Continuous Noise Level Research Articles

Rate representation of tones in noise in the inferior colliculus of decerebrate cats.

Neurons in the central nucleus of the inferior colliculus (ICC) of decerebrate cats show three major response patterns when tones of different frequencies and sound-pressure levels (SPLs) are presented to the contralateral ear. The frequency response maps of type I units are uniquely defined by a narrow excitatory area at best frequency (BF: a unit's most sensitive frequency) and surrounding inhibition at higher and lower frequencies. As a result of this receptive field organization, type I units exhibit strong excitatory responses to BF tones but respond only weakly to broadband noise (BBN). These response characteristics predict that type I units are well suited to encode narrowband signals in the presence of background noise. To test this hypothesis, the dynamic range properties of ICC unit types were measured under quiet conditions and in multiple levels of continuous noise. As observed in previous studies of the auditory nerve and cochlear nucleus, type I units showed upward threshold shifts and discharge rate compression in background noise that partially degraded the dynamic range properties of neural representations at high noise levels. Although the other two unit types in the ICC showed similar trends in threshold shift and noise compression, their ability to encode auditory signals was compromised more severely in increasing noise levels. When binaural masking effects were simulated, only type I units showed an enhanced representation of spatially separated signals and maskers that was consistent with human perceptual performance in independent psychoacoustic observations. These results support the interpretation that type I units play an important role in the auditory processing of narrowband signals in background noise and suggest a physiological basis for spatial factors that govern signal detection under free-field listening conditions.

Noise in the Pediatric Intensive Care Unit

The pediatric intensive care unit is an environment filled with dedicated caregivers, state-of-the-art monitors, and machines that collectively work to provide the best care possible for critically ill children. A previously unappreciated result of having these resources mingled together is the elevated level of environmental noise. Although the levels of noise do not reach OSHA levels (>85 dB for 8 continuous hours) that cause noise-induced hearing loss, the patients are exposed to continuous moderate noise levels that may cause hearing impairment if mixed with other ototoxic agents. In addition, these noise exposures may result in abnormalities in the stress response as well as in sleep patterns. This article reviews the physiology of hearing, the consequences of noise pollution as it relates to pediatric patients, and preventive measures used to reduce noise pollution in a pediatric intensive care setting. Considerations for further research efforts are also discussed.

The presentation level of interrupted pure tone for inducing the pulse phenomenon of continuous white noise

When two sounds which have same spectrum but different amplitude are alternated without silent gap between the sounds, the lower amplitude sound comes to be heard as continuous. This is called "auditory induction" which is one of the most interesting auditory phenomena. The fainter sound is called induce and the louder sound is called inducer. The authors previously reported that the addition of interrupted pure tone to continuous white noise induced the pulsed noise. This pulse phenomenon was thought to be a kind of auditory induction; the inducer is the noise image of white noise with the silent part of interrupted pure tone and the induce is the noise image of white noise with the pure tone part of interrupted pure tone. The authors hypothesized that an auditory filter made with the pure tone of interrupted pure tone generates the two noise images in the continuous white noise. Present experiments were made to investigate the change of "the threshold of interrupted pure tone for inducing the pulse phenomenon" (TIP) when the presentation level of continuous white noise and the frequency of interrupted pure tone are changed. TIP was constant and independent of both the amplitude of continuous white noise and the frequency of interrupted pure tone. All subjects reported that the higher the frequency of interrupted pure tone is, the harder it becomes to hear the pulsed noise image. It is supposed that this phenomenon does not contradict the theory of the auditory filter.

A precise estimation method of probability distribution based on roughly observed data with quantized level and its application to environmental sound noise measurement.

The noise fluctuation is very often measured in a quantized level form at a discrete time interval. By paying attention to this quantization procedure of the actual noise measurement, in this paper, a precise estimation method for the probability distribution form is proposed by using the roughly observed data with quantized levels. That is, we first introduce theoretically a general statistical orthonormal expression of the probability density function for the original noise level fluctuation of continuous level type before passing through the level quantization measurement mechanism. Based on this quantization mechanism and the averaging operation of statistical information, we propose a reasonable estimation method of the distribution parameters in the above statistical expression by using the statistical information on the roughly observed data. The original probability distribution form of the continuous noise level can be precisely estimated by substituting the estimated distribution parameters into the introduced probability expression of general type. On the basis of this estimation theory, a precise evaluation method for the Lx and Leq noise evaluation indices is proposed. The effectiveness of the proposed method is confirmed experimentally too by applying it to first the simulation experiment and then the actual road traffic noise data.

Noise induced hearing loss in military helicopter aircrew--a review of the evidence.

