Hazard from an intense midrange impulse

It has been hypothesized that the ear would become increasingly susceptible to impulses (gunfire) as the spectral peak of the impulse approached the frequency region where the ear was tuned best (about 4 kHz for the cat ear) [G. R. Price, J. Acoust. Soc. Am. Suppl. 1 62, S95 (1977)]. This prediction was counter to the predictions of the world's damage-risk criteria for impulse noise. It has been supported by experiments using exposures to 100-Hz and 800- to 1000-Hz impulses; but no test had been run at the point of predicted maximum susceptibility. In the present experiment, three groups of cats were exposed to 50 impulses produced by a primer explosion (spectral peak at 4 kHz) at peak levels of 135, 140, or 145 dB. Auditory thresholds were electrophysiologically measured from the vertex to 2-, 4-, 8-, and 16-kHz tone pips and losses were determined 30 min after exposure and more than 2 months post-exposure. Losses were greatest at 4 kHz, began to develop at 134-dB peak pressure, and the immediate losses grew at a rate of about 7 dB for every dB increase in peak pressure. About half of the loss measured immediately became permanent. The energy required to begin producing a permanent threshold shift was only about 0.07 J/m2, far lower than that required with continuous noises at lower sound pressures. The data were interpreted as supporting the original hypothesis of greater susceptibility in the midrange.     

Hazard from intense low-frequency acoustic impulses

It was predicted that because the ear is spectrally tuned, it should be most affected by intense impulses with spectral peaks near the frequency where it is tuned best (3.0 kHz for the human ear) and progressively less affected by impulses at lower frequencies [G.R. Price, Scand. Audiol. Suppl. 16, 111-121 (1982)]. This prediction is counter to all the DRCs for impulse noise; therefore an adequate test is essential. In order to augment the data on hearing loss to low-spectral-frequency impulses, three groups of cats (eight, nine, and ten animals) were exposed on one occasion to 50 impulses from a 105-mm howitzer at peak SPLs of 153, 159, and 166 dB. Threshold shifts were measured electrophysiologically on the day of exposure (CTS) and following a 2-month recovery period (PTS). Maximum PTSs appeared at 4 kHz (even though the spectral peak of the impulse had been at about 100 Hz), and CTSs recovered into PTSs about half as large. Furthermore, for group data, even small CTSs tended to have a permanent component. These data raise the question as to whether or not any threshold shift persisting an hour or two after exposure to high levels should be considered tolerable. When compared with data from rifle fire exposures, the data confirmed the earlier prediction that as the spectral frequency drops, hazard declines at the rate of a little more than 3 dB/oct, contrary to the rating by existing DRCs.     

Conditioning-induced protection from impulse noise in female and male chinchillas

Sound conditioning (pre-exposure to a moderate-level acoustic stimulus) can induce resistance to hearing loss from a subsequent traumatic exposure. Most sound conditioning experiments have utilized long-duration tones and noise at levels below 110 dB SPL as traumatic stimuli. It is important to know if sound conditioning can also provide protection from brief, high-level stimuli such as impulses produced by gunfire, and whether there are differences between females and males in the response of the ear to noise. In the present study, chinchillas were exposed to 95 dB SPL octave band noise centered at 0.5 kHz for 6 h/day for 5 days. After 5 days of recovery, they were exposed to simulated M16 rifle fire at a level of 150 dB peak SPL. Animals that were sound conditioned showed less hearing loss and smaller hair cell lesions than controls. Females showed significantly less hearing loss than males at low frequencies, but more hearing loss at 16 kHz. Cochleograms showed slightly less hair cell loss in females than in males. The results show that significant protection from impulse noise can be achieved with a 5-day conditioning regimen, and that there are consistent differences between female and male chinchillas in the response of the cochlea to impulse noise.     

