is necessary.

B‐duration impulse definition: Some interesting results

Abstract: Impulse noise is arbitrarily described as either an A or B duration impulse: (1) an “A” type is a free‐field impulse where there is no reverberation component and the “A” refers to the duration of the maximum overpressure. (2) A “B”‐type impulse is either an inherently reverberant impulse or an A‐type impulse in an acoustically hard environment. The B‐duration is, by definition, the impulse duration to a point on the total wave envelope 10 or 20 dB down from the peak. Two groups of chinchilla were exposed to 40‐μsec A‐duration impulses with a peak PLS of 158 dB. The first group of animals suffered minor TTSmax of about 20 dB at 4 kHz and virtually no PTS or hair cell loss. The second group of animals had large values of TTS (65–70 dB TTSmax at 4 kHz) that resolved to a 25 dB PTS at 4 kHz with concommitant severe cochlear lesions. The increased trauma suffered by the second group is apparently the result of the accidental addition of a reflected wave component that trailed the A‐duration impulse. The reflected wave was 10–20 dB down from the initial peak and, therefore, might be discounted by the existing damage risk criteria or by using the B‐duration measurement scheme. However, the data do suggest a temporal interaction between these two relatively innocuous impulses that is not accounted for within the present framework of the A‐ and B‐duration concepts. Ultimately, these fortuitous results can be important in showing the necessity of specifying the total waveform when describing an impulse noise.

Lärm als Umweltproblem

Abstract: Noise as an Environmental Problem. Anatomical structure and physiological function of the human ear are described. It is shown that constant noise stress leads to damage of certain parts of the inner ear. These damages proceed characteristically and may be diagnosed relatively early by means of audiometric tests. VDI-instructions 2058 (sheet 2) prescribes screening-tests for an earliest possible detection of incipient noise damage. In addition to ear-damaging effects of noise there have also been recorded physiological reactions under noise influence. Such disturbances, however, cannot as yet be described as an “Extraaural disease”. On the other hand, the fact that high sound intensities of 100 dB (A) or more are harmless to human health cannot be proved, as an inverse proportional ratio between the amount of vegetative reaction and the floating away of the threshold of audibility (as indication for potential noise deafness) with hypercritical noise exposure was shown to exist.

Performance of a digital signal processor with impulse noise suppression (meteor radar detection system appl.)

Abstract: Scanning systems which produce records of electromagnetic input signals are usually subject to random impulse interference which may seriously degrade the records. An impulse noise suppressor employing digital techniques has been developed which has general applicability to such systems and which has been employed with success in a meteor radar installation and a thermal infrared scanning system. The results of field tests are presented verifying the usefulness of the suppressor with the meteor radar system. When used in conjunction with a digital signal averager the improvement is equivalent to a tenfold increase in transmitter power, with the added advantage that the signal output is suitably conditioned for further computer processing.