Noise Induced Hearing Loss (NIHL) has been recognised for some time. In the military environment one group of personnel at risk are Army helicopter aircrew who are exposed to continuous noise levels of up to 100 dB(A) in flight. The evidence for the damaging effect of this occupational noise is reviewed and some of the difficulties in drawing conclusions are highlighted. The current protection offered for the Mk 4 helmet is discussed and the incorporation of Active Noise Reduction (ANR) is suggested as a likely way of ensuring that the in-flight noise exposure in Army aircrew is kept as low as possible.

Schumann resonances in magnetic field components

The Institut für Geophysik Göttingen has facilities to record the time varying magnetic components H and D at extremely low frequencies (ELF) (0.1–20 Hz) continuously over several days, with a sampling rate of 100 Hz. The lower ELF-range is characterized by anthropogenic noise and the first two Schumann resonance modes of the Earth-ionosphere cavity. The local observations near Göttingen are disturbed by thunderstorms during the summer that contribute a small part of the energy to the global thunderstorm activity. Transient natural signals with amplitudes of about 10 pT are superimposed on a continuous noise level of about 1 pT; both exhibit the Schumann resonance periodicities. The signals show a tendency to repeat after about 2 s which may suggest excitation by whistler-trains. The Schumann resonances are investigated by an analysis of the autocovariance matrix yielding a robust estimation of the amplitude, damping and frequency. All these parameters show a characteristic directional dependence and variability during the day. The amplitudes represent the excitation with different source locations of thunderstorm activity around the world, generating a dipole field within the resonator, while the damping and centre-frequency are related to solar activity coupling to the lower ionosphere.

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.

Auditory filters in noise‐masked normal and hearing‐impaired listeners

It has been suggested that the poorer‐than‐normal frequency resolution often observed in hearing‐impaired listeners is a direct result of the sensitivity loss and the high stimulus levels required for testing these listeners, rather than an additional processing deficit. The purpose of this study was to determine whether continuous noise masking simulating a hearing impairment in normal‐heating listeners results in auditory filter bandwidths similar to those found in hearing‐impaired listeners. Auditory filter characteristics were determined using a notched noise method, assuming a rounded exponential form of the filter. Measurements were made at center frequencies at 500 and 2000 Hz, under conditions of quiet and for two levels of continuous background noise. At 500 Hz, filter bandwidths remained almost constant with increasing threshold levels, with little filter asymmetry. In contrast, at 2000 Hz, normal‐hearing listeners' bandwidths increased somewhat when thresholds were elevated by the continuous noise, but remained smaller than bandwidths for heating‐impaired listeners with similar thresholds. These data, therefore, do not support the assumption that the impaired frequency resolution of hearing‐impaired listeners is totally attributable to their reduced sensitivity. [Work supported by NIH.]

Seasonal Measurements of Aircraft Noise in a National Park

Jackson Hole Airport is the only air carrier airport located entirely in a National Park, that being Grand Teton. Concerns over noise impacts led to an Airport/Park agreement setting allowable levels, restricting flights and flight paths, and calling for compliance monitoring in the noise sensitive areas. This study documented seven days of continuous noise level measurements made at four sites in the Park during four consecutive seasons in 1985-86. Individual sound exposure level measurements at B-737/200 jets, Swearingen Metro turboprops, and general aviation aircraft permitted computation of “aircraft-only” day/night levels (DNL) at each site for each day. Measurement of half-hourly average sound levels permitted computation of “total” DNL for comparisons. Results showed general compliance with the DNL limits, except when winds necessitated northerly departures, in which case a single B- 737 could cause an exceedance. There were no statistically significant differences in mean aircraft DNL for each sit...

Submarine volcano

An episode of submarine volcanic activity in the northern Mariana Islands, in the northwest Pacific Ocean, has been recorded by the Wake Island hydrophone array. F. Oliveira, C. McCreery and D. Walker of the Hawaii Institute of Geophysics in Honolulu collected the data (see figure) while they were installing upgraded recording equipment for the array.Intense activity began about 02 hours on September 21 UT and ended abruptly about 23 hours that day. During that time several hundred distinct events were observed. Continuous noise levels in the SOFAR channel were at least 20 dB above normal ambient noise levels at frequencies from 5 to 30 Hz. The location of the activity was about 21.9°N, 145.9°E. N. Hamada reported that it was recorded by ocean bottom seismometers of the Japan Meteorological Agency (JMA) sited off the east coast of Japan. The Polynesian Seismic Network also recorded the episode.