Noise dosimeter for monitoring exposure to impulse noise

Commercially available noise dosimeters do not perform properly in impulsive noise environments because they suffer from instrumentation limitations and lack metrics that characterize impulse noise. In this paper, a design concept is proposed for an impulse noise monitoring dosimeter that addresses the current dosimeter’s limited capabilities and describes the various parameters that can appropriately be used to measure and evaluate exposure to impulse noise. The design concept is based on the accurate acquisition and storage of the original impulse waveform. For data analysis (using MATLAB) and calculation of “impulse noise metrics,” National Institute for Occupational Safety and Health (NIOSH) used a prototype impulse noise dosimeter system that consisted of a Bruel&Kjaer 4136 microphone and a Panasonic Digital Audio Tape Recorder. The proposed instrument would enable collection of data for validation of presently defined and yet to be defined metrics quantifying noise-induced permanent threshold shifts (NIPTS) resulting from impulse/impact exposures. It will also enable occupational safety and health professionals to make accurate measurements of ultimately approved metrics.     

Hazard from weapons impulses: histological and electrophysiological evidence

Current methods of rating the hazard of weapons impulses for the ear have recently been challenged by electrophysiological data from experiments with animal ears which indicate that the hazard from low-frequency impulses is much lower than the hazard from higher frequency impulses (Dancer et al., 1981; Price, 1986b). To supplement these data, histological data are reported here for 51 cats that were exposed on one occasion to either rifle or howitzer impulses at peak pressures from 145 to 155 dB or 153 to 166 dB, respectively. Histological procedures (scanning electron and light microscopy) were carried out over 2 months after the exposure and after electrophysiological measures had been made. For both types of impulse the losses tended to be in the middle of the cochlea in focused lesions, even though the spectral peaks of the acoustic stimuli had been at about 80 Hz (howitzer) and 1000 Hz (rifle). Outer hair cells were more susceptible than the inner hair cells and interindividual differences in effects were large. Furthermore, the two impulse sources were equally hazardous when the peak pressure of the rifle impulse was lower than the peak pressure of the howitzer impulse by about 9 dB. In terms of A-weighted energy, the exposures were equally hazardous when the rifle exposure contained about 35 times less energy than the howitzer exposure. The histological data are thus consistent with the electrophysiological data, which indicate that present standards for impulse noise exposure may overrate the hazard of low-frequency impulses relative to impulses in the midrange.     

The importance of "temporal pattern" in traumatic impulse noise exposures

The equal energy hypothesis (EEH) was evaluated for impulse noise. Specifically, the experiments evaluated the importance of the temporal distribution of impulses; the trading relation between the number of impulses and peak level and the difference between continuous and impulse noise. Monaural chinchillas were exposed to one of seven conditions. Their hearing was evaluated before, immediately after, and 30 days after the exposure. Hair cell damage was reported in the form of a cochleogram. The experiments show that the EEH is more appropriate for low-level impulse (135-dB peak); for equal amounts of energy, 150-dB impulses produce more hearing loss and hair cell damage than 135-dB impulses; for equal amounts of energy, impulses presented in rapid bursts cause less damage than impulses presented at "1/s" and 50 microseconds. Pairs of impulses presented at "1/s" produce the largest amount of damage. The results are discussed in terms of implications for the EEH.     

Human temporary threshold shift (TTS) and damage risk

Information regarding the relation of human temporary threshold shift (TTS) to properties of steady-state and intermittent noise published since the 1966 appearance of the CHABA damage risk contours is reviewed. The review focuses on results from four investigative areas relevant to potential revision of the CHABA contours including effects of long-duration exposure and asymptotic threshold shifts (ATS); equivalent quiet and/or safe noise levels; effects of intermittency; and use of noise-induced temporary threshold shift (NITTS) to predict susceptibility to noise-induced permanent threshold shift (NIPTS). These data indicate that two of three major postulates on which the original contours were based are not valid. First, recovery from TTS is not independent of the conditions that produced the TTS as was assumed. Second, the assumption that all exposures that produce equal TTS2 are equally hazardous is not substantiated. The third postulate was that NIPTS produced by 10 years of daily exposure is approximately equal to the TTS2 produced by the same noise after an 8-h exposure. Based upon several TTS experiments showing that TTS reaches an asymptote after about 8 h of exposure, the third CHABA postulate can be reworded to state the hypothesis that ATS produced by sound of fixed level and spectrum represents an upper bound on PTS produced by that sound regardless of the exposure duration or the number of times exposed. This hypothesis has a strong, logical foundation if ATS represents a true asymptote for TTS, not a temporary plateau, and if threshold shifts do not increase after the noise exposure ceases.     

