Hearing threshold level

Audiogram The report produced by an audiometry test, showing measured hearing threshold levels at frequencies of 500, 1000, 2000, 3000, 4000, and 6000 Hz. 

Temporary threshold shift (TTS) A temporary shift in hearing threshold level that goes away after the person has been in a quiet environment for a few hours. It is confirmed by an audiogram retest following a suspected standard threshold shift (STS) after at least 14 hours away from high levels of noise. 

A change of hearing threshold level of 15 dB or greater, in either ear, at any frequency (1000 to 4000 Hz) between the reference audiogram and any subsequent audiogram. SIL Selection... 

Audiogram A chart, graph, or table resulting from an audiometric test showing an individual s hearing threshold levels as a function of frequency. 

Ehidarewicz, A., Toppila, E., Pawlaczyk-Luszczyhska, M. Sliwihska-Kowalska, M. 2010. The influence of selected risk factors on the hearing threshold level of noise exposed employees. Archives of Acoustics, 35, 371-382. 

Suggestive evidence of a lead-related decrease in hearing acuity in children has been reported by Robinson et al. (1985) and Schwartz and Otto (1987, 1991). Hearing thresholds at 2,000 Hertz increased linearly with maximum blood lead levels, indicating that lead adversely affects auditory function. The PbB levels in 75 asymptomatic black children, 3-7 years old, ranged from 6 to 59 tg/dL (mean,... 

Hearing thresholds in children may be affected adversely by lead exposure at low blood lead levels (Robinson et al. 1985 Schwartz and Otto 1987, 1991). Robinson et al. (1985) reported that hearing thresholds increased linearly with maximum historical PbB levels of 6.2-56.0 pg/dL. In the analyses by Schwartz and Otto (1987, 1991), the probability of lead levels studied (NHANES II and HHANES data, respectively), from <4 to >50 pg/dL, with no apparent threshold. There is also some evidence suggesting that lead exposure may cause postural disequilibrium in children (Bhattacharya et al. 1993). The children evaluated in that study had a geometric mean PbB for the first 5 years of life of 11.9 pg/dL, the range was 5.1 to 28.2 pg/dL. 

An absolute sound scale (in dBA for absolute decibels) is defined by establishing an intensity reference. For convenience, the hearing threshold is set at 0 dBA. Table 3-7 contains dBA levels for a variety of common activities. 

As discussed in Section 13.3 concurrent exposure to noise and some organic solvents and solvent mixtures is ototoxic. Styrene is an example of an ototoxic compound, with exposures to it causing permanent hearing threshold shifts and outer hair cell damage. Ethanol alone does not affect auditory sensitivity, yet, when combined with styrene it induces hearing and outer hair cell losses in test animals in levels greater than those caused by styrene alone. The potentiation of the ototoxicity of styrene by ethanol is ascribed to the altering of styrene metabolism by ethanol. 15 ... 

FIGURE 16.12 An example of the masking effect. Based on the hearing threshold of the human ear (dashed line), a 500-Hz sinusoidal acoustic waveform, shown at A on the left graph, is easily audible at relatively low levels. However, it can be masked by adding nearby higher-amplitude components, as shown on the right. Source CCL)... 

The reference sound pressure used here is the sound pressure of a sinus sound with a frequency of 1 kHz the human ear is barely able to perceive it (hearing threshold). The sound energy penetrating through the area unit per unit of time is described as sound intensity, sound density, or sound level J. The value of sound intensity at the hearing threshold is approximately /(,= 10 W/m. This value permits a definition of the sound intensity level with the help of Equation (6-2) ... 

The hearing threshold minimum audibility field = MAF) is decidedly frequency dependent, as demonstrated in Figure 6.25 in Section 6.1.1. Man usually perceives two sounds of differing frequency but with equal sound pressure level, as unequally loud. This is why, for subjective... 

An essential environmental factor which may affect safety is sound. Vibrations of the air of frequencies from between 16 to approximately 20,000 Hz are perceived by the human ear in the form of sound. The hearing threshold lies approximately at 20 iPa (0.2 x 10 bar) of sound pressure, corresponding to a sound intensity level of 0 dB. At a sound pressure (intensity) level of approximately 130 dB, man s pain threshold will be reached. Some actual sound levels, measured at various distances, can be taken from the following table ... 

A standard threshold shift (STS) is a change in hearing threshold, relative to an employee s baseline audiogram (hearing test), averaging 10 decibels (dB) or more at 2000, 3000, and 4000 hertz (Hz) in one or both ears. If an employee s audiogram reveals that a work-related STS has occurred in one or both ears, and the total hearing level is 25 decibels or more above audiometric zero in the same ears as the STS, the case is recordable. 

Results from Otto s replication study also indicated that hearing threshold, a reflection of peripheral auditory system function, increased directly with lead levels. Although hearing threshold did not vary with blood lead level in the five-year follow-up study (Otto et al, 1985), this finding bears further investigation in view of other reports suggesting impaired auditory processing in lead-exposed children (de la Burde and Choate, 1975 Needleman et al, 1979) and in fact, additional new analyses reported elsewhere in this volume by Otto and by Schwartz and Otto (1987) provide further evidence for lead-induced auditory system effects. 

Results of the replication study in the younger children indicated a problem in the BAEP methodology employed in both North Carolina studies. Click intensity (based on audiometric screening) was adjusted so that all subjects would hear clicks at the same sensation level. While this procedure is used widely in psychophysical research, it is not advisable in BAEP testing because wave I latency is very sensitive to click intensity. That is, wave I latency varies inversely with click intensity, an effect that does not necessarily correlate directly with hearing threshold. 

Figure 7 Scatter plots, regression lines, and 95% confidence limits of (a) 2 kHz hearing thresholds in children aged 3 to 7, (b) BAEP wave V latencies in children aged 3 to 7, and (c) in children aged 6 to 12 as functions of maximal PbB levels. Linear relationships of PbB and hearing threshold in younger children and wave V latency in older children were found, while a curvilinear relationship of PbB and wave V latency was observed in younger children...

For airborne sound, the reference pressure is 2 X 10" Pa (29 X psi), which is nominally the human threshold of hearing at 1000 Hz. The corresponding sound pressure level is 0 dB. Conversation is about 50 dB, ana a Jackhammer operator is subject to 100 dB. Extreme levels such as a jet engine at takeoff might produce 140 dB at a distance of 3 m, which is a pressure amplitude oi 200 Pa (29 X 10" psi). These examples demonstrate both the sensitivity and wide dynamic range of the human ear. 

For convenience, the usual measurement of sound is expressed in decibels (dB), and ratings go from threshold of hearing to threshold of pain (135 dB). Figure 34.11 illustrates the common noise criteria, which can be expressed in sound-pressure levels (SPL). The human ear can detect 1 dB but 6 dB represents a doubling of the SPL, although it would need a 10 dB increase to make it sound twice as loud. 

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