Evaluation of impulse noise

The assessment of the risk of hearing loss due to impulse-weighted sound levels in working places leads to wrong results at often intermittent noise exposures, since the energy-equivalent continuous noise level does not consider the recovering of hearing due to noise pauses. In these cases the LAeq without impulse addition can lead to a correct assessment, as Pfeiffer and Maue demonstrated for the construction professions, but it cannot be excluded that LAeq on the one side can lead to underestimation of the risk of hearing loss at continuous exposure to impulse noise, on the other side even to overestimation in cases of height portion of pauses.

Effect of noise masking on the brain-stem and middle-latency auditory evoked potentials: central and peripheral components

Brain-stem (BAEP) and middle-latency (MLAEP) auditory evoked potentials were recorded in zero noise and in 3 levels of continuous ipsilateral broadband noise. New information is presented on the efefcts of noise on BAEP wave I. Latency of wave I was not changed by increasing noise, but wave V latency linearly increased. Amplitude of waves I and V decreased non-linearly. The amplitude decrease was equivalent for both waves and occured only at the higher noise levels. The dissociation of latency effcets fior waves I and V indicates a central component for the effect of noise on latency. The parallel amplitude change for waves I and V suggests a largely peripheral component.. The MLAEP Pa latency also increased with increasing noise further supporting a central mechanism.

Noise exposure of motorcyclists.

This study was designed to evaluate noise exposure of motorcyclists at work. Open and full face safety helmets were compared and the effects of helmet design and the contribution of radio intercoms investigated. Special measuring equipment is described and hearing protection discussed. Previous studies concentrated mainly on the attenuation of noise by safety helmets, and were made under conditions untypical of day-to-day motorcycling. During town driving the sample equivalent continuous noise level (s.leq) ranged from 63 to 90 dB(A), the intercom giving the highest peak levels. On the open road s.leq was up to 105 dB(A); exposure to this level for only 15 min would exceed the present recommended maximum of 90 dB(A) for an 8 h working day.

Noise Levels: A database of industrial noise level measurements

The Noise Levels database has been developed at the Canadian Centre for Occupational Health and Safety for the computerized information system, CCINFO. The database documents measured noise levels from a broad spectrum of industrial settings. Industrial hygienists, noise control engineers, researchers, health and safety committee members, and appropriate government agency personnel are the main intended audience. Noise Levels is a textual‐numeric ‘source’ database of direct measurements, although bibliographic citations are also provided. The data for Noise Levels is gathered from both published and unpublished sources. Each record provides explicit information on the noise source (for example, piece of equipment), the industry, operation associated with the noise production, and the occupational categories. Additional information is provided on the type of noise, exposure duration per day, the presence of engineering controls, and the use of ear protection. Measurement data consists of one or more of the following: Sound Pressure Level (SPL) in dB(A), Time Weighted Average (TWA), Equivalent Continuous Noise Level (ECNL), and the octave band analysis. The primary goals of the database are to encourage sharing of industrial noise data and to promote standardized reporting of noise measurements.

The effect of broadband noise on the human brain-stem auditory evoked response. III. Anatomic locus.

The effects of broadband noise on the brain-stem auditory evoked response (BAER) are reported for two experiments. Experiment 1 used a high-pass subtractive-masking technique and covaried derived bandwidth and continuous broadband noise level. Comparison of responses to half-octave wide derived bands in the presence of within-band noise showed that wave V latency changes were greater than could be explained on the basis of shifts in the cochlear region responsible for generating the response. The magnitude of within-band noise-induced wave V latency shift was independent of the frequency separation of the masker cutoffs. In experiment 2 the effects of noise level and rate on waves I, III, and V of the BAER were evaluated. Peak latencies increased and peak amplitudes decreased with increasing noise level and rate. Higher noise levels and rates produced an increased central (I-V) conduction time in which the wave III-V increase was greater than the wave I-III increase. Together, these results are most consistent with the hypothesis that a nonplace, central auditory mechanism produces most of the noise-induced latency shifts in normal-hearing adults.

Noise in acute patient care areas.

This study was designed to describe the level of sound in acute patient care areas. A convenience sample of 25 subjects from four intensive care and two general care units within three hospitals in a large metropolitan area was studied. Continuous decibel levels [dB(A)] and equivalent continuous sound pressure levels [LEQ] were measured for 24 continuous hours. Two 3-hour observations were made to assess the sources of sound, and patients were interviewed to determine their perceptions of sound effects. Continuous high noise levels were found in the larger hospital's open heart recovery room and intensive care units [48.5-68.5 dB(A), 15-minute LEQ], lower levels in the smaller hospital's intensive care units [32.5-57 dB(A)] and varying levels in the general ward areas [34.25-62.5 dB(A)]. Decibel levels generated from equipment reached 90 dB(A) in some instances. Patients' perceptions ranged from content to highly perturbed. Although some sources of noise were not immediately changeable, other were adaptable, preventable, or reducible.