基于磁开关的重复频率冲击电压发生器

用磁开关取代传统冲击电压发生器的放电球隙,利用磁开关可高重复频率工作和磁压缩陡化的特点和冲击电压发生器“并联充电,串联放电”的特点,可形成重复频率快前沿高压脉冲。建造了基于磁开关的冲击电压发生器,试验表明：其重复频率可达1 kHz,在1 nF的电容负载上可形成17 kV,上升时间小于80 ns的高压脉冲。     

Temporary threshold shift caused by combined steady-state and impulse noises

Temporary threshold shift (TTS) measurements on 11 subjects, resulting from exposures to steady-state noise, impulse noise, and combinations of both types of noise are reported. Twenty minute exposures to wide-band steady-state noise at levels of 78, 84, 90 and 96 dBA, and impulse noise at levels of 96, 102, 108, 114, 120, 126 and 132 dB(peak), and repetition rate of 3·2 pulses/s, were used. When a hazardous level of steady-state noise was combined with various levels of impulse noise, there was a significant reduction in the measured TTS at 4 and 6 kHz. This reduction was greatest when the peak level of the pulse exceeded the r.m.s. level of the steady-state noise by 6–18 dB. When a hazardous level of the pulse was combined with several levels of steady-state noise, no significant reduction in TTS was observed. These findings are interpreted as a result of acoustic reflex stimulation; the pulses superimposed on a hazardous steady-state noise continually re-activated the reflex and prevented fatigue. The converse did not apply, that is, the non-hazardous (but high-level) steady-state noise did not appear to counteract fatigue resulting from hazardous impulse noise.     

Fundamental frequency noise properties of extended cavity erbium fiber lasers

A multimode linear cavity and a single-mode unidirectional ring cavity fiber laser with meter-long cavity lengths are shown to exhibit frequency noise limited by fundamental thermodynamic noise from 100?Hz to 100?kHz. Their measured spectra agree closely with theoretically derived thermodynamic noise and the characteristic dependence of the frequency noise power spectrum on the inverse of the cavity length is observed. The unidirectional ring laser exhibits a frequency noise of 2?Hz/Hz(1/2) at 1?kHz, one of the lowest published values to date from a free-running laser. The multimode linear cavity laser is shown to be a suitable candidate for thermal-noise-limited, meter-long fiber laser strain sensors with a strain resolution of 14?f?/Hz(1/2) at 1?kHz.     

Presbycusis and noise-induced permanent threshold shift.

Bies and Hansen [J. Acoust. Soc. Am. 88, 2743-2754 (1990)] have proposed an alternative formulation of the relationship between noise exposure and noise-induced hearing impairment to that presented in International Standard ISO 1999, in which they assume that presbycusis and noise-induced permanent threshold shift (NIPTS) are additive on an antilogarithm basis. Data concerning deterioration in hearing threshold levels at 4000 Hz due to aging in war veterans with NIPTS do not support the Bies and Hansen assumption but provide support for the formula for combining presbycusis and NIPTS incorporated in International Standard ISO 1999.     

Attenuation of high-level impulses by earmuffs

Attenuation of high-level acoustic impulses (noise reduction) by various types of earmuffs was measured using a laboratory source of type A impulses and an artificial test fixture compatible with the ISO 4869-3 standard. The measurements were made for impulses of peak sound-pressure levels (SPLs) from 150 to 170 dB. The rise time and A duration of the impulses depended on their SPL and were within a range of 12-400 mus (rise time) and 0.4-1.1 ms (A duration). The results showed that earmuff peak level attenuation increases by about 10 dB when the impulse's rise time and the A duration are reduced. The results also demonstrated that the signals under the earmuff cup have a longer rise and A duration than the original impulses recorded outside the earmuff. Results of the measurements were used to check the validity of various hearing damage risk criteria that specify the maximum permissible exposure to impulse noise. The present data lead to the conclusion that procedures in which hearing damage risk is assessed only from signal attenuation, without taking into consideration changes in the signal waveform under the earmuff, tend to underestimate the risk of hearing damage.     

Physiological studies of central masking in man. I: The effects of noise on the 40-Hz steady-state response.

In a typical masking situation, two Békésy waves overlap on the basilar membrane, and each of them initiates a stream of nerve impulses that enters the brain via the auditory nerve. Much is known about the overlapping of the cochlear waves, but much less about where, how, and even if at all, the impulse streams interact once they get inside the brain. In these experiments the incoming impulses are measured electrophysiologically using the auditory brainstem response (ABR), and, simultaneously, using the 40-Hz auditory steady-state response (SSR) to monitor events at a probable site of their interaction, the auditory cortex. The principal finding is that, when progressively increasing levels of continuous noise are presented to the contralateral ear, the SSR to the signal drops to about half its control amplitude. Second, low levels of ipsilateral noise reliably enhance SSR amplitude. Third, moderate levels of ipsilateral noise reduce SSR latency. In none of these cases does the ABR show similar effects. These findings are interpreted to mean that, in each case, impulses excited by the signal interact with impulses excited by the noise, and regardless of ear of origin the interactions take place beyond the brainstem level where ABR wave V is generated, either before the impulses reach the cortex, or in the cortex itself.     

红外探测器的高频测试

研究了长波8~15μm波段,阻值大于440ΩMCT光导红外探测器,探测率在10kHz,14μm大于4×1010 cm·Hz1/2/W,在1kHz和10kHz中心频率下的噪声测试,中波5~8μm红外光伏型InSb器件,探测率在25kHz,8.26μm大于1×1011 cm·Hz1/2/W,在1kHz和255kHz中心频率下的噪声测试,并对器件信号进行了测试。信号和噪声测试是在124A锁相放大器测试系统测试,对124A测试系统的不确定度进行了分析,并与动态信号分析仪35670A对器件在0~50kHz频谱范围的噪声进行了测试和比较。实验结果表明,高阻值的光导器件在1kHz和10kHz中心频率下噪声相差约1.4倍,光伏型InSb器件在1kHz和15kHz中心频率下噪声相差约1.5倍,信号测试结果在1kHz下和3kHz中心频率下变化不超过3%。通过测试和比较,对高频下的测试给出了建议。     

Curie温度附近稀磁半导体(Ga,Mn)As的电学噪声谱性质

建立了自发噪声谱测量系统来研究稀磁半导体（Ga,Mn）As的电学噪声性质.通过测量（Ga,Mn）As材料的自发噪声谱,发现（Ga,Mn）As的自发涨落会随温度升高而逐渐增大,同时,外加磁场会降低（Ga,Mn）As的自发涨落,这来源于外加磁场导致的（Ga,Mn）As磁畴部分有序化.此外,不同频率的噪声随温度的变化规律有很大差异：当频率低于30 kHz的时候,噪声谱和温度的变化关系和热噪声很相似,但数值上明显大于热噪声的值;当频率在30 kHz左右的时候,噪声大小和温度成线性关系;当频率大于30 kHz以后,在相变点附近噪声大小和温度的关系出现了明显的转折,高频高温噪声的大小和热噪声的理论值非常接近.这些结果有助于深入理解（Ga,Mn）As磁性起源的物理机制. 关键词： 自旋电子学 稀磁半导体 自发涨落谱     

Temporary threshold shifts in humans exposed to octave bands of noise for 16 to 24 hours.