Noise levels and hearing thresholds in the drop forging industry.

A-weighted equivalent continuous noise levels for hammer and press operations in a drop-forging industry were determined using both tape recordings of the noise and personal noise dosimeters. The results indicated average A-weighted Leq values of 108 dB for hammer operators and 99 dB for press operators. Comparison of hearing level statistics for 716 hammer and press operators and 293 control subjects indicated the severe hazard to hearing of impact noise exposures. For mean exposure times of less than 10 years, hearing levels for the press (99 dB) and hammer (108 dB) operator age groups are nearly identical, and in the latter case are less than those predicted for exposure to equivalent continuous noise. For long-term exposures of 10 years or more, the results of this study indicate that hearing losses resulting from impact noise in the drop-forging industry are as great or greater than those resulting from continuous noise.

Masked pure‐tone thresholds of the bottlenosed dolphin (Tursiops truncatus) at extended frequencies

Masked underwater pure‐tone thresholds were obtained for a bottlenosed dolphin (Tursiops truncatus) using an “up‐down staircase” method of stimulus presentation and a “go/no‐go” response procedure. Thresholds were determined at two continuous broadband masking noise levels at test frequencies of 30, 60, 90, 100, 110, 120, and 140 kHz. The Critical Ratio (CR), defined as the ratio of signal power to noise spectrum level at masked threshold, was calculated for both noise levels. A psychometric function relating CR's to frequency showed parallel functions with CR's previously reported for the dolphin [Johnson, J. Acoust. Soc. Am. 44, 965–967 (1968)] for frequencies up to 100 kHz. Results for 30, 60, 90, and 100 kHz were 31, 32, 45, and 40 dB, respectively, supporting previous findings for the dolphin. Results beyond 100 kHz, where no other values had previously been obtained, show a sharp increase at 110 kHz (51 dB) followed by a slight decline at 120 kHz (46 dB). Generally, the CR function suggests that the frequency resolving ability of this echolocator is not optimal at these higher frequencies even though peak energies of the outgoing echolocation pulse have been recorded in this higher range.

Comparison between the effects of continuous and impact noise on cochlear potentials in guinea pigs.

Cochlear microphonics (CM), action potentials (AP), and endocochlear potential (EP) were recorded from anesthetized, immobilized guinea pigs during potential (EP) were recorded from anesthetized, immobilized guinea pigs during and following 20 min periods of noise exposure. Changes in cochlear potentials were compared in guinea pigs exposed to continuous broadband noise or mechanically generated impact noise of equal energy content. Over a range of continuous noise levels from 95 to 105 dB SPL it was found that continuous noise produced less suppression of CM and a greater suppression of AP than did impact noise of equal energy. A reduction of EP did not accompany CM suppression with either type of noise exposure. Suppression of CM and AP was also compared in guinea pigs with chronically implanted round window electrodes. In these preparations, AP was suppressed to a similar extent by impact and continuous noise of equal energy, but CM was suppressed to a significantly greater extent by impact noise. The data from both series of experiments indicate that the suppression of cochlear responses is not predicted by an "equal energy" rule when impact and continuous noise are compared.

A multi-parametric study of impact noise-induced TTS.

A series of three experiments with 15 normal-hearing listeners measured temporary threshold shifts (TTS) as a function of various parameters of impact noise exposure. A criterion of 15 dB TTS at 4, 6, and 8 kHz, measured 3 min after exposure was adopted. Results indicated that; (1) the growth of TTS is a power function of the peak level of impact noises, (2) recovery from TTS appears to proceed linearly with the logarithm of the time in quiet, (3) the amounts of TTS3 are exponentially related to the logarithm of the decay time (or e duration: te) of the impact sound pressure envelope, (4) with constant total amounts of energy and a constant repetition rate, increasing the number from 60 to 1000 increases the final amount of TTS; the derived trading relationship involves a 12.7-dB level change for a tenfold change in the number of impacts, (5) repetition rates ranging between 0.5 and 1 pps (pulse per second) are the most harmful to hearing and, (6) daily exposure to 100 impacts (te: 200 ms; repetition rate: 0.5 pps) should be limited to a 130-dB peak level, in order to protect 90% of the exposed people. Results are discussed in terms of the possible use of the equivalent continuous noise level as an index in setting safe limits of occupational impact noise exposure.