Groups of human subjects were exposed in a diffuse sound field for 16--24 h to an octave-band noise centered at 4, 2, 1, or 0.5 kHz. Sound-pressure levels were varied on different exposure occasions. At specified times during an exposure, the subject was removed from the noise, auditory sensitivity was measured, and the subject was returned to the noise. Temporary threshold shifts (TTS) increased for about 8 h and then reached a plateau or asymptote. The relation between TTS and exposure duration can be described by a simple exponential function with a time constant of 2.1 h. In the frequency region of greatest loss, threshold shifts at asymptote increased about 1.7 dB for every 1 dB increase in the level of the noise above a critical level. Critical levels were empirically estimated to be 74.0 dB SPL at 4 kHz. 78 dB at 2 kHz, and 82 dB at 1 and 0.5 kHz. Except for the noise centered at 4.0 kHz, threshold shifts were maximal about 1/2 octave above the center frequency of the noise. A smaller second maximum was observed also at 7.0 kHz for the noise centered at 2.0 kHz, at 6.0 kHz for the noise centered at 1.0 kHz, and at 5.5 kHz for the noise centered at 0.5 kHz. After termination of the exposure, recovery to within 5 dB of pre-exposure thresholds was achieved within 24 h or less. Recovery can be described by a simple exponential function with a time constant of 7.1 h. The frequency contour defined by critical levels matches almost exactly the frequency contour defined by the E-weighting network.     

Reaction of communities to impulse noise

In order to assess the reaction of communities to impulse noise, a sociological survey was conducted in three communities in Ontario, Canada. The dominant industrial noise in these locations is due to drop forging operations. Nearly 600 completed interviews were recorded. Detailed sound level measurements were carried out in the areas surveyed. The results clearly indicate the extent of adverse reaction to the forging noise. This research has been compared with the reaction of the respondents to traffic noise prevailing in their communities. Regression lines are presented showing the relationship between the percent of people disturbed by the forging noise and the sound level of the impulses.     

Impulse duration: A new instrument for its measurement

An instrument is described which, when used with a ‘peak hold’ reading sound level meter, will measure the durations of acoustic impulses in accordance with the Atherley and Martin and CHABA criteria for hearing damage risk to impulsive noise. The instrument is small, lightweight, can be battery powered and is designed for field use.Comparison tests show that the impulse duration meter gives more accurate and repeatable results than the oscilloscope trace photograph method or the digital waveform recorder method of impulse duration assessment.By using the instrument with a suitable impulse source reverberation time, measurements may be conducted.     

Analysis of high-frequency wind-driven ambient noise in shallow brackish water

Ambient noise spectra in a shallow brackish water environment were found to be steeper than expected at frequencies above 10 kHz. The high-frequency behavior of the spectra was resolved by modeling dispersion and noise in bubbly water. Bubble size distributions fitted to the brackish water spectra exhibit a distinctive maximum in the radius range 0.1-0.3 mm, and a substantial drop in bubble density below a radius of 0.1 mm. The brackish water distributions were tied to an oceanic spectrum with a spectral slope of 5.7 dB/octave obtained with a -3 / 2 power law dependence of bubble size density on radius.     

Underwater ambient noise in shallow-water areas of the Indian Ocean within the tropical zone

Frequency spectra of underwater ambient noise were measured in the lagoon of a coral atoll, outside the reef, and on shallow-water banks in the tropical zone of the Indian Ocean. The measurements were performed in the frequency range from 0.003 to 9 kHz with the use of self-contained acoustic buoys and a small boat. In all of the regions studied, continuous underwater noise of biological origin was observed with spectral maxima at frequencies of 1–1.5 and 7–8 kHz. In the lagoon of an atoll, this noise was much more intense than regular dynamic noise in the ocean. The maximum noise levels were observed in the first half of night.